Statistically speaking, global temperatures have risen more slowly over the past 15 years than in the decades prior, leading some to proclaim — incorrectly — that we are experiencing a "hiatus" in global warming, or even that climate change is not real. But two new studies illustrate that global warming remains a problem and that human activities are the source.
The most recent study, published on Nov. 10 in the journal Nature Geoscience, examines the historic effect of ozone-depleting gases such as chlorofluorocarbons, the use of which was phased out around the world beginning in 1988 with the signing of the United Nations' Montreal Protocol on Substances that Deplete the Ozone Layer.
The study looks at historical data related to radiative forcing (the difference between the solar energy received by the Earth and that radiated back into space) and temperature. The researchers found that temperatures and radiative forcing both increased beginning around 1960 and then reduced in the late 1990s. As the researchers write, "our statistical analysis suggests that the reduction in the emissions of ozone-depleting substances under the Montreal Protocol, as well as a reduction in methane emissions, contributed to the lower rate of warming since the 1990s." They also wrote, "we conclude that reductions in greenhouse gas emissions are effective in slowing the rate of warming in the short term."
The researchers — from Universidad Nacional Autónoma de México, the Institute for Environmental Studies in Amsterdam and Boston University — conclude that without the Montreal Protocol in place, today's global surface temperatures would be about one tenth of a degree Celsius more than they are.
Although the reductions in ozone-depleting gases have helped to slow global warming, it alone does not fully explain the "hiatus." For that you need to look to the oceans. Although the warming of global surface temperatures has slowed, a study published on Oct. 31 in the journal Science finds that the middle depths of the Pacific Ocean have warmed 15 times faster over the past 60 years than during the previous 10 millennia.
"We may have underestimated the efficiency of the oceans as a storehouse for heat and energy," study co-author Yair Rosenthal, a climate scientist with Rutgers University, told USA Today. "It may buy us some time — how much time, I don't really know. But it's not going to stop climate change." Co-author Braddock Lindley, a climate scientist with Columbia University, warned that we don't yet understand how all of that heat and energy being absorbed into the ocean will later be released and how it will impact global warming at that point.
This is just the latest study linking the "slowdown" to ocean temperatures, following previous research published in Nature Climate Change and other journals. The importance of the ocean cannot be underestimated in the total climate change picture. As the website Real Climate writes, "The amount of heat stored in the oceans is one of the most important diagnostics for global warming, because about 90 percent of the additional heat is stored there... Thus, heat absorbed by the oceans accounts for almost all of the planet's radiative imbalance."
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