A few weeks ago China admitted that it is the world's biggest greenhouse gas emitter, though this was known for more than a year because already in 2009 China overtook United States and became world's biggest polluter.
China's fast growing economy, mostly based on using coal as primary fuel, is the main reason why China emits about 6,018 million tons of greenhouse gases per year. Though China is doing tremendous efforts to get more power from renewable energy coal will very likely remain dominant fuel for at least next few decades which will certainly not be helpful in global fight against the climate change.
Situation isn't much better in United States. U.S. is currently the second biggest polluter in the world, but you also need to know that China has much more people compared to U.S. so if we look at the pollution per capita then US still leads the way as the world's largest polluter. Coal still generates around 50% of US electricity, and US transition from fossil fuels to renewable energy is anything but fast.
These two countries are the largest polluters in the world followed by Russia, India, and Japan.
Is it realistically to expect that world will actually do something to decrease the global levels of greenhouse gas emissions? Is international climate deal possible at this moment or not?
The climate summit in Cancun looks very likely to fail because not much has happened since the last year's Copenhagen climate talks. The difference in opinion between developed and developing world is still a giant hurdle as on one side United States wants China to commit to emissions cuts and on the other side China argues that developing nations should be exempt from such cuts because they need to grow their economies and lift people from poverty.
You can't really blame China for maintaining this position because United States is the country mostly responsible for climate change issue, and so it should lead the way with example instead of putting the responsibility on others.
World can tackle pollution on global level only by acting as one, united, and world leaders are still far far away from reaching the level of unity needed for the important decisions such as international climate deal.