A study by researchers from the University of California, Berkeley, and Berkeley Earth, a non-profit research institute, disproves what is known as the "global warming hiatus."
The hiatus, which stems from a 2015 paper that concluded there was no detectable slowdown in ocean warming over the previous 15 years, has been cited by those dubious about global warming as evidence that climate change is a hoax.
An analysis indicates that modern buoys now used to measure ocean temperatures tend to report slightly cooler temperatures than older ship-based systems, even when measuring the same part of the ocean at the same time. As buoy measurements have replaced ship measurements, this had hidden some of the real-world warming.
After correcting for this "cold bias," researchers with the U.S. National Oceanic and Atmospheric Administration (NOAA) calculated then that the oceans have actually warmed 0.12 degrees Celsius, or 0.22 degrees Fahrenheit, per decade since 2000, nearly twice as fast as earlier estimates of 0.07 degrees Celsius per decade. This brought the rate of ocean temperature rise in line with estimates for the previous 30 years, between 1970 and 1999.
The new study, published Wednesday in the online, open-access journal Science Advances, uses independent data from satellites and robotic floats as well as buoys, and concludes that the NOAA results were correct.
"Our results mean that essentially NOAA got it right, that they were not cooking the books," said lead author Zeke Hausfather, a graduate student in UC Berkeley's Energy and Resources Group, explaining that years ago, mariners measured the ocean temperature by scooping up a bucket of water from the ocean and sticking a thermometer in it. In the 1950s, however, ships began to automatically measure water piped through the engine room, which typically is warm.
Nowadays, buoys cover much of the ocean and that data is beginning to supplant ship data. But the buoys report slightly cooler temperatures because they measure water directly from the ocean instead of after a trip through a warm engine room.
"Only a small fraction of the ocean measurement data is being used by climate monitoring groups, and they are trying to smush together data from different instruments, which leads to a lot of judgment calls about how you weight one versus the other, and how you adjust for the transition from one to another," Hausfather was quoted as saying quoted as saying in a news release. "So we said, 'What if we create a temperature record just from the buoys, or just from the satellites, or just from the Argo floats, so there is no mixing and matching of instruments?'"
In each case, using data from only one instrument type, either satellites, buoys or Argo floats, the results matched those of the NOAA group, supporting the case that the oceans warmed 0.12 degrees Celsius per decade over the past two decades, nearly twice the previous estimate. In other words, the upward trend seen in the last half of the 20th century continued through the first 15 years of the 21st: there was no hiatus.
"In the grand scheme of things, the main implication of our study is on the hiatus, which many people have focused on, claiming that global warming has slowed greatly or even stopped," Hausfather said. "Based on our analysis, a good portion of that apparent slowdown in warming was due to biases in the ship records."
