Global Warming: What Happens When You Factor Out the Other Factors

Has the warming trend slowed in the 2000s? Yes and no.

The rate of global warming has been the subject of much skepticism among the refudiater set. A good deal of that skepticism has been directed at the claim that there’s been little to no warming since the end of the last millennium, with the main argument being that the warming trends found in countless studies were artifacts of siting, measurement and/or analytical errors.

A red herring? Almost certainly, given the abundance of independent evidence of a globally warming world — the melting of glaciers and permafrost, the shrinking of Arctic sea ice in extent and volume (see also here), earlier bud breaks in spring, to name a few.

Nevertheless, the objections kept coming until a study by none other than a climate skeptic — the temperature trend analysis by the Richard Muller-led Berkeley Earth Surface Temperature (BEST) project put the kibosh on them. The BEST team found a 1.8-degree Fahrenheit (1-degree Celsius) rise in land temps since 1950, pretty much in line with previous investigators.

Looking at the Full Climate Picture

OK, some skeptics say, the latter half of the 20th century saw warming, but so far the 21st century has not. Indeed, a cursory look at the global temperature record does not show any evidence of an increase. So what gives? Has the age of global warming come to a screeching halt? Are we even headed for global cooling? When one’s talking about the future, it is a shaky business to say “never” or “no way.” So I’ll just say, “Not likely.” Here’s why.

(Credit: NASA Earth Observatory/Robert Simmon)

First of all, within the 20th century temperature record there are a number of decade-long periods when warming stalled only to start up again. The current temperature stasis is likely to be of the same ilk.

Moreover, lots of things can influence global temperatures. There are greenhouse gases, yes, but there are also solar variations, volcanic eruptions and the El Niño-La Niña oscillation. All of these can influence the year-to-year variations in global temperatures — conceivably dampening or even at times canceling the warming from greenhouse gases while enhancing it at other times. A number of recent climate-model studies have provided cogent arguments that the current warming stasis can in fact be explained by the transient influence of other factors.

Any Warming in the 2000s? Taking a Statistical Look

Last week, in a paper published in the journal Environmental Research Letters, Grant Foster of Tempo Analytics and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research took a different tack. The authors used statistical methods rather than modeling to tease out the impacts from factors that contribute to shorter-term variability for five of the most-used temperature records. These records include three surface temperature records (from NASA’s Goddard Institute for Space Studies [GISS], NOAA’s National Climatic Data Center [NCDC], and the UK Met Office Hadley Center and the Climatic Research Unit at the University of East Anglia [HadCRU]) and two lower troposphere satellite records (from Remote Sensing Systems in California [RSS] and the University of Alabama, Huntsville [UAH]).

Focusing on 1979 to 2010 — the period of overlap for all five records — the authors used statistical methods, along with data on the temporal variations in the El Niño-La Niña oscillations, solar variations, and volcanic eruptions, to quantify and remove that part of the temperature signal caused by each. The remaining temperature trend, they reasoned, is at least in part, and almost certainly in large part, due to the monotonically increasing concentrations of greenhouse gases. Interestingly enough, once the trio of factors is removed, there is agreement between all five records: steady warming over the whole period, the 2000s included — with “the two hottest years” being the most recent: 2009 and 2010.

In other words, once the influences of shorter-term natural variability are stripped away, there is no evidence that the warming trend has slowed at all.

Natural Climate Variability Versus Human-Driven Climate Changes

This study goes hand in hand with a journal paper by Markus Huber and Reto Knutti of ETH Zurich from last week’s edition of Nature Geoscience, which used modeling to tease out the relative contribution of natural climate variability from human drivers of climate. Their analysis found that it is “extremely likely” that 74 percent of the warming experienced since 1950 is anthropogenic in origin.

Two very interesting papers. Will they put the kibosh on refudiater predictions of imminent global cooling? As I said before, when it comes to the future, never say “never,” but “not likely” seems a good bet.

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