Department of Energy - Critical Review of Impacts of GHG Emissions on the US Climate July 2025

A Critical Review of Impacts of Greenhouse Gas Emissions on the U.S. Climate

Climate Working Group United States Department of Energy July 23, 2025

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A Critical Review of Impacts of Greenhouse Gas Emissions on the U.S. Climate Report to U.S. Energy Secretary Christopher Wright July 23, 2025

Climate Working Group: John Christy, Ph.D. Judith Curry, Ph.D. Steven Koonin, Ph.D. Ross McKitrick, Ph.D. Roy Spencer, Ph.D.

This report is being disseminated by the Department of Energy. As such, this document was prepared in compliance with Section 515 of the Treasury and General Government Appropriations Act for Fiscal Year 2001 (Public Law 106-554) and information quality guidelines issued by the Department of Energy.

Copyright © 2025 United States

Suggested citation: Climate Working Group (2025) A Critical Review of Impacts of Greenhouse Gas Emissions on the U.S. Climate. Washington DC: Department of Energy, July 23, 2025

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Table of Contents SECRETARY’S FOREWORD ....................................................................................................................... VIII EXECUTIVE SUMMARY .............................................................................................................................. IX PREFACE ................................................................................................................................................... X PART I: DIRECT HUMAN INFLUENCE ON ECOSYSTEMS AND THE CLIMATE .................................................... 1 1

CARBON DIOXIDE AS A POLLUTANT .................................................................................................. 2

2

DIRECT IMPACTS OF CO2 ON THE ENVIRONMENT .............................................................................. 3 2.1

CO2 AS A CONTRIBUTOR TO GLOBAL GREENING ................................................................................................... 3

2.1.1

Measurement of global greening .................................................................................................... 3

2.1.2

Photosynthesis and CO2 levels ......................................................................................................... 4

2.1.3

Rising CO2 and crop water use efficiency ........................................................................................ 6

2.1.4

CO2 fertilization benefits in IPCC Reports ........................................................................................ 6

2.2

THE ALKALINE OCEANS................................................................................................................................... 6

2.2.1

Changing pH .................................................................................................................................... 6

2.2.2

Coral reef changes ........................................................................................................................... 7

3

HUMAN INFLUENCES ON THE CLIMATE ........................................................................................... 11 3.1

COMPONENTS OF RADIATIVE FORCING AND THEIR HISTORY .................................................................................. 11

3.1.1

Historical radiative forcing ............................................................................................................ 11

3.1.2

Change in atmospheric CO2 since 1958 ......................................................................................... 13

3.2

FUTURE EMISSION SCENARIOS AND THE CARBON CYCLE ....................................................................................... 14

3.2.1

Emission scenarios ......................................................................................................................... 14

3.2.2

The carbon cycle relating emissions and concentrations .............................................................. 17

3.3

URBANIZATION INFLUENCE ON TEMPERATURE TRENDS ....................................................................................... 20

PART II: CLIMATE RESPONSE TO CO2 EMISSIONS ....................................................................................... 24 4

CLIMATE SENSITIVITY TO CO2 FORCING ........................................................................................... 25 4.1

INTRODUCTION ........................................................................................................................................... 25

4.2

MODEL-BASED ESTIMATES OF CLIMATE SENSITIVITY ........................................................................................... 26

4.3

DATA-DRIVEN ESTIMATES OF CLIMATE SENSITIVITY ............................................................................................. 27

4.4

TRANSIENT CLIMATE RESPONSE ..................................................................................................................... 29

5

DISCREPANCIES BETWEEN MODELS AND INSTRUMENTAL OBSERVATIONS ....................................... 31 5.1

INTRODUCTION ........................................................................................................................................... 31

5.2

SURFACE WARMING ..................................................................................................................................... 33

5.3

TROPOSPHERIC WARMING ............................................................................................................................ 34

5.4

VERTICAL TEMPERATURE PROFILE MISMATCH.................................................................................................... 36

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5.5

STRATOSPHERIC COOLING ............................................................................................................................. 38

5.6

SNOW COVER MISMATCH .............................................................................................................................. 39

5.7

HEMISPHERIC SYMMETRY OF THE PLANETARY ALBEDO ........................................................................................ 41

5.8

U.S. CORN BELT ......................................................................................................................................... 42

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EXTREME WEATHER ....................................................................................................................... 46 6.1

INTRODUCTION ........................................................................................................................................... 46

6.2

HURRICANES AND TROPICAL CYCLONES ............................................................................................................ 48

6.3

TEMPERATURE EXTREMES ............................................................................................................................. 52

6.3.1

Temperatures in the U.S. are becoming less extreme ................................................................... 53

6.3.2

Exceedances of a heat threshold ................................................................................................... 56

6.3.3

Heatwaves ..................................................................................................................................... 57

6.4

EXTREME PRECIPITATION .............................................................................................................................. 61

6.5

TORNADOES ............................................................................................................................................... 66

6.6

FLOODING ................................................................................................................................................. 67

6.7

DROUGHTS ................................................................................................................................................ 68

6.8

WILDFIRES ................................................................................................................................................. 69

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CHANGES IN SEA LEVEL .................................................................................................................. 75 7.1

GLOBAL SEA LEVEL RISE................................................................................................................................. 75

7.2

U.S. SEA LEVEL RISE ..................................................................................................................................... 75

7.3

PROJECTED SEA LEVEL RISE ............................................................................................................................ 79

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UNCERTAINTIES IN CLIMATE CHANGE ATTRIBUTION ....................................................................... 82 8.1

INTRODUCTION ........................................................................................................................................... 82

8.2

ATTRIBUTION METHODS ............................................................................................................................... 83

8.3

ATTRIBUTION OF GLOBAL WARMING ............................................................................................................... 84

8.3.1

Natural climate variability ............................................................................................................. 84

8.3.2

Optimal fingerprinting................................................................................................................... 88

8.3.3

Time series methods ...................................................................................................................... 89

8.4

DECLINING PLANETARY ALBEDO AND RECENT RECORD WARMTH ........................................................................... 90

8.5

ATTRIBUTION OF CLIMATE IMPACT DRIVERS ...................................................................................................... 92

8.6

EXTREME EVENT ATTRIBUTION (EEA).............................................................................................................. 95

8.6.1

Case study – 2021 Western North America heat wave ................................................................. 96

PART III: IMPACTS ON ECOSYSTEMS AND SOCIETY .................................................................................. 103 9

CLIMATE CHANGE AND U.S. AGRICULTURE ................................................................................... 104 9.1

ECONOMETRIC ANALYSES ............................................................................................................................ 104

9.2

FIELD AND LABORATORY STUDIES OF CO2 ENRICHMENT .................................................................................... 105

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10

11

9.3

CROP MODELING META-ANALYSES ................................................................................................................ 106

9.4

CO2 FERTILIZATION AND NUTRIENT LOSS ........................................................................................................ 107 MANAGING RISKS OF EXTREME WEATHER .................................................................................... 110

10.1

SOCIOECONOMIC CONTEXT ..................................................................................................................... 110

10.2

DATA CHALLENGES ................................................................................................................................ 111

10.3

MORTALITY FROM TEMPERATURE EXTREMES .............................................................................................. 111

10.3.1

Heat and cold risks .................................................................................................................. 111

10.3.2

Mortality risks and energy costs ............................................................................................. 113

CLIMATE CHANGE, THE ECONOMY, AND THE SOCIAL COST OF CARBON ......................................... 116 11.1 11.1.1

Overview ................................................................................................................................. 116

11.1.2

Empirical analysis of climate change and economic growth .................................................. 119

11.2

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CLIMATE CHANGE AND ECONOMIC GROWTH .............................................................................................. 116

MODELS OF THE SOCIAL COST OF CARBON ................................................................................................ 120

11.2.1

Estimating the SCC .................................................................................................................. 121

11.2.2

Variations in the SCC ............................................................................................................... 122

11.2.3

Evidence for low SCC ............................................................................................................... 123

11.2.4

Tipping points.......................................................................................................................... 123

11.2.5

Are there alternatives? ........................................................................................................... 125

GLOBAL CLIMATE IMPACTS OF U.S. EMISSIONS POLICIES ............................................................... 129 12.1

THE SCALE PROBLEM.............................................................................................................................. 129

12.2

CASE STUDY: U.S. MOTOR VEHICLE EMISSIONS ........................................................................................... 129

12.3

CONCLUDING THOUGHTS........................................................................................................................ 130

GLOSSARY ............................................................................................................................................. 132 METADATA FOR FIGURES AND TABLES ................................................................................................... 136 ABOUT THE AUTHORS............................................................................................................................ 140

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SECRETARY’S FOREWORD Energy, Integrity, and the Power of Human Potential Over my lifetime, I’ve had the privilege of working as an energy entrepreneur across a range of fields—nuclear, geothermal, natural gas, and more—and I now serve as Secretary of Energy under President Donald Trump. But above all, I’m a physical scientist who sees modern energy as nothing short of miraculous. It powers every aspect of modern life, drives every industry, and has made America an energy powerhouse with the ability to fuel global progress. The rise of human flourishing over the past two centuries is a story worth celebrating. Yet we are told—relentlessly—that the very energy systems that enabled this progress now pose an existential threat. Hydrocarbon-based fuels, the argument goes, must be rapidly abandoned or else we risk planetary ruin. That view demands scrutiny. That’s why I commissioned this report: to encourage a more thoughtful and science-based conversation about climate change and energy. With my technical background, I’ve reviewed reports from the Intergovernmental Panel on Climate Change, the U.S. government’s assessments, and the academic literature. I’ve also engaged with many climate scientists, including the authors of this report. What I’ve found is that media coverage often distorts the science.

Many people walk away with a view of climate change that is exaggerated or incomplete. To provide clarity and balance, I asked a diverse team of independent experts to critically review the current state of climate science, with a focus on how it relates to the United States. I didn’t select these authors because we always agree—far from it. In fact, they may not always agree with each other. But I chose them for their rigor, honesty, and willingness to elevate the debate. I exerted no control over their conclusions. What you’ll read are their words, drawn from the best available data and scientific assessments. I’ve reviewed the report carefully, and I believe it faithfully represents the state of climate science today. Still, many readers may be surprised by its conclusions—which differ in important ways from the mainstream narrative. That’s a sign of how far the public conversation has drifted from the science itself. To correct course, we need open, respectful, and informed debate. That’s why I’m inviting public comment on this report. Honest scrutiny and scientific transparency should be at the heart of our policymaking. Climate change is real, and it deserves attention. But it is not the greatest threat facing humanity. That distinction belongs to global energy poverty.

As someone who values data, I know that improving the human condition depends on expanding access to reliable, affordable energy. Climate change is a challenge—not a catastrophe. But misguided policies based on fear rather than facts could truly endanger human well-being. We stand at the threshold of a new era of energy leadership. If we empower innovation rather than restrain it, America can lead the world in providing cleaner, more abundant energy—lifting billions out of poverty, strengthening our economy, and improving our environment along the way.

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EXECUTIVE SUMMARY This report reviews scientific certainties and uncertainties in how anthropogenic carbon dioxide (CO2) and other greenhouse gas emissions have affected, or will affect, the Nation’s climate, extreme weather events, and selected metrics of societal well-being. Those emissions are increasing the concentration of CO2 in the atmosphere through a complex and variable carbon cycle, where some portion of the additional CO2 persists in the atmosphere for centuries. Elevated concentrations of CO2 directly enhance plant growth, globally contributing to “greening” the planet and increasing agricultural productivity [Section 2.1, Chapter 9]. They also make the oceans less alkaline (lower the pH). That is possibly detrimental to coral reefs, although the recent rebound of the Great Barrier Reef suggests otherwise [Section 2.2]. Carbon dioxide also acts as a greenhouse gas, exerting a warming influence on climate and weather [Section 3.1]. Climate change projections require scenarios of future emissions. There is evidence that scenarios widely-used in the impacts literature have overstated observed and likely future emission trends [Section 3.1].

The world’s several dozen global climate models offer little guidance on how much the climate responds to elevated CO2, with the average surface warming under a doubling of the CO 2 concentration ranging from 1.8°C to 5.7°C [Section 4.2]. Data-driven methods yield a lower and narrower range [Section 4.3]. Global climate models generally run “hot” in their description of the climate of the past few decades − too much warming at the surface and too much amplification of warming in the lower- and midtroposphere [Sections 5.2-5.4]. The combination of overly sensitive models and implausible extreme scenarios for future emissions yields exaggerated projections of future warming. Most extreme weather events in the U.S. do not show long-term trends. Claims of increased frequency or intensity of hurricanes, tornadoes, floods, and droughts are not supported by U.S. historical data [Sections 6.1-6.7]. Additionally, forest management practices are often overlooked in assessing changes in wildfire activity [Section 6.8]. Global sea level has risen approximately 8 inches since 1900, but there are significant regional variations driven primarily by local land subsidence; U.S. tide gauge measurements in aggregate show no obvious acceleration in sea level rise beyond the historical average rate [Chapter 7].

Attribution of climate change or extreme weather events to human CO 2 emissions is challenged by natural climate variability, data limitations, and inherent model deficiencies [Chapter 8]. Moreover, solar activity's contribution to the late 20th century warming might be underestimated [Section 8.3.1]. Both models and experience suggest that CO2-induced warming might be less damaging economically than commonly believed, and excessively aggressive mitigation policies could prove more detrimental than beneficial [Chapters 9, 10, Section 11.1]. Social Cost of Carbon estimates, which attempt to quantify the economic damage of CO2 emissions, are highly sensitive to their underlying assumptions and so provide limited independent information [Section 11.2]. U.S. policy actions are expected to have undetectably small direct impacts on the global climate and any effects will emerge only with long delays [Chapter 12].

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PREFACE This document originated in late March 2025 when Secretary Wright assembled an independent group to write a report on issues in climate science relevant for energy policymaking, including evidence and perspectives that challenge the mainstream consensus. We agreed to undertake the work on the condition that there would be no editorial oversight by the Secretary, the Department of Energy, or any other government personnel. This condition has been honored throughout the process and the writing team has worked with full independence. The group began working in early April with a May 28 deadline to deliver a draft for internal DOE review. The short timeline and the technical nature of the material meant that we could not comprehensively review all topics. Rather, we chose to focus on topics that are treated by a serious, established academic literature; that are relevant to our charge; that are downplayed in, or absent from, recent assessment reports; and that are within our competence. While the report is intended to be accessible to non-experts, we have omitted some introductory or explanatory material that can easily be accessed elsewhere. Nor have we attempted to survey the entire literature related to the topics covered. We have focused as much as possible on literature published since 2020 and referenced previous IPCC and NCA assessment reports.

We have also used data through 2024 where possible. The writing team is grateful to Secretary Wright for the opportunity to prepare this report and for his support of independent scientific assessment and open scientific debate. We are also grateful to a team of anonymous DOE and national lab reviewers whose input helped improve the final report.

John Christy, Ph.D. Judith Curry, Ph.D. Steven Koonin, Ph.D. Ross McKitrick, Ph.D. Roy Spencer, Ph.D.

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PART I: DIRECT HUMAN INFLUENCE ON ECOSYSTEMS AND THE CLIMATE

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1 CARBON DIOXIDE AS A POLLUTANT Chapter summary: Carbon dioxide (CO2) differs in many ways from the so-called Criteria Air Pollutants. It does not affect local air quality and has no human toxicological implications at ambient levels. It is an issue of concern because of its effects on the global climate. The Clean Air Act of 1970 defined six so-called Criteria Air Contaminants subject to regulation (EPA): particulate matter, ground-level ozone, sulfur dioxide, nitrogen dioxide, lead, and carbon monoxide. In 2007, the Supreme Court ruled that greenhouse gases (CO2 among them) were also “pollutants” subject to regulation under Clean Air Act (Mass. v. EPA, 2007). While the definition of “pollutant” is ultimately a legal matter, there are important scientific distinctions between CO2 and the Criteria Air Contaminants. The latter are subject to regulatory control because they cause local problems depending on concentrations that include nuisances (odor, visibility), damage to plants, and, at high enough exposure levels, toxicological effects in humans. In contrast, CO2 is odorless, does not affect visibility and has no toxicological effects at ambient levels. It is a naturally occurring part of the atmosphere and a key component of human and plant respiration. CO2 is essential for plant photosynthesis and higher levels are beneficial for vegetation.

In these aspects, CO2 is similar to water vapor. Ambient outdoor air today contains about 430 parts per million (ppm) CO2, increasing at about 2 ppm per year. The U.S. Occupational Safety and Health Administration issues guidelines for indoor workplaces in which elevated CO2 might be encountered, such as where dry ice is used. The Permissible Exposure Limit is 5,000 ppm over 8 hours (OSHA, 2024). Allen et al. (2015) reported evidence of diminished performance on some cognitive tasks among workers in office cubicles when exposed to CO2 levels above 1,000-1,500 ppm. These levels are far larger than any plausible ambient outdoor value through the end of the 22nd century. The growing amount of CO2 in the atmosphere directly influences the earth system by promoting plant growth (global greening), thereby enhancing agricultural yields, and by neutralizing ocean alkalinity. But the primary concern about CO2 is its role as a greenhouse gas (GHG) that alters the earth’s energy balance, warming the planet. How the climate will respond to that influence is a complex question that will occupy much of this report. References Allen, J., Macnaughton, P., Satish, U., et al. (2015). Associations of cognitive function scores with carbon dioxide, ventilation, and volatile organic compound exposures in office workers: A controlled exposure study of green and conventional office environments.

Environmental Health Perspectives. 124. https://doi.org/10.1289/ehp.1510037 Massachusetts v. Environmental Protection Agency, 549 U.S. 497 (2007). https://www.oyez.org/cases/2006/05-1120 U.S. Environmental Protection Agency. (n.d.). Criteria air pollutants. https://www.epa.gov/criteria-airpollutants U.S. Occupational Safety and Health Administration. (2024). OSHA occupational chemical database: Carbon dioxide. https://www.osha.gov/chemicaldata/183

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2 DIRECT IMPACTS OF CO2 ON THE ENVIRONMENT Chapter summary: CO2 enhances photosynthesis and improves plant water use efficiency, thereby promoting plant growth. Global greening due in part to increased CO2 levels in the atmosphere is well-established on all continents. CO2 absorption in sea water makes the oceans less alkaline. The recent decline in pH is within the range of natural variability on millennial time scales. Most ocean life evolved when the oceans were mildly acidic. Decreasing pH might adversely affect corals, although the Australian Great Barrier Reef has shown considerable growth in recent years.

2.1

CO2 as a contributor to global greening

The growing CO2 concentration in the atmosphere has the important positive effect of promoting plant growth by enhancing photosynthesis and improving water use efficiency. That is evident in the “global greening” phenomenon discussed below, as well as in the improving agricultural yields discussed in Chapter 10. Here we focus just on CO2 fertilization; research on combined effects due to temperature and precipitation changes are discussed in Chapter 10. 2.1.1 Measurement of global greening “Greening” refers to an increase in the fraction of the Earth’s surface covered by plants. It can be quantified by the “Leaf Area Index” (LAI) measured by satellite. Many studies over the past decade have confirmed a global greening pattern (increase in LAI) attributable in part to rising CO2 levels. Zhu et al. (2016) was one of the first studies to report that global greening was detectable using satellite sensors. From 1982 to 2011 they detected greening over 25-50 percent of the Earth versus “browning” over only four percent and attributed 70 percent of the greening to rising CO2 levels (see Figure 2.1). Other contributors included land-use changes, warming and nitrogen. The fraction attributable to CO 2 was largest in the tropics; other factors played more dominant roles in CONUS. Zeng et al.

(2017) confirmed the pattern of greening, noting that over thirty years it had added 8 percent to global leaf area and that greening was mitigating warming. Greening has been observed globally. Chen et al. (2019) show that in China and India much of it is driven by land management changes. Thus, while China accounts for only 6.6 percent of global vegetated area it accounts for 25 percent of global net increase in LAI. Piao et al. (2020) noted that greening was even observable in the Arctic. CO2 fertilization effects are influenced by local temperature and nutrient and water availability, all of which vary regionally. While plant models predict increased photosynthesis in response to rising CO2, Haverd et al. (2020) reported a CO2 fertilization rate much larger than model predictions. That is, CO2 fertilization had driven an increase in observed global photosynthesis by 30 percent since 1900, versus 17 percent predicted by plant models. If true it would indicate that global models of the socioeconomic impacts of rising CO2 have understated the benefits to crops and agriculture. Keenan et al (2023), however, estimated a lower fertilization rate more in line with models. The connection between CO2 fertilization and agriculture will be discussed in Chapter 9. 3

Figure 2.1: Trends in average Leaf Area Index (LAI). Source: Zhu et al. 2016 Figure 3.

Piao et al. (2020) and Chen et al. (2024) report that the greening trend continues with no evidence of slowdown, and CO2 fertilization remains the dominant driver 2.1.2 Photosynthesis and CO2 levels Plants build biomass through photosynthesis, a process that converts carbon dioxide, water, and light into sugar. The plant enzyme responsible for photosynthesis is Ribulose-1,5-bisphosphatecarboxylase/oxygenase or “Rubisco”. Photosynthesis is initiated when CO2 is available at the surface of the Rubisco enzyme where it is converted to a molecule with 3 carbon atoms and thereafter incorporated into plant mass. This is referred to as the “C3” process. Rubisco is estimated to have evolved about 3 billion years ago. Over geological time the Earth’s atmospheric CO2 levels were usually many times higher than they are today. About 400 million years ago CO2 levels were an estimated 2,000-4,000 ppm and were at or above 1,000 ppm for much of the interval from 200 to 50 million years ago (Berner 2006, Judd et al. 2024). Over the past 35 million years the level of atmospheric CO2 has been steadily declining, falling to as low as 170 ppm during glaciations (Gerhart and Ward 2010). While the modern rate of change in CO2 may be high compared to prior intervals, the geological evidence is that plants and animals evolved under much higher CO2 levels than at present.

In response to low-CO2 conditions some plants evolved another photosynthetic pathway called C4, in which CO2 is concentrated in the vicinity of Rubisco, allowing for the C3 process to function more efficiently. For agricultural purposes the plant categories are: • •

C3: rice, wheat, soybeans and most other crops C4: maize (corn), sugar cane, millet, sorghum

Had atmospheric CO2 levels continued declining, plant growth would have declined and eventually ceased. Below 180 ppm, the growth rates of many C3 species are reduced 40-60 percent relative to 350 ppm (Gerhart and Ward 2010) and growth has stopped altogether under experimental conditions of 60— 140 ppm CO2. Some C4 plants are still able to grow at levels even as low as 10 ppm, albeit very slowly (Gerhart and Ward 2010). 4

Figure 2.2: growth of Abutilon theophrasti after 14 days under identical conditions but for the indicated variations in CO2 levels. Source: Gerhart and Ward (2010). Note “Current” corresponds to 1988 in image.

Current CO2 levels are about 430 ppm, up from 280 ppm in the early 1800s. The positive response of plants to extra CO2 is illustrated in Figure 2.2, reproduced from Gerhart and Ward (2010). It shows the growth effect of CO2 on Velvetleaf (Abutilon theophrasti) seedlings over 14 days under controlled conditions where only the CO2 exposure is varied. The gains induced by increasing CO2 from 150 ppm to 350 ppm continue under a further doubling to 700 ppm. Over the past 60+ years there have been thousands of studies on the response of plants to rising CO 2 levels. The overwhelming theme is that plants, especially C3 plants, benefit from extra CO 2. There are two mechanisms by which CO2 confers a growth benefit: • •

Enhanced photosynthesis via the metabolic pathways described above. Increased water use efficiency. This arises because plants draw in CO 2 by opening the stomata (pores) on the leaf surface. When CO2 is scarce the stomata must be kept wide open for long periods, allowing water to evaporate. Under enriched CO2 conditions the stomata remain closed for longer periods, thus helping the plant retain water longer, and so increasing water use efficiency.

Specific effects of climate change on U.S. agriculture will be reviewed in Chapter 9. 5

2.1.3 Rising CO2 and crop water use efficiency Derying et al. (2016) surveyed evidence on crop water productivity (CWP), the yield per unit of water used, drawing attention to the potential for CO2 both to enhance photosynthesis and to reduce leaf-level transpiration (water loss during leaf respiration). They surveyed all available FACE data (Free Air CO2 Enrichment—see Chapter 9) on crop yield changes for maize (corn), wheat, rice, and soybean and combined it with crop model data simulating yield responses as of 2080 under the extreme RCP8.5 emissions scenario in four growing regions (Tropics, Arid, Temperate and Cold) each of which were split into rainfed and irrigated sub-regions. They reported that models without CO2 fertilization predicted CWP losses in every region, but those were more than offset by CO2 fertilization so that all regions showed a net CWP gain. Deryng et al. (2016) also reported that negative impacts of warming on wheat and soybean yields were fully offset by CWP gains and mitigated by up to 90 percent for rice and 60 percent for maize. Similarly, Cheng et al. (2017) noted that increased Gross Primary Production from 1982 to 2011 due to rising CO2 uptake was accompanied by such large gains in CWP that global water use by plants had not increased, despite the extra biomass. Deryng et al. (2016) assumed that climate change would “exacerbate water scarcity”.

Yet while models do predict that drylands will expand under climate warming, current data show the opposite: greening is happening even in arid areas. Zhang et al. (2024) report that due to increased CO2 levels “increasing aridity in drylands won’t lead to a general loss of vegetation productivity”; at most only 4 percent of currently arid areas will see increased desertification. 2.1.4 CO2 fertilization benefits in IPCC Reports The IPCC has only minimally discussed global greening and CO2 fertilization of agricultural crops. The topic is briefly acknowledged in a few places in the body of the IPCC 6 th and earlier Assessment Reports but is omitted in all Summary documents. Section 2.3.4.3.3 of the AR6 Working Group I report, entitled “global greening and browning,” points out that the IPCC Special Report on Climate Change and Land had concluded with high confidence that greening had increased globally over the past 2-3 decades. It then discusses that there are variations in the greening trend among data sets, concluding that while they have high confidence greening has occurred, they have low confidence in the magnitude of the trend. There are also brief mentions of CO2 fertilization effects and improvements in water use efficiency in a few other chapters in the AR6 Working Groups I and II Reports.

Overall, however, the Policymaker Summaries, Technical Summaries, and Synthesis Reports of AR5 and AR6 do not discuss the topic.

2.2

The Alkaline Oceans

2.2.1 Changing pH A neutral aqueous solution has a pH of 7.0, while one with pH greater than 7.0 is alkaline (also termed basic) and with pH less than 7.0 is acidic. The modern-day global average pH of surface sea water is estimated to be 8.04 (Copernicus Marine Service 2025, Figure 2.3), down from an estimated value of 8.2 in pre-Industrial times (Gattuso and Hansson, 2011). As CO2 concentrations in the atmosphere increased, the oceans absorbed more, which decreases their pH. Depending upon the oceans’ buffering capacity, they are expected to become somewhat less alkaline over time, consistent with the observed decrease in pH.

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Figure 2.3: Ocean pH 1985 – 2022. Source: Copernicus Marine Service 2025

While this process is often called “ocean acidification”, that is a misnomer because the oceans are not expected to become acidic; “ocean neutralization” would be more accurate. Even if the water were to turn acidic, it is believed that life in the oceans evolved when the oceans were mildly acidic with pH 6.5 to 7.0 (Krissansen-Totton et al., 2018). On the time scale of thousands of years, boron isotope proxy measurements show that ocean pH was around 7.4 or 7.5 during the last glaciation (up to about 20,000 years ago) increasing to present-day values as the world warmed during deglaciation (Rae et al., 2018). Thus, ocean biota appear to be resilient to natural long-term changes in ocean pH since marine organisms were exposed to wide ranges in pH. 2.2.2 Coral reef changes There are concerns that a decreasing pH of sea water will reduce the calcification rate of coral reefs. But coral reefs already endure large swings in pH, partly due to daily photosynthetic activity in the reef; measured pH values range from 9.4 during the day to 7.5 at night (Revelle and Fairbridge, 1957). De’ath et al. (2009) reported that a portion of Australia’s Great Barrier Reef (GBR, the world’s largest coral reef ecosystem) had experienced a 14 percent decline in calcification since 1990. This was tentatively attributed to increasing water temperature and decreasing pH. But Ridd et al.

(2013) showed that report to have resulted from a biased data analysis that, when corrected, showed no change in calcification rates. Nevertheless, the alarm produced by the original paper has persisted as evidenced by the large number of published citations (541) to the original study compared to only 11 citations to the correction (as of 30 April 2025). The most recent annual summary of GBR conditions from the Australian Institute of Marine Science indicates that coral production has rebounded strongly (AIMS, 2023). Figure 2.4 shows the results of the AIMS surveys of hard coral cover, expressed as a percentage of the reef area. Much of the decline in the GBR before 2011 turned out to be due to intense tropical cyclone activity (Beeden et al., 2015) as well as a string of marine heatwaves, agricultural runoff and invasive species (Woods Hole, 2023). Given the reported declines in GBR calcification between 1990 and 2009 and the continued increase in atmospheric CO2 levels, the rebound has surprised some observers.

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Figure 2.4 Hard coral cover of three regions of the Great Barrier Reef 1985 to 2023. Source: AIMS 2023.

It is being increasingly recognized that publication bias (alarming ocean acidification results preferred by high-impact research publications) exaggerates the reported impacts of declining ocean pH. An ICES Journal of Marine Science Special Issue addressed this problem with an article entitled, Towards a Broader Perspective on Ocean Acidification Research. In the Introduction to that Special Issue, H. I. Browman stated, “As is true across all of science, studies that report no effect of ocean acidification are typically more difficult to publish.” (Browman, 2016). Similarly, a meta-analysis (Clements et al., 2021) of the negative effects of ocean acidification on reef fish behavior found what they called a “decline effect”: initially dramatic conclusions published in prominent journals showing apparently large impacts of acidification tended to be followed up by subsequent studies on larger sample sizes yielding much smaller and typically non-existent effects. They call for their colleagues to improve research practices to counter the “decline effect”: [The] vast majority of studies with large effect sizes in this field tend to be characterized by low sample sizes, yet are published in high-impact journals and have a disproportionate influence on the field in terms of citations.

We contend that ocean acidification has a negligible direct impact on fish behavior, and we advocate for improved approaches to minimize the potential for a decline effect in future avenues of research (Clements et al., 2021). In summary, ocean life is complex and much of it evolved when the oceans were acidic relative to the present. The ancestors of modern coral first appeared about 245 million years ago. CO2 levels for more than 200 million years afterward were many times higher than they are today. Much of the public discussion of the effects of ocean “acidification” on marine biota has been one-sided and exaggerated. References AR6: Intergovernmental Panel on Climate Change Sixth Assessment Report (2021) Working Group I Contribution. www.ipcc.ch. Australian Institute of Marine Science. (2022). Continued coral recovery leads to 36-year highs across two-thirds of the Great Barrier Reef. https://www.aims.gov.au/sites/default/files/202208/AIMS_LTMP_Report_on%20GBR_coral_status_2021_2022_040822F3.pdf Beeden, R., Maynard, J., Puotinen, M., Marshall, P., Dryden, J., Goldberg, J., and Williams, G. (2015). Impacts and recovery from Severe Tropical Cyclone Yasi on the Great Barrier Reef. PLOS ONE, 10, e0121272. https://doi.org/10.1371/journal.pone.0121272 Berner, R. A. (2006). GEOCARBSULF: A combined model for Phanerozoic atmospheric O₂ and CO₂.

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Browman, H. I. (2016). Applying organized scepticism to ocean acidification research. ICES Journal of Marine Science, 73(3), 529.1–536. https://doi.org/10.1093/icesjms/fsw010 Chen, C., Park, T., Wang, X., Piao, S., Xu, B., Chaturvedi, R. K., and Myneni, R. B. (2019). China and India lead in greening of the world through land-use management. Nature Sustainability, 2, 122–129. https://www.nature.com/articles/s41893-019-0220-7 Chen, X., Wang, Y., Liu, Y., and Piao, S. (2024). The global greening continues despite increased drought stress since 2000. Global Ecology and Conservation, 49, e02791. https://www.sciencedirect.com/science/article/pii/S2351989423004262 Cheng, L., Zhang, L., Wang, Y. P., et al. (2017). Recent increases in terrestrial carbon uptake at little cost to the water cycle. Nature Communications, 8, 110. https://doi.org/10.1038/s41467-017-00114-5 Clements, J. C., Sundin, J., Clark, T. D., and Jutfelt, F. (2022). Meta-analysis reveals an extreme “decline effect” in the impacts of ocean acidification on fish behavior. PLOS Biology, 20(2), e3001511. https://doi.org/10.1371/journal.pbio.3001511 Copernicus Marine Service. (2025). Global ocean acidification – Mean sea water pH time series and trend from multi-observations reprocessing. https://data.marine.copernicus.eu/product/GLOBAL_OMI_HEALTH_carbon_ph_area_averaged/desc ription De’ath, G., Lough, J., and Fabricius, K.

(2009). Declining coral calcification on the Great Barrier Reef. Science, 323, 116–119. https://doi.org/10.1126/science.1165283 Deryng, D., Conway, D., Ramankutty, N., Price, J., Warren, R., Jones, R., ... and Elliott, J. (2016). Regional disparities in the beneficial effects of rising CO₂ concentrations on crop water productivity. Nature Climate Change. https://doi.org/10.1038/nclimate2995 Gattuso, J. P., and Hansson, L. (Eds.). (2011). Ocean acidification: Background and history. Oxford University Press. Gerhart, L. M., and Ward, J. K. (2010). Plant responses to low [CO₂] of the past. New Phytologist, 188, 674–695. https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2010.03441.x Haverd, V., B. Smith, J. G. Canadell, et al. (2020). Higher than expected CO₂ fertilization inferred from leaf to global observations. Global Change Biology, 26, 2390–2402. https://doi.org/10.1111/gcb.14950 Keenan, T. F., X. Luo, B. D. Stocker, et al. (2023). A constraint on historic growth in global photosynthesis due to rising CO2. Nature Climate Change 13(12): 1376-1381 DOI: 10.1038/s41558023-01867-2. Judd, E. J., Scotese, C. R., Young, S. A., et al. (2024). A 485-million-year history of Earth’s surface temperature. Science, 385(6715). https://doi.org/10.1126/science.adk3705 Krissansen-Totton, J., Arney, G. N., and Catling, D. C. (2018).

Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model. Proceedings of the National Academy of Sciences, 115(6), 4105–4110. https://doi.org/10.1073/pnas.1721296115 Piao, S., X. Wang, T. Park, et al. (2020). Characteristics, drivers and feedbacks of global greening. Nature Reviews Earth & Environment 1(1): 14-27 DOI: 10.1038/s43017-019-0001-x Rae, J. W. B., Burke, A., Robinson, L. F., et al. (2018). CO₂ storage and release in the deep Southern Ocean on millennial to centennial timescales. Nature, 562, 569–573. https://doi.org/10.1038/s41586018-0614-0 Revelle, R., and Fairbridge, R. W. (1957). Carbonate and carbon dioxide. In J. W. Hedgpeth (Ed.), Treatise on marine ecology and paleoecology (Vol. 1). Geological Society of America. Ridd, P., Silva, E., and Stieglitz, T. (2013). Have coral calcification rates slowed in the last twenty years? Marine Geology, 346, 392–399. https://doi.org/10.1016/j.margeo.2013.09.002 Woods Hole Oceanographic Institution. (2023). Is the Great Barrier Reef making a comeback? https://www.whoi.edu/oceanus/feature/is-the-great-barrier-reef-making-a-comeback/ Zeng, Z., Piao, S. Li, L., et al. (2017). Climate mitigation from vegetation biophysical feedbacks during the past three decades. Nature Climate Change. https://doi.org/10.1038/nclimate3299 9

Zhang, Y., Liu, Y., Chen, X., et al. (2024). Less than 4% of dryland areas are projected to desertify despite increased aridity under climate change. Nature Communications Earth and Environment, 5. https://www.nature.com/articles/s43247-024-01463-y Zhu, Z., Piao, S., Myneni, R. B., et al. (2016). Greening of the Earth and its drivers. Nature Climate Change, 6, 791–795. https://www.nature.com/articles/nclimate3004

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3 HUMAN INFLUENCES ON THE CLIMATE Chapter Summary: The global climate is naturally variable on all time scales. Anthropogenic CO2 emissions add to that variability by changing the total radiative energy balance in the atmosphere. The IPCC has downplayed the role of the sun in climate change but there are plausible solar irradiance reconstructions that imply it contributed to recent warming. Climate projections are based on IPCC emission scenarios that have tended to exceed observed trends. Most academic climate impact studies in recent years are based upon the extreme RCP 8.5 scenario that is now considered implausible; its use as a business-as-usual scenario has been misleading. Carbon cycle models connect annual emissions to growth in the atmospheric CO2 stock. While models disagree over the rate of land and ocean CO2 uptake, all agree that it has been increasing since 1959. There is evidence that urbanization biases in the land warming record have not been completely removed from climate data sets.

3.1

Components of radiative forcing and their history

3.1.1 Historical radiative forcing A changing climate has been the norm throughout the Earth’s 4.6-billion-year history. The Earth’s temperature and weather patterns change naturally over timescales ranging from decades to millions of years. Natural variations in the surface climate originate in two ways. Internal climate fluctuations associated with circulations in the atmosphere and ocean exchange energy, water, and carbon between the atmosphere, oceans, land, and ice. External influences on the climate system include variations in the energy received from the sun and the effects of volcanic eruptions. Human activities influence climate through changing land use and land cover. Humans are also changing the composition of the atmosphere by emissions of CO2 and other greenhouse gases and by altering the concentration of aerosol particles in the atmosphere. The earth is warmed by the sunlight it absorbs and is cooled by the heat it radiates to space. Averaged over the Earth’s surface, each of these processes involve power flows of about 240 Watts per square meter (W/m2). When they are in balance, there are no net external causes of warming or cooling. Both human and natural influences on the climate alter this balance and so cause the climate to change.

Influences on the Earth’s energy balance at the top of the atmosphere are quantified by “radiative forcing”, the extent to which they disturb the warming/cooling balance; a positive forcing warms while a negative forcing cools. The IPCC’s estimated history of major components of radiative forcing since 1750 is shown in the following two figures from its AR6.

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Figure 3.1.1: IPCC estimates of radiative forcing components over time. Shading indicates uncertainty ranges. Source: AR6 WGI Ch2 Fig. 10

Figure 3.1.2 IPCC estimates of radiative forcing component changes from 1750 to 2019. Source: AR6 WGI Ch 7 Fig. 7-6.

These graphs show that the total radiative forcing is comprised of both natural and anthropogenic components. Carbon dioxide is the largest human influence on the climate and the one most relevant to the influence of fossil fuel use. It exerts a warming influence by decreasing the cooling power of the atmosphere. Emissions of CO2 are accumulating in the atmosphere, as described in the following section, so that the warming influence is growing. Other greenhouse gases (methane, nitrous oxide, halogens, and ozone) act similarly, currently adding another 75 percent to CO2’s warming. Aerosols exert an overall cooling effect, although with large uncertainties in the way they catalyze the formation of reflective clouds. As a result, understanding the causes of recent warming requires not just identifying the warming effects of CO2, but also the more uncertain cooling effects of aerosols. The IPCC assesses the change in the radiative forcing by the sun to be negligible, based on their preference for data reconstructions that imply minimal solar change since preindustrial times. But Connolly et al. (2021) reviewed sixteen different Total Solar Irradiance (TSI) reconstructions in the literature 12

covering the years 1600-2000; the reconstructions vary from almost no change in TSI to a relatively large upward trend. Those authors note that the variation in TSI reconstructions combined with variations in surface temperature reconstructions allows for inferences consistent with either no or most 20th century warming being attributable to the sun. A particularly thorny issue is the gap in TSI data between 1989 and 1991 due to a delay in the launch of a monitor following the Space Shuttle Challenger disaster on January 28 1986. This delay prevented a replacement satellite from being launched in time to overlap with, and its readings to be intercalibrated with, the prior system (Zacharias 2014, Scafetta et al. 2019). This is called the ACRIM (Active Cavity Radiometer Irradiance Monitor) gap problem. The question of whether there is an upward trend in TSI over 1978 to 2018 hangs on how the ACRIM data gap is filled. Connolly et al. (2021) found that the IPCC’s consensus statements on solar forcing were formulated prematurely through the suppression of dissenting scientific opinions. Another natural radiative forcing component is volcanic aerosols, which exert an episodic cooling influence. Box 4.1 in the IPCC AR6 Report addresses the climate impact of volcanic eruptions, noting three explosive volcanic eruptions that occurred in the first half of the 19th century.

This included the 1815 Tambora eruption that resulted in the ‘year without summer’, with multiple harvest failures across the Northern Hemisphere. There is uncertainty about the sign of the relatively small forcing due to the submarine volcano Hunga Tonga which erupted in 2022 (Jenkins et al. 2023, Schoeberl et al. 2024). Figure 3.1.1 shows that the anthropogenic forcing component was negligible before about 1900 and has increased steadily since, rising to almost 3 W/m2 today. However, this is still only about 1 percent of the unperturbed radiation flows, making it a challenge to isolate the effects of anthropogenic forcing; stateof-the-art satellite estimates of global radiative energy flows are only accurate to a few W/m2. Natural sources of global energy imbalance other than volcanoes and total solar irradiance (TSI) are not included in these graphs because they remain largely unknown. 3.1.2 Change in atmospheric CO2 since 1958 Carbon dioxide’s warming influence depends on how much “extra” CO 2 accumulates in the atmosphere- i.e., its concentration above the preindustrial value of 280 ppm. The CO2 level as recorded at the Mauna Loa observatory in Hawaii, generally used as the representative global average concentration, is available online at https://gml.noaa.gov/ccgg/trends/index.html.

The concentration was about 316 ppm at the start of the record in 1959 and is now about 430 ppm, a 36 percent increase. At the end of the last glaciation CO2 levels had fallen to about 180 ppm. As discussed in Chapter 2, C3 plants begin dying at CO2 levels below about 140 ppm and C4 plants at levels below 100 ppm, so if CO2 levels had continued falling plant life would have been imperiled.

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Fig. 3.1.3. Yearly average atmospheric CO 2 concentrations (1959-2025) in ppm measured at Mauna Loa (blue). C3 Threshold: Level below which C3 plants begin dying (140 ppm, see Chapter 2). C4 Threshold: Level below which C4 plants begin dying (100 ppm, see Chapter 2). Glacial minimum: Minimum level during recent glaciations (purple arrow). CO2 data source: https://gml.noaa.gov/ccgg/trends/index.html

The annual increase in concentration is only about half of the CO 2 emitted because land and ocean processes currently absorb “excess” CO2 at a rate approximately 50 percent of the human emissions. Future concentrations, and hence future human influences on the climate, therefore depend upon two components: (1) future rates of global human CO2 emissions, and (2) how fast the land and ocean remove extra CO2 from the atmosphere. We discuss each of these in turn.

3.2

Future emission scenarios and the carbon cycle

3.2.1 Emission scenarios Assessing the dangers of future GHG emissions requires assumptions about what those emissions will be. Future emissions, and hence human influences on the climate, will depend upon future demographics, economic activity, regulation, and energy and agricultural technologies. Various assumptions about each of those lead to projections of greenhouse gas emissions and concentrations, aerosol concentrations, and changes in land use, which ultimately can be combined into assumptions about anthropogenic radiative forcing. The great uncertainties about these many factors make it impossible to precisely predict future emissions. Instead, the IPCC has used various sets of scenarios meant to span a plausible range of possibilities for population, economy, and technologies. Recent versions of the scenarios are labeled by a number indicating the anthropogenic radiative forcing expected in 2100 under that scenario. Thus, a scenario labeled with a “6” corresponds to 6 W/m2 of human-induced radiative forcing (warming) at the end of the century. (Recall current anthropogenic radiative forcing is about 2.7 W/m2.) 14

Although the IPCC does not claim its emission scenarios are forecasts, they are often treated as such. Comparisons of past scenario groups against observations show that IPCC emission projections have tended to overstate actual subsequent emissions. For the IPCC Third and Fourth Assessment Reports a set of emission projections from the Special Report on Emission Scenarios was used; these were referred to as the SRES scenarios. McKitrick et al. (2012) showed that, when converted to per capita values, the SRES scenario emissions distribution was skewed upwards compared with observed trends. The bias of the SRES scenarios was confirmed by the later analysis of Hausfather et al. (2019) who showed that observed atmospheric CO2 concentrations tracked the low end of the SRES range and also of subsequent IPCC scenario ranges (Figure 3.2.1). For AR5 the IPCC developed a new set of scenarios called the Representative Concentration Pathways (RCPs). These were identified by a number representing the increase in forcing and were thus called RCP2.6, RCP4.5, RCP6.0 and RCP8.5. RCP2.6 (implying an anthropogenic radiative forcing in 2100 of 2.6 W/m2) describes a GHG concentration pathway leading to warming well below 2°C. At the other end of the scale RCP8.5 is an extreme outcome implying nearly 5°C warming from 1900 to 2100.

RCP8.5 came to be referred to as the no-policy baseline, or “business-as-usual” scenario in both the academic literature and popular media. It was therefore used to generate the reference outcome supposedly representing the 21st century world in the absence of increasingly stringent emission reduction policies. But RCP8.5 was intended as a low-probability high emissions scenario and its use as a business-as-usual baseline has been criticized as grossly misleading.1 Hausfather and Peters (2020a) writing in a commentary in Nature, pointed out that RCP8.5 was developed as an extreme worst-case, and its misuse as a “business as usual” baseline has resulted in a large number of misleading studies and media reporting. The implausibility of the RCP8.5 scenario was examined by Burgess et al. (2021). The implausibility of RCP8.5 should not be interpreted as very unlikely (e.g. 95th percentile) or a “worst case”, but rather as genuinely implausible owing to the implausibility of the inputs required to reach a forcing of 8.5 W/m 2. They noted that RCP8.5 has already diverged from observed trends in energy use and the near future trends diverge sharply from those of the International Energy Agency (IEA), which provides market-based projections of energy use for the coming decades. Pielke Jr. et al.

(2022) further showed that the historic and projected IEA trends run near the bottom of the envelopes of both RCP projections and the more recent Shared Socioeconomic Pathway (SSP) scenario trends. Schwalm et al. (2020) defended the use of RCP8.5 on the grounds that cumulative CO2 emissions over 2005-2020 track it more closely than the lower RCP scenarios. They also argue that a modified version of the IEA scenarios closely track RCP8.5 in the coming decades. Hausfather and Peters (2020b) responded that the skill of RCP8.5 over those 15 years is due to offsetting errors in its representation of CO2 from fuel use and land use change, and the apparent agreement with IEA in coming decades is due to Schwalm et al. adding in very high land use emissions. The IEA’s own projected CO2 emissions track well below RCP8.5. Widespread use of RCP8.5 as a no-policy baseline has created a bias towards alarm in the climate impacts literature. The extent of this problem was confirmed in a literature analysis by Pielke Jr. and Ritchie (2020). They found that some 16,800 scientific papers published between 2010 and 2020 used the RCP8.5 scenario, with about 4,500 of the articles linking RCP8.5 to the concept of “business-as-usual”. Their analysis showed how RCP8.5 was misused not only by individual researchers, but also by influential science agencies like the IPCC and the U.S.

National Climate Assessment (USNCA), which has directly led to misleading coverage in prominent media outlets.

1 This extreme scenario is useful for modelers, since a large forcing generates a large response (warming) making it easier to assess a model’s sensitivity. But that’s very different than claiming it is a plausible future outcome.

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Figure 3.2.1 Since the 1970s successive families of emission and concentration projections (colored lines) have consistently overestimated observations (black line). Source: Hausfather et al. (2019) Figure S4.