China and the United States, the world's two largest economies, are responsible for emitting nearly half the planet's carbon dioxide emissions. China overtook the United States in 2006 as the world's biggest CO2 polluter due to its hardening coal addiction. Per capita, however, America's carbon footprint is far bigger.
Both countries still have large fleets of coal plants and growing, but relatively tiny, renewable electricity sectors. Both have goals for lowering their global warming emissions—though none would match the scale of the climate threat. Scientists say the world's output of greenhouse gases must peak around 2016, and then decline to stop at the critical 2-degree Celsius temperature increase by century's end. Projections show both countries' emissions will peak sometime after the mid-2030s.
Using the latest figures available, InsideClimate News culled federal and international energy data to tell the story of the world's two biggest polluters.

President Obama hasn't publicly drawn a connection between climate change and the Keystone XL pipeline, but new pressure is building on him and other officials to connect those dots.
Protests are springing up from Maine to Washington, D.C. to Oklahoma urging leaders to stop the Keystone XL and other oil sands import projects on climate change grounds. The Texas-bound Keystone XL is the biggest of many projects being proposed to connect Canada's oil sands to U.S. refineries and export ports. Protesters claim the pipelines would commit the United States and other countries to a form of heavy oil that would worsen global warming.
On Jan. 26, some 1,400 people marched through Portland, Maine, against possible plans to move oil from Canada's tar sands mines to local ports. Days earlier, hundreds of people joined solidarity rallies across New England and in Canada, where they picketed outside gas stations, locked arms along bridges and hoisted signs that read "Tar Sands = Game Over for Climate." On Monday, indigenous rights activists In Texas and Oklahoma filled public squares to show support for efforts by Canada's First Nations to block oil sands growth.
"We're trying to build the social movement" against expansion of tar sands oil extraction, said Sophie Robinson, who organized events last week through the Massachusetts chapter of 350.org, a grassroots organization that focuses on climate change.
1/31/13: The story has been updated with comments from industry.
1/30/13: This story has been updated to include information from the EPA that was received after publication.
One of the biggest unknowns in the unfolding Keystone XL debate is the role the U.S. Environmental Protection Agency might play.
Because the Canada-to-Nebraska oil pipeline crosses an international border, the State Department, not the EPA, will decide whether to give the project the federal permit it needs. But the EPA will weigh in during the review, and its opinion will carry new weight now that the Obama administration has vowed to make climate change a national priority.
The EPA's position will become clearer when the State Department releases its Supplemental Environmental Impact Statement (SEIS) for the project, which it is expected to do any day now. Under the Clean Air Act, the EPA is required to review and comment publicly on the SEIS, and the agency has not been shy about criticizing earlier drafts.
"The EPA actually could assert a fair amount of power depending on, basically, how much they want to stick their necks out," said Jim Murphy, senior counsel at the National Wildlife Federation, which opposes the pipeline. "The level of scrutiny this is going to get is pretty intense. With each iteration this goes through, the number of eyes increases."
Two and a half years after the costliest oil pipeline spill in U.S. history, the company responsible for the disaster is balking at digging up oil that still remains in Michigan's Kalamazoo River.
The cleanup has been long and difficult because the ruptured pipeline was carrying bitumen, a heavy oil from Canada's tar sands region. Bitumen is so thick that it can't flow through pipelines until it's mixed with liquid chemicals to form diluted bitumen, or dilbit. When more than one million gallons of dilbit poured out of the broken pipeline in July 2010, the chemicals evaporated and the bitumen began sinking to the riverbed.
Today, regulators and oil spill experts are still struggling to deal with the accident, which was the first major spill of dilbit into a U.S. waterway. The cleanup tools and techniques developed for conventional oil spills—which mostly float on water—are ineffective for submerged bitumen, so experts have had to come up with new methods.
In October, the U.S. Environmental Protection Agency asked Enbridge Inc., the pipeline's Canadian owner, to clean up several miles of the river where submerged oil is still accumulating. The proposed order told Enbridge to dredge 80 to 100 acres of the riverbed. The request was based on the results of a yearlong study the EPA conducted with oil cleanup experts, Michigan state regulators and a committee of about 15 scientists.
The dredging is needed, the agency said, because the oil could spread into uncontaminated areas of the river if it isn't removed.
Sen. John Kerry made it clear Thursday that he will play a pivotal role in deciding the fate of the Keystone XL pipeline if he is confirmed as secretary of state.
“I’ll make the appropriate judgments about it,” he said, referring to the State Department’s ongoing review of the 1,200-mile tar sands oil pipeline. “There are specific standards that have to be met with respect to that review, and I’m going to review those standards and make sure they’re complete.”
For the past decade, the people of Boulder, Colo., have pursued an elusive goal: getting more clean energy into their grid. To do so, they pushed and prodded utility company Xcel Energy to give them a say in electricity decisions.
But nothing satisfied citizens and politicians, so several years ago they organized themselves into a movement for "municipalization," in which the city would split from Xcel and become its own utility. In April, the City Council is expected to vote in favor of pursuing the controversial idea, putting coal-heavy Boulder on the vanguard of efforts to break the monopoly of corporate utility companies.
"Somebody has to stick their neck out and try this," said Boulder Mayor Matt Appelbaum, who believes Boulder will inspire other cities. Already, residents in Minneapolis are preparing a ballot measure by November for municipalization, and advocates in Santa Fe, N.M., are not far behind.
When Amy Hargroves made the rounds in Congress last fall to lobby for an extension of the wind production tax credit, she was often greeted with confusion: Why was she here talking about wind power?
That's because Hargroves wasn't fighting for the credit as a representative of a turbine manufacturer like Vestas or an interest group like the American Wind Energy Association (AWEA). Instead, she was representing Sprint Nextel, a telecommunications giant with no direct ties to the wind game.
Sprint is among dozens of seemingly unrelated corporations, including Starbucks, Levi Strauss and New Belgium Brewing, who lobbied to save the wind tax credit. It's hard to gauge how much effect they had on lawmakers' last-minute decision to give the tax credit a one-year reprieve by putting it in the fiscal cliff tax package. But their involvement shows that the business community has identified a need for renewables and could become an important lobbying force in promoting clean energy.
"This signals a change in the coalition structure," said Clyde Wilcox, a professor in the government department at Georgetown University. "In the past, it would be green energy companies or environmental groups that have either a business interest or a public interest in these issues. But people look up when a new series of players lines up."
The news last week that the New York Times is dismantling its environment desk and reassigning the reporters throughout the newsroom provoked an outpouring of reaction, much of it suggesting that now isn't the time to take risks that could diminish the coverage of climate change.
In an essay after superstorm Sandy, Michael Gerrard, director of the Center for Climate Change Law At Columbia University, proposed that instead of thinking about what Sandy was, we should focus on "what Hurricane Sandy was not," as his title put it.
The question of how an oil spill from the proposed Keystone XL pipeline might affect the Ogallala aquifer was raised again this month, in a report the U.S. State Department will use to help it decide whether to approve or reject the controversial project.
The report concluded that a spill would have little effect on Nebraska's primary source of drinking water, because the oil would spread less than a thousand feet within the High Plains/Ogallala aquifer. The impact on the Ogallala aquifer would be "local," not "regional," said the report, which was prepared by the Nebraska Department of Environmental Quality (DEQ) and HDR Engineering, an Omaha-based consulting firm.
Scientists interviewed by InsideClimate News agreed with the report's conclusions that an underground spill probably wouldn't travel far and that a single accident wouldn't damage the entire Ogallala aquifer. But they also said the report didn't take into account other important factors: