An excess amount of heat trapped by greenhouse gases is being accumulated in the world’s oceans, and therefore the temperature in the ocean is surging upward at a much faster rate than we previously thought. The results which came following an analysis of four recent ocean heating observations further establish the claim of a slowdown or “hiatus” in global warming over the past 15 years to be an unsubstantiated conjecture.

“If you want to see where global warming is happening, look in our oceans,” explained Zeke Hausfather, of the Energy and Resources Group at the University of California, Berkeley, in a statement. “Ocean heating is a very important indicator of climate change, and we have robust evidence that it is warming more rapidly than we thought.”

According to the researchers, greenhouse gases avert an estimated 93 percent of the excess solar energy from spewing out of the atmosphere, and the world’s oceans are the storehouse of that excess heat. Moreover, unlike surface temperatures which are predominantly fueled by climate events like El Nino or volcanic eruption, ocean temperatures are not affected by year-to-year variations caused by events such as those.

Rise in ocean temperature is a critical marker of climate change. And the new analysis shows the trends in ocean heat content to be almost on a par with the ones predicted by leading climate change models.

For the study, the team assumed a “business-as-usual” scenario in which no one makes an effort to cut greenhouse gas emissions, and the Coupled Model Intercomparison Project 5 (CMIP5) models forecast the rise of temperature in the oceans and the chains of events that would follow based on that assumption.

The models predict that the temperature of the first 2000 metres of the oceans will rise up to 1 degree Celsius by the end of this century. The thermal expansion brought about by this bump in temperature would cause sea levels to raise 30 centimeters in addition to already significant sea level rise caused by melting of glaciers and ice sheets. Then violent storms, hurricanes and extreme precipitation would ensue.

“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,” said Hausfather. “The global warming signal is a lot easier to detect if it is changing in the oceans than on the surface.”

A quartet of studies published between 2014 and 2017 did a much better job in assessing past trends in ocean heat content by rectifying discrepancies between different measurement systems of ocean temperature and by estimating the 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. “That was a problem, because of all things, that is one thing we really hope the models will get right.”

“The fact that these corrected records now do agree with climate models is encouraging in that is removes an area of big uncertainty that we previously had,” he said.

Previous measurement of ocean temperature relied on devices called eXpendable BathyThermographs (XBTs) that dove to the depths just once, and continued to transmit data on ocean temperature and stopped on reaching the bottom. But the data accumulated by such gadgetry was rather scanty. However this time, the team used something called Argo – a part of the integrated global observation strategy – and this changes everything.

So what exactly is Argo? And how do they work?

Argo is a fleet of nearly 4,000 floating robots coasting through the world’s oceans.  These ocean monitoring army of robots dive to the depth of 2000 meters every few days and as they rise up, collect information such as the ocean’s temperature, pH, and salinity.

Researchers say the system has provided consistent and widespread data on ocean heat content since the mid-2000s.

Three of the new studies included in this new analysis measured ocean heat content back to 1970. And, in order to steer clear from calibration errors and biases in the both the Argo and bathythermograph data, they avoid using the data on ocean heat content gathered by newer methods – in the assessment.

The fourth one uses the fact that “a warming ocean releases oxygen to the atmosphere” to calculate rise in ocean temperature arising from changes in atmospheric oxygen concentrations. It also accounts for other factors, like burning fossil fuels, that alters the level of atmospheric oxygen.

“Scientists are continually working to improve how to interpret and analyze what was a fairly imperfect and limited set of data prior to the early 2000s,” Hausfather said. “These four new records that have been published in recent years seem to fix a lot of problems that were plaguing the old records, and now they seem to agree quite well with what the climate models have produced.”

The analysis is being published in the journal Science.

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