Natural feedbacks will warm world an extra 20 per cent this century

Wetlands in northern Norway fruttuoso pinato / Alamy Feedbacks that cause ecosystems to emit more carbon will make the climate crisis more severe than expected, amplifying global warming by 20 to 30 per cent this century. It isn’t just fossil fuel emissions that raise temperatures. Warming itself creates more warming. For instance, a hotter atmosphere absorbs more water vapour, which traps more heat in the atmosphere, which then holds more water vapour. Climate models already incorporate most global feedback loops like this one, and they have started to include feedbacks that are disrupting carbon storage in particular ecosystems. But no single model includes all ecosystem feedbacks. Now, a study has shown that these natural feedbacks will heat the planet 0.2°C to 0.4°C by 2100, on top of at least 1°C to 2°C of global warming already expected between now and then. “That suggests we’re going to go further over 1.5°C global warming than we thought, with all the extra risks that brings,” says Tim Lenton at the University of Exeter in the UK, who wasn’t involved in the new research. Christina Schädel at Woodwell Climate Research Center in Massachusetts and her colleagues estimated the emissions from four major natural feedbacks. The first is mainly in the Arctic, in permafrost that once remained frozen year-round. As this ground thaws, microbes begin to decompose plant and animal remains that have been frozen for millennia, emitting carbon dioxide and methane. A second feedback involves wetlands, which also release more methane as temperatures rise. This is because warmer water can hold less gas, and because microbes in the water decompose organic matter at a higher rate. The same processes lead to additional methane emissions from ponds and lakes, a third feedback. The fourth feedback kicks in as the air gets hotter, winters get shorter and soils get drier. All of this increases the frequency and intensity of wildfires, which results in greater CO2 and methane emissions. “We need to fully account for all sources of emissions that contribute to warming, otherwise we plan for the wrong future,” says Schädel. “These emissions are left out of carbon budgets, and we adapt to the wrong targets.” Much of this extra warming is already unavoidable, the modelling suggests. In a climate scenario in which human emissions decline after 2050, the natural feedbacks add an average of 0.4°C of warming by 2100. But even in a scenario in which emissions start declining today, the feedbacks add 0.2°C of extra warming this century, prolonging global warming. “This additional warming keeps rising through 2100… because there is a lag between when emissions occur and when the temperature impacts occur,” says Schädel.