PARIS — The world is about to go on a carbon diet. It won’t be easy — or cheap.
Nearly 200 nations across the world on Saturday approved a first-of-its-kind universal agreement to wean Earth off fossil fuels and slow global warming, patting themselves on the back for showing such resolve.
On Sunday morning, as for many first-day dieters, the reality set in.
The numbers are daunting: More than 7.04 billion tons of carbon dioxide. That’s how much has to stay in the ground instead of being spewed into the atmosphere for the planned reductions to happen, even if you take the easier of two goals mentioned in Saturday’s deal. To get to the harder goal, it’s an even larger number.
The objective of the agreement is to make sure global warming stays ‘‘well below’’ 3.6 degrees Fahrenheit (2 degrees Celsius) and to ‘‘pursue efforts’’ to limit the temperature rise to 2.7 degrees Fahrenheit (1.5 Celsius). Temperatures have already increased by about 1.8 degrees Fahrenheit since preindustrial times.
Scientists who have analyzed the pledges made by nations so far to cut greenhouse gases believe emissions will be reduced only about half the amount necessary to stave off an increase in atmospheric temperatures of 3.6 degrees Fahrenheit.
That is the point at which, scientific studies have concluded, the world will be locked into a future of rising sea levels, severe droughts and flooding, widespread food and water shortages, and more destructive storms.
Ideally, by sometime in the second half of the century, man-made greenhouse gas emissions — which includes methane and other heat-trapping gases as well as carbon dioxide — should not exceed the amount nature can absorb.
In practice, the world has to emit close to zero greenhouse gases by 2070 to reach the easier temperature goal, or by 2050 to reach the harder one, said John Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany.
Probably the best the world can hope for is overshooting that temperature by a few tenths of a degree and then somehow slowly — over decades if not centuries — come back to the target temperature.
That may involve something called negative emissions. That’s when the world — technology and nature combined — take out more carbon dioxide from the air than humanity puts in.
Nearly 90 percent of strategies for establishing a safer temperature in the world involve going backward on emissions, but that’s not very realistic, said Kevin Anderson, deputy director of the Tyndall Centre for Climate Change Research in Britain.
Negative emissions involve more forests, maybe seeding the oceans, and possibly technology that sucks carbon out of the air and stores it underground somehow. More biomass or forests require enormous land areas and direct capture of carbon from air is expensive.
Leading up to the Paris agreement, nearly every nation formed an individual action plan to cut or slow the growth of carbon pollution over the next decade or so. Richer nations that have already developed, including the United States, European countries, and Japan, pledged to cut now.
Developing nations that say they need fossil fuels to pull themselves out poverty pledged to slow the rate of growth for now and to cut later.
China, the world’s top carbon polluter, will eventually have to make the biggest cuts. Overall, for the world to hit its new target, global carbon dioxide emissions will have to peak by 2030, and then fall to near-zero, experts said.
Those levels have been generally rising since the industrial revolution.
Without any efforts to limit global warming, the world would warm by 6.3 degrees Fahrenheit from now by 2100, according to Climate Interactive, a climate modeling group. But China’s submitted plan alone would cut that projected warming by 1.3 degrees, according to Climate Interactive. The US plan trims about six tenths of a degree off the projected warming.
And while China is now the No. 1 carbon dioxide polluter with more than a quarter of the world’s emissions, carbon dioxide stays in the air for at least a century, so historical emissions are important. Since 1870, the United States is responsible for 18 percent of the world’s carbon pollution, compared to 13 percent for China.
That all sounds good, but the goals the nations have set aren’t enough. Taken together, they would still allow temperatures to rise 4.5 degrees Fahrenheit by the end of the century, Climate Interactive found.