To make dent in climate problem, action is needed by the countries with biggest emissions

Six countries produce nearly 60 percent of global carbon dioxide emissions. China and the United States combine for more than two-fifths. The planet's future will be shaped by what these top carbon polluters do about the heat-trapping gases blamed for global warming.

How they rank, what they're doing:

CHINA

It emits nearly twice the amount of greenhouse gases as the United States, which it surpassed in 2006 as the top emitter of carbon dioxide. China accounts for about 30 percent of global emissions. U.S. government estimates show China doubling its emissions by 2040, barring major changes. Hugely reliant on fossil fuels for electricity and steel production, China until recently was reluctant to set firm targets for emissions, which continue to rise, although at a slower rate. That changed when Beijing announced last month in a deal with Washington that it would stem greenhouse gas emission growth by 2030. About a week later, China's Cabinet announced a coal consumption cap by 2020 at about 62 percent of the energy mix. While politically significant, the U.S.-China deal alone is expected to have little effect on the global thermostat.

2013 CO2 emissions: 11 billion tons

2013 Population: 1.36 billion

UNITED STATES

It has never entered into a binding treaty to curb greenhouse gases. Nevertheless, it has cut more carbon pollution than any other nation. It is on pace to meet a 2009 Obama administration pledge to reduce emissions 17 percent from 2005 levels by 2020. Carbon emissions are up, though, as the U.S. rebounds from recession. President Barack Obama has largely leaned on existing laws, not Congress, to make progress — boosting automobile fuel economy and proposing to reduce carbon pollution from new and existing power plants. The White House vowed in the China deal to double the pace of emissions reductions, lowering carbon pollution 26 percent to 28 percent from 2005 levels by 2025. Expect resistance when Republicans control Congress in January.

2013 CO2 emissions: 5.8 billion tons

2013 Population: 316 million

INDIA

The U.S.-China agreement puts pressure on the Indian government, which could announce new targets during a planned Obama visit in January. Meantime, India plans to double coal production to feed a power grid still suffering blackouts. Its challenge: to curb greenhouse gases as its population and economy grow. In 2010, India voluntarily committed to a 20 percent to 25 percent cut in carbon emissions relative to economic output by 2020 against 2005 levels. It has made recent strides installing solar power, which it is expected to increase fivefold to 100 gigawatts by 2030. Under current policies, its carbon dioxide emissions will double by then, according to the International Energy Agency.

2013 CO2 emissions: 2.6 billion tons

2013 population: 1.2 billion

RUSSIA

It never faced mandatory cuts under the 1997 Kyoto Protocol because its emissions fell so much after the Soviet Union collapsed. A major oil and gas producer, Russia in 2013 adopted a domestic greenhouse gas target that would trim emissions 25 percent from 1990 levels by 2020. Russia's carbon dioxide emissions today average 35 percent lower than 1990 levels. To meet its goal, Russia has set a goal for 2020 of boosting energy efficiency 40 percent and expanding renewable energy 4.5 percent. The state-owned gas company Gazprom has energy conservation plans, as has the federal housing program. But in 2006, Russia announced a move to more coal- and nuclear-fired electricity to export more oil and natural gas.

2013 CO2 emissions: 2 billion tons

2013 population: 143.5 million

JAPAN

The shuttering of its nuclear power plants after the 2011 Fukushima nuclear disaster forced a drastic change in plans to curb carbon pollution. In November, Japanese officials said they would now reduce greenhouse gases 3.8 percent from 2005 levels by 2020. With more fossil fuels in the mix, Japan's emissions will be up 3 percent from 1990 levels, its benchmark for its pledge at a 2009 United Nations summit in Copenhagen to reduce emissions 25 percent. Beginning in 2012, Japan placed a carbon tax based on emissions of fossil fuels, with the proceeds going to renewable energy and energy-saving projects.

2013 CO2 emissions: 1.4 billion tons

2013 population: 127 million

GERMANY

It has outperformed the 21 percent reduction in greenhouse gases it agreed to in 1997. Emissions are down 25 percent against 1990 levels. To comply with 2020 European Union-set goals, Germany must reduce greenhouse gases 40 percent by 2020. On Wednesday, it boosted subsidies for energy efficiency to help it get there. Germany has in recent years seen back-to-back emissions increases due to higher demand for electricity and a switch to coal after Fukushima, which prompted a nuclear power phase-out. Coal use is down this year and renewables continue to gain electricity market share. Renewables already account for a quarter of Germany's electrical production. The country plans to boost that share to 80 percent by 2050 — and put a million electric cars on the road by 2020.

2013 CO2 emissions: 836 million tons

2013 population: 80.6 million

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Sources: World Bank, Global Carbon Project, AP Research