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Another Earth Day has come and gone and despite a theme of "the face of climate change," this year's event was a muted affair. Yet unmitigated climate change has potentially catastrophic consequences for the world. For example, in the United States, 2012 was marked by a host of unusual weather patterns — including droughts, wildfires, and Hurricane Sandy — as well as the warmest year on record in the lower 48.

As the world's largest economy, the U.S. is an easy target for activists seeking to mitigate climate change. But there are actually multiple metrics for determining national responsibility for climate change and the U.S. is not the worst offender across the board. Here's a look at different criteria and the leader in each category.

1. Current Carbon Dioxide Emissions Worst Offender: China


"The simplest and most common way to compare the emissions of countries is to add up all the fossil fuels burned and cement produced in each nation and convert that into CO2," The Guardian notes. Surprise! In the standard metric for climate change responsibility, China is the worst offender, not the U.S. According to European Commission estimates, China alone was responsible for almost 29 percent of global carbon dioxide emissions in 2011. The U.S. was a distant second at 15 percent. In fact, energy-related CO2 emissions in the U.S. fell in both 2011 and 2012, bringing emissions levels down to a two-decade low. Meanwhile, China now roughly burns as much coal as the rest of the world combined.

2. Carbon Dioxide Emissions Per Capita Worst Offender: Qatar

When looking at CO2 emissions on a per-person basis, the usual suspect, the U.S., once again loses out on the crown. Qatar's level of carbon emissions per capita (about 50 tons) is the highest in the world - three times as high as the United States' and about ten times the global average. (India clocks in at just 1.4 tons per person). Energy usage in the Middle Eastern country is "sky-high," National Geographic notes, with citizens provided free electricity and water. "Energy demand is rising by 7 percent a year to run the desalinators and air conditioners that maintain life in the desert and the natural gas production equipment that funds it." Ironically, last year's United Nations climate change conference took place in Doha, Qatar's capital.

3. All Greenhouse Gas Emissions Worst Offender: China

According to World Resources Institute data, when accounting for the emission of greenhouse gases other than carbon dioxide, such as methane and nitrous oxide, as well as emissions from land-use (i.e. deforestation), China once again takes the lead. The dubious distinction has led to infamously bad air pollution and a recent study linking, for the first time, the burning of fossil fuels to a country's increasing daily temperature spikes.

4. Historical Carbon Dioxide Emissions: United States

We're number one! We're number one! Finally, the U.S. takes a win. If you thought the world's sole superpower, leader in such metrics as obesity and most McDonalds's - hey, might those two be related? — would take top marks in at least one metric for climate change responsibility, you were right. Carbon dioxide can stay in the atmosphere for centuries and according to World Resources Institute estimates, the U.S. is responsible for almost 30 percent of cumulative carbon dioxide emissions since 1850. China and Russia are a distant second and third at 9 and 8 percent, respectively.

There are other metrics to consider — according to the Guardian, for example, when looking at historical CO2 emissions per capita, Luxembourg wins out, while Belgium takes top billing on the basis of a country's "consumption footprint" — CO2 output including imports and excluding exports. But the most important takeaway from the variety of possible metrics is the shared global responsibility for both causing and mitigating climate change. All countries have contributed or are contributing to the problem and its impact is felt everywhere and as such, climate change mitigation must be a truly global effort.