Let’s be clear: The planet is still getting hotter. The so-called pause, or hiatus, in global warming means the rate of temperature rise has slowed. The average global temperature is still going up, but in the past 10 to 15 years it hasn’t been going up as quickly as it was in the decades before.
Although the ongoing increase is trouble, a slower rate is preferable. The question is: Why did the slowdown occur—and how long will it last? We now have an answer. Three well-known climate researchers have combined actual temperature readings from 1880 to 2010 with a slew of climate models and have concluded that the slowdown is caused by the timing of two large ocean cycles, known as the Pacific multidecadal oscillation and the Atlantic multidecadal oscillation. And their analysis, published online today in Science, suggests that the slowdown will end in the next few decades.
The temperature of the Pacific and Atlantic oceans, particularly the upper layers, goes through natural cycles of warmer and cooler, driven by large circulations of water across these and the rest of the world’s seas. Warmer and cooler periods can last several decades. The analysis shows that usually, when the northern Pacific is warming, the northern Atlantic is cooling, and vice versa—offsetting one another in their impact on atmospheric temperatures in the northern hemisphere. But the cycles, and their magnitude, don’t match exactly. For the past decade, the magnitude of northern Pacific cooling has been greater than that of northern Atlantic warming, resulting in a net slowdown in temperature rise, according to an email sent to me by Byron A. Steinman, assistant professor of earth and environmental sciences at the University of Minnesota in Duluth, who led the new study.
Understanding these patterns matters because they can counteract or accelerate warming due to human activities. The paper concludes by noting that the two ocean oscillations have “offset anthropogenic warming over the past decade.” However, the authors go on to say that, based on the natural cycles over the past 130 years, the offset trend “will likely reverse…adding to anthropogenic warming in the coming decades.” The oscillations have slowed the warming due to human activities for a while, but when that effect inevitably ends the oscillations will instead add to human warming, raising the rate of increase.
So when will the super heat-up begin? The researchers did not design the study to create a precise timeline. But the historical patterns “suggest that right now we’re near the peak negative excursion, and very close to a turning point,” according to an email from Michael Mann, director of the Earth System Science Center at Pennsylvania State University, and one of the paper’s three authors. Once that happens, Steinman noted, “warming will accelerate as a result.”
Separate work by Mann, presented in a Scientific American article he wrote last April, also indicates that the pause will not last long. Mann calculated that if the world continues to burn fossil fuels at the current rate, global warming would rise to two degrees Celsius by 2036 (compared with preindustrial levels), crossing a threshold that would harm human civilization. And even if the pause persists for longer than expected, the world would cross the line in 2046. The article includes a monumental graph showing all the details. Mann also published the data sources and formula he used, on Scientific American's Web site, so anyone could replicate his calculations.
The lesson in all this is that even though the oceans run through natural cycles of warming and cooling, pumping more and more greenhouse gases into the atmosphere makes the entire system hotter over time. In the past decade the oceans have saved us, to an extent, from rapid atmospheric warming, but when the current ocean effect reverses, our emissions will come back to haunt us.
Image of Pacific Ocean and clouds courtesy of NASA