‘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?
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.
+ Author Affiliations
1Large Lakes Observatory and Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN, USA.
2Department of Meteorology and Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA, USA.
The recent slowdown in global warming has brought into question the reliability of climate model projections of future temperature change and has led to a vigorous debate over whether this slowdown is the result of naturally occurring, internal variability or forcing external to Earth’s climate system. To address these issues, we applied a semi-empirical approach that combines climate observations and model simulations to estimate Atlantic- and Pacific-based internal multidecadal variability (termed “AMO” and “PMO,” respectively). Using this method, the AMO and PMO are found to explain a large proportion of internal variability in Northern Hemisphere mean temperatures. Competition between a modest positive peak in the AMO and a substantially negative-trending PMO are seen to produce a slowdown or “false pause” in warming of the past decade.
No, climate change is not experiencing a hiatus. No, there is not currently a “pause” in global warming.
It is true that Earth’s surface warmed a bit less than models predicted it to over the past decade-and-a-half or so. This doesn’t mean that the models are flawed. Instead, it points to a discrepancy that likely arose from a combination of three main factors (see the discussion my piece last year in Scientific American). These factors include the likely underestimation of the actual warming that has occurred, due to gaps in the observational data. Secondly, scientists have failed to include in model simulations some natural factors (low-level but persistent volcanic eruptions and a small dip in solar output) that had a slight cooling influence on Earth’s climate. Finally, there is the possibility that internal, natural oscillations in temperature may have masked some surface warming in recent decades, much as an outbreak of Arctic air can mask the seasonal warming of spring during a late season cold snap. One could call it a global warming “speed bump”. In fact, I have.
Some have argued that these oscillations contributed substantially to the warming of the globe in recent decades. In an article my colleagues Byron Steinman, Sonya Miller and I have in the latest issue of Science magazine, we show that internal climate variability instead partially offset global warming.