CHILDREN born now could live to witness the planet warming more than 4掳C, even if we cut greenhouse emissions by a fair amount. That鈥檚 one of the terrifying implications of a study that adds to the growing evidence that the 鈥渙fficial鈥� projections underestimate future warming.
鈥�,鈥� says of Stanford University in California.
The biggest problem for global warming forecasts is that we don鈥檛 know how much more carbon dioxide and other climate-altering stuff will be released. Even if we assume greenhouse gases reach a specific level, climate models still produce a wide range of results.
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To narrow that range, Brown and , also at Stanford, tried to pick the climate models whose projections to date best match real-world data.
They chose several measures: for instance, how much heat is escaping from the top of Earth鈥檚 atmosphere. This is a direct measure of how much the total heat content of the sea, air and land surface is changing. Models that handle this well should be better at forecasting long-term temperature change.
鈥淲arming is fundamentally a result of these radiation changes,鈥� says Brown.
Brown also looked at how well models predict monthly shifts in temperature. The idea here is that such changes in regions like the tropics are determined by clouds. Because clouds respond strongly to changes in temperature, models that are good at predicting these short-term changes should also be good over the long term.
Once Brown had picked the best-performing models, he found they tend to project more warming (Nature, ). 鈥淭he models that warm the most are also the ones doing the best right now,鈥� he says.
For instance, for a scenario called RCP8.5 in which emissions continue unabated, the models used by the project an average of 4.3掳C warming by 2081 to 2100, plus or minus 0.7掳C. But the best-performing models project 4.8掳C, plus or minus 0.4掳C.
鈥淚t is now completely clear that our best models predict more warming than the average model鈥�
For a scenario that assumes more action is taken to combat climate change, called RCP6, the IPCC models project 2.8掳C on average, compared with 3.2掳C for the best models. The world is currently on a course somewhere between these two scenarios.
鈥淚t is now completely clear that our best models predict more warming than the average model,鈥� says of the University of New South Wales in Australia. In 2014, he drew this conclusion by a different method.
Clouds are key to these models鈥� stronger warming response, says at Columbia University in New York. 鈥淭hey project that clouds will block less sunlight in the future,鈥� she says.
Still, even the best-performing models may be wrong, Brown says. They may even underestimate warming.
For instance, models tend to assume soils will take up more carbon as the planet heats up. But a recent study found that soils will release a lot of carbon.
鈥淭here鈥檚 a lot we don鈥檛 know, but that doesn鈥檛 mean everything鈥檚 going to be OK,鈥� says Marvel.
This article appeared in print under the headline 鈥淲e will get roasting with shock rise in warming鈥�
