Energy Information Administration's (EIA) latest study of global emission figures by countries showed that China has officially overtaken US and became world's biggest CO2 emitter. US was number one in 2006 with with 5.70 billion tonnes compared to China's 5.61, but since then China took over with with 6.01 billion tonnes of CO2 compared to US 5.90 billion tonnes. This is mostly because of China's recent economic boom and the fact that China's economy is dominantly based on coal, with very little percentage of country's energy coming from renewable energy sources.
It has to be said though that these numbers focus strictly on emissions of CO2 from fossil fuels (oil, coal and gas), the burning of which causes around 57% of total global greenhouse emissions, and do not include non-fossil fuel CO2 emissions like for instance emissions from deforestation. Also it is important to know that China isn't the one mostly responsible for global warming phenomenon because responsibility for causing it is primarily on US shoulders. For instance when we look at the data from 1850 the US has emitted 1088 tonnes of CO2 since 1850 for each of today's Americans which looks gigantic compared to just 68 historical tonnes for each living Chinese person. However one thing is sure, both these countries need to cut down their emissions if we want to save our planet for future generations.