Five IPCC Scenarios ‘Show How Much Suffering Can Be Averted’

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Even the most optimistic scenario in this week’s science assessment by the Intergovernmental Panel on Climate Change (IPCC) shows average global warming pushing past the crucial boundary of 1.5°C and staying there for most of the century, before gradually beginning to decline. All the other scenarios point to a far worse future.

But the differences among the five pathways make all the difference in the world.

The scenarios for future global warming form the foundation of the IPCC’s analysis, each of them based on its own assumptions about future greenhouse gas emissions and how quickly humanity can scale them back. Across the series of calculations, the IPCC’s best estimate is that average warming will increase to between 1.5 and 1.6°C by 2040, a sharp rise from today’s threshold of nearly 1.1°C. The scenarios show a “very likely” range of 1.2 to 1.9°C, depending on how quickly carbon dioxide, methane, nitrous oxide, and other atmospheric pollutants are phased out.

Even in the most optimistic scenario, warming stays just above 1.5° through at least 2080, before dialing back to 1.4° by 2100.

**IPCC Science Assessment, August 2021**

And the gaps among the scenarios get wider over time. Between 2041 and 2060, the scientists’ best estimates put average warming at 1.6 to 2.4°C, with a range between 1.2 and 3.0°. For the last two decades of this century, the best estimate is 1.4 to 4.4°C, with a range as low as 1.0°C or as high as 5.7°C.

“Global surface temperature will continue to increase until at least the mid-century under all emissions scenarios considered,” the report states. “Global warming of 1.5°C and 2°C will be exceeded during the 21st century unless deep reductions in CO2 and other greenhouse gas emissions occur in the coming decades.”

Which means that hopes of holding average warming to 1.5°C by mid-century are rapidly slipping out of reach. But at a moment when “every tenth of a degree of warming is a choice between life or death,” just as it was when the IPCC last issued its last major report in 2018, urgent action to reduce emissions is still possible. And essential.

“Every fraction of a degree averted is suffering (human and non-human) averted,” Climate Interactive Co-Director Elizabeth Sawin tweeted Friday. “Just because you can’t stop all suffering is not a reason to not prevent what suffering you can. There’s no giving up, my friends.”

“1. Yes, it’s scary. 2. No, you’re not overreacting. 3. No, the fight is not over. 4. No, you’re not alone,” added @RealHotTake co-creator Mary Annaïse Heglar.

The IPCC’s latest findings are based on improved understanding of climate processes, the climate’s response to increased warming, and ancient data from paleoclimatology, all acquired since the panel’s last science assessment eight years ago. The estimate of atmospheric warming, or “radiative forcing”, increased 19% between 2011 and 2019. The rate of warming caused by human activity rose nearly 60% between 2006 and 2018 compared to the previous 35 years, and the world’s oceans took in more than nine-tenths of the additional heat, with land warming, ice loss, and atmospheric warming accounting for just 5%, 3%, and 1%.

The massive amount of heat absorbed by the oceans, in turn, accounted for half of all sea level rise between 1971 and 2018, compared to 22% from glacier loss and 20% from diminished polar ice sheets.

With all of that data in hand, the scientists conclude that a doubling in atmospheric CO2 since pre-industrial times would produce 2.0 to 5.0°C of global warming, with a best estimate of 3.0°. The five scenarios project the even more devastating warming that would result from high or very high emissions, a more optimistic future in which emissions reach net-zero around or after 2050, and a low range that factors in various methods of removing carbon dioxide from the atmosphere.

The 1.5°C threshold “would be exceeded during the 21st century under the intermediate, high, and very high scenarios considered in this report,” the scientists write, and with varying levels of probability, it’s likely to happen by 2040. But in the lowest scenario—the one that depends on effective atmospheric carbon removal—“it is more likely than not that global surface temperature would decline back to below 1.5°C toward the end of the 21st century, with a temporary overshoot of no more than 0.1°C” before concentrations begin to decline.”

But that relatively optimistic picture still includes dramatic ecosystem and human impacts. “With every additional increment of global warming, changes in extremes continue to become larger,” the IPCC states. Those impacts “include increases in the frequency and intensity of hot extremes, marine heat waves, and heavy precipitation, agricultural and ecological droughts in some regions, and proportion of intense tropical cyclones, as well as reductions in Arctic sea ice, snow cover, and permafrost.”