Heat trapped by greenhouse gases is raising ocean temperatures faster than previously thought, according to a new research which shows that earlier claims of a slowdown or "hiatus" in global warming over the past 15 years were unfounded.
Ocean heating is critical marker of climate change because an estimated 93 per cent of the excess solar energy trapped by greenhouse gases accumulates in the world's oceans.
Unlike surface temperatures, ocean temperatures are not affected by year-to-year variations caused by climate events like El Nino or volcanic eruptions.
"If you want to see where global warming is happening, look in our oceans," said Zeke Hausfather, a graduate student at the University of California (UC) Berkeley, in the US.
"Ocean heating is a very important indicator of climate change, and we have robust evidence that it is warming more rapidly than we thought," said Hausfather.
The new analysis, published in the journal Science, shows that trends in ocean heat content match those predicted by leading climate change models, and that overall ocean warming is accelerating.
Assuming a "business-as-usual" scenario in which no effort has been made to reduce greenhouse gas emissions, the Coupled Model Intercomparison Project 5 (CMIP5) models predict that the temperature of the top 2,000 metres of the world's oceans will rise 0.78 degrees Celsius by the end of the century.
The thermal expansion caused by this bump in temperature would raise sea levels 30 centimeters on top of the already significant sea level rise caused by melting glaciers and ice sheets. Warmer oceans also contribute to stronger storms, hurricanes and extreme precipitation.
"While 2018 will be the fourth warmest year on record on the surface, it will most certainly be the warmest year on record in the oceans, as was 2017 and 2016 before that," Hausfather said.
"The global warming signal is a lot easier to detect if it is changing in the oceans than on the surface," he said.
Four studies, published between 2014 and 2017, provide better estimates of past trends in ocean heat content by correcting for discrepancies between different types of ocean temperature measurements and by better accounting for gaps in measurements over time or location.
"The Intergovernmental Panel on Climate Change's (IPCC) Fifth Assessment Report, published in 2013, showed that leading climate change models seemed to predict a much faster increase in ocean heat content over the last 30 years than was seen in observations," Hausfather said.
A fleet of nearly 4,000 floating robots drift throughout the world's oceans, every few days diving to a depth of 2,000 metres and measuring the ocean's temperature, pH, salinity and other bits of information as they rise back up.
This ocean-monitoring battalion, called Argo, has provided consistent and widespread data on ocean heat content since the mid-2000s.
Prior to Argo, ocean temperature data was sparse at best, relying on devices called expendable bathythermographs that sank to the depths only once, transmitting data on ocean temperature until settling into watery graves.
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