Fig. 1. Global temperature relative to 1880-1920 based on the GISS analysis.¹ ²

Reflections on Time Scales and Butterflies

12 July 2024

James Hansen, Makiko Sato, Pushker Kharecha

Abstract. Breathless reporting on when the present global heat anomaly will begin to fall is understandable, given heat suffering around the world. However, fundamental issues are in question and a reflection on time scales is in order, for the sake of understanding ongoing climate change and actions that need to be taken.

June was the 13 th consecutive record monthly global temperature (Fig. 1). The changing gap between this current string and prior records is revealing. The gap is smallest in Northern Hemisphere winter, the months when a strong El Nino gives global temperature the biggest kick, consistent with direct evidence that the recent El Nino was far from a super El Nino. Thus, more than the recent modest El Nino is needed to explain the uniquely large rise of temperature in the past year (Fig. 2).

Reanalyses (computer simulations of global weather post facto, using available observations) show that the first week of July this year was cooler than in 2023 (Fig. 3). Nevertheless, we suggest caution in predictions about July and August for Fig. 1. Almost a dead-heat (sorry, no pun intended) with the 2023 July and August records is possible, even likely. In September, global temperature surely will fall well below the unusually-high September record (Fig. 1); with that, the 12-month running-mean global temperature (Fig. 2) will decline noticeably.


Fig. 2. Global temperature relative to 1880-1920 based on the GISS analysis. Warming rate is 0.18°C/decade for 1970-2010, 0.32°C/decade for 2010-present.


Fig. 3. Global surface air temperature from ECMWF reanalysis, provided by Univ. of Maine.³

Tropical surface air temperature (Fig. 4) during the next few months is expected to fall below its El Nino-enhanced 2023 value – but how far below? Sea surface temperature (SST) is a less “noisy,” powerful, climate diagnostic because it serves as a measure of the entire depth of the ocean surface “mixed layer,” which is well-mixed almost daily by winds. The mixed layer accumulates effects of all the forcings at the ocean surface, capturing effects 24/7/365, not only at the moments of satellite sampling. Thus, SST reduces effects of weather noise, cloud variability, and sampling biases in satellite observations.

Global SST (Fig. 5) reveals the huge warming in the transition from La Nina at the beginning of 2023 to El Nino by mid-2023, exceeding the SST warming in even the strongest El Ninos. The magnitude and stability of the warming imply the need for a substantial mechanism(s) in addition to the El Nino, especially given the modest strength of the El Nino. Our suggestion4 for a large component of this additional forcing is aerosol forcing that grew especially during the 2020s. A sharp increase of aerosol forcing is expected in January 2020 due to regulations on sulphates in ship fuels. Aerosol forcing occurs mainly via effects on clouds that are highly nonlinear, being most effective in less-polluted air; thus, ship aerosols have a substantial impact, even though the ship portion of total human-made aerosols is small.


Fig. 4. Tropical surface air temperature from ECMWF reanalysis, provided by Univ. of Maine.

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¹ Lenssen NJL, Schmidt GA, Hansen JE et al. Improvements in the GISTEMP uncertainty model, J Geophys Res Atmos 124(12), 6307-26, 2019
² Hansen J, Ruedy R, Sato M et al. Global surface temperature change. Rev Geophys 48:RG4004, 2010
³ This ECMWF reanalysis graphic is provided by the Climate Change Institute, University of Maine.

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Atmos. Chem. Phys., 16, 3761–3812, 2016
www.atmos-chem-phys.net/16/3761/2016/
doi:10.5194/acp-16-3761-2016
© Author(s) 2016. CC Attribution 3.0 License.

Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming could be dangerous

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Received: 11 June 2015 – Published in Atmos. Chem. Phys. Discuss.: 23 July 2015
Revised: 17 February 2016 – Accepted: 18 February 2016 – Published: 22 March 2016

Abstract. We use numerical climate simulations, paleoclimate data, and modern observations to study the effect of growing ice melt from Antarctica and Greenland. Meltwater tends to stabilize the ocean column, inducing amplifying feedbacks that increase subsurface ocean warming and ice shelf melting. Cold meltwater and induced dynamical effects cause ocean surface cooling in the Southern Ocean and North Atlantic, thus increasing Earth’s energy imbalance and heat flux into most of the global ocean’s surface. Southern Ocean surface cooling, while lower latitudes are warming, increases precipitation on the Southern Ocean, increasing ocean stratification, slowing deepwater formation, and increasing ice sheet mass loss. These feedbacks make ice sheets in contact with the ocean vulnerable to accelerating disintegration. We hypothesize that ice mass loss from the most vulnerable ice, sufficient to raise sea level several meters, is better approximated as exponential than by a more linear response. Doubling times of 10, 20 or 40 years yield multi-meter sea level rise in about 50, 100 or 200 years. Recent ice melt doubling times are near the lower end of the 10–40-year range, but the record is too short to confirm the nature of the response. The feedbacks, including subsurface ocean warming, help explain paleoclimate data and point to a dominant Southern Ocean role in controlling atmospheric CO₂, which in turn exercised tight control on global temperature and sea level. The millennial (500–2000-year) timescale of deep-ocean ventilation affects the timescale for natural CO₂, change and thus the timescale for paleo-global climate, ice sheet, and sea level changes, but this paleo-millennial timescale should not be misinterpreted as the timescale for ice sheet response to a rapid, large, human-made climate forcing. These climate feedbacks aid interpretation of events late in the prior interglacial, when sea level rose to +6–9 m with evidence of extreme storms while Earth was less than 1 ◦C warmer than today. Ice melt cooling of the North Atlantic and Southern oceans increases atmospheric temperature gradients, eddy kinetic energy and baroclinicity, thus driving more powerful storms. The modeling, paleoclimate evidence, and ongoing observations together imply that 2 °C global warming above the preindustrial level could be dangerous. Continued high fossil fuel emissions this century are predicted to yield (1) cooling of the Southern Ocean, especially in the Western Hemisphere; (2) slowing of the Southern Ocean overturning circulation, warming of the ice shelves, and growing ice sheet mass loss; (3) slowdown and eventual shutdown of the Atlantic overturning circulation with cooling of the North Atlantic region; (4) increasingly powerful storms; and (5) non-linearly growing sea level rise, reaching several meters over a timescale of 50–150 years. These predictions, especially the cooling in the Southern Ocean and North Atlantic with markedly reduced warming or even cooling in Europe, differ fundamentally from existing climate change assessments. We discuss observations and modeling studies needed to refute or clarify these assertions.

1 Introduction
Humanity is rapidly extracting and burning fossil fuels without full understanding of the consequences. Current assessments place emphasis on practical effects such as increasing extremes of heat waves, droughts, heavy rainfall, floods, and encroaching seas (IPCC, 2014; USNCA, 2014). These assessments and our recent study (Hansen et al., 2013a) conclude that there is an urgency to slow carbon dioxide (CO₂ ) emissions, because the longevity of the carbon in the climate system (Archer, 2005) and persistence of the induced warming (Solomon et al., 2010) may lock in unavoidable, highly undesirable consequences.

Despite these warnings, fossil fuels remain the world’s primary energy source and global CO₂ emissions continue at a high level, perhaps with an expectation that humanity can adapt to climate change and find ways to minimize effects via advanced technologies. We suggest that this viewpoint fails to appreciate the nature of the threat posed by ice sheet instability and sea level rise. If the ocean continues to accumulate heat and increase melting of marine-terminating ice shelves of Antarctica and Greenland, a point will be reached at which it is impossible to avoid large-scale ice sheet disintegration with sea level rise of at least several meters. The economic and social cost of losing functionality of all coastal cities is practically incalculable. We suggest that a strategy relying on adaptation to such consequences will be unacceptable to most of humanity, so it is important to understand this threat as soon as possible.

We investigate the climate threat using a combination of atmosphere–ocean modeling, information from paleoclimate data, and observations of ongoing climate change. Each of these has limitations: modeling is an imperfect representation of the climate system, paleo-data consist mainly of proxy climate information usually with substantial ambiguities, and modern observations are limited in scope and accuracy. However, with the help of a large body of research by the scientific community, it is possible to draw meaningful conclusions.

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Opinion: I’m a climate scientist. If you knew what I know, you’d be terrified too

Opinion by Bill McGuire
4 minute read — Updated 9:56 AM EST, Thu March 7, 2024

Editor’s Note: Editor’s Note: Bill McGuire is professor emeritus of geophysical & climate hazards at University College London and author of “Hothouse Earth: An Inhabitant’s Guide.” The views expressed in this commentary are his own. Read more CNN opinion here.

CNN
—
Are you frightened by climate change? Do you worry about what sort of world we are bequeathing to our children and grandchildren? In the words of science writer and author of “The Uninhabitable Earth” David Wallace-Wells, “No matter how well informed you are, you are surely not alarmed enough.”

I would put it even more strongly.

If the fracturing of our once stable climate doesn’t terrify you, then you don’t fully understand it. The reality is that, as far as we know, and in the natural course of events, our world has never — in its entire history — heated up as rapidly as it is doing now. Nor have greenhouse gas levels in the atmosphere ever seen such a precipitous hike.

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Review Article | Published: 05 March 2024
Projections of an ice-free Arctic Ocean
Alexandra Jahn, Marika M. Holland & Jennifer E. Kay
Nature Reviews Earth & Environment (2024)

Abstract
Observed Arctic sea ice losses are a sentinel of anthropogenic climate change. These reductions are projected to continue with ongoing warming, ultimately leading to an ice-free Arctic (sea ice area ﹤1?million?km²). In this Review, we synthesize understanding of the timing and regional variability of such an ice-free Arctic. In the September monthly mean, the earliest ice-free conditions (the first single occurrence of an ice-free Arctic) could occur in 2020–2030s under all emission trajectories and are likely to occur by 2050. However, daily September ice-free conditions are expected approximately 4?years earlier on average, with the possibility of preceding monthly metrics by 10?years. Consistently ice-free September conditions (frequent occurrences of an ice-free Arctic) are anticipated by mid-century (by 2035–2067), with emission trajectories determining how often and for how long the Arctic could be ice free. Specifically, there is potential for ice-free conditions in May–January and August–October by 2100 under a high-emission and low-emission scenario, respectively. In all cases, sea ice losses begin in the European Arctic, proceed to the Pacific Arctic and end in the Central Arctic, if becoming ice free at all. Future research must assess the impact of model selection and recalibration on projections, and assess the drivers of internal variability that can cause early ice-free conditions.

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Fig. 1. Daily surface temperature analysis from the ECMWF reanalysis version 5 (ERA5).¹

04 January 2024

James Hansen, Makiko Sato, Pushker Kharecha

Abstract. December was the 7th consecutive month of record-shattering global temperature, driven by the combination of a moderately strong El Nino and a large decrease of Earth’s albedo. The El Nino will fade in the next few months, but we anticipate that the string of record monthly temperatures will continue to a total of 12 and possibly 13 months because of Earth’s unprecedented energy imbalance. By May the 12-month running-mean global temperature relative to 1880-1920 should be +1.6-1.7°C and not fall below +1.4 ± 0.1°C during the next La Nina minimum. Thus, given the planetary energy imbalance, it will be clear that the 1.5°C ceiling has been passed for all practical purposes.

Zeke Hausfather memorably termed the record September global temperature as “gobsmackingly bananas.” Subsequent monthly temperature anomalies have not been much smaller. These records coincide with a moderately strong El Nino, but they exceed expectations for even the strongest El Nino, if that were the only driving factor. Warming is also being driven by another factor, one that

Fig. 2. Global absorbed solar radiation (W/m² ) relative to mean of the first 120 months of CERES data. CERES data² are available at http://ceres.larc.nasa.gov/

Fig. 3. 12-month running-mean of Earth’s energy imbalance from CERES satellite data normalized to 0.71 W/m² mean for July 2005 – June 2015 (blue bar) from in situ data.

is ultimately more consequential: a large decrease of Earth’s albedo. In other words, Earth has become darker, absorbing more of the sunlight incident on the planet (Fig. 2). The increase of absorbed solar radiation (1.4 W/m² ) is a decrease of Earth’s albedo (reflectivity) of 0.4% (1.4/340).³ This reduced albedo is equivalent to a sudden increase of atmospheric CO₂ from 420 to 530 ppm.

We conclude in our Pipeline paper⁴ that the decreased albedo is spurred by reduced atmospheric aerosols and enhanced by feedbacks. Given that NASA decided in the early 1990s⁵ not to make precise measurements of the global aerosol forcing and cloud feedbacks, we are faced with a difficult task of sorting out how much of the increased solar absorption is aerosol forcing and how much is from feedbacks. The two major feedbacks expected to darken Earth as the planet warms are reduced sea ice cover and reduced cloud cover. The recent spike of absorbed solar radiation to almost 3 W/m² (Fig. 2) may be related in part to the fact that it occurred during the season when solar insolation was rising in the region of Southern Hemisphere sea ice when sea ice cover was at its lowest point in the period of satellite data. Large variability of clouds, unforced and forced, complicates interpretation of anomalies, but spatial variations may help untangle the situation.

Feedbacks associated with ice melt are particularly important for reasons discussed in our Ice Melt paper⁶ (that’s the paper blackballed by IPCC because it disagrees with IPCC about the imminent threat of shutdown of the overturning ocean circulations and large sea level rise). We find in that paper that freshwater injection into the ocean mixed layer by melting polar ice is already a significant climate forcing that acts to slow down the production of deepwater in the North Atlantic and production of bottom water in the Southern Ocean.

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¹ Daily Surface Air Temperature graph based on ERA5 available via Climate Reanalyzer, Climate Change Institute, University of Maine https://climatereanalyzer.org/wx/todays-weather/?var_id=t2&ortho=1&wt=1
² Loeb NG, Johnson GC, Thorsen, TJ et al. Satellite and ocean data reveal marked increase in Earth’s heating rate. Geophys Res Lett 2021;48:e2021GL09304
³ The average solar energy incident on Earth is about 340 W/m².
⁴ Hansen J, Sato M, Simons L et al. Global warming in the pipeline. Oxford Open Clim Chan 2023;3(1):kgad008, doi.org/10.1093/oxfclm/kgad008
⁵ Hansen J, Rosa W, Fung I. Long-term monitoring of global climate forcings and feedbacks. Washington: NASA Conference Publication 3234, 1993
⁶ Hansen J, Sato M, Hearty P et al. Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 C global warming could be dangerous. Atmos Chem Phys2016;16:3761-812

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07 December 2023

James Hansen, Pushker Kharecha, Makiko Sato

The COP28 Chairman and the United Nations Secretary General say that the goal to keep global warming below 1.5°C is alive, albeit barely, implying that the looser goal of the 2015 Paris Agreement (to keep warming well below 2°C) is still viable. We find that even the 2°C goal is dead if policy is limited to emission reductions and plausible CO₂ removal. IPCC (the Intergovernmental Panel on Climate Change, which advises the UN) has understated global warming in the pipeline and understated fossil fuel emissions in the pipeline via lack of realism in the Integrated Assessment Models that IPCC uses for climate projections. Wishful thinking as a policy approach must be replaced by transparent climate analysis, knowledge of the forcings that drive climate change, and realistic assessment of policy options. The next several years provide a narrow window of time to define actions that could still achieve a bright future for today’s young people. We owe young people the knowledge and the tools to continually assess the situation and devise and adjust the course of action.

Our approach to analysis of global climate change, as described in Global Warming in the Pipeline,¹ puts comparable emphasis on (1) Earth’s paleoclimate history, (2) global climate models (GCMs), (3) modern observations of climate processes and climate change. One purpose of the Pipeline paper was to distinguish between this approach and that of IPCC, which puts principal emphasis on GCMs. GCMs are an essential tool, but the models must be consistent with Earth’s history and the projections of future climate must employ plausible scenarios for energy use and for the climate forcings that drive climate change.

Policy implications of climate science can be grasped from a basic understanding of the human-made forcings that are driving Earth’s climate away from the relatively stable climate of the Holocene (approximately the past 10,000 years). Our task is to provide understandable quantification of climate forcings and changes that will be needed to maintain a hospitable climate. Concerned public, including policymakers, must learn to appreciate basic graphs that summarize real-world data, because these must provide the basis for policy discussion.

1. CLIMATE SCIENCE

There are two major climate forcings: human-made greenhouse gases (GHGs) and aerosols (fine airborne particles). GHGs reduce Earth’s thermal (heat) radiation to space and are the main cause of global warming. Aerosols reflect sunlight to space, mainly via their effect as condensation nuclei for clouds; more nuclei lead to smaller cloud drops and brighter, longerlived, clouds. Aerosols thus cause a global cooling that partially offsets GHG warming.

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Oxford Open Climate Change, 2023, 3(1), kgad008
https://doi.org/10.1093/oxfclm/kgad008
Perspective article

Global warming in the pipeline
James E. Hansen¹*, Makiko Sato¹, Leon Simons², Larissa S. Nazarenko³,⁴, Isabelle Sangha¹, Pushker Kharecha¹, James C. Zachos⁵, Karina von Schuckmann⁶, Norman G. Loeb⁷, Matthew B. Osman⁸, Qinjian Jin⁹, George Tselioudis³, Eunbi Jeong¹⁰, Andrew Lacis³, Reto Ruedy³,¹¹, Gary Russell³, Junji Cao¹², Jing Li¹³

¹ Climate Science, Awareness and Solutions, Columbia University Earth Institute, New York, NY, USA
² The Club of Rome Netherlands, ‘s-Hertogenbosch, The Netherlands
³ NASA Goddard Institute for Space Studies, New York, NY, USA
⁴ Center for Climate Systems Research, Columbia University Earth Institute, New York, NY, USA
⁵ Earth and Planetary Science, University of CA, Santa Cruz, CA, USA
⁶ Mercator Ocean International, Ramonville St., -Agne, France
⁷ NASA Langley Research Center, Hampton, VA, USA
⁸ Department of Geosciences, University of AZ, Tucson, AZ, USA
⁹ Department of Geography and Atmospheric Science, University of KS, Lawrence, KS, USA
¹⁰ CSAS KOREA, Goyang, Gyeonggi-do, South Korea
¹¹ Business Integra, Inc, New York, NY, USA
¹² Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
¹³ Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
* Correspondence address. Director of Climate Science, Awareness and Solutions, Earth Institute, Columbia University, 475 Riverside Drive, Ste. 401-O, New York, NY 10115, USA. E-mail: jeh1@columbia.edu

Abstract
Improved knowledge of glacial-to-interglacial global temperature change yields Charney (fast-feedback) equilibrium climate sensitivity 1.2 ± 0.3°C (2r) per W/m², which is 4.8°C ± 1.2°C for doubled CO₂. Consistent analysis of temperature over the full Cenozoic era—including ‘slow’ feedbacks by ice sheets and trace gases—supports this sensitivity and implies that CO₂ was 300–350 ppm in the Pliocene and about 450 ppm at transition to a nearly ice-free planet, exposing unrealistic lethargy of ice sheet models. Equilibrium global warming for today’s GHG amount is 10°C, which is reduced to 8°C by today’s human-made aerosols. Equilibrium warming is not ‘committed’ warming; rapid phaseout of GHG emissions would prevent most equilibrium warming from occurring. However, decline of aerosol emissions since 2010 should increase the 1970–2010 global warming rate of 0.18°C per decade to a post-2010 rate of at least 0.27°C per decade. Thus, under the present geopolitical approach to GHG emissions, global warming will exceed 1.5°C in the 2020s and 2°C before 2050. Impacts on people and nature will accelerate as global warming increases hydrologic (weather) extremes. The enormity of consequences demands a return to Holocene-level global temperature. Required actions include: (1) a global increasing price on GHG emissions accompanied by development of abundant, affordable, dispatchable clean energy, (2) East-West cooperation in a way that accommodates developing world needs, and (3) intervention with Earth’s radiation imbalance to phase down today’s massive human-made ‘geo-transformation’ of Earth’s climate. Current political crises present an opportunity for reset, especially if young people can grasp their situation.

Keywords: Aerosols; Climate Sensitivity; Paleoclimate; Global Warming; Energy Policy; Cenozoic

Background information and structure of paper
It has been known since the 1800s that infrared-absorbing (greenhouse) gases (GHGs) warm Earth’s surface and that the abundance of GHGs changes naturally as well as from human actions [1, 2].¹ Roger Revelle wrote in 1965 that we are conducting a ‘vast geophysical experiment’ by burning fossil fuels that accumulated in Earth’s crust over hundreds of millions of years [3] Carbon dioxide (CO₂ ) in the air is now increasing and already has reached levels that have not existed for millions of years, with consequences that have yet to be determined. Jule Charney led a study in 1979 by the United States National Academy of Sciences that concluded that doubling of atmospheric CO₂ was likely to cause global warming of 3 ± 1.5°C [4]. Charney added: ‘However, we believe it is quite possible that the capacity of the intermediate waters of the ocean to absorb heat could delay the estimated warming by several decades.’ After U.S. President Jimmy Carter signed the 1980 Energy Security Act, which included a focus on unconventional fossil fuels such as coal gasification and rock fracturing (‘fracking’) to extract shale oil and tight gas, the U.S. Congress asked the National Academy of Sciences again to assess potential climate effects. Their massive Changing Climate report had a measured tone on energy policy—amounting to a call for research [5]. Was not enough known to caution lawmakers against taxpayer subsidy of the most carbon-intensive fossil fuels? Perhaps the equanimity was due in part to a major error: the report assumed that the delay of global warming caused by the ocean’s thermal inertia is 15 years, independent of climate sensitivity. With that assumption, they concluded that climate sensitivity for 2 × CO₂ is near or below the low end of Charney’s 1.5–4.5°C range. If climate sensitivity was low and the lag between emissions and climate response was only 15 years, climate change would not be nearly the threat that it is. Simultaneous with preparation of Changing Climate, climate sensitivity was addressed at the 1982 Ewing Symposium at the Lamont Doherty Geophysical Observatory of Columbia University on 25–27 October, with papers published in January 1984 as a monograph of the American Geophysical Union [6]. Paleoclimate data and global climate modeling together led to an inference that climate sensitivity is in the range 2.5–5°C for 2 × CO₂ and that climate response time to a forcing is of the order of a century, not 15 years [7]. Thus, the concept that a large amount of additional human-made warming is already ‘in the pipeline’ was introduced. E.E. David, Jr, President of Exxon Research and Engineering, in his keynote talk at the symposium insightfully noted [8]: ‘The critical problem is that the environmental impacts of the CO₂ buildup may be so long delayed. A look at the theory of feedback systems shows that where there is such a long delay, the system breaks down, unless there is anticipation built into the loop.’

Thus, the danger caused by climate’s delayed response and the need for anticipatory action to alter the course of fossil fuel development was apparent to scientists and the fossil fuel industry 40 years ago.² Yet industry chose to long deny the need to change energy course [9], and now, while governments and financial interests connive, most industry adopts a ‘greenwash’ approach that threatens to lock in perilous consequences for humanity. Scientists will share responsibility if we allow governments to rely on goals for future global GHG levels, as if targets had meaning in the absence of policies required to achieve them.

The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 to provide scientific assessments on the state of knowledge about climate change [10] and almost all nations agreed to the 1992 United Nations Framework Convention on Climate Change [11] with the objective to avert ‘dangerous anthropogenic interference with the climate system’. The current IPCC Working Group 1 report [12] provides a best estimate of 3°C for equilibrium global climate sensitivity to 2 × CO₂ and describes shutdown of the overturning ocean circulations and large sea level rise on the century time scale as ‘high impact, low probability’ even under extreme GHG growth scenarios. This contrasts with ‘high impact, high probability’ assessments reached in a paper [13]—hereafter abbreviated Ice Melt—that several of us published in 2016. Recently, our paper’s first author (JEH) described a long-time effort to understand the effect of ocean mixing and aerosols on observed and projected climate change, which led to a conclusion that most climate models are unrealistically insensitive to freshwater injected by melting ice and that ice sheet models are unrealistically lethargic in the face of rapid, large climate change [14].

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