The United States-China agreement on climate change is a huge political triumph, possibly “historic,” as its supporters say. Whether it much alters the world’s climate is a more open question.

Recall the agreement’s outlines. By 2030, China pledges to reach peak emissions of global greenhouse gases and also to increase its reliance on non-fossil fuels to 20 percent of its total energy. For its part, the United States committed to reducing its greenhouse gas emissions in 2025 by 26 percent to 28 percent from 2005 levels.

Environmentalists correctly see three big gains.

First, it makes China a major player in the climate debate. Writing in the Wall Street Journal, Fred Krupp, president of the Environmental Defense Fund, put it this way: “The argument that the U.S. can’t act because China won’t act has finally begun to fade. A very understandable anxiety — that America can’t cut carbon emissions while our biggest competitor keeps burning dirty energy with no end in sight — can now be put to rest.”

Second, it pushes other countries to make similar emission-cut pledges before the next international negotiation, scheduled for 2015 in Paris. “[This] sends a powerful signal to those who have been sitting on the sidelines, like Australia and Canada and some developing nations,” said Jake Schmidt of the Natural Resources Defense Council. Smaller countries would have less fear that their emission cuts would simply be offset by increases from the two largest emitters. In 2012, Chinese and American emissions represented 42 percent of the main greenhouse gas — carbon dioxide — according to the International Energy Agency (IEA). China’s share was 26 percent; the U.S. share, 16 percent.

Third, the emphasis on non-fossil fuels raises the possibility of technological breakthroughs transforming energy use. There are big advantages to being a “first-mover in technological innovation,” notes Paul Joffe of the World Resources Institute. This is the crux of the matter, because fossil fuels — coal, oil, natural gas — still provide roughly 80 percent of the world’s energy. The quandary of climate change is how to wean the world from these fuels without causing a global economic collapse.

In this light, the U.S.-China deal looks less impressive. For starters, it’s voluntary. If either the United States or China doesn’t meet its goals, there are no penalties and no enforcement.

Just how the United States will cut emissions by roughly one-quarter from 2005 levels is unclear. Based on government data, emissions in 2012 were already 10 percent below 2005 levels. These cuts are usually attributed to a weak economy that uses less energy; switching from coal to natural gas for electricity generation (natural gas’s carbon dioxide emissions are about half coal’s); and more fuel-efficient vehicles meeting tougher government standards.

Some of these trends might continue; others might not. All might be amplified by new subsidies for wind and solar, a cap-and-trade system and an energy tax. Or targets might be missed.

As for China, the agreement allows unlimited emissions expansion until 2030, when they are already projected to stabilize. Not that China wants unlimited expansion. It’s trying to limit coal-fired electricity plants, because their choking pollution is unhealthy and widely unpopular. Logically, China has also promoted renewables; it already had a goal that they will represent 15 percent of energy use by 2020. The agreement with the United States mostly covers these existing policies, not new ones.

Despite its best efforts, China still needs more fossil fuels to feed a growing economy. By 2030, its greenhouse gas emissions could increase by one-fifth from 2012 levels, estimates the IEA. (Chinese news reports last week suggested the country wants to hold growth to 16 percent by 2020.) Other developing countries, led by India, are in a similar position. But even if all cut emissions growth, the specter of climate change would remain.

The reason is this: The planet’s warming results from the concentration of greenhouse gases in the atmosphere, not annual volumes of new emissions. Lower increases in annual emissions or modest declines don’t solve the problem. The gases’ atmospheric concentrations continue to grow — just more slowly — and greater concentrations increase the likelihood of warming. With present policies, concentrations will reach levels in 2040 that gradually raise temperatures by 2 degrees Celsius (3.6 degrees Fahrenheit), says the IEA. And concentrations would continue to rise.

Confronting global warming is so difficult because doing anything meaningful requires radical changes to all the world’s major economies — virtually abandoning fossil fuels when there is no easy substitute. Also, there is legitimate uncertainty over the extent of future temperature movements. Even many believers in climate change acknowledge that, while skeptics assert that the danger is overstated. History may judge the U.S.-China agreement a significant turning point in this struggle — or just a fleeting act of political symbolism.

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