Climate Sensitivity

Do Researchers Really Know Why Global Warming Is On Pause and When It Will End?


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The science is settled. Maybe. Let's take look at some recent studies that aim to explain why projected increases in global average temperature have "paused" even as global greenhouse gas concentrations in the atmosphere have been going up siginficantly. It has generally been acknowledged that there has been an "hiatus' in warming that has lasted for the past 15 to 16 years so far. In August, Canadian statistician Ross McKitrick published a paper in the Open Journal of Statistics in which he lengthened the period of the pause:

In the surface data we compute a hiatus length of 19 years, and in the lower tropospheric data we compute a hiatus length of 16 years in the UAH [University of Alabama in Huntsville satellite data] series and 26 years in the RSS [Remote Sensing Systems satellite data]series.

It's fair to say that this pause is something of an embarrassment to many in the climate research community, since their computer models failed to indicate that any such thing could happen. Spurred by the mismatch between computer projections and empirical data, lots of climate scientists have been trying to figure out why the average global temperature has not been increasing significantly.

For example, a 2010 study in Science attributed part of the temperature slowdown to increases decreases in stratospheric water vapor.

A 2011 article in Atmospheric Chemistry & Phyics suggested that a prolonged solar minimum combined with atmospheric aerosols left over from volcanic eruptions reduced the amount of heat reaching the surface of the planet.

But by far, the most popular explanation for why the atmosphere was not warming even as greenhouse gas concentrations were rising was that the excess heat is hiding in the oceans. Some researchers in March of this year argued in Nature Climate Change that the Pacific Ocean trade winds have speeded up thus pushing heat beneath the waves.

In August, other researchers countered in Science that the real reason the atmosphere is not warming is that changes in North Atlantic Ocean circulation are burying the extra warmth. The researchers reported that this process could go on for as long as another 20 years before the ocean begins releasing the stored heat, greatly boosting future rates of warming.

In late August, yet another set of researchers, in Nature Climate Change, suggested that natural variations in Pacific trade winds account for nearly half of the changes temperature seen over the past three decades. The bad news is that natural variation is now being overwhelmed by climate change caused by increasing greenhouse gas concentrations in the atmosphere. Consequently, they predict that man-made warming will dominate future temperature trends soon and the hiatus will end.

Also in August, researchers associated with the Pacific trade winds theory report in Geophysical Research Letters the results of 31 climate models. They find that "under high rates of greenhouse gas emissions there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption."

Just how long the temperature pause must last before it would falsify the more catastrophic versions of man-made climate change obviously remains an open question for many researchers. For the time being, most are betting that it will get real hot real fast when the hiatus ends.