Average surface temperatures in 2017 were 1.1 degrees Celsius above pre-industrial times, creeping towards 1.5 degrees, the most ambitious limit for global warming set by almost 200 nations under the 2015 Paris climate agreement. The agreement has been weakened by a plan by US President Donald Trump to pull out of the Paris agreement. Trump doubts mainstream scientific findings that warming is driven by man-made greenhouse gases. NOAA's National Centres for Environmental Information said that 2017 achieved a temperature of 0.84 degrees Celsius (1.51 degrees Fahrenheit) above the average temperature seen in the 20th century. "From 1900 to 1980, a new temperature record was set on average ever 13.5 years; however, since 1981 it has increased to every three years," NOAA said.

The years 2014, 2015 and 2016 had set new all-time temperature records - culminating in a dramatic new high in 2016 - and NASA and NOAA had both agreed on their rankings as they occurred. By contrast, 2017 merely stayed within the elevated temperature range that these prior years had already established. Temperatures in both 2016 and 2015 were lifted by an El Nino, a natural event that releases heat from the tropical Pacific Ocean into the atmosphere that can disrupt weather patterns worldwide every few years. The WMO said 2017 was the warmest year without an El Nino since records began in the 19th century. It said 17 of the warmest 18 years since records began have now happened since 2000, confirming a warming trend driven by man-made greenhouse gases.

"When even the 'colder' years are rewriting the warmest year record books, we know we have a problem," Professor Dave Reay, chairman in carbon management at the University of Edinburgh, said. Among extreme weather events last year, the Caribbean and the United States suffered a battering from hurricanes, the Arctic ended 2017 with the least sea ice for mid-winter and tropical coral reefs suffered from high water temperatures. "Arctic warmth has been especially pronounced and this will have profound and long-lasting repercussions on sea levels, and on weather patterns in other parts of the world," WMO Secretary-General Petteri Talaas said. Gavin Schmidt, who directs NASA's Goddard Institute for Space Studies, said that 2017 was 1.12 degrees Celsius above late 19th-century temperatures. It's the third straight year in NASA's records that temperatures have eclipsed 1 degree Celsius above temperatures in the late 19th century.

"This year governments are due to start the process of assessing the size of the gap between their collective ambitions for reducing greenhouse gas emissions and the goals of the Paris Agreement," said Bob Ward, policy and communications director at the Grantham Research Institute on Climate Change and the Environment at the London School of Economics and Political Science. "The record temperature should focus the minds of world leaders, including President Trump, on the scale and urgency of the risks that people, rich and poor, face around the world from climate change." NASA and NOAA, which both keep independent records of the earth's temperature, have adopted a practice in recent years of jointly announcing their numbers, even though they can differ. In addition to the official US agencies, a number of additional expert outlets have tracked temperatures and found results consistent with those of NOAA and NASA. Zeke Hausfather, a climate scientist who also tracks annual temperatures with his group Berkeley Earth, also found that 2017 was the second hottest year on record.

"The Arctic has warmed 2½ degrees since the middle of the century," he said. "It's really warming faster than anywhere on earth. So much of the difference in 2017 between the groups that find it in second place and third place has to do with how the Arctic is handled. "It's unlikely we'll ever see temperatures as cool as we had back before 2014 again," Hausfather said. NOAA, NASA, and Berkeley Earth track temperatures at the surface of the earth, over both land and oceans. But another way to track the planet's warming is to analyse the temperature of the atmosphere at a significantly higher elevation, in the so-called "lower troposphere" extending from a little above the planet's surface to several kilometres into the air. The Washington Post, Reuters