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Science
| 06/11/15

NOAA says ‘hiatus’ disappears once data corrected

TCE

The ongoing conversation as to why the planet had apparently experienced what has been termed a ‘hiatus’ or pause in global warming since just before the turn of the millennium, has received another contribution, this time with US National Oceanographic and Atmospheric Administration (NOAA) suggesting that there actually was no global warming hiatus.

Researchers with NOAA say in a new paper out this week in the journal Science that once adjustments to data are made correcting for errors, the slowdown disappears. That is, the hiatus is in fact nothing more than an artifact of inaccurate data. Instead, the research suggests that global warming may now be accelerating.

Up to now, land-based and marine records had shown that average global temperatures from 1998 to 2014 had increased at just two thirds the rate that had been experienced from the middle of the last century, even though carbon emissions had continued to soar. The slowdown presented something of a puzzle for climate scientists, while climate sceptics and conservative politicians jumped on the phenomenon with glee. The consensus amongst researchers is that a combination of factors had conspired to produce the observed effect—solar radiation decreases, more particulate from volcanic eruptions and the predominance of La Niña events in the Pacific—a regular cooling surface ocean waters along the western coast of South America. Above all, the researchers concluded, world was still warming, but more heat was being taken up by the world’s oceans.

Thanks to increased international cooperation between scientists in recent years, the researchers were able to integrate land-based measurements of temperatures from more sites around the world, particularly Asia, South America, and Africa, which had earlier been under-represented in data sets. In addition, the data were corrected for errors in marine temperature records introduced by both ship-board sensors, which tend to be warmer than readings from buoys as a result of heat from the ship’s engine. Corrections were also made for variation in historic temperature readings made by less-than-scientifically-diligent sailors dipping buckets into the water. The practice of plopping buckets with thermometers over the side of a ship to measure sea surface temperature dates back to long before the Second World War, but did not always produce the most accurate readings.

Once these corrections were made, the hiatus all but disappears, with warming increasing at roughly the same rate as in earlier decades. And after temperatures from the abnormally warm 2013 and 2014 are incorporated, the hiatus vanishes completely.

According to the researchers, had it not been for the volcanoes, La Niña events and ocean heat uptake, the rate of warming would likely have been steeper. These factors appear to have run their course, and so we should expect an acceleration in warming in the coming years. Already, 2014 was the hottest year on record, and 2015 is on track to beat even that record.

Not all researchers agree with the conclusions however, noting that while climate sceptics have been wrong to suggest that the hiatus shows that global warming has stopped, more caution is warranted before declaring the hiatus to have disappeared. The Intergovernmental Panel on Climate Change has also considered observational uncertainty, but still acknowledged a slow-down since 1998.

In response to the hiatus controversy, PICS earlier produced a briefing note for policy makers and the public that clarifies the issue.

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