Ocean temperature data shows warming is accelerating faster than we thought – Science News
The rate of ocean warming today has accelerated significantly since 1991, and is increasing much faster than previously recorded, according to a new study of ocean temperature research.
Higher sea-level rise and more extreme tropical cyclones are just some of the consequences forecast as ocean temperatures increase.
Argo — a global system of satellite-linked ocean temperature measuring devices — has allowed researchers to better calculate our ocean warming trajectory.
The results published in Science today show that previous ocean warming data has significantly undercalculated how steep that trajectory is, according to study co-author Zeke Hausfather from the University of California.
In the period between 1991 to 2010, the ocean warmed, on average, more than five times faster than in the 1971 to 1990 period, according to the research.
When water heats up, it expands. Under worst-case-scenario projections, the expansion of the oceans due to warming will add around 30 centimetres to sea-level rise by 2100.
That’s on top of the sea-level rise that will be caused by melting ice caps.
This latest analysis, headed by Lijing Cheng from the Chinese Academy of Sciences, looked at four different studies published since 2013, that used improved statistical and analytical methods to estimate historic warming.
Each of these studies independently came to the same conclusion, according to Dr Hausfather.
“There’s been four different estimates of ocean-heat content published since the 2013 IPCC report,” he said.
“They all show more warming than previous projections.”
What is Argo and how has it helped?
The reason that the rate of ocean warming has appeared to be lower, until recently, is because we have lacked historical temperature data for large areas of the ocean.
Where those data points have been lacking, scientists previously have had to conservatively estimate “zero change” in ocean temperature in those areas.
In other words, they’ve had to assume that the temperature at those points 100 years ago, for instance, was the same as it is today, Dr Cheng said.
“This is simply not accurate,” he said.
Since 2005, data from Argo has provided a much more comprehensive picture of ocean temperatures, and is now allowing scientists to accurately gauge the historical warming for parts of the ocean where we don’t have historical data.
Argo is a network of more than 3,900 free-drifting measuring probes launched in the early 2000s. Each Argo probe is a pill-shaped device with an antenna on top.
The probes drift at a “parking depth” of around 1,000 metres, and every 10 days they dive to 2,000 metres before ascending to the surface. All the while, they’re taking around 200 temperature, pressure, depth and conductivity measurements.
When they hit the surface, they beam their measurements to satellites, then deflate their buoyancy bladder and sink back down to 1,000 metres to repeat the cycle.
Data, not models, at fault
Since Argo data became accessible, contemporary ocean warming measurements have been tracking in sync with modelled ocean warming projections.
But climate models have previously struggled to match up with historical data generated before Argo, according to Dr Hausfather.
“It’s pre-2005 where all the uncertainty lies,” Dr Hausfather said.
So the question has been, is the historical data inaccurate or is there something wrong with the accuracy of climate models in tracking historic ocean warming?
Today, using the massive data coverage of Argo, scientists are able to fill in the gaps in historic data points, according to the CSIRO’s Steve Rintoul, who wasn’t involved in this study.
“We can then say, ‘well, we didn’t measure in this particular location back in the ’50s, but our more recent measurements show that when a change happens in one place, this [previously] unmeasured place warms up too’.”
This allows scientists to replace “zero change” data with more accurate estimates.
93 per cent of heat energy stored in oceans
Most significantly though, the adjusted historic temperature records used in these four studies now track in sync with climate models, according to Dr Rintoul.
“In the past when [the models and records] didn’t agree so well, part of that was a problem with the observations, not the models,” he said.
Water takes much more energy to heat up than the atmosphere, and holds onto that energy far more effectively than air. The oceans contain 93 per cent of the extra energy that is stored in the climate system as a result of climate change.
Even if we were able to cut our emissions to zero today, the oceans will act like a hot-water bottle — they’ll be warming our atmosphere for decades or centuries to come.
And with that warming comes consequences that scientists are still working to predict and understand, according to Dr Rintoul.
“Warming in the ocean affects things like coral bleaching which we know a lot about. But more recently it’s becoming clear that there are other important impacts including melting of the ice sheets in Greenland and Antarctica and even the amount of rain that falls in a hurricane.”