Fossil fuel air pollution aerosol cooling offsets industrial GHG pollution warming
and can lead to cooling
2013
There is now proof in causation that the lull in global temperature of 1940 – 1960) followed by a renewed
increase since 1960 has been largely due to air pollution aerosol cooling and its abatement with regulation.
Cleaner air: Brightening the pollution perspective? AIP Conf. Proc. 1527, pp. 579-582;
NUCLEATION AND ATMOSPHERIC AEROSOLS: 19th International Conference
June 2013
Colin O'Dowd1, Darius Ceburnis1, Aditya Vaishya1, S. Gerard Jennings1, and Eoin Moran2 1School of Physics &
Centre for Climate and Air Pollution Studies, National University of Ireland Galway, University Road, Galway,
Ireland
2Met Éireann, Glasnevin, Dublin 6, Ireland
Clean-air policies in developing countries have resulted in reduced levels of anthropogenic atmospheric
aerosol pollution. Reductions in aerosol pollution is thought to result in a reduction in haze and cloud layers,
leading to an increase in the amount of solar radiation reaching the surface, and ultimately, an increase in
surface temperatures. There have been many studies illustrating coherent relationships between surface solar
radiation and temperature however, a direct link between aerosol emissions, concentrations, and surface
radiation has not been demonstrated to date. Here, we illustrate a coherence between the trends of reducing
anthropogenic aerosol emissions and concentrations, at the interface between the North-East Atlantic and
western-Europe, leading to a staggering increase in surface solar radiation of the order of ∼20% over the last
decade.
Cleaner air: Brightening the pollution perspective?” by O’Dowd et al. suggests that European warming of the
last decade(s) is (also) caused by a sky brightening: the good results of air-pollution control lead to a rise in the
negative radiative forcing of the aerosols (i.e. the negative forcing becomes less negative), and as a
consequence solar irradiance at ground level increases. This increase is at least partially responsible for the
observed warming.
Colin O’Dowd and co-authors have published a similar free-access article in issue 11 of the bulletin of Royal
Irish Academy’s Climate Change Sciences Committee (here). Wang et al published in 2012 a paper
“Atmospheric impacts on climatic variability of surface incident solar radiation” which essentially follows the
same path.
What makes me uncomfortable, is that both authors clearly affirm that we have an ongoing solar brightening
in Europe (brightening and dimming is a seen as a decadal change in solar irradiance). Here is a figure from
Wang’s paper showing the situation in Europe:
The green line shows the anomaly (5
years smoothing applied) of solar irradiance, the blue of the inverted atmospheric optical depth (-AOT) and
the red of the free (= cloudless) sky fraction. Clearly AOT declines after 2000, and solar irradiance increases.
O’Dowd et al conclude their investigation with “…the trends of reducing anthropogenic aerosol emissions and
concentrations, at the interface between the North-East Atlantic and western-Europe, lead(ing) to a
staggering increase in surface solar radiation of the order of ∼20% over the last decade.”
These are strong words, and paint a situation that is the contrary of what I have been measuring since about
15 years. Actually, in Diekirch (Luxembourg) we have an ongoing solar dimming since 1998, which has even
become stronger after 2003 (the heat-wave year I would not dare to take as a starting point for a regression
line!)
There also is no warming here, but we even measure a slightly cooling since 2002 (see here). What gives me
some comfort that our measurements are not picked out of the blue, are the satellite data from Helioclim for
Luxembourg (which alas stop at 2005):
The Helioclim plot shows a long-time
dimming over two decades for Luxembourg!
Martin Wild from ETH (Zürich, Switzerland) is one of the world specialists in global dimming/brightening, and
co-author of the Wang and Norris papers. He asserts that there was a dimming in Europe from 1950 to 1980,
followed by two brightening periods of different magnitude from 1980 to 2000 and after 2000 (see here). So
he is in consensus with the two other authors.
Assuming these authors correct, global European trends may clearly be very different from those of smaller
specific regions. So one should be cautious to generalize a broad picture to regional scale, where the exact
opposite might go on.
Let us conclude with another paper by Norris et al. “Trends in aerosol radiative effects over Europe inferred
from observed cloud cover, solar ‘‘dimming,’’ and solar ‘‘brightening’’” which use the GEBA archive (Global
Energy Balance Archive) and which give this picture:
The blue dots correspond to decreasing,
the red to increasing irradiance. Now watch the right part and the legends: all crossed circles correspond to
statistically not significant trends, and clear circles to a situation where 25% or more data are missing. If we
leave out these measurements, not much remains in the 1987-2002 part which allows to conclude to a
brightening. Here the right part magnified:
If you keep only the statistically significant points and those with enough data you have 3 cooling points
(Germany, France, Bulgaria) and three brightening (2 in northern Italy and one in Finland), see the arrows. Not
so impressive!
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References:
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Colin O’Dowd, Darius Ceburnis, Aditya Vaishya, S. Gerard Jennings, Eoin Moran : Cleaner air:
Brightening the pollution perspective? AIP Conf. Proc. 1527, pp. 579-582;
doi:http://dx.doi.org/10.1063/1.4803337 (4 pages)
K. C. Wang, R. E. Dickinson, M. Wild, S. Liang: Atmospheric impacts on climatic variability of surface
incident solar radiation. Atmos. Chem. Phys., 12, 9581–9592, 2012. http://www.atmos-chemphys.net/12/9581/2012/ doi:10.5194/acp-12-9581-2012 (link)
Joel R. Norris, Martin Wild: Trends in aerosol radiative effects over Europe inferred from observed
cloud cover, solar ‘‘dimming,’’ and solar ‘‘brightening’’. JOURNAL OF GEOPHYSICAL RESEARCH, VOL.
112, D08214, doi:10.1029/2006JD007794, 2007 (link)