Independent Study Confirms No Hiatus in Ocean Warming

Zeke Hausfather discusses changes in how sea surface temperatures have been measured over the past two decades, and the impact of these changes on warming estimates. | Produced by Peter Sinclair, GreenMan Studios

Independent measurements of sea surface temperatures in the last two decades support a recent government analysis that found an increase in sea surface warming, according to a new study in the 4 January issue of the journal Science Advances.

The government dataset, called the National Oceanic and Atmospheric Administration's Extended Reconstructed Sea Surface Temperature version 4, increased the sea surface temperature trend estimate over the last 18 years from 0.07° Celsius per decade to 0.12° Celsius per decade, partly because of adjustments for different types of measuring instruments. These results further refute the existence of a 21st century global warming slowdown described in studies including the 2013 Intergovernmental Panel on Climate Change assessment.

In June 2015, NOAA researchers led by Thomas Karl published a paper in the journal Science comparing the new and previous NOAA sea surface temperature datasets, finding that the rate of global warming since 2000 had been underestimated and there was no so-called "hiatus" in warming in the first fifteen years of the 21st century. The paper proved controversial with some members of Congress and prompted the chairman of the House Science and Technology Committee to initiate an investigation into scientists at NOAA .

"There has been a lot of skepticism of government temperature records expressed by some in the U.S. Congress … with a number of unwarranted accusations of scientists 'cooking the books,'" said the study's lead author Zeke Hausfather, a research scientist at Berkeley Earth, an independent, non-profit research group, and Ph.D. candidate in the Energy and Resources Group at the University of California, Berkeley.

Hausfather and the other authors of the study looked at data collected in the last two decades from three instruments separately — floating buoys, autonomous battery-powered robots that collect ocean data called Argo floats, and satellites — and found that the measurements agree with NOAA's revised dataset.

"Our findings should help put to rest the controversy surrounding the new NOAA ERSSTv4 record in recent years, as it agrees quite well with instrumentally homogenous sea surface records from buoys, satellites and Argo floats," Hausfather said.

Obtaining accurate sea surface temperatures is important for a range of applications — from weather prediction to climate modeling to understanding marine ecosystem fluctuations.

In recent years, buoy-based measurements, thought to be more accurate as buoy sensors are in direct contact with the ocean surface, have grown in frequency, while ship-based measurements have become less common.

"The way we take ocean measurements has changed massively in the past two decades," Hausfather said. "Buoy measurements are generally considered higher quality than ship-based measurements. They send their measurements in automatically via satellite uplink, and all have pretty much the same type of measurement system. They also sit directly in the water."

In NOAA's v4 dataset, buoy measurements are given more weight than ship-based measurements. "NOAA's old record simply combined ship and buoy measurements without any correction for this offset, leading to a lower [temperature] trend in the combined record than would have occurred if the ships were adjusted down to match the buoys or the buoys were adjusted up to match the ships," Hausfather explained.

The study also suggests two other widely-used sea surface temperature datasets, the Hadley Centre's HadSST3 record and the Japanese COBE-SST record, have significant "cool biases" due to treating all measuring instruments equally.

This means the new NOAA record is likely the most accurate sea surface temperature record in recent years, the authors say on a site presenting their study , hosted by author Kevin Cowtan and the University of York.

"I do hope these results provide a strong message that the scientists at NOAA are doing solid, unbiased research," Hausfather said. "Especially with the new administration coming in and likely making a lot of changes to various government agencies involved in earth science."

[Credit for associated image: NOAA/ OSPO]