The world is about to go on a carbon diet. It won't be easy — or cheap.

Nearly 200 countries 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, like for many first-day dieters, the reality sets in. The numbers — like calorie limits and hours needed in the gym — are daunting.

How daunting? Try more than 7.04 billion tonnes. That's how much carbon dioxide needs to stay in the ground instead of being spewed into the atmosphere for those 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 even larger numbers.

In the pact, countries pledged to limit global warming to about another one degree Celsius from now (or 2 C measuring against the pre-industrial average global surface temperature) — and if they can, only half that.

Another, more vague, goal is that by sometime in the second half of the century, human-made greenhouse gas emissions won't exceed the amount that nature absorbs. Earth's carbon cycle, which is complex and ever-changing, would have to get back to balance.

Ship has sailed, scientist says

In practice, that means the world has to emit close to zero greenhouse gases by 2070 to reach the easier one-degree goal, or by 2050 to reach the harder half-degree one, said John Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany.

Oh and by the way, the harder goal is probably already impossible, said Joeri Rogelj at the International Institute for Applied Systems Analysis in Austria. Most likely 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, coming back to the target temperature.

Canada's new emissions reduction goal

Most scenarios for how to establish safer global temperatures involve someday reaching 'negative emissions' — when technology and nature combined take out more carbon from the air than humanity puts in.

That may involve something called negative emissions. That's when the world — technology and nature combined — takes out more carbon dioxide from the air than humanity puts in. Nearly 90 per cent of scenarios of how to establish a safer temperature in the world involve going backward on emissions, but it is also so far 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, but with a serious sustained research effort, costs can probably be brought below $100 a tonne, said engineering and policy professor Granger Morgan of Carnegie Mellon University.

Much hinges on China

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, maybe earlier, and then fall to near-zero, experts said. Those levels have been generally rising since the Industrial Revolution. A new study suggests emissions may have fallen slightly this year, but that may be a blip.

Without any efforts to limit global warming, the world would have warmed by 3.5 degrees Celsius from now by 2100, according to Climate Interactive. But China's submitted plan alone would cut that projected warming by 1.3 degrees, the Washington-based organization says. 

Climate demonstrator in Paris

An NGO activist protests against the funding of fossils during the UN conference on climate change on the outskirts of Paris. A pair of climate NGOs say countries' current emissions-reduction commitments aren't enough to keep further global warming to below 1 C. (Dominique Faget/AFP/Getty)

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 U.S. is responsible for 18 per cent of the world's carbon pollution, compared to 13 per cent for China.

That all sounds good, but the goals countries have set aren't enough. Taken together, they would still allow temperatures to rise 2.5 C by the end of the century from now, so to reach the goals agreed on this weekend countries will need to do more, Climate Interactive found.

Another climate modelling group, Climate Action Tracker, is slightly more optimistic, but still finds countries' plans would miss the goal of limiting temperature rise to one more degree. It says the current proposals would allow a rise of 1.7 C.

"Clearly," said Rachel Cleetus, climate policy manager for the Union of Concern Scientists, "countries must be exercising their low-carbon muscles more."