Enough carbon dioxide has already entered the atmosphere to cause global warming in excess of 2 degrees Celsius regardless of future efforts to reduce greenhouse-gas emissions, according to scientists from the Lawrence Livermore National Laboratory and two research universities.

Researchers from LLNL, Nanjing University, and Texas A&M University used observations and climate model simulations to evaluate how much warming is “in the pipeline” from past CO2 emissions. Their estimate, published in Nature Climate Change, takes into account changes in the geographic pattern of surface warming and is higher than previous estimates.

“Typically, committed warming is estimated assuming that changes in the future will pretty much follow changes in the past,” said LLNL atmospheric scientist Mark Zelinka. “But we now know that this is a bad assumption.”

While most of the planet’s surface has warmed, several important regions, like the Southern Ocean, have not, Zelinka said. The lack of surface warming in those regions favors the buildup of low-level clouds, which reflect sunlight back to space and cool the planet. But eventually those regions will warm up, and when they do, the low clouds will decrease, which will allow more sunlight to be absorbed by Earth.

Specifically, the team found that future warming has a most likely value of +2.3°C above pre-industrial levels.

"The bad news is that our results suggest that we have most likely already emitted enough carbon dioxide to exceed 2°C,” the goal set in the Paris Agreement, said Andrew Dessler, Texas A&M researcher and co-author of the study.

But Dressler pointed out that there is good news. Once net emissions get to near zero, the rate of continued, committed warming will be very slow. “So if we can get net emissions to near zero soon, it may take centuries to exceed 2°C.”

The LLNL portion of the research was funded by the Department of Energy’s Office of Science.