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The country accounts for over a quarter of global greenhouse gas emissions.

Even assuming that China’s population does not grow at all over the next 30 years, that the energy efficiency of its economy increases at a faster pace than most developed and developing countries and that it manages to decarbonize its energy sources faster than pretty much anybody else, China would still be emitting a lot more carbon in 2040 than it does today, according to E.I.A. calculations.

Can the United States or anybody else do anything to speed China down a low-carbon path?

The latest report from the United Nations Intergovernmental Panel on Climate Change, issued in April, suggested several ways to allot responsibilities. If one starts counting in the 18th century and counts only emissions from industry and energy generation, the United States is responsible for more than a quarter of all greenhouse gases that humanity has put into the air. China, by contrast, is responsible for 10 percent.But if one starts counting in 1990, when the world first became aware that CO2 was a problem, and includes greenhouse gases emitted from changes in land use, the United States is responsible for only 18 percent, and China’s share rises to 15 percent. Rich and poor countries, unsurprisingly, disagree on the proper measure. Photo

Not everybody will meet their Copenhagen pledges. Japan, which unplugged its nuclear energy after the disaster at the Fukushima nuclear power plant, will fall behind. So will Canada and Australia, whose new conservative governments have lost interest in the pledges of their predecessors.

The methane is like a hangover that you can get over if you stop drinking,” said Raymond T. Pierrehumbert, a climate scientist at the University of Chicago and the author of a textbook on planetary atmospheres. “CO2 is more like lead poisoning — it sticks around, you don’t get rid of it, and it causes irreversible harm.”

Aggressively controlling methane, they say, would help slow the warming sharply over the coming decades.

By contrast, “our success in controlling CO2 emissions is likely to make very little difference on temperature over the next 40 years,” said Drew Shindell, a longtime NASA climate scientist who is leaving for Duke University.

Experts say that, looking at the more distant future, it is hugely important to keep carbon dioxide out of the atmosphere now, even if that requires burning more gas. Dr. Pierrehumbert and Dr. Shindell largely agree on this point, with Dr. Pierrehumbert discounting the gas-is-worse-than-coal argument as “bunkum.”

To do so, CO2 emissions from industry and energy use would have to fall to at most 1.6 tons a year for every person on the planet by midcentury.

That is less than a tenth of annual American emissions per person today and less than a third of the world average

Lacking any understanding of the feasibility of the exercise, governments postured and jockeyed over which country should be responsible for what

This is not achievable by going after low-hanging fruit, such as replacing coal with natural gas in power plants.

The decarbonization paths rely on aggressive assumptions about our ability to deploy new technologies on a commercial scale economically.

Russia, for instance, hit the target. But Oleg Lugovoy of the Environmental Defense Fund, who worked on the Russian plan, observed that “if we don’t have carbon capture and storage we would have to reconsider.”

Worse still, the aging natural-gas infrastructure is riddled with holes and seeps; early this year, a survey of gas mains along Boston’s 785 miles of road, the first-ever such examination, found 3,356 leaks.

As an example, typical solar cells today have an EROEI of about 10—better than tar sands but worse than most oil and gas.

One recent estimate put the EROEI of Spain’s extensive solar-power network at less than 3.

When renewables supply 20 to 30 percent of all electricity, many utility-energy engineers predict, the system will no longer be able to balance supply and demand. Brownouts will ripple across the landscape

To ask utilities to take in large amounts of solar power

is like asking a shipping firm to replace its huge, professionally staffed container ships with squadrons of canoes paddled by random adolescents.

But even if such techniques work in the way researchers hope, the infrastructure transformation ahead is daunting in scale and scope. It’s like setting up a second Industrial Revolution, only all over the world and in one-third the time.

Externalities do not fix themselves. In the absence of effective individual property rights they require government action, in this case the action of close to 200 governments.

Why should they do this? The answer is: because a low-carbon atmosphere is a global public good.

It is by now impossible to be optimistic that anything like this will happen. This is partly because the needed agreement must be long-term and global. That, in turn, raises difficult questions of intragenerational and intergenerational equity.

Suppose that, despite all the logic, it proves impossible to achieve a relevant global agreement. Does it make sense for any country or group of countries to take determined action on their own? If the aim is to deal with climate change, the answer is: absolutely not, unless the countries are China or the US.

But it might be possible for a country to demonstrate proof of concept:

Today renewable energy supplies only about 6 percent of American demand. And most of that comes from water flowing through dams. Solar energy contributes next to nothing.

The arithmetic is merciless. To make it likely that the world’s temperature will rise no more than 2 degrees Celsius above the average of the preindustrial era — a target agreed to by the world’s governments in 2010 — humanity must spew no more than 900 billion more tons of carbon dioxide into the air from now through 2050 and only 75 billion tons after that, according to an authoritative new study in Britain.

The United States Energy Information Administration forecasts that global energy consumption will grow 56 percent between now and 2040.

The only way around this is to put something in coal’s place, at a reasonably competitive price. Neither the warm glow of the sun nor the restless power of the wind is going to do the trick, at least not soon enough to make a difference in the battle to prevent climate change.