There Is No 1.5°C Climate Cliff

The world will not come to its end in 2030 because of climate change.

Dubai, United Arab Emirates—"The North Star of the COP28 Presidency is to keep 1.5°C within reach," has been the frequent refrain of Sultan Ahmed Al Jaber, the COP28 president overseeing the current U.N. climate change negotiations in Dubai. Al Jaber is reflecting the oft-chanted activist slogan "Keep 1.5 Alive!" The idea is that humanity must reduce its emissions of globe-warming greenhouse gases from burning fossil fuels sufficiently to keep the planet from warming more than 1.5 degrees Celsius above the pre-industrial baseline (1850-1900).

It is worth tracing the history of where the 1.5 C "North Star" originated and what the consequences of breaching it would likely be. The 1.5 C threshold was officially enshrined as a goal under the United Nations Framework Convention on Climate Change with the adoption of the Paris Climate Change Agreement in 2015. Article 2 in the Paris Agreement commits signatories to strengthening the global response to the threat of climate change by "holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 °C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change."

In his 2007 article in Energy Policy, University of Sussex economist Richard S.J. Tol traces the germination of the 2 C target back to a 1995 report on a workshop convened by the German Advisory Council on Global Change. It is notable that only one member of the eleven-member advisory council was a meteorologist, but, on the other hand, there were four economists.

The advisors adopted two principles to guide their work. The first was the "preservation of Creation in its present form" achieved chiefly by staying within their guess of what would be "a tolerable 'temperature window'." The second was the "prevention of excessive costs." Their analysis of what would constitute a tolerable temperature window occupies a single paragraph. There they reckoned that the mean maximum temperature during the last interglacial period was 16.1 C to which they arbitrarily added a further 0.5 C to establish tolerable maximum temperature of 16.6 C. They then assumed that in 1995 the current global mean temperature was around 15.3 C which would be only 1.3 C below their tolerable maximum. Finally, they presupposed the 1995 average was 0.7 C above the preindustrial average which yields an overall 2.0 C threshold. (For what it's worth, the advisors just as sketchily calculated that if global average temperatures rose by 2.0 C above preindustrial levels that global GDP would be 5 percent lower than it would otherwise have been.)

Tol notes that the 2.0 C target was adopted by the Council of the European Union (CEU) just a year later when it stated that it "believes that global average temperatures should not exceed 2°C above pre-industrial level." As Tol points out the CEU reaffirmed that target in 2004. Ultimately Tol persuasively argues that "the official documents that justify the 2°C warming target for long term climate policy have severe shortcomings. Methods are inadequate, reasoning sloppy, citations selective, and the overall argumentation rather thin." He adds, "This does not suffice for responsible governments, answerable to the people, when deciding on a major issue."

Nevertheless, other countries involved in climate negotiations could not ignore the European Union's push for a 2 C warming target. As it happens, the 2 C target was internationally recognized in the Copenhagen Accord which was hastily cobbled together at the last minute in order to prevent COP15 from total collapse in 2009. In the Accord, countries agreed that deep cuts in global emissions would be needed "so as to hold the increase in global temperature below 2 degrees Celsius."  In addition, the last paragraph of the Copenhagen Accord is where the lower 1.5 C was also first officially incorporated.

In their 2023 article in WIREs Climate Change, two researchers with the French Center for Scientific Research, Béatrice Cointe and Hélène Guillemot, trace the history of diplomatic jockeying that led to the order to achieve this goal, the Intergovernmental Panel on Climate Change (IPCC) report concludes that humanity must cut greenhouse gas emissions—chiefly carbon dioxide emitted by the burning of fossil fuels—in half by 2030, and reach net zero emissions by 2050. The researchers note that the lower threshold was initially championed in climate negotiations by the Alliance of Small Island States (AOSIS) who are concerned about the effects of rising sea levels owing to glacier and ice sheet melt caused by increasing temperatures. AOSIS persuaded the Least Developed Countries (LDC) bloc to sign onto the goal, resulting in its inclusion in the Paris Agreement in 2015. Cointe and Guillemot do tellingly note that "before the Paris Agreement, the 1.5°C limit was outside the range of explored pathways. It was deemed unrealistic."

However, the Paris Agreement set up a process through which the IPCC would commission a report specifically probing the effects of a temperature increase of 1.5 C and exploring possible emissions pathways to get there. The result was the IPCC's 2018 special report Global Warming of 1.5 C. That report concluded that to hold temperature increases below that threshold, humanity would have to cut greenhouse gas emissions—chiefly carbon dioxide emitted by the burning of fossil fuels—in half by 2030, and reach net zero emissions by 2050. In fact, it is the special report that is largely responsible for embedding the concept of "net zero" into current climate negotiations. "The world is going to end in 12 years if we don't address climate change," is how Rep. Alexandria Ocasio-Cortez (D-NY) in 2019 infamously mischaracterized the report's findings. And the congresswoman is far from alone. Earlier this year, President Joe Biden asserted, "If we don't keep the temperature from going above 1.5 degrees Celsius raised, then we're in real trouble. That whole generation is damned. I mean, that's not hyperbole, really, truly in trouble."

Cambridge University climate researcher Michael Hulme forcefully rejected this kind of catastrophizing deadline-ism in his 2019 editorial in WIREs Climate Change. "The rhetoric of deadlines and 'it's too late' does not do justice to what we know scientifically about climate change," observed Hulme. "It is as false scientifically to say that the climate future will be catastrophic as it is to say with certainty that it will be merely lukewarm. Neither is there a cliff edge to fall over in 2030 or at 1.5°C of warming."

Pennsylvania State University climate researcher Michael Mann also rebuffs this kind of climate doomerism. In a post on X (formerly Twitter) Mann called out Biden's comment as "Unhelpful rhetoric, unsupported by the science. It's a continuum not a cliff." He further noted, "If we miss the 1.5C exit ramp, we still go for 1.6C exit rather than give up."

To keep the global average temperature below 1.5 C, a recent U.N. report calculated that the world must cut by 2030 its greenhouse gas emissions 43 percent below their levels in 2019. The world is very unlikely to achieve such steep cuts during the next six years. So, it is good, although not surprising, news that when the world passes through the 1.5 C target, it will not be plunging to its death over a climate cliff. The upshot is that Al Jaber, climate activists, and COP28 negotiators are pursuing a fake North Star.