Earth’s oceans have been acting like a giant heat sink, say a group of climate experts who argue that the idea of a slowdown or hiatus to the global warming phenomenon is more fiction than fact, once temperature and energy trends are viewed from a global perspective.
In a new report published in the journal Earth’s Future, a group of climate scientists led by Xia-Hai Yan of the University of Delaware’s College of Earth, Ocean and Environment discuss the topic of the so-called global warming hiatus, the term given to the time period between 1998 and 2013 during which the warming trend that had been charted over the previous fifty years appeared to have ceased – just at the time when experts were claiming that global temperatures should be climbing in sync with increases in the production of atmospheric carbon dioxide.
Indeed, the Intergovernmental Panel on Climate Change (IPCC), the world’s foremost body on the science of climate change, spoke in its 2013 Assessment Report of the hiatus, stating that over the previous 15 years, the rate of global warming was smaller (0.05 degrees Celsius per decade) than it had been during the period since 1951 (0.12 degrees Celsius per decade).
Needless to say, then, the topic of the global warming hiatus has been a going concern both for climate change specialists, who were left trying to explain where all that heat went to, if it were true that rising CO2 levels really are heating up the planet, as well as for climate change deniers, who have been wont to point to the hiatus as Reason #1 to doubt the very idea of global warming.
One approach by the experts has been to explain away the hiatus and argue that the data used in the 2013 IPCC report was based on miscalculations of surface temperatures. A 2015 study in the journal Science, for example, claimed that after adjusting for biases – such as differences between ocean temperature readings taken on ships versus from floating buoys – the global rate of increase in temperature has actually remained constant since mid-20th century.
Another tactic has been to admit to the slowdown but to insist that it merely represents a short term deviation in an otherwise longer-term trend towards a warmer Earth. Earlier this year, a study led by John Fyfe from the Canadian Centre for Climate Modelling and Analysis in Victoria, BC. Fyfe’s claim was that we can’t ignore the “mismatch” between what the climate models, based on measurements of CO2 concentrations and past trends, were predicting for the 1998 – 2013 period and what the actual temperature readings were showing.
But, then, if carbon dioxide levels are increasing (and, yes, this part is unchallenged), why the hiatus?
The answer lies deep in the oceans, according to the new report, which points to the fact that the prevailing indicator used by scientists to track global warming has been the global mean surface temperature (GMST) – it was a major contributor to the IPCC’s 2013 conclusions, for example. But as the name reads, GMST only represents the heat accumulation on the planet’s surface and therefore doesn’t accurately take into account the redistribution of heat energy within the entire Earth system. Such a deeper reading would incorporate changes in energy distribution not only within the atmosphere but also in the land, the oceans and ice coverage, too.
The oceans are especially adept at absorbing heat energy, say the scientists, who point out that over 90 per cent of the excess heat in the Earth’s system gets taken up by the oceans. “Not only does the ocean have a much higher heat capacity than the atmosphere, allowing it to hold more heat energy within the same volume,” reads the report, “but the motion of the ocean, the constant horizontal and vertical advection and mixing, removes water from direct contact with the atmosphere, sequestering heat at depths far from surface interaction and direct influence on GMST.”
Thus, for those of us keeping track, the global warming hiatus is real in that the global mean surface temperature did go through a slowdown in its upward trend over the past 15 years. But at the same time – and more accurately – the hiatus is not real, since the GMST doesn’t explain the whole picture of how heat energy is being distributed throughout the planet and its various systems. For that calculation, we need to take the absorption of heat energy into the oceans into account, say the report’s authors, something we need to study much more intensely if we want to get a clear picture of how global warming is truly shaping up.
“To better monitor the Earth’s energy budget, and its consequences, the ocean is most important to consider because the amount of heat it can store is extremely large when compared to the land or atmospheric capacity,” says Yan.