Ice age climate analysis reduces worst-case warming expected from rising CO₂

[Jim__]

From phys.org

The left panel shows the sea surface temperature map during the most recent ice age, 21,000 years ago, compared to modern preindustrial temperatures. This new, more detailed analysis shows that the strong cooling over the northern oceans, caused by the North American ice sheet, contributed substantially to total global cooling. The right panel shows that the warming of the ocean's surface expected under future doubling of atmospheric CO2 displays a different pattern of temperature change, with a lower expectation for globally averaged warming than previous worst-case estimates. Credit: Science Advances (2024). DOI: 10.1126/sciadv.adk9461
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As carbon dioxide accumulates in the atmosphere, the Earth will get hotter. But exactly how much warming will result from a certain increase in CO2 is under study. The relationship between CO2 and warming, known as climate sensitivity, determines what future we should expect as CO2 levels continue to climb.

New research led by the University of Washington analyzes the most recent ice age, when a large swath of North America was covered in ice, to better understand the relationship between CO2 and global temperature. It finds that while most future warming estimates remain unchanged, the absolute worst-case scenario is unlikely.

The open-access study was published April 17 in Science Advances.

"The main contribution from our study is narrowing the estimate of climate sensitivity, improving our ability to make future warming projections," said lead author Vince Cooper, a UW doctoral student in atmospheric sciences. "By looking at how much colder Earth was in the ancient past with lower levels of greenhouse gases, we can estimate how much warmer the current climate will get with higher levels of greenhouse gases."

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[NickB79]

1. Summary: we are at risk of "only" 4C of warming, not 5C, by century's end

Based on the assumption we'll hit double the CO2 levels compared to preindustrial times (280 ppm) by 2100. We're currently at 425 ppm, rising around 2.5 ppm per year now. Throw in all the extra methane, and we might be flirting with 475 ppm equivalent.

Three points:

1: Methane levels 21,000 yr ago were not nearly as high as they are today, and I don't know if the study took this into consideration.

2: 4C of warming is catastrophic no way you slice it. All coral reefs will go extinct, the Arctic will permanently lose it's ice sheets, sea levels will eventually rise 50-75' as vast swaths of Greenland and Antarctica melt away, the permafrost will thaw, all coastal cities will flood, vast areas of farmland will be inundated by the ocean, the Amazon will die and burn, most boreal forests will die and burn, and half the planet will become uninhabitable. There's a good chance global trade comes to a screeching halt and most advanced civilization collapses.

2: Due to positive feedback loops like the burning of the Amazon and the boreal forests and the thawing of the permafrost, there's very little guarantee that CO2 levels will stop rising by 2100, even if humans transition to 100% renewables. If they keep rising for centuries to come, and CO2 levels eventually level out at 650-700 ppm, 5-6C of warming is back in play.

[Jim__]

2. I believe a better summary: we have improved the ability to model climate sensitivity.

As the lead author is reported in the phys.org article to have said: "The main contribution from our study is narrowing the estimate of climate sensitivity, improving our ability to make future warming projections".

Which is also the emphasis given in the paper's abstract:

Abstract

Here, we show that the Last Glacial Maximum (LGM) provides a stronger constraint on equilibrium climate sensitivity (ECS), the global warming from increasing greenhouse gases, after accounting for temperature patterns. Feedbacks governing ECS depend on spatial patterns of surface temperature (“pattern effects”); hence, using the LGM to constrain future warming requires quantifying how temperature patterns produce different feedbacks during LGM cooling versus modern-day warming. Combining data assimilation reconstructions with atmospheric models, we show that the climate is more sensitive to LGM forcing because ice sheets amplify extratropical cooling where feedbacks are destabilizing. Accounting for LGM pattern effects yields a median modern-day ECS of 2.4°C, 66% range 1.7° to 3.5°C (1.4° to 5.0°C, 5 to 95%), from LGM evidence alone. Combining the LGM with other lines of evidence, the best estimate becomes 2.9°C, 66% range 2.4° to 3.5°C (2.1° to 4.1°C, 5 to 95%), substantially narrowing uncertainty compared to recent assessments.

Improving climate modeling is a good thing.