"It's been hot," said Gavin Schmidt, director of the NASA Goddard Institute for Space Studies (GISS). "The last five years have been the record warmest years that we've seen in a record that goes back to the 19th century and possibly even hundreds of years before that."
2018 marks the fourth warmest year since 1880 and ranks behind 2016, 2017 and 2015, according to NASA and the National Oceanic and Atmospheric Administration (NOAA).
Temperatures in both 2016 and 2015 were lifted by an El Niño, a natural event which can disrupt weather patterns worldwide every few years and releases heat from the tropical Pacific Ocean into the atmosphere.
"The key issues in reducing emissions are moving towards renewable fuels for electricity, moving away from internal combustion engines for transport, sequestering carbon more efficiently in agriculture, making better decisions about how we organise our world so that we don't need so much energy," Schmidt said.
"But most of all, reducing the amount of coal, natural gas, and other fossil fuels, and deforestation," he added.
Weather extremes in 2018 included wildfires in California and Greece, drought in South Africa and floods in Kerala, India. Record levels of man-made greenhouse gas emissions, mainly from burning fossil fuels, trap ever more heat.
The outlook is for more sizzling heat approaching levels that most governments view as dangerous for the Earth.
"What kind of planet is a planet that's four or five degrees warmer than it is now? Well, we haven't seen that on Earth since about 3 million years ago in the Pliocene. At that point, we had forests all the way up to the Arctic Circle, there wasn't any ice," Schmidt informed.
"There was no Greenland and sea level was about 25 meters higher. That was a very different planet, and that's kind of where we're headed unless we do something about emissions."