Nature’s Emissions Could Add Up to 0.4°C of Warming

A new estimate suggests greenhouse gases released as natural systems warm could add 0.2°C to 0.4°C to global temperatures by 2100. The study combines projections for thawing permafrost, intensifying wildfires, warmer wetlands and inland waters, four sources that are not represented together in the climate models most often used to guide policy.

Quick Summary

• The four feedbacks could amplify post-2020 human-caused warming by roughly 20% to 30% this century.
• The central estimate is about 0.2°C of extra warming under strong climate action and 0.4°C under a higher-emissions pathway.
• The combined estimate has a large uncertainty of about ±0.2°C at one standard deviation.
• Cutting human emissions still reduces the size of these natural feedbacks.

The result is a modeled risk estimate, not evidence that an extra 0.4°C is unavoidable.

The paper, published in Environmental Research Letters on September 10, derives relationships between temperature and additional carbon dioxide and methane emissions from process-based studies. The researchers then ran those relationships through the reduced-complexity MAGICC climate model under three Shared Socioeconomic Pathways.

Permafrost contributes the largest share in the central estimates. As frozen ground thaws, microbes can convert old organic matter into carbon dioxide or methane. Hotter and drier conditions can also increase wildfire emissions, while warmer wetlands, lakes and rivers can accelerate methane-producing microbial activity.

Why current projections can miss the effect

None of the 11 Earth-system models used in the latest major UN climate assessment included all four sources, according to the researchers. Two represented permafrost, five included wildfire, and none included the modeled wetland and freshwater responses together with the other sources.

That omission matters because carbon budgets translate a temperature limit into how much more human-caused carbon dioxide can be released. If warming itself unlocks additional greenhouse gases, the remaining budget can be smaller than a model without those feedbacks implies. This adds a distinct mechanism to the warning in the recent UN assessment of climate overshoot.

Reality Check

The 20% to 30% figure describes amplification of warming after 2020, not a measured 30% jump in today’s emissions. It also carries wide uncertainty. The estimates combine separate process models, and nonlinear effects mean the contribution from each source cannot simply be added. Some possible warming-induced emissions, including extra nitrous oxide from soils, were excluded for lack of reliable global estimates, while cooling feedbacks were outside the study’s scope.

The analysis does not predict an inevitable runaway climate. Under the lower-emissions SSP1-2.6 pathway, the feedback is smaller than under SSP4-6.0. Rapidly reducing fossil-fuel and other human emissions therefore limits both the initial warming and the natural emissions it triggers. A new international model-comparison project is intended to test the estimates with a broader set of models.