It may have been the most controversial climate change study in years.
In the summer of 2015, a team of federal scientists at the National Oceanic and Atmospheric Administration published a blockbuster paper in Science that appeared to wipe away one of global warming doubters' favorite arguments. The skeptics had for years suggested that following the then-record warm year of 1998 and throughout the beginning of the 21st century, global warming had slowed down or "paused." But the 2015 paper, led by NOAA's Thomas Karl, employed an update to the agency's influential temperature dataset, and in particular to its record of the planet's ocean temperatures, to suggest that really, the recent period was perfectly consistent with the much longer warming trend.
This didn't merely surprise some scientists (who had been busily studying why global warming had appeared to moderate its rate somewhat in the early 21st century). It actually led to a congressional subpoena from Rep. Lamar Smith, chair of the House Committee on Science, who charged that "NOAA's decision to readjust historical temperature records has broad national implications" and requested more information on why NOAA had made the dataset adjustment, including data and communications from the scientists involved.
That controversy is likely to be stirred anew in the wake of a new study, published Wednesday in Science Advances, that finds the NOAA scientists did the right thing in adjusting their dataset. In particular, the new research suggests that the NOAA scientists correctly adjusted their record of ocean temperatures in light of known biases in some observing systems – and indeed, that keepers of other top global temperature datasets should do likewise.
"We pretty robustly showed that NOAA got it right," said study author Zeke Hausfather, a Ph.D. student at the University of California-Berkeley and a researcher with Berkeley Earth, a nonprofit consortium that has reanalyzed the Earth's temperatures. "There was no cooking of the books, there's no politically motivated twisting of the data."
Hausfather completed the study with scientists based at York University in the U.K., George Mason University, and NASA, as well as an independent researcher.
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To understand the new study – which gets complicated fast, as it dances back and forth between different datasets – you first need to understand the biggest issue underlying the original NOAA analysis. This involved reconciling the data from two separate ways of measuring temperatures at or near the surface of the planet's oceans (which are the largest component of determining its overall temperature).
One data source was global ships, which draw in ocean water in their engine rooms and take its temperature. Key parts of the past ocean temperature record are based on these reports. The other data source is buoys, which float in the water, take measurements, and relay the results to satellites. In general, buoys have been relied upon more for measurements beginning in the 1990s, as they have become more widely deployed. They are, naturally, a more direct measurement, one less mediated by physical ships and fallible humans.
But the increasing use of buoys created an issue of reconciling the two data sources to piece together a seamless and continuous record – and NOAA was, essentially, siding with the buoys when it comes to accuracy. "The ship data are systematically warmer than the buoy data," NOAA explained in the controversial study. (After all, ship engines are relatively warm places.) It also said that the buoy data are "more accurate and reliable."
Failing to account for this difference, once the shift from ship data to buoy data occurred, had led NOAA's temperature record to be too cold – and also appeared to dampen the overall rate of global warming. So to better patch together a long term temperature record necessarily reliant on both data sources, NOAA used a "bias correction" to take this into account, and more generally gave greater weight to the buoy data, in updating its dataset.
This highly technical switch, in turn, had the effect of increasing the overall warming of the oceans in the new dataset – and helping to wipe out claims that there'd been any recent slowdown in the rate of climate change.
So with all of that background in place, what Hausfather and his colleagues have done in the new study is create separate records of ocean temperatures from three different, standalone measuring systems – satellites, Argo floats, and buoys once again. They then compared these measurements to the original and revised NOAA records (and to some other international records), and found that the new NOAA dataset aligns better with the standalone sources, especially for buoys and satellites. The old and uncorrected dataset, in contrast, they say suffers from a "cold bias."
For Argo floats, there were three separate independent records and two also favored the new NOAA temperature record, although the third one did not. (These floats also haven't been in operation that long, which complicates using them.)
Buoys in this context count as an independent dataset, Hausfather explained by email, because "a lot of the uncertainty in the NOAA record comes [from] the merging of ships and buoys. By looking only at buoys (which pretty much everyone agrees are higher quality more homogenous instruments), we avoid all of the issues associated with merging the data."
Based on this evidence, the new study therefore concludes that the NOAA correction was probably. . .well, correct. It better aligns with independent measurements of what the oceans are actually doing.
Moreover, the study also finds that for another major ocean temperature record – kept by the U.K.'s Hadley Center – the correction from ships to buoys is adequate. However, this dataset fails to weight buoys more than ships, and so is still not warm enough, the study concludes. This agency, the paper therefore suggests, ought to emulate NOAA's path.
The upshot, says Hausfather, is that "at least globally, it's impossible to differentiate the rate of warming over the last say 18 years from the longer term rate of warming, over the last 30 or 50 years. . .I don't think we can say any more that we have evidence that global warming has slowed down in any way."
"There are still questions about why the rate of warming in one decade could be a little slower than the rate of warming in another, or a little faster," he continues. "But I don't think we can really say that there was a discernible long-term hiatus."
At least one author of the original NOAA study heralded the new work in a comment to the Post.
"Given how rigorously we evaluated our ERSST v4 work, these results are not surprising but they are gratifying," said Thomas Peterson, one of the authors of the NOAA study (who has since retired from the agency). ("ERSST v4" refers to NOAA's more recent, adjusted sea surface temperature dataset at the root of the controversy.)
"Not working on any of the major long-term global sea surface temperature data sets allowed the authors to perform an unbiased analysis," he continued.
But Gerald Meehl, a climate scientist at the National Institute for Atmospheric Research who has published on the "hiatus" and attributed it to changes in the Pacific Ocean, said that notwithstanding this work, something really did happen, temporarily, to the planet's climate in the 2000s that could be called "hiatus" and is worth understanding.
"The early-21st century 'slowdown' (or 'hiatus') was mostly a product of internally-generated naturally occurring climate variability," said Meehl by email. "It was notable compared to the previous 20 years when there was accelerated warming, also with a big contribution from internally generated variability."
Therefore, for Meehl, it's not an either-or. "To say the slowdown never occurred is to ignore the important aspects of internal variability, and to say that global warming stopped in the early 2000s ignores the important long-term warming trend due to increasing [greenhouse gases]."
The global warming "pause" debate had, itself, paused somewhat in light of the temperatures of 2014, 2015, and now 2016. These three years, in descending order, appear very likely to have been the warmest, second warmest, and third warmest years on record (the record for 2016 is not yet official, at least per NOAA and NASA). In this context, debating whether global warming was slowing down when it kept setting new records seemed a rather odd pursuit.
But if anything can reawaken it, well, it's new evidence about whether the pause existed at all as a substantial global phenomenon.