You may have seen the news reporting the latest research on climate sensitivities. Some outlets have happily declared that the worst-case scenarios are now off the table. Others have sounded the alarm that our lower-temperature targets have been pushed out of reach. Both these interpretations are flawed. To get the real picture, we need to consider climate sensitivity.
Climate sensitivity concerns how responsive the climate system is to changes in forcings (e.g. human produced greenhouse gases). Future temperature rises depend both on climate sensitivity and on future concentrations of greenhouse gases. Climate sensitivity alone will not tell us how physical and human systems will respond to global warming. Nevertheless, climate sensitivity can provide us with important insights for our climate future.
Over 100 years ago, Swedish chemist Svante Arrhenius estimated sensitivity to a doubling of atmospheric CO2 at around 3-4 ˚C. His estimate was relatively accurate, but he lacked some of the specific details. In the 1970s, with a deeper understanding of climate system dynamics, scientists produced the Charney Report, which arrived at a similar value, suggesting that doubling CO2 would raise temperatures between 1.5 ˚C and 4.5 ˚C.
Research over the past decade seems to indicate that doubling CO2, CO2equivalent to be precise (CO2eq), leads to about 1 ˚C of direct warming. After the direct warming, it is critical to consider feedbacks in the climate system. Unlike forcings, feedbacks are internal to the climate system and can amplify (positive feedbacks) or mitigate (negative feedbacks) changes. Higher levels of CO2 lead to some rapid positive feedbacks, predominantly through increases in atmospheric water vapor (itself a potent greenhouse gas) that is responsible for a good deal more of the warming. Then, longer-term positive feedbacks, including changes in albedo related to ice melting, add about another degree. All of this results in a sensitivity close to Arrhenius’ original 3-4 ˚C estimate.
What we are really seeing in this latest report is a tightening of the error bars around that sensitivity. We have been able to do this for a few reasons. First, the understanding of feedbacks has improved significantly in recent years. Next, climate models are also better able to incorporate these feedbacks into their estimates. Finally, climate scientists also just have a lot more data from years of constantly raising the CO2 levels, and we are of course closer to a doubling of pre-industrial levels than at any point in modern human history. The closer the planet gets to that doubling, the easier it will be to empirically demonstrate the sensitivity.
As far as what this means, it is hard to consider this good or bad news. There is no question we are far away from our climate goals, and we are not going to get much help from a less sensitive atmosphere. However, few were expecting that result. For those creating integrated assessment models (IAMs), a lower sensitivity would have allowed for higher levels of CO2eq for the same amount of warming and thus given a boost to carbon budgets. This would have meant that a transition could proceed at a slightly slower pace (although still far faster than it currently is) and still let us hit our Paris targets. Most IAMs and other energy models use a carbon budget that is implied by a mid-range sensitivity anyway, so these latest results are unlikely to vastly change these IAMs.
While we can rule out super high sensitivities, such a finding says nothing about how hot things will get and for how long. Climate change impacts are a bit like an ice cube in an oven. To understand how bad the impacts will be you need to know both how hot the oven and how long the ice cube stays in. In this analogy sensitivity is almost how quickly the oven heats up. It matters how many GHGs we plan to emit in the future and for how long. For example, even with lower sensitivities, continued emissions may take us above 4 ˚C in 2120 rather than 2100. That hardly feels comforting and is one reason relying on a fixed time horizon like 2100 inherently limits the discussion.
In short, the latest research marks an advance in our understanding of how climate systems behave. The climate emergency is just as serious as ever and the need for climate action is equally urgent.
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