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Beware of unintended consequences » Yale Climate Connections


“Keep 1.5 alive” emerged as the haunting refrain of the recent United Nations climate conference in Glasgow. Although a well-intentioned rallying cry, it raises important questions about how the chant is to be interpreted. Unfortunately, 1.5° centigrade is often presented as an immutable crisis point, rooted in established scientific consensus. It’s implied that beyond this point, climate-induced damages increase dramatically.

Given the prospect that the 1.5°C target may not be met, proponents might come to rue their choice of mantra. Not only will it likely cause unnecessary despair, but oversimplification of the underlying science provides those resolutely opposed to acting on climate change with opportunities for further mischief.

This is not to argue that danger points don’t exist. There is ample evidence in paleoclimate records of sudden and dramatic shifts in Earth’s climate system, though the conditions that triggered them are not fully understood and are not directly comparable to present-day conditions.

Another reason for concern comes from computer models of the climate system. Models also alert us to the possibility that continued warming may cause rapid climate changes that cannot be easily reversed for centuries or longer. Examples of such changes include abrupt sea-level rise and slowing or even shutdown of a key part of the ocean’s system for circulating heat. While few studies suggest that these changes are imminent, models and “deep time” climate records both tell us that somewhere out there, beyond the 1.1°C of warming experienced to date, dangerous tipping points exist. We don’t know exactly where they are, just that further warming makes it more likely we will cross them.

This understanding often contrasts sharply with the statements made by public officials and the press. For example, United Nations Secretary-General Guterres implicitly attributes the politically-set 1.5°C temperature target to the science. He refers to the 2021 release of the Physical Science section of the latest IPCC Assessment Report as a “code red for humanity” and notes that “The internationally agreed threshold of 1.5°C is perilously close.” But nowhere does the IPCC special report on the impacts of 1.5°C warming suggest that a critical, well-established threshold exists at that precise level of warming: It simply concludes that limiting warming to 1.5°C, if it could be achieved, would be better than limiting warming to 2°C.

Likewise, climate stories in the news media commonly refer to a 1.5°C critical threshold beyond which the planet will experience increasingly deadly floods, wildfires, and enormous storms – and then add “scientists say.”  Crediting the climate science with establishing a specific damage threshold at 1.5°C of warming, which scientists did not do, risks erosion of public trust in their work. It also provides ammunition for those who would criticize the scientific enterprise for hyping the issue, or for using scare tactics that go beyond the available evidence.

Limiting global warming to 1.5°C is a daunting task. The climate system has not fully adjusted to the levels of greenhouse gases currently in the atmosphere, so some additional warming is already baked in, even if all emissions were to cease overnight.

There are also inherent impediments to how quickly existing energy-producing systems and capital stock (power plants, transport, and buildings) can be replaced or renovated to create nonpolluting alternatives. Together, these lags – in both the physical and human systems – suggest that 1.5°C may well already be in the rearview mirror.

Strong language, metaphors understandable…but carry risks

These are dramatic times for Earth’s climate. It is not surprising that those trying to spur public effort to counter the real and serious threat of global climate change use dramatic language and metaphors, try to anchor political targets in scientific proof, and stress frightening consequences of inaction. But motivating action on climate change is a delicate undertaking. While the scale of coming climate damage clearly calls for urgent action to transform the energy economy, there are risks in painting too bleak a picture of the challenge, and in presenting 1.5°C as the well-established climate equivalent of an end-of-the-world prediction.

One must be aware of the possibility of unintended and unwanted consequences: Paralysis and despair may arise if millions believe that exceeding the 1.5°C target inevitably signals climate Armageddon, beyond which all is lost. Such despair would imperil the continued energy and attention needed to sustain the global effort to cut greenhouse emissions.

While useful as a spur to action, “Keeping 1.5 Alive” must not be allowed to obscure the fact that it’s worth fighting to prevent every 0.1°C of additional warming – up to and (importantly!) beyond 1.5°C. And that every 0.1°C of warming avoided is cause for celebration and hope.

Richard Richels directed climate change research at the Electric Power Research Institute (EPRI). He served on the National Assessment Synthesis Team for the first U.S. National Climate Assessment and as lead author for multiple chapters for the IPCC from 1992 through 2014.

Henry D. Jacoby is the William F. Pounds Professor of Management, Emeritus in the M.I.T. Sloan School of Management and former co-director of the M.I.T. Joint Program on the Science and Policy of Global Change.

Ben Santer is a climate scientist and John D. and Catherine T. MacArthur Fellow. He was also the lead author of Chapter 8 of the 1995 IPCC report and has been a contributor to all six IPCC reports.

Gary Yohe is the Huffington Foundation Professor of Economics and Environmental Studies, Emeritus, at Wesleyan University in Connecticut. He served as convening lead author for multiple chapters and the Synthesis Report for the IPCC from 1990 through 2014 and was vice-chair of the Third US National Climate Assessment.



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