"The climate time-bomb is ticking," declared United Nations Secretary-General António Guterres at the press conference on Monday launching the Intergovernmental Panel on Climate Change's (IPCC) AR6 Synthesis Report. He called it "a survival guide for humanity." The report is supposed to be a comprehensive summary of the scientific, economic, and policy findings of six earlier IPCC climate reports.
The report provoked dire headlines. "World is on brink of catastrophic warming," warned The Washington Post. The New York Times proclaimed, "World Has Less Than a Decade to Stop Catastrophic Warming." And The Guardian starkly asserted, "Scientists deliver 'final warning' on climate crisis: act now or it's too late."
What is the supposed looming climate catastrophe? Exceeding the threshold in which global average temperature rises 1.5 degrees Celsius above the 1850-1900 baseline. That threshold was established in the 2015 Paris Climate Change Agreement, which aims to hold "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." In order to have a 50/50 chance of achieving that goal, the new report calculates humanity must cut its greenhouse gas emissions (chiefly carbon dioxide) basically in half by 2030. Secretary-General Guterres asserted that the report shows that "the 1.5-degree limit is achievable."
Will humanity inevitably suffer a catastrophic fall if we go over the supposed 1.5 degrees Celsius climatic cliff in 2030? No, argues University of Cambridge climate researcher Mike Hulme in his October 2019 editorial introducing a special issue of the journal WIREs Climate Change devoted to the question, "Is it too late (to stop dangerous climate change)?"
Hulme notes, "There is a long history of climate deadlines being set publicly by commentators, politicians and campaigners…and then of those deadlines passing with the threat unrealized." As an example, he cites Secretary-General Guterres' September 2018 assertion, "If we do not change course by 2020, we risk missing the point where we can avoid runaway climate change, with disastrous consequences for people and all the natural systems that sustain us."
Hulme pointedly observes that "deadline-ism" as embodied in such claims "does not do justice to what we know scientifically about climate change." Climate change prediction science reports "a range of possible values for future global warming. 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." He adds, "Neither is there a cliff edge to fall over in 2030 or at 1.5°C of warming."
It is the case that the world's average temperature is about 1.1 degrees Celsius higher than it was between 1850 and 1900. The bulk of that temperature increase largely stems from burning fossil fuels that have loaded up the atmosphere with extra heat-trapping carbon dioxide. Atmospheric carbon dioxide stood at about 285 parts per million around 1850, rising to about 316 ppm by 1958 and is now at 420 ppm.
The report states that the evidence has "strengthened" that man-made global warming is responsible for observed changes in extremes such as heat waves, heavy precipitation, droughts, and tropical cyclones. Recent studies do show that the intensity, frequency, and duration of heat waves have increased since the 1950s and that the frequency of heavy rainfall events has also risen. On the other hand, clear evidence for changes in global trends in meteorological drought is lacking and global tropical cyclone accumulated energy (a measure of the combined duration and strength of tropical cyclones) is not increasing.
"Risks and projected adverse impacts and related losses and damages from climate change will escalate with every increment of global warming (very high confidence). They are higher for global warming of 1.5°C than at present, and even higher at 2°C (high confidence)," states the report. "Deep, rapid, and sustained mitigation and accelerated implementation of adaptation actions in this decade would reduce future losses and damages related to climate change for humans and ecosystems (very high confidence)." Deep, rapid, and sustained mitigation means cutting global greenhouse gas emissions in half by 2030.
Interestingly, the report does not put a dollar figure on the losses that are projected to result from unmitigated climate change. Perhaps, as the report asserts, that is because "cost-benefit analysis remains limited in its ability to represent all avoided damages from climate change (high confidence)." Still, the report does note, "Even without accounting for all the benefits of avoiding potential damages the global economic and social benefit of limiting global warming to 2°C exceeds the cost of mitigation in most of the assessed literature (medium confidence)." A discreet footnote observes, "The evidence is too limited to make a similar robust conclusion for limiting warming to 1.5°C." So the costs of trying to keep temperatures from increasing by 1.5 degrees Celsius might be greater than the benefits?
While the researchers associated with the IPCC are reluctant to explicitly calculate the costs and benefits of deep, rapid, and sustained mitigation, other analysts have not been. University of Sussex economist Richard S.J. Tol has spent most of his career working on the economics of climate change. He finds in his most recent study, Costs and Benefits of the Paris Climate Targets, that the costs of implementing climate policies that aim to keep average global temperature by 2100 below the two Paris threshold temperatures of 2.0 and 1.5 degrees Celsius would respectively cost 3.8 to 5.6 percent of global GDP. In comparison, the benefits of climate policy would amount to 2.8 to 3.2 percent of GDP.* The upshot is that the costs outweigh the benefits of steeply cutting greenhouse gas emissions in order to keep average temperatures below the Paris Agreement thresholds.
Let's make these numbers more concrete. Without climate change and assuming that the world's GDP of $107 trillion grows at a relatively modest rate of 2 percent annually for the next 77 years, world GDP would be nearly $500 trillion by 2100. Average incomes then for the world's 9 billion inhabitants would be around $55,000 per capita. (Current GDP per capita is just over $12,000.) Implementing policies to keep temperatures below 2 degrees Celsius would cut global GDP in 2100 to $472 trillion ($52,400 per capita). In contrast, allowing global temperatures to rise would reduce global GDP to $484 trillion ($53,800 per capita). If Tol is right, the costs of mitigating climate change would make people a bit poorer than they would otherwise have been.
In another 2022 analysis of 61 estimates, from 33 studies, of the total economic impact of climate change, Tol reports that "a global warming of 2.5°C would make the average person feel as if she had lost 1.7% of her income."
So how much warming is likely to occur? University of Colorado climate change policy researcher Roger Pielke Jr. and his colleagues conclude in their 2022 Environmental Research Letters study that IPCC's worst-case scenarios are highly implausible. Consequently, the good news is that global average temperature by 2100 is likely to be between 2 and 3 degrees Celsius higher than the 1850-1900 baseline with a median estimate of 2.2 degrees Celsius. That is only slightly higher than the Paris Agreement's 2.0 degree Celsius threshold.
These calculations and projections do not suggest that humanity has "less than a decade to stop catastrophic warming."
*Keep firmly in mind that both the IPCC and Tol are combining estimates from climate models and economic models to make projections about the state of the world 77 years from now. That would be like people living in 1946 predicting the state of the world we live in now.
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