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Boston University’s Robert Kaufmann and colleagues recently published a paper in the Proceedings of the National Academy of Sciences examining the causes of the recent dearth of “global warming.” They concluded that it’s simply natural variability, augmented by increasing sulfate emissions from dramatically growing coal consumption by China.
Of course, it is the latter conclusion that has drawn all the attention, for it allows the possibility that greenhouse gases are continuing to impart an as‐expected warming influence on the global climate. And then once China gets its air pollution under control (and we are talking about true air pollution here, i.e., not carbon dioxide), global temperatures will rise rapidly. Thus the dream of alarming climate change lives.
If China’s sulfate emissions are not having much of an impact of global temperatures, then, the dearth of warming in recent years supports the hypothesis — now made by many (unpopular) folks in the climate business — that the “sensitivity” of temperature to carbon dioxide has been guessed (we choose our words carefully here) to be too high by climate modelers. In this scenario, we wake up from the alarmist nightmare and resume our normal lives.
There are two reasons why we think it is wrong for Kaufmann et al. to attribute a reduced rate of global warming to Chinese sulfates:
1) China’s cooling sulfates do not readily make their way into the Southern Hemisphere, yet, from 1999–2010, temperatures actually fell there, while they rose in the Northern Hemisphere. This is exactly the opposite of what should have happened if sulfates are exerting a relative cooling primarily in the Northern Hemisphere
2) Chinese coal consumption increased in 2009 and 2010 (in fact, 2010 had the biggest year‐over‐year increase recorded) — yet, the global temperature rose sharply in 2009 and in 2010. Because Kaufmann’s climate model responds instantaneously to sulfates (as opposed to a decades‐long lag to adjust to carbon dioxide changes) this is contrary to his hypothesis.
Let’s look at the first one.
The link below shows the march of weather systems around the globe for several months. Notice that the weather systems passing through China quickly move into the north Pacific Ocean, and don’t mix into the Southern Hemisphere. Since sulfates only have an atmospheric lifetime of about a week or so, they are hard pressed to cause any cooling impact beyond the areas to the immediate east of China.
So, if a dramatic increase in Chinese sulfur emissions during the past decade or so has been responsible for the observed slowdown in the rate of global temperature increase, then the Northern Hemisphere should be doing most of the work — that is, the rate of warming in the Northern Hemisphere should have slowed by much more than the rate of warming in the Southern Hemisphere. This situation is easy to check.
Figure 1 shows the Northern and Southern Hemisphere temperature history from 1980 through 2010 according to the surface temperature data set compiled and maintained by the Climate Research Unit at the University of East Anglia. During the period of time during which Chinese sulfate emissions rose (1998–2010), the warming in the Southern Hemisphere went negative (i.e. became a cooling) while the Northern Hemisphere warmed. From Figure 1 it is obvious that the Southern Hemisphere is driving the global temperature slowdown, not the Northern — a result completely contrary to Kaufmann et al.‘s Chinese sulfate hypothesis.
Now let’s turn to our Reason #2. Simply put, during the past 2 years (which were not part of the Kaufmann et al. dataset), global temperatures rose as did Chinese coal consumption. According to Kaufmann et al.‘s hypothesis, the increase in Chinese coal consumption should act to drive down the rate of global temperature rise, but that is not what happened. Again, temperatures are behaving in an opposite fashion, compared to what the hypothesis predicts.
The top panel in Figure 2 shows Chinese coal consumption from the BP Statistical Review of World Energy from 1998 through 2010. Notice that it increased substantially in both 2009 and 2010 — two years not included in the Kaufmann et al. analysis. The year‐over‐year increase from 2009 to 2010 was the highest annual increase on record. If Chinese coal consumption were having a large impact on global temperature, we would expect that global temperatures would remain suppressed in 2009 and 2010. But the bottom panel in Figure 2 shows what really happened — global temperatures rose in both 2009 and 2010, contrary to the Kaufmann et al. hypothesis.
It is clear that natural variability, not sulfate emissions, is the cause of the lack of recent warming. We arrived at this very same conclusion several years ago, however, despite repeated attempts, we were unable to find a journal even interested in considering our work for publication.
Our analysis was remarkably similar to Kaufmann’s: