Climate scientists have been trying to work out why new computer models have begun projecting a potentially much hotter future as CO2 levels rise. A new analysis gives our best idea yet – it seems to be to do with clouds. Ahead of the next major UN climate science panel reports in 2021, researchers have found their sixth generation of climate models show a much wider range for the future temperature than before, up from 1.5 to 4.5 C to 1.8 to 5.6 C. Those estimates are for when "equilibrium climate sensitivity" (ECS) occurs, a theoretical point when the climate system comes into equilibrium after CO2 levels have doubled. "There is definitely not one single common cause. But quite a lot of the models at the high end have introduced new, more sophisticated models of clouds and aerosols. That does seem to be the driver of the new, higher sensitivity," says Catherine Senior at the UK's Met Office.
A major new study of the relationship between carbon dioxide and global warming lowers the odds on worst-case climate change scenarios while also ruling out the most optimistic estimates nations have been counting on as they attempt to implement the Paris agreement. A group of 25 leading scientists now conclude that catastrophic warming is almost inevitable if emissions continue at their current rate, even if there's less reason to anticipate a totally uninhabitable Earth in coming centuries. The research, published Wednesday in the journal Reviews of Geophysics, narrows the answer to a question that's as old as climate science itself: How much would the planet warm if humanity doubled the amount of carbon dioxide in the atmosphere? That number, known as "equilibrium climate sensitivity," is typically expressed as a range. The scientists behind this new study have narrowed the climate-sensitivity window to between 2.6 and 3.9 degrees Celsius.
Global climate model (GCM) estimates of the equilibrium global mean surface temperature response to a doubling of atmospheric CO2, measured by the equilibrium climate sensitivity (ECS), range from 2.0 to 4.6 C. Clouds are among the leading causes of this uncertainty. Here we show that the ECS can be up to 1.3 C higher in simulations where mixed-phase clouds consisting of ice crystals and supercooled liquid droplets are constrained by global satellite observations. The higher ECS estimates are directly linked to a weakened cloud-phase feedback arising from a decreased cloud glaciation rate in a warmer climate. We point out the need for realistic representations of the supercooled liquid fraction in mixed-phase clouds in GCMs, given the sensitivity of the ECS to the cloud-phase feedback.
A host of global climate models developed for the United Nations's next major assessment of global warming, due in 2021, are now showing a puzzling but undeniable trend: They are running hotter than they have in the past. In earlier models, doubling atmospheric carbon dioxide over preindustrial levels led models to predict somewhere between 2 C and 4.5 C of warming once the planet came into balance. But in at least eight of the next-generation models, produced by leading centers in the United States, the United Kingdom, Canada, and France, that "equilibrium climate sensitivity" has come in at 5 C or warmer. Many scientists, including the model developers, are doubtful this increased warming is likely to be real. Over the next year, they will be comparing notes on what happened in their models, which in many cases simulate the Earth system better than ever before.