Is cleaner air responsible for recent global warming?

Is the clue in the Stratosphere?

Every once in a while, a paper comes along that make you think out of the box, and that’s good. Even if it proves to be unsubstantiated, it is a useful exercise that all scientists should go through from time to time. Carbon dioxide is a greenhouse gas accumulating in our atmosphere, and physics grants it a role in global warming. Climate models predict it causes surface warming and stratospheric cooling.

The overall climatic response is very complex and, in a recent paper, Qing-Bin Lu of Waterloo University in Canada looks at regional and global upper stratosphere temperature (UST) and surface temperature, as well as various climate drivers other than carbon dioxide, including ozone, aerosols, solar variability, snow cover extent, and sea ice extent. It is well known that simulations by mainstream climate models often consider that these shorter-term climate forcings likely cancel out, and warming is pretty much controlled by just carbon dioxide emissions. Accepting that, Lu is clearly thinking outside the box labelled “carbon dioxide”.

Carbon dioxide-based climate models in the IPCC AR6 2 show that although these short-term effective radiative forcings did essentially cancel out during the period 1970–2010, they have more recently led to a positive net forcing of 0.4–0.5 W/m2 from around 2010 up to the present. Lu notes another climatic pattern in addition to the predicted continuous surface warming and upper stratospheric cooling by carbon dioxide-based climate models. The observed data, he maintains, show that the UST at altitudes of 35–40 km demonstrates warming trends in the polar regions but no significant trends in non-polar regions since 2002.

The surface temperature exhibits cooling trends in the Antarctic since 2002 and in the Arctic since 2016, and no statistically significant trend in the tropics since 2016. It is the significant surface warming trends at SH and NH mid-latitudes that is responsible for the rise in global mean surface temperature in the last two decades.  

To explain all this, Lu believes that this warming is well explained by the radiative forcings of aerosols and ozone as a result of improved air quality. According climate models, the observed UST trends is evidence that the total greenhouse effect has been decreasing in the polar regions and not significantly increased in non-polar regions in the last two decades.

Such a view will be unpopular, but at least Professor Lu is making a prediction. With observations of rapidly lowered aerosol loading, he predicts a reversal in global mean surface temperature, in perhaps as early as 5–10 years.

Curiously, another recent paper postulates the same point. A new analysis of carbon dioxide in ice cores shows rapid swings in the 1800s that were clearly not driven by human emissions. The researchers say this also indicated flaws in estimating global carbon dioxide levels from measurements made in one location, Antarctica.