According to Nasa, the US space agency, the other warmest years since 1850, when records began, have been 2005, 1998, 2007 and 2006.
The data was released as negotiators at the two-week summit worked to craft a global deal to tackle climate change.
US emissions move
The Copenhagen talks, under the banner of the UN Framework Convention on Climate Change (UNFCCC), are aimed at outlining an agreement that sets down pledges by major emitters of greenhouse gases to curb pollution.
On Monday the US government boosted talks by saying it would start to regulate carbon dioxide as a dangerous pollutant.
The move means Barack Obama, the US president, can sidestep congress to order cuts in emissions.
The announcement by the US Environmental Protection Agency (Epa) was clearly timed to build momentum towards an agreement at the international conference on climate change.
Environmentalists hailed Monday's announcement and welcomed the timing, saying it will help the US president convince delegates at the Copenhagen conference that the US is serious about addressing the problem.
The conference is also hoping to set down the principles of long-term financing to help wean poor countries off high-carbon technology and bolster their defences against climate change.
Once ratified, the accord would take effect from 2013.
Delegates said the next few days would see countries lay out their positions before some 110 world leaders, including Obama, China's Wen Jiabao and India's Manmohan Singh, arrive for the climax.