The O’Dowd paper on aerosols and global warming.

O_Dowd_Air_pollution_2013

There is a lot of discussion in the blogosphere on the paper Cleaner air: Brightening the pollution perspective?” by  O’Dowd et al. (paywalled!) which 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:

Wang_Climate_variabilityThe 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!)

solar_energy_trend_1998_2012

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):

Helioclim_solar_dimming_Luxbg_1985_2005The 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:

GEBA_global_radiation_trendsGEBA_global_radiation_trends_legendThe 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:

GEBA_global_radiation_trends_1987_2002

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!

_______________________________

References:

  • 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-chem-phys.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)

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