Deciphering Natural Variability

“There are two components influencing temperatures over time,” Easterling says. “You have a trend due to increasing greenhouse gases and you have natural variability superimposed on that trend.”

Beyond our control, natural variability includes large-scale oceanic changes affecting regional and global temperatures. Cooling between 2006 and 2008, for instance, has likely been driven by La Niña. Opposite of El Niño conditions, the area of cooler-than-average sea surface temperatures that defines La Niña conditions can push global temperatures downward, if the phenomenon is strong enough.

Schlesinger, who has served as a co-author for the Intergovernmental Panel on Climate Change, knows quite a bit about natural variability. In 1994, he described the Atlantic Multi-decadal Oscillation, an alternating pattern of heat-distributing ocean circulation that brings warmed waters from the tropics to high latitudes. “On average, ocean circulation transports a petawatt of heat,” Schlesinger explains. “A petawatt is the numeral 1 with 15 zeros after it, or a million billion watts of heat energy. If you could just acquire one percent of that heat, you could satisfy all your energy needs for planet Earth.” One cycle of the oscillation takes about 65 to 70 years. Over that time, the amount of heat moved northward along the western side of the North Atlantic Ocean waxes and wanes, increasing and decreasing temperatures in the North Atlantic Ocean and the surrounding continental margins.

Although not directly dictated by this decades-long phenomenon, global temperatures are influenced by the oscillation, just as average temperatures can be driven up or down by El Niño and La Niña events. The transport of such a massive quantity of heat helps explain some counterintuitive temperature trends during the twentieth century. Despite the increase in greenhouse gas concentrations that began in the mid-1800s, between 1850 and 1900, global temperatures showed little significant change. Between 1900 and 1940, temperatures rose. After 1940, temperatures declined for 35 years. The lack of a direct correlation between greenhouse gas concentration and global temperature over this time span “was confounding understanding of the observed temperature record,” Schlesinger recalls, “and it was allowing people to make the argument, ‘If there was this early twentieth-century warming when humanity was putting relatively fewer greenhouse gases into the atmosphere, why wasn’t there a larger temperature increase later when humanity was putting vastly more greenhouse gases into the atmosphere?’ By discovering this oscillation, we were able to refute this argument.”

This graph shows Global Average Temperature compared with the Atlantic Multi-decadal Oscillation (AMO) Index. The AMO Index is the average sea-surface temperature over the North Atlantic Ocean. The roughly parallel curves of the two parameters show that they are related: the increase in global temperature over time, coincident with the increase in greenhouse gases observed since the Industrial Revolution, is alternately obscured and enhanced by the AMO.

Like Schlesinger, Easterling is also familiar with natural variability, and his knowledge of such variability caused him to question the cooling-since-1998 argument. Working with his co-author Michael Wehner, Easterling examined both observed temperatures and model simulations. Confident that the “cooling” was a short-term trend, Easterling wanted to see if he could find periods of no trend or with slight cooling over other time spans. He could.

For observed temperatures, Easterling relied on a data set generated by NOAA’s National Climatic Data Center, incorporating globally averaged surface air temperatures from 1901 to 2008. In that data set, he found two periods—1977 to 1985 and 1981 to 1989—that showed slight cooling, similar to what appears in the Hadley Centre’s data set from 1998 to 2008.

Global temperature anomalies calculated by NOAA for 1975 through 2008. Despite the long-term warming trend evident in the graph at the bottom, global temperature decreased slightly from 1977 to 1985 and from 1981 through 1989, as shown by blue arrows in the insets. Graph modified from Easterling, 2009.

For model simulations, Easterling used a database of predicted temperatures, validated by its ability to retroactively “predict” temperatures for years past. He assumed a “business-as-usual” scenario in terms of greenhouse gas emissions, with little future reduction. When projecting temperatures for the twenty-first century, he found two more periods—2001 to 2010 and 2016 to 2031—that showed no trend, again similar to 1998 to 2008. Every one of these no-trend periods occurred against a backdrop of rising temperatures.

“The supposed cooling is really a non-issue,” Easterling explains. “It’s just natural variability.”