There is no statistically significant warming trend since November of 1996 in monthly surface temperature records compiled at the University of East Anglia. Do we now understand why there's been no change in fourteen and a half years?
If you read the news stories surrounding a recent paper in the Proceedings of the National Academy of Sciences by Boston University's Robert Kaufmann and three colleagues, you'd say yes, indeed. It's China's fault. By dramatically increasing their combustion of coal, they have increased the concentration of fine particles in the atmosphere called sulphate aerosols, which reflect away solar radiation, countering the warming that should be occurring from increasing atmospheric carbon dioxide.
Further, if this is true, then (as is usual in climate-world), "it's worse than we thought." After all, China will eventually reduce their sulfate emissions as their population becomes affluent enough to demand something better than miasmic air. Indeed, they are already beginning to clean things up, and when they finally do, all the cooling particles will be gone and the earth will warm substantially.
Reality may be a bit simpler, or much more complicated. But the reason this is all so important is that if there is no good explanation for the lack of warming, then an increasingly viable alternative is that we have overestimated the gross sensitivity of temperature to carbon dioxide in our computer models.
One problem is that we really don't know how much cooling is exerted by sulfates, or whether they are just a convenient explanation for the failure of the forecasts of dramatic warming. The United Nations' Intergovernmental Panel on Climate Change, which grants itself climate authority, states that our "Level of Scientific Understanding" of the effects range between "low" and "very low," with a possible cooling between zero (none) and a whopping 3.5 degrees (C) when the climate comes to equilibrium (which it will never do). That's a plenty large range from which to pick out a number to cancel about as much warming as you'd like.
Kaufmann's team looked into how sulfate uncertainty impacted its results and decided that it was relatively minor. However, we can't find any independent test showing that the geographic "fingerprint" of a dramatic recent increase in sulfate cooling is actually being observed. More on this in a minute.
The other problem — and climate flatliners hate me for pointing this out — is that the beginning of the period of "no warming" includes the warmest year in the instrumental record, caused by the great El Niño of 1997-1998. In a modestly warming world, starting off at or near an anomalously high point pretty much assures little or no warming for years afterward.
Kaufmann's team (and others) have duly noted that El Niño cycles are one factor partially responsible for the lack of recent warming. There's little doubt of this. Further, if you back out solar changes and volcanism, as they did, you can convince yourself that there is still an underlying "residual" warming trend, but it is masked by all these variables. This has been done repeatedly in the scientific literature, which, until now, did not include increasing the sulfate effect on recent temperatures.
Where is the test of the hypothesis that sulfates are indeed responsible for the lack of warming? In this paper, it's simply "modeled-in" as it fits the data well. That's correlation, not causation.
There is very little exchange of air between the northern and southern hemispheres, and basic climate science shows that most sulfates from China will rain out before they get across the thermal equator. In fact, there is a great deal of literature out there published by luminaries like the Department of Energy's Ben Santer and NASA's James Hansen claiming relative cooling of the northern hemisphere from sulfates, compared to the southern.
So, if it is indeed sulfates cooling the warming, given that there is no net change in global temperature, then the northern hemisphere should be cooling since 1998 (the first year in Kaufmann's paper) while the southern warms. Here are the sad facts:
The opposite is occurring. Why this test was not performed eludes me. Perhaps that is because it provides yet another piece of evidence supporting the hypothesis that we have simply overstated the sensitivity of surface temperature to changes in carbon dioxide.