Paris: The global average temperature has risen 0.74 degrees Celsius (1.3 deg Fahrenheit) in the century to 2005, according to a benchmark 2007 report of the UN's Intergovernmental Panel on Climate Change (IPCC).
The decade 2001-2010 was the hottest since records began in 1850, says the World Meteorological Organisation. 2011 was the hottest year.
Most scientists believe the main culprit is greenhouse gas released by burning fossil fuel, trapping solar heat in the atmosphere.
The IPCC, which is to bring out a new, updated report in 2013/14, has predicted further warming of between 1.1 deg C and 6.4 deg C by 2100 from 1980-1999 levels depending on greenhouse gas emission levels.
UN countries agreed in Copenhagen in 2009 to reduce greenhouse gas pollution to limit warming to 2 deg C from pre-industrial levels.
The UN Environmental Programme said Wednesday that that goal has moved further out of reach and temperatures will rise by 3-5 deg C on current reduction pledges.
The WMO says greenhouse gases in Earth's atmosphere reached record concentrations in 2011, and reported Tuesday a 30 percent increase in "radiative forcing" the warming effect on climate, between 1990 and 2011.
Since the start of the industrial age in about 1750, some 375 billion tonnes of carbon have been released into the atmosphere and will stay there for centuries, according to the WMO.
Scientists say climate change is already visible in sea-level rise, loss of alpine glaciers and snow cover, shrinking Arctic summer sea ice, thawing permafrost, poleward migration of animals and plants and an increase in intense tropical cyclone activity in the North Atlantic.
The planet has witnessed a rise in extreme weather events in the past decade that some have blamed on climate change prolonged droughts, heat waves and torrential storms recently superstorm Sandy which ravaged Haiti and the US east coast.
The trend is likely to continue, and the IPCC predicts that 20-30 percent of plant and animal species will risk extinction once temperatures rise by 1.5-2.5 deg C from 1980-99 levels (IPCC)
Up to 250 million people in Africa may be exposed to water stress due to climate change by 2020, and crop yields could be slashed in half in some countries.
A global temperature rise of 3-4 deg C could displace more than 300 million people through sea level rise and flooding inundating coastal cities and small islands, according to UN figures.
The IPCC has predicted that sea levels will rise by between 18 and 59 cm by 2100, but some scientists say a surge in Antarctic and Greenland ice sheet melt could boost levels by as much as a metre (38 inches).
In September, US scientists said Arctic sea ice has shrunk to its smallest surface area since record-keeping began.