There was no global warming hiatus

Researchers confirm continuing rising sea temperatures.

Between 1998 and 2013 global warming was believed to have slowed down or paused. This belief was disproven two years ago when a controversial paper showed that sea temperatures have actually continued to rise and there was in fact no “global warming hiatus”. This has now been confirmed again in a new study published in Science Advances.

Zeke Hausfather and colleagues from University of California, Berkeley showed that differences in measurement techniques was to blame for the apparent hiatus. The use of modern buoys to measure ocean temperatures tended to report slightly cooler temperatures than older ship-based systems because the buoys measure the water directly from the ocean rather than after a trip through a warm engine room.

We spoke to Hausfather about the work.

ResearchGate: There was a controversial paper two years ago that concluded that there was no “global warming hiatus.” Can you tell us what the researchers found at the time?

Zeke Hausfather: Two years ago, researchers led by Tom Karl of NOAA published a paper in Science where they argued that the rate of warming had been significantly underestimated over the past 18 years, and that much of the apparent "pause" or "hiatus" in warming was due to biases in ocean measurements that they claimed to correct. This paper proved highly controversial in the public realm, leading to efforts by the U.S. congress to demand access to all the emails of scientists involved. From a scientific standpoint, the new NOAA record ended up being notably warmer than the commonly used UK Hadley Centre record. This difference (and to a lesser extent the controversial nature of the original paper) led us to undertake a project to determine which record was most accurate in recent years.

RG: How did you feel about those results? 

Hausfather: There are always challenges when combining measurements from different types of instruments into a single long-term record, and researchers have to make judgement calls about how to best correct for differences between instruments. We've long respected the hard work done by NOAA to maintain and update the global temperature record, but we were also somewhat skeptical of their results due to the difference between their new record and the Hadley record.

Credit: Zeke Hausfather, UC Berkeley
Credit: Zeke Hausfather, UC Berkeley.

RG: What made you want to study the “hiatus?” 

Hausfather: The apparent "hiatus" in warming in the early part of the 21st century raises a number of important scientific questions. If the hiatus is real, it might have implications for our understanding of multidecadal variability or climate sensitivity. While the recent record high temperatures in 2014, 2015, and 2016 should put to bed any notion that global warming has somehow stopped, the relatively slow rate of warming in prior years is still of scientific interest.

RG: What did you study?

Hausfather: The differences between the old NOAA, new NOAA, and Hadley datasets over the past 18 years occur during a period of unprecedented expansion in our ocean monitoring capabilities. Before the mid-1990s almost all our ocean temperature measurements came from ships, mostly through engine room intake valves. Ship measurements can be problematic, as they will be affected by the depth of the hull, the speed of the ship, and other factors that will change over time.

Today we have thousands of buoys floating around the ocean taking temperatures and sending the data up to satellites automatically. We have thousands of robotic Argo floats that dive deep down into the ocean, and take measurements as they come back up. We also have multiple advanced radiometer satellites that can measure the heat radiating off the ocean's surface.

The challenge with the NOAA and Hadley records is that both are trying to combine measurements from different instruments and different types of ships into a single continuous record, and this requires a number of judgement calls. In our study we created three new instrumentally homogenous sea surface temperature records, one using only data from buoys, one using only data from Argo floats, and one using only data from satellite radiometers. These records are all from one type of instrument, and do not require any adjustments for changing instrument types.

RG: What did you find?

Hausfather: It turns out that data from buoys, satellites, and Argo floats all largely agree with the new NOAA ocean temperature record. They show a strong cool bias in the old NOAA record and a modest, yet significant, cool bias in the Hadley record and the Japanese record. These results provide a strong independent validation of the controversial updates that NOAA made to their temperature record in 2015.

RG: What can you do to prevent biases like this in the future? 

Hausfather: Our results suggest that there is a cool bias in ship data in recent years that does not show up in the higher-quality buoy data or satellite or Argo data and that needs to be addressed in the Hadley record moving forward. In general, we should try and rely more on buoys and other automated measurement systems that sit directly in the water going forward.

Featured image courtesy of flickr.