2017 Was One Of The Hottest Years On Record

NOAA has released the latest State of the Climate report, its annual checkup on our planet.

So, how did Earth fare in 2017?

Greenhouse gases in the atmosphere: highest concentrations ever. Global surface temperature: near-record high. Sea surface temperature: near-record high. Global sea level: highest on record.

Warm global temperatures have been a strong trend in recent years: the four warmest years on record all occurred since 2014, and last year was among them. In fact, 2017 was the warmest non-El Niño year ever recorded.

The past three years were "substantially warmer than the previous — kind of establishing a new neighborhood in terms of global temperature," said Deke Arndt, a climatologist at NOAA and the lead editor of the report. "And 2017 reinforced that."

Several countries reported record high annual temperatures: Argentina, Uruguay, Spain and Bulgaria. And Mexico had record high annual temperatures for the fourth year in a row.

NOAA's report, based on contributions from 500 scientists in 60 countries, was released today on the website of the American Meteorological Society.

The findings show the extent to which humans have already changed key aspects of our climate as we've increased concentrations of greenhouse gases in the atmosphere.

The Arctic continued to warm, and preliminary data show that the world's glaciers have continued to diminish, with the average glacier losing 72 feet off its top since 1980.

A coral bleaching event from 2014 to 2017 was "the longest, most widespread, and almost certainly most destructive on record," the report notes. Mass coral bleaching used to occur at a rate of once every 25–30 years in the 1980s. But now it happens about every six years, and it's expected to accelerate as the oceans keep warming. Severe bleaching is now occurring faster than reefs can recover.

The report shows a number of small shifts in our climate metrics – shifts that can drive extreme events like the heat waves, downpours and wildfires we've seen in recent weeks.

"We've had something on the order of one degree [Celsius] or so of global warming," says Radley Horton, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory, who wasn't involved in the new report. "It doesn't sound like much, but already it's producing more frequent heat waves. We've had globally less than a foot of sea level rise in the last century. Again it doesn't sound like much, but for certain regions it's already causing a fourfold increase in the frequency of coastal flooding."

What's most surprising in the report, he says, are the hints that we may be reaching tipping points where change accelerates or becomes irreversible.

"The further we push the climate system by increasing greenhouse gas concentrations, the greater the potential that we get a sudden surprise — something that climate models and their predictions aren't able to prepare us for," Horton says.

Last year saw a new record low in the extent of Arctic sea ice in the winter. As sea ice melts, there is less of its white surface reflecting the sun's rays, and more dark blue ocean absorbing the sunlight. That can create a positive feedback loop in which warmer oceans drive a faster decline of Arctic sea ice than climate models had predicted.

Just a decade ago, Horton says, the models predicted that the Arctic could be ice-free in the summer by 2100, while more recent models suggested that could happen by 2050. "Scientists are now concerned it could happen in the next couple decades," he says.

However, not every aspect of climate was record-breaking last year. Tropical cyclones were only slightly above average. Only the North Atlantic basin had an above-normal season, with hurricanes Harvey, Irma and Maria all causing huge destruction, in the basin's seventh most active season in 164 years.

And fire activity was at its lowest globally since at least 2003 – but the U.S. had by far its most expensive fire season ever, with more than $18 billion in damages.

The report "highlights the urgency for us as a society to reduce greenhouse gas emissions," says Horton, "and prepare our most vulnerable communities for some of these climate changes that we're locked into."

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