A reconstruction of 66 million years of climate history indicates global temperature may be even more sensitive to carbon dioxide levels than current models estimate
By Alison George

7 December 2023

It is possibly the toughest question in climate science: how much warming does carbon dioxide cause? A new analysis of 66 million years of Earth’s climate history suggests the planet is much more sensitive to greenhouse gases than current climate models predict – meaning we could get far more warming in the long term.

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There is a big caveat, however. This new insight into Earth’s deep climate history covers trends over hundreds of thousands of years, not the shorter timescales of decades or centuries that are pertinent to humans today, so it doesn’t tell us what global temperatures are likely to be in 2100. “There’s a sluggish, cascading effect that slowly kicks in,” says Hoenisch.

The vast timescales covered in the study also mean it can’t detect the finer details of climate sensitivity. Michael Mann at the University of Pennsylvania says the climate sensitivity may have been different at other times in Earth’s history compared with now, and this probably explains why the study arrived at a higher estimate than those based on more recent periods.

“In short, the estimates of climate sensitivity from this study are probably not applicable to current human-caused warming,” says Mann. “Nonetheless, the study confirms the very close relationship between CO2 and global temperatures, underscoring the threat of continued fossil fuel burning.”

A Massive Study Sharpens the Outlook on Greenhouse Gases and Climate

BY KEVIN KRAJICK |DECEMBER 7, 2023

A massive new review of ancient atmospheric carbon-dioxide levels and corresponding temperatures lays out a daunting picture of where the Earth’s climate may be headed. The study covers geologic records spanning the past 66 million years, putting present-day concentrations into context with deep time. Among other things, it indicates that the last time atmospheric carbon dioxide consistently reached today’s human-driven levels was 14 million years ago—much longer ago than some existing assessments indicate. It asserts that long-term climate is highly sensitive to greenhouse gas, with cascading effects that may evolve over many millennia.

The study was assembled over seven years by a consortium of more than 80 researchers from 16 nations. It appears today in the journal Science.

“We have long known that adding CO2 to our atmosphere raises the temperature,” said Bärbel Hönisch, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, who coordinated the consortium. “This study gives us a much more robust idea of how sensitive the climate is over long time scales.

Mainstream estimates indicate that on scales of decades to centuries, every doubling of atmospheric CO2 will drive average global temperatures 1.5 to 4.5 degrees Celsius (2.7 to 8.1 Fahrenheit) higher. However, at least one recent widely read study argues that the current consensus underestimates planetary sensitivity, putting it at 3.6 to 6 C degrees of warming per doubling. In any case, given current trends, all estimates put the planet perilously close to or beyond the 2 degrees warming that could be reached this century, and which many scientists agree we must avoid if at all possible.

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ECS defined by Charney is a gedanken concept that helps us study the effect of human-made and natural climate forcings. If knowledge of ECS were based only on models, it would be difficult to narrow the range of estimated climate sensitivity—or have confidence in any range—because we do not know how well feedbacks are modeled or if the models include all significant real-world feedbacks. Cloud and aerosol interactions are complex, e.g. and even small cloud changes can have a large effect. Thus, data on Earth’s paleoclimate history are essential, allowing us to compare different climate states, knowing that all feedbacks operated.

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Earth’s surface change is the other forcing needed to evaluate ECS: (1) change of surface albedo (reflectivity) and topography by ice sheets, (2) vegetation change, e.g. boreal forests replaced by brighter tundra, and (3) continental shelves exposed by lower sea level. Forcing by all three can be evaluated at once with a GCM. Accuracy requires realistic clouds, which shield the surface. Clouds are the most uncertain feedback [52]. Evaluation is ideal for CMIP [53] (Coupled Model Intercomparison Project) collaboration with PMIP [54] (Paleoclimate Modelling Intercomparison Project); a study of LGM surface forcing could aid GCM development and assessment of climate sensitivity. Sherwood et al. [21] review studies of LGM ice sheet forcing and settle on 3.2 ± 0.7 W/ m², the same as IPCC AR4 [55]. However, some GCMs yield efficacies as low as ~0.75 [56] or even ~0.5 [57], likely due to cloud shielding. We found [7] a forcing of –0.9 W/m² for LGM vegetation by using the Koppen [58] scheme to relate vegetation to local climate, but we thought the model effect was exaggerated as real-world forests tends to shake off snow albedo effects. Kohler et al. [59] estimate a continental shelf forcing of −0.67 W/m². Based on an earlier study [60] (hereafter Target CO₂), our estimate of LGM-Holocene surface forcing is 3.5 ± 1 W/m². Thus, LGM (1821 kyBP) cooling of 7°C relative to mid-Holocene (7 kyBP), GHG forcing of 2.25 W/m², and surface forcing of 3.5 W/m² yield an initial ECS estimate 7/(2.25 + 3.5) = 1.22°C per W/m². We discuss uncertainties in Equilibrium climate sensitivity section.

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Climate response time
In this section we define response functions for global temperature and Earth’s energy imbalance that help reveal the physics of climate change. Cloud feedbacks amplify climate sensitivity and thus increase eventual heat uptake by the ocean, but cloud feedbacks also have the potential to buffer the rate at which the ocean takes up heat, thus increasing climate response time.

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