Even under the best-case scenarios, rising CO2 levels in the Earth's atmosphere will cause unstoppable effects to the climate for at least the next 1,000 years, say Canadian researchers.
They predict a collapse of the West Antarctic ice sheet by the year 3000, and an eventual rise in the global sea level of at least four metres.
The study is the first full climate model simulation to make such long-term predictions, and will no doubt be challenged as a result. It is based on best-case, 'zero-emissions' scenarios developed by the Canadian Centre for Climate Modelling and Analysis and the University of Calgary.
"We created 'what if' scenarios," says Professor Shawn Marshall of Calgary. "What if we completely stopped using fossil fuels and put no more CO2 in the atmosphere? How long would it then take to reverse current climate change trends and will things first become worse?"
The northern hemisphere comes out of the computer simulations rather better than the south, with patterns of climate change reversing within the 1,000-year timeframe in places like Canada and the northern US.
At the same time, though, parts of North Africa experience desertification as land dries out by up to 30 percent, and ocean warming of up to 5°C off of Antarctica is likely to trigger widespread collapse of the West Antarctic ice sheet.
The researchers suggest that one reason for the variability between the north and south is the slow movement of ocean water from the North Atlantic into the South Atlantic.
"The global ocean and parts of the southern hemisphere have much more inertia, such that change occurs more slowly," says Marshall. "The inertia in intermediate and deep ocean currents driving into the Southern Atlantic means those oceans are only now beginning to warm as a result of CO2 emissions from the last century. The simulation showed that warming will continue rather than stop or reverse on the 1,000-year time scale."
Wind currents in the southern hemisphere may also have an impact. Marshall says that these winds tend to strengthen and stay strong without reversing. "This increases the mixing in the ocean, bringing more heat from the atmosphere down and warming the ocean," he says.
The team now plans to investigate more thoroughly the impact of atmosphere temperature on ocean temperature to help work out how fast West Antarctica could destabilize and how long it could take to fully collapse into the water.