Scientists have solved a puzzling break in continuity of ocean warming records that sparked much controversy after climate data was published in the journal Science in 2015.
The latest research from the Universities of York, UK, and California, Berkeley, US, confirms that the conclusions of the National Oceanic and Atmospheric Administration's (NOAA) research paper, which sparked wide debate following the suggestion that there was no detectable slowdown in ocean warming, were in fact accurate.
The 2015 analysis by NOAA scientists showed that ocean buoys now used to measure water temperatures tend to report slightly cooler temperatures than older ship-based systems. As buoy measurements have replaced ship measurements, this had hidden some of the real-world warming.
Scientists corrected this 'cold bias' and concluded that oceans have actually warmed 0.12 degrees Celsius per decade since 1997, 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 50 years, between 1950 and 1999.
Many scientists, including the International Panel on Climate Change, acknowledged the 'global warming hiatus', while those dubious about the science pointed to it as evidence that climate change is a hoax. The new study, which uses independent data from satellites and robotic floats as well as buoys, concludes that the NOAA results were correct.
Dr Kevin Cowtan, from the University of York's Department of Chemistry, said: "Replication is an important part of science, but is often unrewarded - everyone wants to get the big new result rather than checking old ones. In this case the political controversy which was manufactured around the NOAA paper provided a strong motivation for doing the study.
"We were initially sceptical of the NOAA result, because it showed faster warming than a previous updated record from the UK Met Office. So we set out to test it for ourselves, using different methods and different data. We now think NOAA got it right, and a new dataset from the Japan Meteorological Agency also agrees."
Historically mariners measured the ocean temperature by scooping up a bucket of water from the ocean and sticking a thermometer into it. In the 1950s, however, ships began to automatically measure water piped through the engine room, which is typically warm.
Nowadays, buoys cover much of the ocean and that data is beginning to supplant ship data. The buoys report slightly cooler temperatures because they measure water directly from the ocean instead of after a trip through a warm engine room.
Zeke Hausfather, a graduate student in UC Berkeley's Energy and Resources Group, said: "Only a small fraction of ocean measurements are being used by climate monitoring groups, and they are trying to combine data together from different instruments, which leads to a lot of judgement calls about how you weight one over the other, and how you adjust for the transition from one to another.
"So we created a temperature record just from the buoys, or just from the satellites, or just from the Argo floats, so there was no mixing and matching of instruments."
Using data from only one instrument type - either satellite, 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.
This means that the upward trend seen in the last half of the 20th century continued through the first 15 years of the 21st - there was no sudden 'break' from global warming.
The research, funded by Berkeley Earth, is published in Science Advances.