Oops: Got the Sign Wrong Trying to Explain Away the Global Warming “Pause”

Global Science Report is a feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

A couple of years ago, when it was starting to become obvious that the average global surface temperature was not rising at anywhere near the rate that climate models projected, and in fact seemed to be leveling off rather than speeding up, explanations for the slowdown sprouted like mushrooms in compost.

We humbly suggested a combination of natural variability and a lower “sensitivity” of surface temperature to rising carbon dioxide.

Now, several years later, the “pause” continues. Natural variability is now widely accepted as making a significant contribution and our argument for a lowered climate sensitivity—which would indicate that existing climate models are not reliable tools for projecting future climate trends—is buoyed by accumulating evidence and is gaining support in the broader climate research community. Yet is largely rejected by federal regulators and their scientific supporters.  These folks prefer rather more exotic explanations that seek to deflect the blame away from the climate models and thus preserve their over-heated projections of future global warming.

The problem with exotic explanations is that they tend to unravel like exotic dancers.

Such is the case for the explanation—popular with the press when it was first proposed—that an increase in aerosol emissions, particularly from China, was acting to help offset the warming influence of anthropogenic carbon dioxide emissions.

The suggestion was made back in 2011 by a team of researchers led by Boston University’s Robert Kaufmann and published in the Proceedings of the National Academy of Sciences. Shortly after it appeared, we were critical of it in these pages, pointing out how the explanation was inconsistent with several lines of data.

Now, a new paper appearing in the peer-reviewed scientific literature takes a deeper view of aerosol emissions during the past 15 years and finds that, in net, changes in aerosol emissions over the period 1996-2010 contributed a net warming pressure to the earth’s climate.

Kühn et al. (2014) write:

Increases in Asian aerosol emissions have been suggested as one possible reason for the hiatus in global temperature increase during the past 15 years. We study the effect of sulphur and black carbon (BC) emission changes between 1996-2010 on the global energy balance. We find that the increased Asian emissions have had very little regional or global effects, while the emission reductions in Europe and the U.S. have caused a positive radiative forcing. In our simulations, the global-mean aerosol direct radiative effect changes 0.06 W/m2 during 1996–2010, while the effective radiative forcing (ERF) is 0.42 W/m2.

So in other words, rather than acting to slow global warming during the past decade and a half as proposed by Kaufmann et al. (2011), changes in anthropogenic aerosol emissions (including declining emissions trends in North America and Europe) have acted to enhance global warming (described as contributing to a positive increase in the radiative forcing in the above quote).

This means that the “pause,” or whatever you want to call it, in the rise of global surface temperatures is even more significant than it is generally taken to be, because whatever is the reason behind it, it is not only acting to slow the rise from greenhouse gas emissions but also the added rise from changes in aerosol emissions.

Until we understand what this sizeable mechanism is and how it works, our ability to reliably look into the future and foresee what climate lies ahead is a mirage. Yet, somehow, the Obama Administration is progressing full speed ahead with regulations about the kinds of cars and trucks we can drive, the appliances we use, and the types of energy available, etc., all in the name of mitigating future climate change.

As we repeatedly point out, not only will the Obama Administration’s actions have no meaningful impact on the amount of future climate change, but it is far from clear that the rate of future change will even be enough to mitigate—or even to worry about.

References

Kaufmann, R. K., et al., 2011. Reconciling anthropogenic climate change with observed temperature 1998–2008. Proceedings of the National Academy of Sciences. doi: 10.1073/pnas.1102467108

Kühn, T., et al., 2014. Climate impacts of changing aersol emission since 1996. Geophysical Research Letters, doi: 10.1002/2014GL060349