Methane is a powerful greenhouse gas and an important precursor of tropospheric ozone; both are key air pollutants and short-lived climate forcers (SLCFs). Several factors in addition to rising anthropogenic methane emissions have influenced the evolution of atmospheric methane from its pre-industrial level of ~700?ppb to its present-day value of over 1900?ppb. The Intergovernmental Panel on Climate Change's Sixth Assessment Report (Szopa et al., 2021) assessed how changes in emissions of nitrogen oxide (NO_x), carbon monoxide (CO), and non-methane volatile organic compounds (NMVOCs) have contributed to historical changes in methane via their impacts on OH, the main sink for methane. A range of modelling studies have explored these indirect impacts on methane (e.g. Shindell et al., 2005, 2009; Stevenson et al., 2013; Thornhill et al., 2021). For example, the Atmospheric Chemistry and Climate Model Intercomparison Project found that 1850-2000 increases in anthropogenic NO_x emissions had reduced year 2000 methane levels by 955?ppb, whilst growing emissions of CO and NMVOCs had increased methane by 150 and 59?ppb respectively (Table 7 of Stevenson et al., 2013). These results have quite large uncertainties (at least ±10?%, based on the model range in Stevenson et al., 2013) but indicate that non-methane (especially NO_x) emissions have had very significant impacts on methane. Better understanding of what controls methane and its evolution is vital for progress towards the Paris Climate Agreement target that seeks to limit warming to 1.5?^°C above pre-industrial levels.

Following the onset of the COVID-19 pandemic in early 2020, the trace gas composition of the global atmosphere changed substantially. Atmospheric NO_x levels decreased as surface and aviation NO_x emissions fell (Bauwens et al., 2020; Cooper et al., 2022), whilst the measured growth rate of methane (CH₄) rose sharply in 2020 (Laughner et al., 2021). The observed NO_x changes are clearly linked to falls in emissions resulting from lockdowns, but the driver of the methane increases is less clear, with some studies discussing causes related to decreases in OH (e.g. Weber et al., 2020; Laughner et al., 2021), whereas others suggest rises in sources (e.g. Feng et al., 2022). Methane, NO_x, CO, and NMVOCs are linked via the oxidising capacity of the atmosphere, specifically by the abundance of the hydroxyl (OH) radical. The response of global atmospheric chemistry to the large lockdown perturbation since early 2020 provides an opportunity to explore the sensitivity of the NO_x-CO-NMVOC-OH-CH₄ system and to compare models and observations. Here, we use model-derived sensitivities of global methane to NO_x, CO, and NMVOC emissions as well as the estimated changes in anthropogenic emissions of these species related to the COVID-19 lockdowns to calculate estimated impacts from lockdown emission changes on the growth rate of global methane, and we compare this to observations.

The possibility of points-of-no-return in the climate system has been discussed for two decades^1,2,3. A point-of-no-return can be seen as a threshold which, once surpassed, fundamentally changes the dynamics of the climate system. For example, by triggering irreversible processes like thawing of the permafrost, drying of the rainforests, or acidification of surface waters. Recently, Lenton et al.⁴ summarized the global situation and warned that thresholds may be closer in time than commonly believed.

The purpose of this article is to report that we have identified a point-of-no-return in our climate model ESCIMO - and that it is already behind us. ESCIMO is a "reduced complexity earth system" climate model⁵ which we run from 1850 to 2500. In ESCIMO the global temperature keeps rising to 2500 and beyond, irrespective of how fast humanity cuts the emissions of man-made greenhouse gas (GHG) emissions. The reason is a cycle of self-sustained thawing of the permafrost (caused by methane release), lower surface albedo (caused by melting ice and snow) and higher atmospheric humidity (caused by higher temperatures). This cycle appears to be triggered by global warming of a mere?+?0.5 °C above the pre-industrial level.

Despite its high abundance in the Earth's crust, iron concentrations are very low in the ocean. This is due to the low solubility of iron in the oxic and slightly alkaline seawater of today's oceans, uptake by microorganisms and limited input from external sources. Consequently, the bioavailability of this element is very low in many oceanic regions [1]. Iron is essential for cell metabolism, especially for electron transport. Photosynthesis, respiration, and nitrogen fixation require high cellular concentrations of iron [2].

Microorganisms, particularly bacteria, have evolved several different iron uptake mechanisms [3], [4]. Inorganic iron can be acquired either in its reduced ferrous (Fe²⁺) [5], or oxidized ferric (Fe³⁺) state [6]. However, in well-oxygenated seawater, the free ion concentrations of both forms are low, and bacteria have developed alternative strategies to access the organically complexed pool. The synthesis and uptake of siderophores, strong chelators of ferric iron, is one example [7], but bacteria can also acquire iron from free heme (a prosthetic group of porphyrin containing an atom of iron) or heme-containing proteins using both direct uptake and hemophores [8]. Such strategies have been identified in diverse marine bacteria [9].

Many recent studies have highlighted the strategies used by diatoms and cyanobacteria to minimize their iron demand, such as reducing the expression of "expensive" iron genes in the photosystem I complex [10], [11] and nitrogenase [12], or else increasing flavodoxin production [13], [14]. Genomic analysis of Prochlorococcus clades isolated from iron-depleted oceanic regions showed that several genes encoding for iron-containing proteins were absent, thereby reducing the cellular iron quota [15].

The relation between genomic iron uptake strategies and oceanic environments has recently been investigated in cyanobacteria, the most abundant photosynthetic group of marine bacteria. Many Synechococcus genomes from strains isolated in the open ocean lack most known genes for iron stress, while genomes from strains isolated in coastal and upwelling areas contain many such genes, suggesting that maintaining multiple iron limitation compensation strategies is not a selective advantage in the open ocean [16]. Consistent with this, the light-harvesting gene isiA of Synechococcus has been proposed as a biomarker of HNLC regions [17]. However, heterotrophic bacteria also compete for iron at low concentrations, and can account for up to 50% of the total planktonic iron uptake [18]. Additionally, they may modify iron chemistry through the production of organic ligands and thereby regulating phytoplankton production [19]. Because metagenomes contain the gene content of the community of most abundant and therefore most successful microorganisms, they provide a complementary glimpse to organismal studies into the genetic basis of adaptation to the environment.

We set up a database of 2319 sequences, corresponding to 140 gene families and 10 different iron-related metabolic pathways, to explore the mechanisms involved in iron acquisition in the ocean. The caveats of using Blast Best Hits and evalue cutoffs have been discussed previously [20]. Here, we defined a stringent criterion to discard false positives: we only considered Reciprocal Best Hits with coverage and identity threshold empirically estimated from our database. We investigate the link between genomic iron metabolism strategies of microbial communities and iron concentrations among 54 worldwide distributed marine metagenomes (Figure 1). Iron concentrations at each location were taken from a biogeochemical model that incorporates information about the sources and cycling of iron in the ocean and were compared to independently acquired observed iron concentrations. We thus established the relationships between microbial communities' iron-related gene prevalence, taxonomic affiliation and iron concentrations in different marine habitats.

These largely unsupported presumptions distort risk assessments and discount the urgent need to develop a viable mitigation strategy. Due to political pressures, many critical scientific concerns are ignored or preemptively dismissed in international negotiations. As a result, the present and growing crisis and the level of effort and time that will be required to control and rebalance the climate are severely underestimated.

In conclusion, the paper outlines the key elements of a realistic policy approach that would augment current efforts to constrain dangerous warming by supplementing current mitigation approaches with climate cooling interventions.

In the early 1970s in the United Kingdom (UK) and elsewhere research into ‘alternative’ energy sources was growing partly as a consequence of emergent theories of global warming and the oil crisis. The latter arose out of the Arab producers of the Organization of Petroleum Exporting Countries (OPEC) putting in place an embargo on oil exports to the United States in October 1973 and threatening to cut back overall production 25% (Udall 1973). Any research into a new, untested and unorthodox energy technology that could displace traditional and polluting energy supply methods required at least some realistic start-up funding. Given the size of funding needed the most likely source for such untested technologies would be governmental. The new technology discussed in this case study was one method for extracting energy from sea waves: the Edinburgh Wave Power Project led by Professor Stephen Salter who was head of the Department of Mechanical Engineering at Edinburgh University (ERA 2021).

Given that our concerns for this book relate to the ethical issues that can arise in such situations a series of questions immediately present themselves: What are the immediate consequences of giving funding to new technologies that could undermine previous (traditional) ways of producing energy? What range of ‘interests’ are involved? What broader policies influence the research and the views of funders and government? Does it matter who funds the research? Is there too much opportunity for misconduct depending on how funds are disbursed? And does it matter who analyses the results? These are the kind of questions addressed in the following case study.

At this point I must declare an interest of my own. I am a supporter of alternative energy technologies that could reduce all forms of environmental pollution and, thereby, help minimize human-induced global warming. More specifically the origins of such a view lie in my experience with the Wave Power Project itself from 1974 to 1977. My wife was then a Personal Assistant (P.A.) to Stephen Salter and told me of her frustration in getting the main source of funding to deliver the correct funding in good time. She constantly had to phone the funder and was regularly offered unreasonable excuses for delays—the consequence of which were that the salaries for the young engineering researchers were rarely paid in time and money for materials always delayed. Such a tactic seemed problematic to me at the time until she explained the funding was disbursed by the UK Atomic Energy Authority—one of the ‘old’ polluting technologies that alternative energy projects were directly challenging. Perhaps this was a rationale for the tactic that could only be described as ‘undermining’?

For my college teaching post at the time I was tasked with delivering a series of lectures of a ‘general’ nature and thought something on alternative energy technologies would be of interest to my experienced mature students—who were all ‘older adults’ entering further and higher education for the first time. I invited both the Wave Power Project and the Atomic Energy Authority to come and give a talk about the merits of their approach. The Wave Power Project sent one of their researchers who gave an informed and stimulating talk about the balance in terms of cost and effectiveness of different energy sources and showed some photographs to illustrate their ongoing work. The Atomic Energy Authority sent no-one but delivered (at their considerable expense) boxes of hundreds of reprinted articles to be distributed freely for the benefit of my students. All the articles were in support of nuclear energy without providing a balanced perspective on the controversies surrounding nuclear energy. I cannot say I suffered a ‘confirmation bias’ prior to those experiences, but it certainly grew subsequently as my suspicions of at least some lack of fairness in funding were aroused.

These details, in relation to a specific research activity, illustrates that ‘who funds and how?’ raises a series of ethical questions along with whether a balanced assessment of alternative options—in any field of research—is vital to honest, reliable and trustworthy research. Trust in the funding source becomes part of transparency of practice that can help the public and policymakers decide on the best ways of proceeding with an innovative, and potentially challenging, technology.

Go to:

8.2. More Background and Context

Research into marine wave power really began with Salter’s experiments using a dynamically shaped float (the ‘Duck’) that linked via a spine to a series of other floating ‘Ducks’ which bobbed up and down in the waves. The Duck was a 300-tonne floating canister designed to drive a generator from the motion of bobbing up and down on waves like a duck. It is still regarded as the most efficient of any wave power system produced, converting up to 80% of the wave energy to electricity which was to be then cabled ashore. All the experiments were successful until 1982 when the work suddenly stopped. Salter (2016) provides a full discussion of both the key technical issues and the emergent governance problems.

The funding problem arose from the control of all renewable energy research—not just wave power—during the 1970s and 1980s coming from an organisation that was part of the United Kingdom Atomic Energy Authority. The Department of Energy’s research and development advisory council (ACORD) operated at long range from all the projects and its staff was recruited largely from the nuclear and the depletable energy industries. In other words, wave power research was funded and controlled by the regulators of the nuclear, coal and gas industries.

It is far easier to modify Earth's albedo than to determine whether it should be done or what the consequences might be of such an action. One serious concern is that such an action could be unilaterally undertaken by a small nation or smaller entity for its own benefit without international sanction and regardless of international consequences. Transparency in discussing this subject is critical. In the spirit of that transparency, Climate Intervention: Reflecting Sunlight to Cool Earth was based on peer-reviewed literature and the judgments of the authoring committee; no new research was done as part of this study and all data and information used are from entirely open sources. By helping to bring light to this topic area, this book will help leaders to be far more knowledgeable about the consequences of albedo modification approaches before they face a decision whether or not to use them.

Contributor(s): National Research Council; Division on Earth and Life Studies; Board on Atmospheric Sciences and Climate; Ocean Studies Board; Committee on Geoengineering Climate: Technical Evaluation and Discussion of Impacts

Increased greenhouse gas concentrations combined with reductions in aerosol pollution have led to rapid increases in human-induced effective radiative forcing, which has in turn led to atmosphere, land, cryosphere and ocean warming (Gulev et al., 2021). This in turn has led to an intensification of many weather and climate extremes, particularly more frequent and more intense hot extremes, and heavy precipitation across most regions of the world (Seneviratne et al., 2021). Given the speed of recent change, and the need for evidence-based decision-making, this Indicators of Global Climate Change (IGCC) update assembles the latest scientific understanding on the current state and evolution of the climate system and of human influence to support policymakers whilst the next Intergovernmental Panel on Climate Change (IPCC) assessment is under preparation. This first annual update is focused on indicators related to heating of the climate system, building from greenhouse gas emissions towards estimates of human-induced warming and the remaining carbon budget. In future years, this effort could be expanded to encompass other indicators, including global precipitation changes and related extremes.

We adopt the Global Carbon Budget ethos of a community-wide inclusive effort that synthesises work from across a large and diverse global scientific community in a timely fashion (Friedlingstein et al., 2022a). Like the Global Carbon Budget, this initiative arises from the international science community to establish a knowledge base to support policy debate and action to meet the Paris Agreement temperature goal.

This update complements other international efforts under the auspices of the Global Climate Observing System (GCOS) and the World Meteorological Organization (WMO). Annual state-of-the-climate reports are released by the WMO which use much of the same data analysed here for surface temperature and energy budget trends. The Bulletin of American Meteorological Society (BAMS) releases annual state-of-the-climate reports covering many essential variables including temperature and greenhouse gas concentrations. However, these reports focus on statistics from the previous year and make slightly different choices over datasets and analysis compared to the IPCC (see Sect. 5). The Global Carbon Project publishes updated carbon dioxide datasets which are used directly in this report. There is no similarly structured activity that provides all the necessary datasets to update the assessment of human influence on global surface temperature annually.

The update is based on methodologies for key climate indicators assessed by the IPCC Sixth Assessment Report (AR6) of the physical science basis of climate change (Working Group One (WGI) report; IPCC, 2021a) as well as Chap. 2 of the WGIII report (Dhakal et al., 2022) and is aligned with the efforts initiated in AR6 to implement FAIR (Findable, Accessible, Interoperable, Reusable) principles for reproducibility and reusability (Pirani et al., 2022; Iturbide et al., 2022). IPCC reports make a much wider assessment of the science and methodologies - we do not attempt to reproduce the comprehensive nature of these IPCC assessments here.

The IPCC Special Report on Global Warming of 1.5?^°C (SR1.5), published in 2018, provided an assessment of the level of human-induced warming and cumulative emissions to date (Allen et al., 2018) and the remaining carbon budget (Rogelj et al., 2018) to support the evidence base on how the world is progressing in terms of meeting aspects of the Paris Agreement. The AR6 WGI Report, published in 2021, assessed past, current and future changes of these and other key global climate indicators, as well as undertaking an assessment of the Earth's energy budget. It also updated its approach for estimating human-induced warming and global warming level. In AR6 WGI and here, reaching a level of global warming is defined as the global surface temperature change, averaged over a 20-year period, exceeding a particular level of global warming, for example, 1.5?^°C global warming. Given the current rates of change and the likelihood of reaching 1.5?^°C of global warming in the first half of the 2030s (Lee et al., 2021, 2023; Riahi et al., 2022), it is important to have robust, trusted and also timely climate indicators in the public domain to form an evidence base for effective science-based decision-making.

When making their assessments, authors of IPCC reports assess published literature but also apply established published analysis methods to assessed datasets, such as the dataset produced by the latest climate model intercomparison projects (Lee et al., 2021). The authors combine and analyse both model and observational data as part of their expert assessment, making assessments of the trustworthiness and error characteristics of different datasets. It is this synthetic analysis by IPCC authors that derives the estimates of key climate indicators. Wherever possible, these same assessed methodological approaches are implemented here to provide the updates with variations clearly flagged and documented. The same approach, using the same datasets (updated by 2 years) and methods as employed in WGI, was used in the AR6 Synthesis Report (2023) (AR6 SYR; Lee et al., 2023) to provide an updated assessment of the latest atmospheric well-mixed greenhouse gas concentrations (up to 2021) and decadal average change in global surface temperature (+1.15?^°C [1.00-1.25?^°C] in 2013-2022 for global surface temperature). However, the assessment of human-induced warming was not updated (and therefore only covers warming up to the decade 2010-2019), nor was the remaining carbon budget updated, so the related information in the AR6 SYR report remained based on data up to the end of 2019.

The indicators in this first annual update give important insights into the magnitude and the pace of global warming. This paper provides the basis for a dashboard of climate indicators grounded in IPCC methodologies and directly traceable to reports published as part of the AR6 cycle. We employ datasets that can be updated on a regular basis between the publication of IPCC reports. Note that there are other similar initiatives underway to update other AR6 cycle products; for example, the evolution of the WGI Interactive Atlas (Gutiérrez et al., 2021) is being developed under the Copernicus Climate Change Service (C3S) and has potential connections and synergies with this initiative that will be explored in the future.

Our longer-term ambition is to rigorously track both climate system change and methodological improvements between IPCC report cycles, thereby building consistency and awareness. An example of why tracking methodological change is important was the updated estimate for historic warming (the increase in global surface temperature from 1850-1900 to 1986-2005). This was 0.08 [-0.01 to 0.12]?^°C higher in the AR6 than in the fifth assessment report (AR5) and SR1.5. Datasets and methods of evaluating global temperature changes altered between the AR5 and AR6, leading to a small shift in the historical temperature. This was reflected in changes between AR5 and AR6, whereas SR1.5 mostly relied on methodologies from AR5 (see AR6 WGI Cross Chap. Box 2.3, Gulev et al., 2021). Annual updates provide indications of possible future methodological shifts that subsequent IPCC reports may make as science advances and can detail their impact on perceived trends.

The update is organised as follows: emissions (Sect. 2) and greenhouse gas (GHG) concentrations (Sect. 3) are used to develop updated estimates of effective radiative forcing (Sect. 4). Observations of global surface temperature change (Sect. 5) and Earth's energy imbalance (Sect. 6) are key global indicators of a warming world. The global surface temperature change is formally attributed to human activity in Sect. 7, which tracks human-induced warming. Section 8 updates the remaining carbon budget to policy-relevant temperature thresholds. Section 9 gives an example of global-scale indicators associated with climate extremes of maximum land surface temperatures.

An important purpose of the exercise is to make these indicators widely available and understood. Plans for a web dashboard are discussed in Sect. 10 and code and data availability in Sect. 11, and conclusions are presented in Sect. 12. Data are available at https://doi.org/10.5281/zenodo.8000192 (Smith et al., 2023a).

Increased greenhouse gas concentrations combined with reductions in aerosol pollution have led to rapid increases in human-induced effective radiative forcing, which has in turn led to atmosphere, land, cryosphere and ocean warming (Gulev et al., 2021). This in turn has led to an intensification of many weather and climate extremes, particularly more frequent and more intense hot extremes, and heavy precipitation across most regions of the world (Seneviratne et al., 2021). Given the speed of recent change, and the need for evidence-based decision-making, this Indicators of Global Climate Change (IGCC) update assembles the latest scientific understanding on the current state and evolution of the climate system and of human influence to support policymakers whilst the next Intergovernmental Panel on Climate Change (IPCC) assessment is under preparation. This first annual update is focused on indicators related to heating of the climate system, building from greenhouse gas emissions towards estimates of human-induced warming and the remaining carbon budget. In future years, this effort could be expanded to encompass other indicators, including global precipitation changes and related extremes.

We adopt the Global Carbon Budget ethos of a community-wide inclusive effort that synthesises work from across a large and diverse global scientific community in a timely fashion (Friedlingstein et al., 2022a). Like the Global Carbon Budget, this initiative arises from the international science community to establish a knowledge base to support policy debate and action to meet the Paris Agreement temperature goal.

This update complements other international efforts under the auspices of the Global Climate Observing System (GCOS) and the World Meteorological Organization (WMO). Annual state-of-the-climate reports are released by the WMO which use much of the same data analysed here for surface temperature and energy budget trends. The Bulletin of American Meteorological Society (BAMS) releases annual state-of-the-climate reports covering many essential variables including temperature and greenhouse gas concentrations. However, these reports focus on statistics from the previous year and make slightly different choices over datasets and analysis compared to the IPCC (see Sect. 5). The Global Carbon Project publishes updated carbon dioxide datasets which are used directly in this report. There is no similarly structured activity that provides all the necessary datasets to update the assessment of human influence on global surface temperature annually.

The update is based on methodologies for key climate indicators assessed by the IPCC Sixth Assessment Report (AR6) of the physical science basis of climate change (Working Group One (WGI) report; IPCC, 2021a) as well as Chap. 2 of the WGIII report (Dhakal et al., 2022) and is aligned with the efforts initiated in AR6 to implement FAIR (Findable, Accessible, Interoperable, Reusable) principles for reproducibility and reusability (Pirani et al., 2022; Iturbide et al., 2022). IPCC reports make a much wider assessment of the science and methodologies - we do not attempt to reproduce the comprehensive nature of these IPCC assessments here.

The IPCC Special Report on Global Warming of 1.5?^°C (SR1.5), published in 2018, provided an assessment of the level of human-induced warming and cumulative emissions to date (Allen et al., 2018) and the remaining carbon budget (Rogelj et al., 2018) to support the evidence base on how the world is progressing in terms of meeting aspects of the Paris Agreement. The AR6 WGI Report, published in 2021, assessed past, current and future changes of these and other key global climate indicators, as well as undertaking an assessment of the Earth's energy budget. It also updated its approach for estimating human-induced warming and global warming level. In AR6 WGI and here, reaching a level of global warming is defined as the global surface temperature change, averaged over a 20-year period, exceeding a particular level of global warming, for example, 1.5?^°C global warming. Given the current rates of change and the likelihood of reaching 1.5?^°C of global warming in the first half of the 2030s (Lee et al., 2021, 2023; Riahi et al., 2022), it is important to have robust, trusted and also timely climate indicators in the public domain to form an evidence base for effective science-based decision-making.

When making their assessments, authors of IPCC reports assess published literature but also apply established published analysis methods to assessed datasets, such as the dataset produced by the latest climate model intercomparison projects (Lee et al., 2021). The authors combine and analyse both model and observational data as part of their expert assessment, making assessments of the trustworthiness and error characteristics of different datasets. It is this synthetic analysis by IPCC authors that derives the estimates of key climate indicators. Wherever possible, these same assessed methodological approaches are implemented here to provide the updates with variations clearly flagged and documented. The same approach, using the same datasets (updated by 2 years) and methods as employed in WGI, was used in the AR6 Synthesis Report (2023) (AR6 SYR; Lee et al., 2023) to provide an updated assessment of the latest atmospheric well-mixed greenhouse gas concentrations (up to 2021) and decadal average change in global surface temperature (+1.15?^°C [1.00-1.25?^°C] in 2013-2022 for global surface temperature). However, the assessment of human-induced warming was not updated (and therefore only covers warming up to the decade 2010-2019), nor was the remaining carbon budget updated, so the related information in the AR6 SYR report remained based on data up to the end of 2019.

The indicators in this first annual update give important insights into the magnitude and the pace of global warming. This paper provides the basis for a dashboard of climate indicators grounded in IPCC methodologies and directly traceable to reports published as part of the AR6 cycle. We employ datasets that can be updated on a regular basis between the publication of IPCC reports. Note that there are other similar initiatives underway to update other AR6 cycle products; for example, the evolution of the WGI Interactive Atlas (Gutiérrez et al., 2021) is being developed under the Copernicus Climate Change Service (C3S) and has potential connections and synergies with this initiative that will be explored in the future.

Our longer-term ambition is to rigorously track both climate system change and methodological improvements between IPCC report cycles, thereby building consistency and awareness. An example of why tracking methodological change is important was the updated estimate for historic warming (the increase in global surface temperature from 1850-1900 to 1986-2005). This was 0.08 [-0.01 to 0.12]?^°C higher in the AR6 than in the fifth assessment report (AR5) and SR1.5. Datasets and methods of evaluating global temperature changes altered between the AR5 and AR6, leading to a small shift in the historical temperature. This was reflected in changes between AR5 and AR6, whereas SR1.5 mostly relied on methodologies from AR5 (see AR6 WGI Cross Chap. Box 2.3, Gulev et al., 2021). Annual updates provide indications of possible future methodological shifts that subsequent IPCC reports may make as science advances and can detail their impact on perceived trends.

The update is organised as follows: emissions (Sect. 2) and greenhouse gas (GHG) concentrations (Sect. 3) are used to develop updated estimates of effective radiative forcing (Sect. 4). Observations of global surface temperature change (Sect. 5) and Earth's energy imbalance (Sect. 6) are key global indicators of a warming world. The global surface temperature change is formally attributed to human activity in Sect. 7, which tracks human-induced warming. Section 8 updates the remaining carbon budget to policy-relevant temperature thresholds. Section 9 gives an example of global-scale indicators associated with climate extremes of maximum land surface temperatures.

An important purpose of the exercise is to make these indicators widely available and understood. Plans for a web dashboard are discussed in Sect. 10 and code and data availability in Sect. 11, and conclusions are presented in Sect. 12. Data are available at https://doi.org/10.5281/zenodo.8000192 (Smith et al., 2023a).

Datasets of global temperatures vary a little depending on method, but two of the most significant are Berkeley Earth which put 2023 at 1.54°C above the pre-industrial (1850-1900) level, and Copernicus/ECMWF at 1.48°C.

Berkeley said that “a single year exceeding 1.5°C is a stark warning sign of how close the overall climate system has come to exceeding this Paris Agreement goal. With greenhouse gas emissions continuing to set record highs, it is likely that climate will regularly exceed 1.5°C in the next decade.”

2023 was notable for:

Global average warming hitting the 1.5°C mark, and new monthly records for global temperature every month from June to December. The October to December period was 1.74°C.

New national record high annual averages for an estimated 77 countries.

The first year that global average ocean surface temperatures exceeded 1°C, with once-in-a-century levels of warmth in the North Atlantic.

Two days in November when global average temperature, for the first time ever, reached 2°C above the pre-industrial levels.

Catastrophic flooding from Greece to Beijing to Vermont, and earlier in the year major flooding in New Zealand associated with a rain bomb and then cyclone Gabrielle.

Severe wildfires in Europe, Russia, Maui and North America; fires in Canada burned 18.5 million hectares of land.

The 2023 extremes were a shock. Prof. Katharine Hayhoe told the Guardian that: “We have strongly suspected for a while that our projections are underestimating extremes, a suspicion that recent extremes have proven likely to be true… We are truly in uncharted territory in terms of the history of human civilisation on this planet.”

Explanations for 2023 are incomplete, but warming is accelerating and 2024 is likely to be hotter

What happened in 2023 was not what scientists’ models anticipated at the beginning of the year and fell well outside the confidence intervals of any of the estimates. Carbon Brief says that “while there are a number of factors that researchers have proposed to explain 2023’s exceptional warmth, scientists still lack a clear explanation for why global temperatures were so unexpectedly high… researchers are just starting to disentangle the causes of the unexpected extreme global heat the world experienced in 2023”.

One person who has a clear view is the former NASA climate chief James Hansen who says that “the 1.5 degree limit is deader than a doornail” and warns that warming will accelerate to 1.7°C by 2030 and “2°C will be reached by the late 2030s”.

Bellouin, N., O. Boucher, J. Haywood, C. Johnson, A. Jones, J. Rae, and S. Woodward, 2007: Improved representation of aerosols for HadGEM2. Hadley Centre Tech. Note 73, 42 pp. [Available online at http://www.metoffice.gov.uk/media/pdf/8/f/HCTN_73.pdf.]

Still, the fact that more research is needed is true for all geoengineering techniques. So even though the scientists within our group focus most closely on stratospheric aerosol injection, we believe that MCB research is valuable.

In recent years, discussions of climate change have shown growing interest in “tipping elements” of the Earth system, also imprecisely referred to as “tipping points.” This refers to Earth system components like the tropical rainforests of Amazonia or the Greenland and Antarctic ice sheets which may exhibit large-scale, long-term changes upon reaching critical global warming, greenhouse gas, or other thresholds. Once such thresholds are passed, some tipping elements could in turn produce additional greenhouse gas emissions or change the Earth's energy balance in ways that moderately reinforce warming. In this review, we summarize the current state of scientific knowledge on 10 systems that some have referred to as potential tipping elements of the climate system. We describe the mechanisms important to each system, highlight the response of these systems to climate change so far, and explain the dynamics of potential future changes that these systems could undergo in response to further climate change. Overall, even considering remaining scientific uncertainties, tipping elements will influence future climate change and may involve major impacts on ecosystems, climate patterns, and the carbon cycle starting later this century. Aggressive efforts to stabilize climate change could significantly reduce such impacts.

Atomic chlorine (Cl) is a very reactive radical, known to destroy stratospheric ozone (O₃) through catalytic cycles^1,2. In the lower troposphere, it can initiate the oxidation of volatile organic compounds (VOCs), increase the levels of conventional radicals (OH, HO₂ and RO₂), and produce O₃ and secondary organic aerosols (SOA)^3,4,5,6,7 which are air pollutants and also alter the Earth's radiation budget and climate. Cl reacts rapidly with methane, the most abundant hydrocarbon and the second-most important greenhouse gas emitted into the atmosphere^8,9. Molecular chlorine (Cl₂) is an important Cl precursor. It can be photolyzed quickly to release two Cl atoms during the daytime, and its production through heterogeneous reactions is a key step in the O₃ destruction over Antarctica during austral spring¹⁰. Previously, Cl₂ has been measured in the lower troposphere in locations such as at the Arctic surface^11,12, the marine boundary layer^13,14,15, and continental sites^16,17. Cl₂ was found to typically peak during nighttime, but elevated levels (17-450 pptv) have also been observed during daytime^{6,11,12,18,19,20,21}. The daytime occurrence of Cl₂ is of great importance as it may have a profound impact on atmospheric photochemistry and oxidation capacity^6,19. Such observations also reveal the existence of a significant Cl₂ source that compensates or even overcomes its fast photolytic loss. Although daytime Cl₂ can be emitted from various sources, such as from coal combustion¹⁶ or water treatment facilities¹⁵, it can also be produced through some photochemical processes^11,18,19,20. However, the underlying photochemical mechanisms remain uncertain. As a result, current state-of-the-art air quality models do not typically implement such chemistry, and therefore, cannot reproduce the observed high daytime Cl₂ levels in polluted regions^7,22. Consequently, the impact of Cl₂ on atmospheric oxidation is currently underestimated. Finally, as there were just only handfuls of Cl₂ observations outside the polar regions to date^{13,14,15,16,17,19,20,21}, our ability to assess the Cl₂ and Cl impact in different parts of the world is still very limited.

In this work, we report atmospheric observations of Cl₂ and other chemicals obtained at a polluted coastal site in southern China during autumn 2018. The Cl₂ concentrations are much higher than those previously measured outside polar regions. We show that previously proposed Cl₂ production mechanisms cannot account for the large Cl₂ daytime source at this site and this source positively correlates with solar radiation, particulate nitrate, and particulate surface area. Laboratory experiments show that illuminating solution of sodium chloride and nitrate under acidic condition and ambient particulates can produce a large amount of Cl₂, which can explain a large fraction of the observed Cl₂ at our site. We propose that nitrate photolysis at high aerosol acidity is an important pathway for activating inert chloride to produce photoliable Cl₂ during daytime in the polluted troposphere. Model calculations demonstrate significant enhancement of conventional radical levels, hydrocarbon oxidation, and ozone production by the high levels of Cl₂ at the study site. We suggest that the same Cl₂ production pathway may exist in other places of the world and call for more attention to the role of Cl₂ in tropospheric chemistry and air quality of polluted regions.

In the past 150 years, Earth's climate has been warming at a rate that is unprecedented in at least the last 2000 years¹. This human-caused warming has caused widespread adverse impacts and related losses and damages to nature and people, which will continue in the future as global climate continues to warm². Detecting changes in the rate of warming is crucial for informed decision-making in international climate negotiations, with the aim of limiting global warming to specific levels. However, it remains a significant challenge to detect such changes due to the substantial internal variability of the climate system on a decadal scale (e.g., ref. ³). In this paper, we address this challenge by examining the global heat accumulation rate across the entire climate system, including the ocean, atmosphere, cryosphere, and land. By focusing on this integrated view, rather than solely relying on changes in global mean surface temperature, we can mitigate the impact of variability and gain a more comprehensive understanding^4,5.

Global heat accumulation in the climate system, resulting from the current positive Earth's Energy Imbalance (EEI) at the top of the atmosphere, is primarily dominated by changes in Global Ocean Heat Content (GOHC)⁴. GOHC changes account for approximately 90% of the total heat increase in the past fifty years, while land heating, ice melting, and atmospheric warming contribute around 5%, 3%, and 1% respectively^6,7,8. Several studies have indicated an increase in the global heat accumulation rate in recent decades, with values rising from 0.50 [0.32 to 0.69] W/m² during the period 1971-2006 to 0.79 [0.52 to 1.06] W/m² (90% confidence interval) for the period 2006-2018 (ref.^{4,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22} and Fig. 1). Some studies have even suggested a potential doubling of EEI in the last decade compared to the previous one^6,17.

years1

. Tis human-caused warming has caused widespread adverse impacts and related losses and damages to

nature and people, which will continue in the future as global climate continues to warm2

. Detecting changes in

the rate of warming is crucial for informed decision-making in international climate negotiations, with the aim

of limiting global warming to specifc levels. However, it remains a signifcant challenge to detect such changes

due to the substantial internal variability of the climate system on a decadal scale (e.g., ref. 3

). In this paper, we

address this challenge by examining the global heat accumulation rate across the entire climate system, including

the ocean, atmosphere, cryosphere, and land. By focusing on this integrated view, rather than solely relying on

changes in global mean surface temperature, we can mitigate the impact of variability and gain a more comprehensive understanding4,5

.

Global heat accumulation in the climate system, resulting from the current positive Earth’s Energy Imbalance

(EEI) at the top of the atmosphere, is primarily dominated by changes in Global Ocean Heat Content (GOHC)4

.

GOHC changes account for approximately 90% of the total heat increase in the past ffy years, while land heating, ice melting, and atmospheric warming contribute around 5%, 3%, and 1% respectively6–8

. Several studies

have indicated an increase in the global heat accumulation rate in recent decades, with values rising from 0.50

[0.32 to 0.69] W/m2

during the period 1971–2006 to 0.79 [0.52 to 1.06] W/m2

(90% confdence interval) for the

period 2006–2018 (ref.4,6–22 and Fig. 1). Some studies have even suggested a potential doubling of EEI in the last

decade compared to the previous one6,17.

Despite this suggestive body of evidence, no study has conducted an analysis of heat accumulation acceleration since 1960 to date. While the results presented in Fig. 1 provide insights, they represent accumulation rates

computed over varying time spans, with higher rates calculated over decadal periods and lower rates calculated

over multi-decadal periods. Tis variation in time spans makes it challenging to make defnitive and quantifed

statements about acceleration. Additionally, the use of diverse datasets and methodologies can signifcantly

impact the calculated accumulation rates. Te only notable climate variable where acceleration has previously

been detected in past decades is Global Mean Sea Level (GMSL)23–31. Tis GMSL acceleration has been attributed

to factors such as increasing GOHC leading to thermal expansion of seawater, declining land water storage, or

increasing land ice melt25–27,31,32.

In this paper, we present the frst observation-based quantifcation of the acceleration of Earth’s system heating. Our study adopts a systematic approach, incorporating multiple datasets and employing various methods.

We estimate the rate of change and acceleration of Earth’s heat content using a collection of GOHC time series

Gulf Stream volume transport through Florida Straits declined by 1.2 ± 1.0 Sv during the past 40 years (95% credible interval)

We find a weakening trend in the Gulf Stream by applying Bayesian methods to synthesize cable, in situ, and satellite data sets congruently

Plain Language Summary

The Gulf Stream is a major ocean current located off the East Coast of the United States. It carries a tremendous amount of seawater and along with it heat, carbon, and other ocean constituents. Because of this, the Gulf Stream plays an important role in weather and climate, influencing phenomena as seemingly unrelated as sea level along coastal Florida and temperature and precipitation over continental Europe. Given how important this ocean current is to science and society, scientists have tried to determine whether the Gulf Stream has undergone significant changes under global warming, but so far, they have not reached a firm conclusion. Here we report our effort to synthesize available Gulf Stream observations from the Florida Straits near Miami, and to assess whether and how the Gulf Stream transport there has changed since 1982. We conclude with a high degree of confidence that Gulf Stream transport has indeed slowed by about 4% in the past 40 years, the first conclusive, unambiguous observational evidence that this ocean current has undergone significant change in the recent past. Future studies should try to identify the cause of this change.

The stratosphere (see NOAA image below) is a layer of the Earth’s atmosphere that ranges between 7 to 31 miles above the ground between the Troposphere and the Mesophere. The stratosphere is an ideal target for atmospheric geoengineering because it is relatively isolated from human populations, is accessible by planes (and other transport/delivery methods), and doesn’t have weather such as rain that would cause aerosol spray particles to fall quickly to the ground

Other analyses are broadly consistent:

The International Energy Agency finds that stated national policies will result in oil and gas production in 2050 as high as 2020; with coal halved.

The OECD finds that a world economy four times larger than today is projected to need 80% more energy in 2050; and without new policy action and the global energy mix in 2050 will not differ significantly from today.

The intentions of the world’s five largest fossil fuel producers are clear — and civilisation-threatening — as reported by the UN:

In China, oil production is projected to be flat to 2050, but gas will increase more than 60 percent from 2020 to 2050, while coal use will remain high till 2030 then decline sharply.

In the United States, oil production will grow and then remain at record levels to 2050, and gas is projected to continuously and significantly increase to 2050; whilst coal will drop by half.

Projections for Russia are available only to 2035, with coal and gas production projected to increase significantly, while oil remains flat.

In Saudi Arabia, oil production is projected to grow by 26 to 47 percent by 2050, with gas up 40 percent between 2019 and 2050. Together they make up half of the Saudi economy.

And in Australia, one of the world’s top two liquified natural gas and coal exporters, gas production is projected to stay above the current level for the next 15 years, with coal remaining high over the same period, above 450 million metric tons annually.

We are heading towards 3–4°C.

This outlook suggests Earth is heading towards 3°C of warming and perhaps a good deal more, because current climate models which project warming of around 2.7°C do not adequately account for all the system-level reinforcing feedbacks.

In 2021, the pre-eminent UK international affairs think-tank Chatham House said a “plausible worst-case scenario” is 3.5°C or more, which could be an underestimate if tipping points are reached sooner than the orthodox science suggests. This now seems to be the reality.

A clear majority of scientists expected warming of more than 3°C, and 82% expected to see catastrophic impacts of climate change in their lifetime, according to a 2021 survey by the journal Nature.

Questions about the size of the aerosol forcing, and the related issue of how sensitive the climate is to changes in greenhouse gases, remain an issue of scientific contention.

New climate history research published in December 2023, based on a study of the last 66 million years, concluded that global temperature may be more sensitive to CO2 levels than current models estimate. It showed that the last time CO2 levels were as high as today was around 14 million year ago, which is longer than previous estimates, and that climate sensitivity — the amount of warming resulting from a doubling of atmospheric CO2 — may be between between 5°C and 8°C, compared to the IPCC orthodoxy of 1.5–4.5°C.

The level of greenhouse gases is currently around 560 parts per million, double the pre-industrial level. Some of those gases such as methane are short-lived so this level of forcing is not written in stone, but nevertheless if Hansen et al. are right that a doubling may lead to around 4–5°C of warming, then another 30 years of high emissions means humans will have created an increasingly unliveable planet.

Has the impact of aerosols been widely understood? In what the New York Times described as “an eye-opening Nature commentary”, Geeta Persad and her colleagues wrote in late 2022 that “overall, vast emissions of aerosols since the start of the industrial age have had a profound cooling effect” and that without them “the global warming we see today would be 30 to 50 percent greater”, warning that “the impacts of aerosols on climate risk are often ignored”.

In 2018, a group of eminent scientists explored the potential — once warming had exceeded the 1.5–2°C range — for self-reinforcing positive feedbacks in major elements of the climate system to push passed a planetary threshold that would prevent temperature stabilisation, and drive the system to a “Hothouse Earth”. They warned that “we are in a climate emergency… this is an existential threat to civilisation”.

The 2023 State of the Climate Report: Entering uncharted territory warned of: “potential collapse of natural and socioeconomic systems in such a world [of 2.6°C warming] where we will face unbearable heat, frequent extreme weather events, food and fresh water shortages, rising seas, more emerging diseases, and increased social unrest and geopolitical conflict.”

Whatever the words, the understanding is widely shared that contemporary nations and societies, and likely the global social system, are heading towards collapse. “If we carry on the way we are going now, I can’t see this civilisation lasting to the end of this century”, says Professor Tim Lenton. The US Defence Secretary Lloyd Austin III calls the risks “existential”.

Opening the Innovation Zero Congress in London in May 2023, Potsdam Institute Director Prof. Johan Rockstrom described the path we are on:

“2.5°C global mean surface temperature rise is a disaster. It’s something that humanity has absolutely no evidence that we can cope with… [There] would be a 10-metre sea-level rise. There would be a collapse of all the big biomes on planet Earth – the rainforest, many of the temperate forests – abrupt thawing of permafrost, we will have complete collapse of marine biology… Over one-third of the planet around the equatorial regions will be uninhabitable because you will pass the threshold of health, which is around 30°C. It’s only in some parts of the Sahara Desert today that has that kind of average temperature.”

Chatham House’s Climate Risk Assessment 2021 concludes that by 2050 global food demand would be 50% higher, but crop yields may drop by 30%. As desertification spreads across the dry sub-tropics, and one-third of the planet experiences unprecedented heat, it is not difficult to see why they concluded that cascading climate impacts will “drive political instability and greater national insecurity, and fuel regional and international conflict”.

What is worse is the setback to climate action posed by current conflicts and military posturing in Europe, the Middle East and east Asia, which are huge political distractions from dealing with the greatest threat to humanity, and all of which have the potential to spread more widely.

To maintain military flexibility, the US insisted in 1997 that direct military carbon emissions be excluded from international carbon accounting. Those emissions, around 5 percent of the total global, are far less than the indirect emissions from conflict, as recent estimates here and here indicate.

Projections show that by 2100 the expansion of the Sahara due to desertification will embrace Israel/Palestine, as well as spreading across the Mediterranean into Spain, Italy, Greece and Turkey (see map).

The Australian Prime Minister has finally spoken out about the escalating climate threat whilst inspecting damage from the recent Queensland floods: “All of this is a reminder that the science told us that climate change would mean there would be more extreme weather events and they would be more intense. And unfortunately, we are seeing that play out with the number of events that we’re having to deal with right around Australia”.

Just so, except that in common with leaders globally, the Australian government continues to have its head stuck in the sand about the real risks climate change now represents. It refuses to release an intelligence assessment of climate-security risks, and has fumbled a domestic climate risk assessment.

As a result, the community remains ill-informed and unprepared for what is coming.

Here, we explore potential future trajectories of the Earth System by addressing the following questions.

Is there a planetary threshold in the trajectory of the Earth System that, if crossed, could prevent stabilization in a range of intermediate temperature rises?

Given our understanding of geophysical and biosphere feedbacks intrinsic to the Earth System, where might such a threshold be?

If a threshold is crossed, what are the implications, especially for the wellbeing of human societies?

What human actions could create a pathway that would steer the Earth System away from the potential threshold and toward the maintenance of interglacial-like conditions?

Addressing these questions requires a deep integration of knowledge from biogeophysical Earth System science with that from the social sciences and humanities on the development and functioning of human societies (5). Integrating the requisite knowledge can be difficult, especially in light of the formidable range of timescales involved. Increasingly, concepts from complex systems analysis provide a framework that unites the diverse fields of inquiry relevant to the Anthropocene (6). Earth System dynamics can be described, studied, and understood in terms of trajectories between alternate states separated by thresholds that are controlled by nonlinear processes, interactions, and feedbacks. Based on this framework, we argue that social and technological trends and decisions occurring over the next decade or two could significantly influence the trajectory of the Earth System for tens to hundreds of thousands of years and potentially lead to conditions that resemble planetary states that were last seen several millions of years ago, conditions that would be inhospitable to current human societies and to many other contemporary species.

Risk of a Hothouse Earth Pathway

Limit Cycles and Planetary Thresholds.

The trajectory of the Earth System through the Late Quaternary, particularly the Holocene, provides the context for exploring the human-driven changes of the Anthropocene and the future trajectories of the system (SI Appendix has more detail). Fig. 1 shows a simplified representation of complex Earth System dynamics, where the physical climate system is subjected to the effects of slow changes in Earth’s orbit and inclination. Over the Late Quaternary (past 1.2 million years), the system has remained bounded between glacial and interglacial extremes. Not every glacial–interglacial cycle of the past million years follows precisely the same trajectory (7), but the cycles follow the same overall pathway (a term that we use to refer to a family of broadly similar trajectories). The full glacial and interglacial states and the ca. 100,000-years oscillations between them in the Late Quaternary loosely constitute limit cycles (technically, the asymptotic dynamics of ice ages are best modeled as pullback attractors in a nonautonomous dynamical system). This limit cycle is shown in a schematic fashion in blue in Fig. 1, Lower Left using temperature and sea level as the axes. The Holocene is represented by the top of the limit cycle loop near the label A.

Fig. 1.

A schematic illustration of possible future pathways of the climate against the background of the typical glacial–interglacial cycles (Lower Left). The interglacial state of the Earth System is at the top of the glacial–interglacial cycle, while the glacial state is at the bottom. Sea level follows temperature change relatively slowly through thermal expansion and the melting of glaciers and ice caps. The horizontal line in the middle of the figure represents the preindustrial temperature level, and the current position of the Earth System is shown by the small sphere on the red line close to the divergence between the Stabilized Earth and Hothouse Earth pathways. The proposed planetary threshold at ~2 °C above the preindustrial level is also shown. The letters along the Stabilized Earth/Hothouse Earth pathways represent four time periods in Earth’s recent past that may give insights into positions along these pathways (SI Appendix): A, Mid-Holocene; B, Eemian; C, Mid-Pliocene; and D, Mid-Miocene. Their positions on the pathway are approximate only. Their temperature ranges relative to preindustrial are given in SI Appendix, Table S1.

The current position of the Earth System in the Anthropocene is shown in Fig. 1, Upper Right by the small ball on the pathway that leads away from the glacial–interglacial limit cycle. In Fig. 2, a stability landscape, the current position of the Earth System is represented by the globe at the end of the solid arrow in the deepening Anthropocene basin of attraction.

Fig. 2.

Stability landscape showing the pathway of the Earth System out of the Holocene and thus, out of the glacial–interglacial limit cycle to its present position in the hotter Anthropocene. The fork in the road in Fig. 1 is shown here as the two divergent pathways of the Earth System in the future (broken arrows). Currently, the Earth System is on a Hothouse Earth pathway driven by human emissions of greenhouse gases and biosphere degradation toward a planetary threshold at ~2 °C (horizontal broken line at 2 °C in Fig. 1), beyond which the system follows an essentially irreversible pathway driven by intrinsic biogeophysical feedbacks. The other pathway leads to Stabilized Earth, a pathway of Earth System stewardship guided by human-created feedbacks to a quasistable, human-maintained basin of attraction. “Stability” (vertical axis) is defined here as the inverse of the potential energy of the system. Systems in a highly stable state (deep valley) have low potential energy, and considerable energy is required to move them out of this stable state. Systems in an unstable state (top of a hill) have high potential energy, and they require only a little additional energy to push them off the hill and down toward a valley of lower potential energy.

The Anthropocene represents the beginning of a very rapid human-driven trajectory of the Earth System away from the glacial–interglacial limit cycle toward new, hotter climatic conditions and a profoundly different biosphere (2, 8, 9) (SI Appendix). The current position, at over 1 °C above a preindustrial baseline (10), is nearing the upper envelope of interglacial conditions over the past 1.2 million years (SI Appendix, Table S1). More importantly, the rapid trajectory of the climate system over the past half-century along with technological lock in and socioeconomic inertia in human systems commit the climate system to conditions beyond the envelope of past interglacial conditions. We, therefore, suggest that the Earth System may already have passed one “fork in the road” of potential pathways, a bifurcation (near A in Fig. 1) taking the Earth System out of the next glaciation cycle (11).

In the future, the Earth System could potentially follow many trajectories (12, 13), often represented by the large range of global temperature rises simulated by climate models (14). In most analyses, these trajectories are largely driven by the amount of greenhouse gases that human activities have already emitted and will continue to emit into the atmosphere over the rest of this century and beyond—with a presumed quasilinear relationship between cumulative carbon dioxide emissions and global temperature rise (14). However, here we suggest that biogeophysical feedback processes within the Earth System coupled with direct human degradation of the biosphere may play a more important role than normally assumed, limiting the range of potential future trajectories and potentially eliminating the possibility of the intermediate trajectories. We argue that there is a significant risk that these internal dynamics, especially strong nonlinearities in feedback processes, could become an important or perhaps, even dominant factor in steering the trajectory that the Earth System actually follows over coming centuries.