Attribution of Recent Climate Change - Internal Climate Variability and Global Warming

Internal Climate Variability and Global Warming

One of the issues that has been raised in the media is the view that global warming "stopped in 1998". This view ignores the presence of internal climate variability, an example of which is the El Niño-Southern Oscillation (ENSO). The El Niño in 1998 was particularly strong, possibly one of the strongest of the 20th century.

Cooling between 2006 and 2008, for instance, has likely been driven by La Niña, the 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. Even accounting for the presence of internal climate variability, recent years rank among the warmest on record. For example, every year of the 2000s was warmer than the 1990 average.

Another example of internal climate variability is the Atlantic Multi-decadal Oscillation (AMO), which is an alternating pattern of heat-distributing ocean circulation that brings warmed waters from the tropics to high latitudes. 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 varies, 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 counter-intuitive 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. According to Michael Schlesinger, of the University of Illinois at Urbana-Champaign, the Atlantic Multi-decadal Oscillation helps to explain the lack of a direct correlation between CO₂ concentrations and global temperatures over this time period.

David Easterling (Chief of the Scientific Services Division at NOAA’s National Climatic Data Center) and co-author Michael Wehner have examined observed temperatures and model simulations of future temperatures. 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.

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.