There is no such thing as a “pause” or “hiatus” in global warming.
There is, however, variation as the earth’s surface temperature steadily rises as the result of the human release of greenhouse gas pollution. Every now and then that variation results in a period of several years when the rise in global temperature is relatively slow, and a recent such period has been termed a “hiatus” or “pause.” But that signifier mainly comes from those who deny the reality of global warming, and is often used by them as an argument that global warming is somehow not real. It is real, and they are wrong.
Looking at an upward shift in global surface temperatures, seeing some variation, and claiming that this variation obviates the long term and rather startling trend can only mean one of two things. One possibility is that the person making that observation is ignorant of both the nature of the Earth’s climate system and of the nature of measurements of this kind. The other possibility is that the person making that observation is willfully obfuscating the science, in an effort to distract from the reality of human caused climate change. Neither speaks well of the individual making this observation.
Having said all that, variations in the upward trend of surface temperatures are both interesting and important. There are several possible causes, and understanding these causes is important in understanding the overall system. Part of this variation may be difficult to quantify or track natural variation in the system. Part of this variation may be issues with the measurement system. After all, the long term trend is derived from the stringing together of different data sets collected with different methods, so we would expect some measurement effects. A good part of this variation, maybe most of it, is thought to be the shifting of heat between the global surface (the sea surface and the bottom of the atmosphere) and the deeper ocean, through a variety of mechanisms. A long term trend has recently been nailed down by several studies including work by Michael Mann and his colleagues, and has to do with the ocean-air interaction as well. From a post by Mann on Real Climate:
In an article my colleagues Byron Steinman, Sonya Miller and I have in the latest issue of Science magazine, we show that internal climate variability instead partially offset global warming….
[That] natural cooling in the Pacific is a principal contributor to the recent slowdown in large-scale warming is consistent with some other recent studies, including a study … showing that stronger-than-normal winds in the tropical Pacific during the past decade have lead to increased upwelling of cold deep water in the eastern equatorial Pacific. Other work by Kevin Trenberth and John Fasullo of the National Center for Atmospheric Research (NCAR) shows that the there has been increased sub-surface heat burial in the Pacific ocean over this time frame, while yet another study by James Risbey and colleagues demonstrates that model simulations that most closely follow the observed sequence of El Niño and La Niña events over the past decade tend to reproduce the warming slowdown.
I discussed that research here.
And now, there is a paper just out in Science that explores the measurement part of the variation in increasing global surface temperature. Possible artifacts of data biases in the recent global surface warming hiatus by Thomas R. Karl, Anthony Arguez, Boyin Huang, Jay H. Lawrimore, James R. McMahon, Matthew J. Menne, Thomas C. Peterson, Russell S. Vose, and Huai-Min Zhang looks at changes in various measurements used to generate the basic data to track global surface temperatures. From the abstract:
Much study has been devoted to the possible causes of an apparent decrease in the upward trend of global surface temperatures since 1998, a phenomenon that has been dubbed the global warming “hiatus.” Here we present an updated global surface temperature analysis that reveals that global trends are higher than reported by the IPCC, especially in recent decades, and that the central estimate for the rate of warming during the first 15 years of the 21st century is at least as great as the last half of the 20th century. These results do not support the notion of a “slowdown” in the increase of global surface temperature.
An example of a measurement issue has to do with how sea surface temperatures are obtained. In the old days, most temperature measurements taken from boats involved the “bucket technique” which involves directly sampling the surface water. Later, this shifted to using thermometers measuring water temperature at intakes on board ship. The two methods measure the same thing, but because they are slightly different measurements, produce results slightly biased in relation to each other. Adjustments to these measurements apparently assumed that all the ship measurements had gone to the intake method, while in fact, some had not. This requires a small adjustment in how the numbers are used in the surface temperature estimate. The authors assert that similar small changes in the data are required for some other measurements as well.
The new analysis produced in this paper shows a consistant difference between the data as previously adjusted and how the authors feel it should be adjusted, with the older methods generally resulting in lower temperatures than the newer adjusted data. The difference between the two is especially large during the so-called “Hiatus” period. Here is the fancy graphic from the paper that shows this:
This is important and valid work. But, with respect to the public and policy-related conversation about anthropogenic global warming, this is really dotting of i’s and crossing of t’s. The difference between a temperature curve without this new adjustment and with it is very small. This graphic at the top of the post is the top part of the paper’s Figure 2, comparing the proposed new corrections and the previous corrections for the instrumental record from 1880 to the present. You can see that the better estimate of temperatures is in fact higher for the so-called hiatus, and varies from the older method noticeably here and there, but there is nothing in this new curve that changes anything important.
The end result is that the temperature trends over the past 17 or so years has continued to increase with no halt. In fact, it has increased at approximately the same rate as it had for the prior five decades. But the authors went further by trying to cherry-pick the start and end dates. For instance, they stacked the cards against themselves by purposefully picking a very hot year to start the analysis and a cool year to terminate the study (1998 and 2012, respectively). Even this cherry-picked duration showed a warming trend. Furthermore, the warming trend was significant.
I’ve heard about a number of commentaries from the denialist community. See the Hot Whopper link above for some of that. But really, who cares what a bunch of science deniers say about science?