A frequent refrain of many who dispute evidence of human caused climate change is that the climate stopped warming in the late 1990s. Some even assert that the planet is now cooling and that this disproves the mainstream scientific view that, by increasing the concentrations of greenhouse gases in the atmosphere, humans are warming the Earth’s climate. Advocates of the proposition that humans are not warming the planet point to estimates of annual global mean surface temperature for the period from the late 1990s to the present, a period for which there is not a trend that is statistically significant.

Global temperature anomalies from 1992 through 2008 as estimated by the Hadley Centre. Temperatures are shown as the difference from the 1961-1990 average. Source: Scott, ClimateWatch Magazine.

What does this mean? There is a good article by Michon Scott in NOAA’s online ClimateWatch Magazine about the recent temperature data and the implications for climate change. A few key points to bear in mind are summarized here. But do access and read the full article – also search out what others have to say on this topic, take a look for yourself at the observational record, and become better informed.

As a starting point, it is important to recognize that annual variation in global average temperature is large relative to trends that might typically be observed over a period of 10 or so years – whether those trends are driven by natural or human causes. Research by Easterling and Wehner, as summarized by Scott, shows that a number of decades in the period 1901 to 2009 had no statistically significant warming or cooling trend. Yet these occur within a longer-term context in which temperatures at the end of the 20th century and beginning of the 21st century are significantly higher, in statistical terms, than in the early years of the 20th century.

The lack of a warming trend in some decades is not inconsistent with longer term warming. In fact, it is entirely expected, and even predicted by climate models, that there will be some decades of little or no warming set within a period of accumulating greenhouse gases in the atmosphere that drive warming over a multi-decadal time scale. Note also that, while there is no clear warming trend in recent years, the decade that just ended was exceptionally warm – the warmest decade on record according to NASA’s Goddard Institute for Space Studies.

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. Source: Scott, ClimateWatch Magazine.

So the recent temperature record by no means disproves human caused climate change. The longer-term record amply demonstrates that the climate has warmed and that the climate is significantly warmer now than in the past 100 years. The record also gives compelling evidence to expect that the climate will continue to warm in the future.

Neil Leary

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