This year is likely to be the fifth warmest on record and the first decade of this century the hottest since records began, the World Meteorological Organisation said.
Speaking on the sidelines of a UN climate conference in Copenhagen, WMO head Michel Jarraud pointed to extreme hotspots this year - Australia had its third warmest year since record dating began in 1850, "with three exceptional heatwaves".
"I could go on. There was the worst drought in five decades which affected millions of people in China, a poor monsoon season in India causing severe droughts, massive food shortages associated with a big drought in Kenya," he told reporters.
Jarraud also highlighted extreme floods, including one which broke a 90-year record in Burkina Faso. 2009 marked the third lowest summer Arctic sea ice on record, after the two previous years, he added.
Vicky Pope, head of climate change advice at Britain's MetOffice Hadley Centre, which supplied some of the WMO data, agreed that 2009 is likely to be the fifth warmest year.
"Essentially what's happened is we've gone into an El Nino," she added, referring to a natural weather pattern which drives abnormal warming in the eastern Pacific Ocean and can unleash wider havoc in global weather.
Global warming doubters have pointed to some recent years of cooling, but Mark Maslin, a professor at the University College London Environment Institute said "the weight of scientific evidence for man-made climate change is now irrefutable."
Paal Presterud, a scientist at the Center for International Climate and Environmental Research in Oslo, said: "Scientists have not been good enough telling the public we may have periods without warming" amid a long-term trend of higher temperatures.
Presterud, also speaking on the sidelines of the Copenhagen summit, warned that even if countries managed to eliminate all carbon emissions, global temperatures decrease extremely slowly.
US scientist Robert Correll said that the world needed aggressive action to avoid 3 metres of sea level rise this century and a 3.5 degree Celsius temperature rise from pre-industrial levels - far above the 2-degree barrier many have warned should not be breached.
The hottest year on record, 1998, coincided with a powerful El Nino, and a new El Nino developed this year.
"It's getting warmer and warmer. The warming trend is increasing," Jarraud told Reuters. "It's difficult to say (when the record will be broken) because of the variability. The first time there will be a strong El Nino the temperature will be greater than before."
The fact that the record for the hottest year has not been broken since 1998 has helped fuel arguments from a small minority of scientists that climate change may not be as severe as feared.
But MetOffice Hadley Centre's Pope said that temperatures had "climbed slightly" in the past decade. "There hasn't been a cooling (since the 1998 spike)," she said.
The decade 2000-2009 was 0.4 degrees Celsius above the 1961-1990
average, while the 1990s decade was 0.23 degrees higher, said