A remarkably warm December helped drive global surface temperatures in 2015 to the hottest year in records going back to 1880, the US National Atmospheric and Oceanic Administration said.
2015: Hottest year on record
An international report confirming that 2015 was the world's hottest year should act as a wakeup call, warns Australia's Climate Council. Courtesy ABC News 24.
Temperature records were broken for 10 of the 12 months of last year but it was December's spike that marked the sharpest leap for any month, surging 1.11 degrees above the 20th-century average.
It was the first time a monthly temperature anomaly had exceeded 1 degree, NOAA said.
Land temperatures last month were 1.89 degrees above average, smashing the previous record departure from the norm, set in December 2006, by almost half a degree (0.48 degrees).
Sea temperatures were 0.83 degrees warmer than the 20th-century average, beating the previous record anomaly for December 2009 by 0.36 degrees.
The December surge came during a month when leaders from almost 200 nations gathered in Paris to set a new agreement to keep temperature increases to less than 2 degrees and reduce the risk of dangerous climate change.
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The NOAA report on Thursday coincided with a paper in Nature by Swiss and Australian researchers that outlined how a 2-degree target would still see some parts of the world - notably the Arctic - rise by as much as 6 degrees because of regional differences.
As a whole, 2015 was a standout year, with surface temperatures 0.9 degrees above the 20th-century average.
The tally was 0.16 degrees above the previous record year - set only in 2014 - making it the largest margin by which an annual temperature record had been broken, NOAA said.
David Jones, head of climate monitoring at the Bureau of Meteorology, said previous stand-out years, such as 1998, had also come at a time of a big El Nino event in the Pacific.
"Now we've emphatically broken [all the previous records]," Dr Jones said. "Decade after decade, the planet is accumulating more heat because of greenhouse gas emissions."
Temperatures are rising about 0.17 degrees per decade since 1970, NOAA said.
During El Nino years, the Pacific Ocean's normally westward-blowing winds stall or reverse, leading to a warming of the central and eastern Pacific. Global temperatures get a kick of about 0.1-0.2 degree during such events.
The current El Nino - among the three strongest on record - probably peaked at the end of last year. The temperature impacts on the globe, though, typically lag about six months so it is likely that the start of 2016 will also be exceptionally warm, Dr Jones said.
The gap between last year and all previous record hot years was wide and widening by the end of 2015, as the following NOAA chart shows:
All of the 16 warmest years on record have happened since 1998, NOAA said.
The unusual warmth was widespread, with the Indian Ocean among those regions posting record heat.
Asia and South America had their hottest year on record, while Africa and Europe posted their second hottest years.
The US and Australia had their fifth hottest years on record, NOAA said.
Australia was about 0.8 degrees above average in 2015, or not far short of the global anomaly.
Land temperatures worldwide, though, were 1.33 degrees above average, beating the previous record departure from the norm by a quarter of a degree.
Australia's famously variable climate can cause annual temperatures to gyrate about 1 degree compared with the long-term average, while global shifts are in the order of 0.2-0.3 degrees, Dr Jones said.
While temperature changes were worth highlighting, people may also experience climate change through altered rainfall patterns and rising extremes, such as fire weather, he said.
Australia's fire season is already a particularly active one, with widespread fires in Victoria, South Australia, Western Australia and Tasmania - with at least another month of high fire risk to come.