2018 takes the podium as one of the hottest years on record. Let’s look deeper.

"We've known since the 1980s that Earth has had a fever," stressed Sarah Green, an environmental chemist, in a recent interview.

That fever continued, undiminished, in 2018.

NASA's Goddard Institute for Space Studies keeps track of Earth's changing temperatures with a data bank that reaches back to the 1880s. This year will end up as the 4th-warmest year in recorded history, Gavin Schmidt, the director of the NASA program, said over email.

"I’d emphasize that any one year’s temperature is not that important, but the long-term trends are — and they are unmistakable and furthermore, exactly in line with predictions made years ago," Schmidt, a climate scientist, said. "The trends are due almost entirely to us (and specifically the fossil-fuel related increases in CO2)."

Original image has been replaced. Credit: Mashable

And with 2018, the longer-term trends have again grown increasingly certain.

"The last four years were the warmest years on Earth since systematic record-keeping began in the 19th century," Stefan Rahmstorf, head of Earth System Analysis at the Potsdam Institute for Climate Impact Research, said over email.

What's more, 18 of the last 19 years are now Earth's warmest years on record. This makes sense, as Earth's concentrations of carbon dioxide — a potent heat-trapping gas — are the highest they've been in some 15 million years.

Overall, Earth is now the warmest it's been since a balmy period 120,000 years ago, when hippopotamuses roamed Europe.

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The consequences, or symptoms, of the warming are really only in their beginning stages, but they're evident everywhere around the globe, from the warming Arctic plains where depleted reindeer populations roam to the charred hills of Malibu, California, where mansions dot the land.

"We have identified many additional symptoms, from decreasing sea ice to extreme storms and droughts," said Green, a professor at Michigan Technological University. "An abundance of evidence shows that the disease is an overdose of greenhouse gases in the atmosphere."

Scientists know, specifically, that the carbon saturating the atmosphere comes from humans burning fossil fuels — oil, coal, and natural gases composed of long-dead plants, trees, and marine life. When burned for energy, they give off a specific carbon signature, the lighter form of carbon that all of Earth's plants breathe.

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While Earth's temperature gradually increased during the 20th century, it was really around the early 1980s that an accelerated warming trend began, and it's only continued.

The early '80s is when the influence of carbon dioxide really began to overshadow other influences in the global climate, such as naturally occurring greenhouse gases and Earth's natural internal variability, which is the way weather and short-term climate wavers, especially year to year.

"Those other factors go up and down, but CO2 keeps on going in one direction and so it wasn’t until the 1980s that the signal from CO2 started to clearly come out of the noise of everything else," said NASA's Schmidt. "And as the signal has increased, the irrelevance of everything else (for the global mean at least) has become ever clearer."

In 2018, the signal keeps up its accelerated pace.

"Global heating continues relentlessly and will do so as long as we continue raising the greenhouse gas levels in our atmosphere," said Rahmstorf.

UPDATE: Feb. 6, 2019, 11:15 a.m. EST: NASA and NOAA confirmed that 2018 was the fourth warmest year on record in a 2019 update.