Hansen’s revolutionary call to action

The publication of James Hansen et al’s ‘Global Warming in the Pipeline’ paper in November 2023 (previously released as a preprint) was a watershed moment for many in the climate movement. Together with rapidly accelerating global temperatures and a rising toll of catastrophic weather events, the accumulation of scientific evidence is challenging our simple narratives.

Some have concluded that not only is runaway warming and societal collapse now inevitable, but that attempting to put pressure on governments to cut carbon emissions is a waste of valuable time. Time that would be better spent attending to the needs of our communities, so that we can build resilience for what is to come.

But while the news is bleak and the evidence is gathering that we are getting close to many important climate tipping points, the passing of which would trigger catastrophic outcomes for humanity, nothing about the collective response to our situation is certain or pre-determined. The fact is that we cannot know the future (good or bad) and in that uncertainty we still have room to believe that civil resistance can make a difference to outcomes.  

This is not the same as blind faith in the power of government or in the potential of magical technologies, otherwise known as ‘hopium’. It is not the cheery Micawberish belief that “something will turn up”. But neither is it the pessimistic, deterministic certainty that we are utterly fucked and there’s nothing anyone can do.  Any of these positions is an excuse for inaction and that is the last thing any of us needs.

    So while attending to the needs of our communities and building resilience is vital work, it is something that must happen alongside, and as part of, the building of a revolutionary movement to challenge our broken political and economic system rather than as an alternative.

    Yes, as things stand, with another COP meeting ending in disappointment and failure, its hard not to conclude that we are fucked but there is something we can do to change where we stand and that is a message that Hansen seems to agree with. So in that spirit here is a summary of Hansen’s key findings.

    The lag between cause and effect

    “The climate on our remarkable home planet is characterised by delayed response and amplifying feedbacks, which is a recipe to lock in intergenerational injustice”  – James Hansen

    The Hansen paper recalls that we have known since the late 1970s that there is a delay between rising CO2 concentrations in the atmosphere, caused by the burning of fossil fuels, and rising global temperatures, because of the slow response time and immense heat absorbing capacity of the oceans. 

    While previously thought to be of the order of 15 years, by the mid 1980s paleoclimate data and global climate modelling together led to estimates that the delay is of the order of a century.

    The paper quotes E.E. David, Jr, President of Exxon Research and Engineering, talking at a symposium in 1982: ‘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.’

    In other words, the fossil fuel industry was aware 40 years ago of the existence of a considerable lag in the global temperature response to increased CO2 concentrations in the atmosphere. They had already identified this lag between cause and effect as a key threat to humanity, but subsequently chose to weaponise the “uncertainty” to push the required phase out of fossil fuels into the long grass. 

    The authors note: “A climate characterised by delayed response and amplifying feedbacks is especially dangerous because the public and policymakers are unlikely to make fundamental changes in world energy systems until they see visible evidence of the threat. Thus, it is incumbent on scientists to make this situation clear to the public as soon as possible. That task is complicated by the phenomenon of scientific reticence.”

    A new estimate for Equilibrium Climate Sensitivity

    The sensitivity of the Earth’s climate to increases in atmospheric CO2 concentration is a question that sits at the heart of climate science. Sensitivity estimates are either derived from detailed physics based models of how the climate works or estimated from instrumental records or from examination of paleo climate data.

    Equilibrium climate sensitivity (ECS) is a standard measure of how much the global mean temperature is expected to rise for a doubling of atmospheric CO2 concentrations compared with pre-industrial levels after taking account of various feedback effects. It was first proposed in a 1979 United States National Academy of Sciences study led by Jule Charney which concluded that a doubling of atmospheric CO2 would likely cause eventual global warming of 3°C (with a range of  1.5°C – 4.5°C). 

    Charney’s calculations take account of fast feedbacks – elements of the climate system that show rapid  responses to rising temperature – such as clouds, aerosols, water vapour, snow cover and sea ice, but did not incorporate slow feedbacks such as melting ice sheets and changes in vegetation or the impact of the long-lived non-CO2 GHGs. 

    45 years after Charney’s estimate was first published, estimates of climate sensitivity have not changed significantly. The 2021 Intergovernmental Panel on Climate Change (IPCC) report concludes that 3°C for a doubling of CO2 is the best estimate for ECS. Hansen et al suggests that the IPCC  has underestimated this climate sensitivity and therefore the threat of large-scale sea level rise and the potential for a shutdown of ocean overturning circulations. 

    For reference, the current level of CO2 is 422.38 ppm (October 2024) compared with around 280 ppm prior to the Industrial Revolution (and for almost 6,000 years of human civilization). Thus we are about halfway towards a doubling of CO2. According to Hansen et al, if you add in Methane and the other greenhouse gases the actual climate forcing is already equivalent to a doubling of CO2 . (CO2e is currently about 534 ppm). Hansen notes “Humanity is now taking its first steps into the period of consequences”.  

    The Hansen paper reviews a wide range of climatological information to quantify a new estimate for the  ECS at 4.8°C (with a range of 3.6°C- 6.0°C) , incorporating fast feedback effects.  A higher ECS  means there is more warming “in the pipeline” than previously assumed. The eventual climate response under this new ECS estimate is +5.5°C. Since we have experienced about 1.5°C of warming already, Hansen is saying that there is 4°C of warming in the pipeline from fast feedback effects alone. The paper suggests that it would take around 100 years to reach 63% of the effect. 

    One major point of difference between Hansen and IPCC is the magnitude of the role of aerosols in masking the impact of rising CO2 concentrations on global temperatures

    Aerosols and the acceleration in global heating

    Aerosols have a cooling effect on the climate by reducing the amount of energy from the sun that reaches the Earth’s surface. Hansen et al notes that aerosols have likely been having a significant cooling effect for as long as humans have been burning wood and fossil fuels. However, since the Industrial Revolution, humans have sharply increased the amount of aerosols in the air, which has masked the warming that would have otherwise occurred. 

    More recently there has been a reduction in the level of aerosols brought about by the phasing out of sulphur in diesel and marine fuels. This has increased the amount of solar radiation hitting the earth’s surface and therefore Earth’s energy imbalance and resulted in an acceleration of global warming in the past decade. The paper estimates that the decline of aerosol emissions since 2010 has increased the global warming rate from 0.18°C per decade in 1970–2010 to at least 0.27°C per decade since 2010. 

    This leads to Hansen’s prediction that the global average temperature rise above pre-industrial levels will pass 1.5°C in the 2020s and reach 2°C before 2050.  He is quoted elsewhere as suggesting the world will pass 2°C in the 2030s.

    The authors note that since neither climate sensitivity nor aerosol forcing are directly measured, many combinations of climate sensitivity and aerosol forcing can fit the observed pattern of global warming to date. The models referred to in IPCC analysis have lower climate sensitivity and a smaller role for aerosol cooling, while Hansen has higher sensitivity and a larger role for aerosol cooling. Both are consistent with the observed temperature rise prior to the last couple of years, but only Hansen’s explanation appears fully consistent with the “gobsmackingly bananas” temperature rises we’ve seen through 2023 and 2024. 

    Earth system sensitivity and slow feedbacks

    The paper also introduces the concept of Earth System Sensitivity (ESS). This is looking at the earth’s response over much longer time scales. These “slow feedbacks” include changes in carbon sinks and sources as the planet warms as well as a decrease in the amount of solar radiation reflected back into space (ice albedo effect) caused by the melting of Greenland and Antarctica, over 1000 year timescales. ESS also includes the amplifying feedback of non-CO2 GHGs.

    The headline conclusion is that:  “Equilibrium global warming for today’s GHG amount is 10°C, which is reduced to 8°C by today’s human-made aerosols.”

    In other words, if we do nothing, and the level of greenhouse gas concentrations remains unchanged at today’s level,  then the eventual long term global heating in the atmosphere will be 10°C above pre-industrial levels.  Since we have experienced about 1.5°C of warming already, Hansen is saying that there is 7-9°C of warming in the pipeline. 

    However, Hansen is very clear that: “Equilibrium warming is not ‘committed’ warming; rapid phaseout of GHG emissions would prevent most equilibrium warming from occurring.”

    These new equilibrium warming calculations assume atmospheric concentration of GHGs remains fixed at today’s level over the long term and they incorporate the effects of all the fast and slow feedbacks due to that initial concentration level. Hansen is clear that if we can reduce GHG concentrations in the atmosphere we can avoid most of this increase in global heating. But cautions that even if human emissions ceased entirely it would take millennia for CO2 to decline to pre-industrial levels. This underscores the difficulty of restoring Earth’s energy balance via emission reductions alone. 

    What is in store?

    Hansen goes on to pose the question with climate in a state of extreme energy imbalance, how much time do we have before we pass the point of no return, the point where major climate impacts are locked in, beyond our ability to reverse on any time scale humans care about? The prime point of no return the paper identifies is the collapse of the West Antarctic ice sheet. 

    In a previous paper sidelined by the scientific community, Hansen and colleagues had used evidence of observed ice sheet changes and paleoclimate evidence of sea level rise by several metres in a century within a climate model and found that ice sheet collapse is an exponential process. The growth of meltwater and GHG emissions led to shutdown of the North Atlantic and Southern Ocean overturning circulations, amplified warming at the foot of the ice shelves that buttress the ice sheets, and drove other feedbacks consistent with accelerating sea level rise, reaching several metres in 50–150 years.

    In the current paper, based on examination of paleoclimate data, Hansen found that AMOC shutdown is not unusual and occurred in the last interglacial period, the Eemian, about 130,000 years ago. At that time, global temperature was similar to today and sea level rose a few metres within a century with the likely source being the collapse of the West Antarctic ice sheet. Hansen concludes that the equilibrium response to current CO2 concentrations is a nearly ice-free Antarctica, with enough ice in contact with the ocean to provide around 25m of sea level rise. 

    The paper notes that while these impacts are expected to occur over a longer time scale than most people care about, the danger is that it will become impossible to prevent large sea level rise, if deglaciation is allowed to get well underway. The present huge energy imbalance means that the climate will become less tolerable to humanity, with greater climate extremes, before it is possible to reverse the trend. But reversing the trend is essential for the sake of preserving shorelines and saving the world’s coastal cities. This means cooling the planet. 

    Policy implications

    Hansen proposes: “Reduction of greenhouse gas emissions as rapidly as practical has highest priority, but that policy alone is now inadequate and must be complemented by additional actions to affect Earth’s energy balance. The world is still early in this ‘vast geophysical experiment’—as far as consequences are concerned—but time has run short for the ‘anticipation’ that E.E. David recommended.”

    The paper concludes that the enormity of consequences demands a return to a climate like that in which civilization developed with a Holocene-level global temperature and that it is still feasible to do that without passing through irreversible disasters such as many-metre sea level rise. It recommends action to reduce emissions by imposing a global carbon price as well as increased global cooperation on emissions reduction and renewable energy and action to reduce and reverse Earth’s energy imbalance.

    The highest priority is to phase down emissions, but it is no longer feasible to rapidly restore energy balance by emission reductions alone. The authors note that carbon capture cannot be viewed as the solution, although it may play a role in a portfolio of policies, if its cost is driven down. Instead additional action is needed to prevent rapid escalation of climate impacts including lock-in of sea level rise that could destroy coastal cities world-wide. Temporary solar radiation management (SRM) through purposeful injection of atmospheric aerosols will likely be needed.

    The authors note that it will take many years to define and gain acceptance of an approach for climate restoration but this effort should not deter action on emissions reduction – indeed discussion of intervention in the climate may drive greater acceptance of emissions reduction. 

    Vehicles swept away in the Valencia floods. Over 200 people have died with many still reported missing.

    Conclusion: Hansen’s revolutionary call to action

    To conclude, here are the authors’  words about the implications of the Global Warming in the Pipeline paper taken from an email to the scientific community on 29 March 2024 entitled Global Warming Acceleration: Hope vs Hopium:

    “We have shown that the world is approaching a point of no return in which the overturning ocean circulation may shut down as early as mid century and sea level rise of many metres will occur on a time scale of 50-150 years. Time is running short to make the public and policymakers aware of the threat posed by the delayed response of our climate system and of the actions that should replace present wishful thinking (hopium). This education will not happen instantly, but it is realistic to hope that we can greatly improve understanding this decade, a period that should be long enough to expose the fruitlessness of present policies, as well as to verify the physics of ongoing climate change.”

    Finally,  this conclusion and call to action from the Pipeline paper itself:

    “It is asking a lot to expect young people to grasp the situation that they have been handed—but a lot is at stake. As they realise that they are being handed a planet in decline, the first reaction may be to stamp their feet and demand that governments do better, but that has little effect. Nor is it sufficient to parrot big environmental organisations, which are now part of the problem, as they are partly supported by the fossil fuel industry and wealthy donors who are comfortable with the status quo. Instead, young people have the opportunity to provide the drive for a revolutionary third party that restores democratic ideals while developing the technical knowledge that is needed to navigate the stormy sea that their world is setting out upon.”