A team of government scientists has revised its estimate for how much the planet has been warming.
The new results, published in the journal Science, may dispel the idea that Earth has been in the midst of a "global warming hiatus" — a period over the past 20 years where the planet's temperature appears to have risen very little.
"We think the data no longer supports the notion of having a hiatus," says Tom Karl, a scientist with the National Oceanographic and Atmospheric Administration and coauthor of the new study.
If the entire problem of global warming could be summed up in a single number, that number would be the average temperature of the entire surface of the earth, land and sea, at a moment in time. It's also called the global mean surface temperature.
During the 20th century, that number shot up. But then something happened.
"Since about 1998, through to about 2013, the rate of increase was a lot less," says Kevin Trenberth, a scientist at the National Center for Atmospheric Research in Boulder, Colo.
The slowdown was so dramatic that it appeared that global warming might have stopped altogether. Scientists called it the global warming hiatus.
Skeptics of climate change have seized on the idea of the hiatus. "They're using this as a ploy to say, 'Oh, there is no global warming; we don't have to worry about climate change,' " Trenberth says.
But few scientists have believed a hiatus meant climate change had stopped. Trenberth says the slower warming is, in part, caused by unusual currents in the Pacific Ocean. Others have cited volcanic activity.
Now Karl's team, which is directly responsible for taking the Earth's temperature, says a technological shift in the way the measurements are taken has also obscured the temperature's climb.
Here's why: The single number — average global temperature — comes from tens of thousands of independent temperature readings. And, in recent decades, the technology for getting those readings has gradually shifted.
On land those measurements are made by weather stations; on the sea, the job has generally been done by commercial and military ships for decades. But starting in the 1980s, governments also began dropping buoys into the ocean to do independent measurements.
Karl and his colleagues decided to look at stretches of water where ships pass very near buoys, to compare the two temperatures. And they made a surprising discovery.
"The buoys actually read colder than the ships," Karl says.
Even though the two thermometers were in the same place, they gave different readings. And it was happening all over the world. As more buoys were dropped into the sea — all delivering measurements that were consistently cooler than a ship would show in that same spot — the warming trend in the average global temperature seemed to slow dramatically.
But Karl and his colleagues believe what looked like a flattening of the warming trend actually just reflected a change in the way the temperature was taken. When the team factored in a correction to the historical data that reconciled the buoys with the ships, they found that what had seemed to be a hiatus in warming disappeared.
Earth's average temperature has, indeed, maintained a steady climb through the past decades.
Trenberth, who wasn't involved in this analysis, says critics may be skeptical of these revisions, but they shouldn't be.
"You see this kind of thing also with the stock market and various other economic indicators: 'Oh they've revised the estimates of what the unemployment rate was in the last quarter,' or something like that, and that's exactly what's going on here," Trenberth says.
And the warming trend may be accelerating even more. The calendar year 2014 was the warmest on record, and Trenberth says the past 12 months — midyear to midyear — have been even warmer than that.
AUDIE CORNISH, HOST:
Now a revision on how much the planet has been warming. That came today from a team of government scientists as NPR's Geoff Brumfiel reports.
GEOFF BRUMFIEL, BYLINE: If the entire problem of global warming could be summed up in one single number, that number would be the global mean surface temperature. It's the temperature of the entire surface of the Earth at a moment in time. During the 20th century, that number shot up. But then, something happened. Kevin Trenberth is at the National Center for Atmospheric Research.
KEVIN TRENBERTH: Since about 1998 through to 2013, the rate of increase was a lot less.
BRUMFIEL: So much less that it appeared global warming may have stopped altogether. Scientists called it the global warming hiatus. Climate change skeptics used the hiatus in the political fight.
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TUCKER CARLSON: We haven't seen a rise in global temperatures in almost 20 years. Why is that? We're not sure. And so until we figure out why - what's happening with the global climb, you probably shouldn't hike energy taxes for average Americans in response to a threat you don't understand.
BRUMFIEL: That's commentator Tucker Carlson on Fox News last year. Trenberth says slower warming is in part caused by unusual changes to Pacific Ocean currents. But today in the journal Science, a team directly responsible for taking the Earth's temperature says there may also be a more fundamental problem. It turns out global surface temperature is really hard to measure.
TOM KARL: The concept itself is quite simple. Actually trying to do it is quite complex.
BRUMFIEL: Tom Karl is at the National Oceanographic and Atmospheric Administration. The single number comes from tens of thousands of independent temperature measurements. On land, they're made by weather stations. On the sea, it's ships.
KARL: All kinds of ships. They include military ships. They include commercial ships.
BRUMFIEL: The ship's measurements have been used for decades, but starting in the 1980s, governments began dropping buoys into the ocean to do independent measurements. Karl and his colleagues decided to look where ships passed near buoys and compared the two temperatures, and they made a surprising discovery.
KARL: The buoys actually read colder than the ships.
BRUMFIEL: Even though the two thermometers were in the same place. It was happening all over the world, and as more buoys delivered more cooler temperature readings, they made global temperature look cooler. Karl and his colleagues corrected decades of data so that ships and buoys in the same place agree on the temperature. The result...
KARL: We think the data no longer supports the notion of having a hiatus.
BRUMFIEL: In other words, the Earth's temperature kept right on warming for the past 20 years. Kevin Trenberth, who wasn't involved in this analysis, says critics may be skeptical of these revisions, but they shouldn't be.
TRENBERTH: You see this kind of thing also with the stock market or with various other economic indicators that, oh, they have revised the estimates of what the unemployment rate was in the last quarter or something like that, and that's exactly what's going on here.
BRUMFIEL: And the warming may be speeding up even more. 2014 was the warmest year on record, and Trenberth says the past 12 months have been even warmer than that. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.