If you want to limit the risk of exceeding 2 degrees C global warming to one in four, or 25 percent, then total CO2 emissions over the first half of the 21st century have be kept below 1,000 billion tons,” said Malte Meinshausen, a climatologist at the Potsdam Institute for Climate Impact Research in Germany . . . . Between 2000 and 2006, human activities emitted about 236 billion tons of carbon dioxide, the researchers estimate. “Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming,” Meinshausen says. . . . The researchers also estimate that limiting cumulative carbon dioxide emissions between now and 2050 to no more than 1 trillion tons would actually leave three-fourths of the world’s known reserves of oil, gas and coal in the ground unburned — unless techniques for capturing and sequestering carbon dioxide underground rather than dumping it into the atmosphere become commonplace in the future.
We have 764 billion tons left to emit over the next 41 years, which implies a rate of a 18.6 billion tons per year. We have been emitting at a rate of about 34 billion tons per year. So, we need to reduce emissions by more than 50% to ward off catastrophic global warming by 2050 with a reasonable probability. If the developing world and third world are to make any development progress over the next four decades, moreover, that means that the fossil fuel use per unit of economic productivity, must decline by much more than 50%.
A switch away from fossil fuels means producing electric power with a far larger share of nuclear power and renewables, and means a wholesale shift from gasoline and diesel fueled cars and trucks, through a combination of reduced travel, more fuel efficient vehicles, public transportation, rail rather than road travel, and alternative fuel vehicles.