Did Global Warming Really ‘Pause’ During the 2000s?

It is the first year of the new Republican president’s term. He has taken over a healthy economy from his Democratic predecessor, and, with it, the freedom to branch out beyond the typical Reaganism. He has also inherited a slew of environmental policies, many of which combat global warming.

Most important among these is a fledgling UN treaty, a global agreement to restrict greenhouse-gas emissions. It was never ratified by the Senate, so the new president—whose Cabinet members have deep ties to the oil and gas industry—must decide whether to stay in the agreement or abandon it.

Global warming does not obsess most Americans, but it frightens the scientists who study it. Just before the new president took office, an unprecedented and monstrous El Niño, the largest ever recorded, set a new annual global temperature record—“the hottest year ever measured,” as the newspapers put it. Ocean temperatures surged around the world, bleaching the Great Barrier Reef and inducing a mass coral die-off . Great cracks are even appearing in ancient Antarctic ice shelves. Climate change seems to be already under way.

You know where this is going. The president, of course, was George W. Bush. In his first months in office, he decided to abandon negotiations over the Kyoto Protocol, which required the United States to cut its greenhouse gas emissions five percent below 1990 levels. He promised to supply a different climate policy later in his term. Environmentalists warned that soon global warming would become too devastating to ignore.

And then it did something inconvenient. For about a decade following the El Niño of 1998, “the world seemed hardly to warm,” as a new paper puts it. With some marked exceptions—such as the devastating European heat wave of 2003 or the continued melting of mountain glaciers—the global climate sent up few of the temperature sirens of the late 1990s.

This period—termed the global-warming hiatus, slowdown, or pause—became one of the most controversial events in modern climatological history.

Most scientific accounts of the hiatus say it was over by 2009. It has now ended by any measure. 2014, 2015, and 2016 all set new global temperature records, and eight of the 10 hottest years in the 120-year meteorological record have occurred in the last decade. A new paper takes stock of the global-warming hiatus, comparing scientific and popular accounts of the phenomenon. It finds that—just as scientists predicted throughout the episode—the existence of the hiatus did not disprove climate change. Nor did it indicate that global warming had halted or even slowed down. And more importantly, they find that the existence of the hiatus does not change end-of-century predictions for global warming.

One of the problems with the “hiatus” idea is that it was never well defined. Did a global-warming hiatus mean that Earth had stopped warming, or only that it was warming less than scientists expected? The paper’s authors examine three different definitions of the hiatus: that Earth stayed the same temperature or even cooled, that Earth’s rate of warming slowed down, and that climate models predicted Earth would warm more than it did.

That first definition of the hiatus was the one that first caught on. In 2009, the global temperature data set put out by the Hadley Center for Climate Prediction, the U.K. government’s climate-research office, appeared to show that temperatures over the previous nine years had very slightly decreased. To be precise, it said that the global mean temperature decreased by 0.01 degrees Celsius between 1998 and 2009. Researchers noted this was due mostly to a drop in ocean temperatures—land temperatures continued to warm during this period.

Over the next several years, scientists revisited that period in the data. They found that its underlying measurements masked a huge changeover in the technology used to actually record temperatures, especially of the oceans. In the early 2000s, ocean temperatures were mostly measured in hot engine rooms; by the late 2000s, they were mostly measured by standalone buoys and robotic floaters working alone in the open sea. This would have caused ocean temperature measurements at the early part of the decade to be biased toward warmth in a way that late-decade measurements weren’t.

Once this bias was corrected, the apparent decrease in temperatures from 1998 to 2009 went away. The first definition of the hiatus doesn’t hold water.

This correction carried a good deal of controversy: Thomas Karl, a longtime scientist at the National Oceanic and Atmospheric Association, first recommended that change in a landmark 2015 paper. Shortly thereafter, he was subpoenaed by Lamar Smith, a Republican congressman of Texas, who alleged the decision to “readjust historical temperature records has broad national implications.” Karl’s office was subjected to a congressional investigation, but the scientific community has tested and upheld his team’s findings. In an independent analysis released earlier this year, Zeke Hausfather, a climate scientist at Berkley Earth, found that when you isolate temperature records taken by individual types of instruments—for example, from only buoys, or from only satellites—all the data sets follow the trend first identified by Karl.

The second and third definitions of the hiatus rely on the two different ways that scientists talk about the global warming trend. The first warming trend is the rate of warming we’ve already observed—the 1.1 degree Celsius rise in temperatures that’s been measured by thermometers, buoys, and ships over the last 120 years. The second warming rate is the one that we predict—the eventual 1.5 to 4 degrees Ceslius of warming that computer models say will most likely occur by 2100.

The second definition of the hiatus—that temperatures increased, just by less than the observed trend—also proves weak. For instance, whether a certain 15-year period after 1997 “underperforms” depends greatly on whether you’re measuring a long-term trend from the years between 1951 and 2000, or the years between 1951 and 2012. And no matter what, the difference between all those trends is not statistically significant.

The final definition is the strongest of the three: Climate change did underperform the predicted trend during the hiatus.

But that hiatus goes away, too, when the models are made to more accurately reflect the world as it was between 1998 and 2009. The hiatus period, for instance, is short—less than a decade long—so many small contingencies can affect its mean temperature. For example, if volcanos happen to belch a lot of sun-blocking gases into the atmosphere, temperatures will go down. If the Pacific Ocean has a number of La Niña events but no strong El Niños, temperatures will also look low. These kinds of variations even out at the century-scale, but not within decades.

If you update 40 of the world’s top climate models so that they contain the right number of gases and the right frequency of El Niño events, then they do a much better job of predicting the hiatus. But their long-term predictions don’t change. They still find that Earth is still on track for 1.5 to 4 degrees Celsius of warming over the next 80 years—just as they did before the update.

In other words, you can “tune” the models to make them better mimic a certain span of years, but that tuning doesn’t seem to teach them anything essential about the underlying Earth system.

Climate scientists who were not connected to the study says the paper “closes the book” on the hiatus period, summarizing the best science and allowing researchers to move on. They also noted that the hiatus has ended now by any measure. “It’s 2014, 2015, and 2016 that killed the hiatus, and not any adjustment to the data. And the same thing is true if you look at the raw data without any fixes,” Hausfather told me last week.

“In fact, if you look at 1998 to 2016—which is more of a fair comparison, because you start and end with an El Niño—you see that the warming continues at the same trend. That’s why, as climate scientists, we tend to focus on 30-year trends,” he said.

Kim Cobb, a paleoclimatologist at Georgia Tech, noted that the hiatus may contain a more ominous lesson for humankind. “There’s no reason why our world should walk at the model mean,” she told me. “It’s very important for the public to understand there are two sides to the model mean. We’ve found ourselves on the cooler side of that, but we might find ourselves on the warmer side of that as well. And that would be very challenging.”

To that point, some assessments of the climate literature—such as Gernot Wagner and Martin Weitzman’s book, Climate Shock—have found a 10-percent chance that the world will experience more than six degrees Celsius (11 degrees Fahrenheit) of warming by 2100.

Chris Colose, a researcher at NASA’s Goddard Institute for Space Studies, says that controversies like the hiatus will keep frustrating scientists as long as there is public dispute over the authority of climate science.

“I may be pessimistic, but I think bad-faith actors will seize upon anything they can find to cast doubt on climate science reliability,” he said in an email.

“Because of the big El Niño event in 2016, I suspect that temperatures five to 10 years from now will be statistically comparable to 2016, even though 2016 to 2025 will be a warmer decade than the previous one,” he added. “If climate science is still very polarized, I have little doubt people will be talking about ‘how global warming ended in 2017,’” Colose told me. “It will be just as silly then, and hopefully all the hiatus talk this time around will serve as a compelling reason to ignore them.”