By David Spratt
Prof. James Hansen is sometimes affectionately referred to as the ”godfather” of modern climate science, so when he drops a bomb, there is bound to be shock and awe.
And that’s what has happened with the recent release by Hansen and his colleagues of a draft of a new paper which finds that the climate is much more sensitive to increases in greenhouse gas that generally thought. This new analysis means that the current level of greenhouse gases, if maintained, would be enough in the longer term to melt all ice sheets and push up sea-levels by more than 60 metres.
Now the proposition that the current level of carbon dioxide has been enough in past climates to raise sea levels by tens of metres is not new. In “Atmospheric CO2 over the past 66 million years from marine archives” (2021), James W.B. Rae et al found that the last time CO2 was as high as it is today enough ice melted to raise sea level by 20 metres and it was warm enough for beech trees to grow on Antarctica. And in 2020, “A 23 m.y. record of low atmospheric CO2” concluded that today's carbon dioxide (CO2) levels are actually higher than they have been for the past 23 million years.
But back to the new draft paper, which is long and complex, and is primarily concerned with “climate sensitivity”, so that is where we should start.
Climate sensitivity
Climate sensitivity is an estimate of the temperature response to a doubling of atmospheric CO2 levels, in the present case from the pre-industrial level of 280 parts per million (ppm) to 550 ppm.
Studies vary, but Equilibrium Climate Sensitivity (ECS) has been generally estimated to be around 3°C. The respected Sherwood paper of 2020 found a range of 2.6–3.9°C.
However ECS is a measure of sensitivity with short-term feedbacks only, which are active on a decadal basis, such as changes in clouds, water vapour, changing sea-ice extent and so on.
Including factors such as “slow” feedbacks (carbon stores) and albedo changes (reflectivity), warming may be as high as 5–6°C for a doubling of carbon dioxide for a range of climate states between glacial conditions and ice-free Antarctica.
This longer-term sensitivity, on a century-to-millennium basis was termed by Hansen as Earth System Sensitivity (ESS). With unprecedented rates of change in the levels of greenhouse gases, such long-term feedbacks are becoming active in much shorter time frames, that is decades rather than centuries.
[For background, see “Carbon budgets, climate sensitivity and the myth of ‘burnable carbon’" and “New study on climate sensitivity not what poor media headlines, deniers are saying”.]
Hansen and colleagues, basing their analysis on the study of climate history (paleoclimatology), have for two decades consistently estimated ECS at around 3°C, and the longer term ESS at around 6°C, saying that ESS is around double ECS. See, for example, the 2013 paper, “Climate sensitivity, sea level and atmospheric carbon dioxide”.
By trapping heat reflecting from the Earth's surface, a doubling of CO2 increases net radiation (the climate forcing) by an average 4 watts per metre (W/m2) over the Earth’s surface. So with an ECS of 3°C, in the short term an energy imbalance of 1 W/m2 is equivalent to about 0.75°C of warming, a figure that Hansen has used on many occasions.
[Climate forcing may be understood as a physical process affecting the Earth’s climate through one or more forcing factors. For example at present, a perturbation (change) in the Earth’s energy system resulting from an imbalance between incoming and outgoing radiation caused by many factors but principally an increase in the “blanket” of human-caused greenhouse gases, with associated warming.]
As a side point, it is worth noting that Earth’s current energy imbalance compared to pre-industrial, including short-lived gases such as methane, is > 4 W/m2, suggesting equilibrium warming of 3°C without long-term feedbacks. Of course, some of that forcing from short-lived gases may diminish, and decrease that figure, but human-cause CO2 emissions are still being emitted at an undiminished rate.
So the big issue with the new Hansen et al. (not yet peer-reviewed) draft paper, “Global warming in the pipeline”, is the conclusion that “Fast-feedback equilibrium climate sensitivity (ECS) is 1.2 ± 0.3°C (2σ) per W/m2”.
That’s a big change, from 0.75 W/m2 to 1.2 ± 0.3°C W/m2, an increase of half, with big implications for the level of future warming.
That figure means that today’s energy imbalance of ~4 W/m2 would eventually lead an equilibrium to around 5°C of warming (short feedbacks only), and 10°C in the long term: “Equilibrium global warming including slow feedbacks for today’s human-made greenhouse gas (GHG) climate forcing (4.1 W/m2) is 10°C, reduced to 8°C by today’s aerosols” the draft says.
Hansen has made it clear that this is not a statement about committed warming [“The word “committed” or “commit” does not appear in our paper…”], but a warning.
The draft paper also points to data that the observed rate of decadal warming of 0.18°C per decade from 1970 to 2010 has doubled to 0.27°C since 2010, a trend that will continue due to the decreased impact of sulfate aerosols. The draft paper says: “Under the current geopolitical approach to GHG emissions, global warming will likely pierce the 1.5°C ceiling in the 2020s and 2°C before 2050.”
This should not be contentious given the amount of data available supporting such outcomes (some of it summarised here), but some scientists have been critical, leading Hansen to declare:
“Their claim that current scientific literature points to eventual global warming being kept ‘well below 2°C’ as being consistent with real world trends and policies is egregious, an uncritical acceptance of models and the assumptions that went into them.”
[For background on the role of decreasing aerosols, see for example “The case for climate cooling, and some eye-watering charts” (chart 3).]
Lessons from climate history
But back to the main point, the draft’s estimate of higher climate sensitivity. This is based on a new analysis of climate forcing and temperature variations in past climates, by comparing glacial and interglacial equilibrium climate states:
“One merit of our analysis of Cenozoic (past 66 million years) climate is that it reveals that the present human-made GHG (greenhouse gas) forcing is already greater than the GHG forcing at the transition from a nearly unglaciated Antarctica to a glaciated continent. Yes, if we leave atmospheric composition as it is today, sea level will eventually rise about 60 metres (200 feet). However, it’s not the new equilibrium at +200 feet that’s of most concern, it’s the chaos that ensues once ice sheet collapse begins in earnest.”
In their Pipeline draft, Hansen et al. say that “our principal motivation in this paper is concern that IPCC has underestimated climate sensitivity and understated the threat of large sea level rise and shutdown of ocean overturning circulations, but these issues, because of their complexity, must be addressed in two steps. Our present paper addresses climate sensitivity and warming in the pipeline, concluding that these exceed IPCC’s best estimates.”
So what’s the new paleoclimatological evidence on which they rely, noting that this paper is a draft and not yet peer-reviewed?
The paper draws together some new evidence of conditions during two recent glacial–interglacial periods: from the Last Glacial Maximum to the Holocene (LGM–Holocene) from 20,000 to 7000 years ago, and from the Eemian interglacial compared with the prior glacial maximum (PGM–Eemian), from around 140,000 to 123,000 years ago.
“We evaluate ECS by comparing stable climate states before and after a glacial-to-interglacial climate transition. GHG amounts are known from ice cores and ice sheet sizes are known from geologic data”, says the draft paper. This leads to conclusions that the initial ECS estimate is 1.22°C per W/m2 for the LGM–Holocene, and 1.36°C per W/m2 for the PGM–Eemian.
These higher results appear to be principally driven by higher estimates of the total temperature change in each case, based on recent research: “Recent advanced analysis techniques allow improved estimates of paleo temperatures.” This is estimated as 7°C of warming for the LGM–Holocene between 20,000 and 7000 years ago, higher than the existing literature.
ECS can vary with the Earth’s state. If the planet is already ice-free, for example, then the ice–albedo feedback — in which heat melts ice sheets, and their reflective surface is replaced by a darker terrain that absorbs more heat — is no longer operable. However, the author’s note that “ECS is rather flat between today’s climate and warmer climate, based on a study covering a range of 15 CO2 doublings using an efficient GCM developed by Gary Russell.”
A 60-metre sea-level rise?
The draft paper then has a long section looking at climatology over the Cenozoic era, from 66 million years ago till today. One focus is the Paleocene–Eocene Thermal Maximum (PETM), an interval of global warming that occurred around 56 million years ago, when the planet was ice-free. [The Antarctic ice sheet did not start to form until about 35 million years ago, as the planet proceeded along its long-term cooling descent from the PETM.]
The authors say that: “The PETM event provides an invaluable benchmark for assessing the eventual impact of the human-made climate perturbation and the time scale for natural recovery of the climate system.” Their conclusions are startling:
“We conclude that human-made climate forcing has reached the level that drove PETM climate change; today’s 1.2°C global warming is but a fraction of the equilibrium response to gases now in the air. The greater warming in the pipeline and its impacts are not inevitable… because climate’s delayed response allows preventative actions. Better understanding of the PETM will aid policy considerations, but we must bear in mind two major differences between the PETM and human-made climate change. First, there were no large ice sheets on Earth in the PETM era. Today, ice sheets on Antarctica and Greenland make the Earth system sensitivity (ESS) greater than it was at the time of the PETM… Equilibrium response to today’s human-made climate forcing includes deglaciation of Antarctica and Greenland, with sea level 60 metres (about 200 feet) higher than today and the potential for chaotic climate change this century. The second major difference between the PETM and today is the rate of change of the climate forcing. Most of today’s climate was introduced in a century, which seems to be 10 times or more faster than the PETM forcing growth.”
In summary, today’s climate forcing is similar to that of the PETM when the planet was ice-free. So if today’s forcing is maintained or increases, and stays that way for long enough, the planet is likely to be ice-free, raising sea-levels by more than 60 metres.
Aerosols and the need for cooling
Finally, the paper returns to the vexed question of how large is the temporary cooling (masking of warming) provided by atmospheric aerosols which have a strong, short-lived cooling effect. I have discussed this many times, including in this post: “Renowned climate scientist warns rate of global warming during next 25 years could be double what it was in the previous 50”.
Hansen et al. conclude that “Our best estimate for ECS, 1.2°C per W/m2, yields a gap of 1.5°C between expected and actual warming in 2022. Aerosols are the likely cooling source.” If that is the case, Hansen’s worry, expressed over decades — that the aerosols forcing is likely to be higher rather than lower — will have been realised, and the “Faustian bargain” even more deadly that we thought.
In its final section, the paper draws the conclusion that should be obvious to everyone: to protect the Earth and its ecosystems, and human society, atmospheric conditions need to return to those that made that fixed-settlement society possible, the Holocene, where CO2 levels were ~ 280 ppm, not the 420 ppm at present: “The enormity of consequences demands a return to Holocene-level global temperature”. This requires a dramatic drawdown of CO2 to cool the planet:
“A new plan is essential. The plan must cool the planet to preserve our coastlines. Even today’s temperature would cause eventual multimeter sea level rise, and a majority of the world’s large and historic cities are on coastlines. Cooling will also address other major problems caused by global warming. We should aim to return to a climate close to that in which civilization developed, in which the nature that we know and love thrived. As far as is known, it is still feasible to do that without passing through an irreversible disaster such as many-meter sea level rise.”
Hansen has an unsurpassed record as a climate scientist: from his ground-breaking 1988 Congress testimony, his leadership of NASA’s climate work, his big-picture analysis drawing together observations, models and paleo studies, and his fearlessness in saying what he thinks and calling out politically-mediated scientific reticence. He has often been ahead of his time and made many big calls, sometimes dismissed at the time, but later found to be right on the money.
This draft paper will be subject to a lot of scrutiny and some revision, but I doubt if the big conclusions will be substantially changed. It will be dismissed by some, as was the “superstorms” paper of 2016, but the projections in that paper of rapid increase in surface ocean heat content and slowing of the Southern Ocean overturning circulation have been vindicated by recent data.
Hansen has not so far been known to be significantly wide of the mark. If that remains true, this paper has profound implications for climate science, policymaking and strategy. If current climate models are too insensitive to climate forcings —with problems around aerosol forcing, ocean mixing and cloud feedbacks —and their outputs are of limited use, then ideas of remaining carbon budgets for 2°C are even further out the window.
Of course, amongst the most important findings is the fact that “present human-made GHG (greenhouse gas) forcing is already greater than the GHG forcing at the transition from a nearly unglaciated Antarctica to a glaciated continent”, or in other words, the current level of greenhouse gases is enough in the longer term to create an ice-free planet with sea levels 60 metres higher than today.
I wonder if policymakers could even get their heads around that proposition, let alone act on it?
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