When China and the United States agreed a year ago to scale back greenhouse gas emissions, they set the stage for this month's international climate talks in Paris. But Paris is not just about these two powerhouses, even though they account for one-third of fossil fuel emissions. Curbing the threat posed by climate change truly takes global action.

So far, more than 150 countries – from Sudan to Suriname and from Kiribati to Kyrgyzstan – have outlined for United Nations negotiators just how, when, and by how much each would cut carbon dioxide over the next several decades. If an agreement is reached during the talks that start Nov. 30, it would mark the first serious global commitment to reduce the pollution that is warming the planet, souring the oceans, and causing seas to rise.

So who are the climate heroes? Which countries are doing the most (or least) to tackle greenhouse gases? It's not easy to discern. For starters, these pledges are not binding. Any country can change its mind at any point, though the hope is that international pressure would make that difficult. And some countries are a bigger part of the problem than others. Cuts by big polluters have more direct impact than those by smaller countries.

Plus, these pledges come in many varieties. Some nations, such as tiny Bhutan, made exceptionally detailed projections, complete with footnotes and dates, explaining precisely how they plan to realign their country’s agriculture, forestry, energy, and transportation systems to reduce their carbon footprint. On the other hand, the Russian Federation – one of the world's largest polluters ­– submitted three confusing pages, which, by some interpretations, proposed no cuts at all.

Comparing efforts is tricky. Should a country be judged on how much it cuts emissions over all or by how much it reduces carbon pollution, on average, for each citizen? Should poor countries get extra credit for trying, since most are not huge contributors to climate change? Should it matter if experts think a country's commitment is unlikely to be achieved? All of this is subjective.

Yet climate experts tend to agree which countries deserve praise for their ambition, which must do more, and which are just a welcome addition at all. Here are some of each:

AMBITIOUS

MEXICO: Mexico, which is responsible for about 1.4 percent of global greenhouse gas emissions, was one of the first developing nations to adopt a law committing to cut CO2. Then the plans it gave to U.N. negotiators showed how high it set the bar. Most countries want to slow the rate of increase in greenhouse gases over coming decades but “Mexico is pledging an absolute reduction," says Drew Jones, co-director of Climate Interactive, aiming for a 50 percent cut by 2050. "They're not saying emissions will be less than they would have under a 'business as usual' scenario. They're saying emissions will go down."

In addition, Mexico plans to revamp how it handles forests, which store carbon, proposing a replanting program and a reduction in logging, hoping to bring its overall rate of deforestation to zero by 2030. Doug Boucher, a forestry expert with the Union of Concerned Scientists, calls Mexico's aggressive plans "striking."

View Images Trucks and machinery move through the Suncore Oil Sands site near Fort McMurray in Northern Alberta. The Athabasca oil sands are responsible for roughly 10 percent of Canada’s emissions, and have risen nearly 80 percent in just the past decade. Photograph by David Levene, eyevine/Redux

MOROCCO: Several African countries, including South Africa, earned praise from climate experts, but Morocco is proposing some of the most detailed steps to reduce CO2 from land-clearing, deforestation, and agriculture. It would rehabilitate 200,000 hectares of woodlands, alter how it raises sheep and cattle, and change the types of crops it grows to limit emissions.

The country, responsible for less than a quarter of a percent of global emissions, plans to reduce the growth of those emissions by up to 13 percent by 2030. It will cut 19 percent more if it can get international help with financing. The country is committed to getting 42 percent of electricity from wind, solar and water by 2030. It already is building Africa's largest solar-power project.

INDIA: While China and the U.S. get most of the attention, India – where 300 million people still live without electricity – is the globe's third greatest emitter of greenhouse gases, and one of the globe's fastest-growing coal consumers. But it also plans to increase the electricity it can get from renewable energy to about 40 percent by 2030.

One climate expert called that proposal "nothing less than gargantuan." Because India is trying to make most of its investments in clean energy over the next seven years, that could quickly help drive down the cost of renewable technology and "spur a transition toward cleaner sources," wrote climate experts at World Resources Institute. It also plans to replant its forests. Some experts believe India may end up cutting emissions even more than it pledged.

COSTA RICA: This Central American rainforest jewel already emits less CO2 than Rhode Island, thanks to hydropower. But it is straining to reduce emissions more by focusing on its traffic-clogged roads. To eliminate some of the more than 1 million older cars on its streets, the country is considering a tax that would target the most-polluting older vehicles. Some lawmakers also are hoping to jumpstart a transition to electric vehicles. And the country is seeking financing for an electric commuter train, while adding buses and bike lanes. (Read more about Costa Rica’s plans.)

ETHIOPIA: Fewer than 16 percent of its citizens has access to electricity, yet Ethiopia plans to increase development while cutting the growth of its greenhouse gas emissions by half. Meeting its goals would require international financial support, which is not guaranteed. But the country wants to "leapfrog" the fossil fuel economy and grow using green building technology, electric transportation, and renewable power. "Here's a country with a low, low capacity to do something, and yet they have a very good plan," says Niklas Hohne, with the NewClimate Initiative. "It's exceptional."

View Images Saudi Arabia, with its massive ties to fossil fuels, including the Shaybah oil field,often has been the spoiler at international climate talks. But this month the country formally pledged to cut emissions, signaling its intent to join the fight against climate change. Photograph by Reza, National Geographic

NEEDS IMPROVEMENT

RUSSIA: The world's largest country proposed reducing greenhouse gases by up to 75 percent from 1990 levels by 2030. It sounds impressive, except for two things: First, Russia's boreal forests are a massive storage place for carbon, and it's not clear how the country plans to account for that. Second, emissions in Russia in 1990 were among the highest they've been in decades. That, according to Kelly Levin at the World Resources Institute, means Russia could keep its pledge and still increase emissions by 40 percent or more from today. And since Russia is the world's fifth biggest greenhouse-gas polluter, that means "the rest of the world is going to have to make up the difference."

CANADA: Under former Prime Minister Stephen Harper, Canada proposed cuts that, like Russia, look better than they are. Canada counts the carbon stored in its gigantic swaths of forest to achieve cuts in CO2. But if forests are ignored, Canada's emissions actually rise up to 8 percent by 2030 from 2005 levels. (The U.S., by contrast, pledged to reduce emissions up to 28 percent over that time.) Greenhouse gases from Alberta’s oil sands already are up nearly 80 percent since 2005, which is about one-tenth of the country's emissions. The U.S. decision to reject the Keystone Pipeline may dent that growth, but more pipelines are under consideration. New Prime Minister Justin Trudeau may propose more meaningful cuts.

AUSTRALIA: One of the world's largest coal exporters, Australia recently repealed its own carbon tax, then proposed cutting emissions less than its own government's climate agency recommended. The agency's head said the proposal put Australia "at or near the bottom of the group of countries we generally compare ourselves with." It was weaker than the U.S.'s and Canada's. And it’s not even clear Australia could achieve the goals it set. Prime Minister Tony Abbott since has been ousted, and the French ambassador to Australia called his replacement good news for the planet. (Read more about Australia’s plans.)

JAPAN: Despite an earlier pledge to step back from carbon emissions, more than half of Japan's energy would still come from fossil fuels in 2030. Less than a quarter would come from renewable energy. Japan also proposes to rely on carbon trading, and to allow its emissions per capita to increase in the short term. Overall, says Hohne, Japan’s plan is weaker than during 2009’s climate talks in Copenhagen. "They are actually moving backwards in ambition from one year to the next."

WILD CARD

SAUDI ARABIA: For years the Gulf States leader and oil giant appeared most eager when it was slowing international climate negotiations. "Their position for a long time was to not have ambition and try not to encourage ambition," says Wael Hmaidan, a Lebanon-based director for Climate Action Network. The Saudis expanded into renewables but also "were protecting the oil trade." Yet this month, as the country of 40 million deals with economic fallout from low oil prices, Saudi Arabia is proposing emissions cuts for the first time. The magnitude of the cuts aren’t as significant as the fact that they were proposed at all. It suggests that one of top 15 polluters in the world now recognizes that being a part of the move to curb emissions is essential. The shift in position also could free its neighbors to be more aggressive. "Saudi Arabia's participation is big news," Hohne says.