Fake skeptics of anthropogenic global warming love to set up the straw man that mainstream climate science believes that CO2 is the one and only driver of climate change. They can then use it in many different attacks, such as gee whiz isn’t it stupid that they haven’t even thought of the sun’s influence. This is of course patently false as even the most cursory survey of actual scientific content will quickly reveal. This straw man is also an implicit part of the argument that the “16 year pause” in global warming proves that CO2 is not a climate driver. If CO2 has risen and temperature has not, then the theory of AGW has failed, they say. Well, this would only be true if CO2 were believed to be the only possible driver of the surface temperature record. (For this post I am ignoring the two facts that, one, 16 years is not enough to unambiguously show a trend and two, there actually is an upward trend).
Well, some recent research out of the University of Colorado, Boulder has found that despite the lack of any major eruptions, like that of Pinatubo in 1991, there has in fact been a significant increase in volcanic aerosols in the stratosphere over the last decade. This increase is large enough that its cooling effect could mask as much as 25% of the greenhouse warming we would expect to see if CO2 really were on its own as a climatic influence.
Between 2000 and 2010, the average atmospheric concentration of carbon dioxide — a planet-warming greenhouse gas — rose more than 5%, from about 370 parts per million to nearly 390 parts per million. If that uptick were the only factor driving climate change during the period, global average temperature would have risen about 0.2°C, says Ryan Neely III, an atmospheric scientist at the University of Colorado, Boulder. But a surge in the concentration of light-scattering particles in the stratosphere countered as much as 25% of that potential temperature increase, he notes.
Further modeling indicates that the Asian Brown Cloud, unpleasant and influential as it may be regionally, does not explain the global data being recorded by satellite observation.
Now, by using a computer model that includes processes due to global atmospheric circulation and atmospheric chemistry, Neely and his colleagues show that the human contribution of aerosols to the stratosphere was minimal between 2000 and 2010. In one set of simulations, the researchers estimated the effects of all known volcanic eruptions, including the quantity of aerosols produced and the heights to which they wafted, on the month-to-month variations in particulate concentrations.
The pattern of stratospheric particulate variations during the past decade “shows the fingerprint of volcanoes, with the right episodes showing up at the right time,” says William Randel, an atmospheric scientist at the National Center for Atmospheric Research in Boulder.
This is an excellent illustration of why what the IPCC publishes are not called predictions, nor can they fairly be treated as such. They are projections of what would happen given assumed trajectories of key climatic forcing agents. When those agents, be they insolation, CO2 concentrations, stratospheric aerosol concentrations, ozone or albedo changes, do not actually change the way it was assumed in a climate model run, this is not the fault of the model.
So in summary, we have now yet another reason to dismiss the suggestions that recent surface temperature trends require the scrapping of the last 150 years of research and we must abandon the idea that humans are changing the global climate. We are changing the global climate, and the temporary amelioration of our disastrous warming influence by other natural factors is not likely to last long.