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Global warming and local dimming: the statistical evidence.
This is joint work with Jan Magnus and Bertrand Melenberg. It was submitted for publication
in September 2009.
Two effects largely determine global warming: the well-known greenhouse effect and the
less well-known radiation effect. An increase in concentrations of carbon dioxide and other
greenhouse gases contributes to global warming: the greenhouse effect. In addition, small
particles, called aerosols, reflect and absorb sunlight in the atmosphere. More pollution causes
an increase in aerosols, so that less sunlight reaches the Earth (global dimming). Despite its
name, global dimming is primarily a local effect. Because of the dimming the Earth becomes
cooler: the radiation effect. Global warming thus consists of two components: the (global)
greenhouse effect and the (local) radiation effect, which work in opposite directions. Only the
sum of the greenhouse effect and the radiation effect is observed, not the two effects separately.
Our purpose is to identify the two effects. This is important, because the existence of the
radiation effect obscures the magnitude of the greenhouse effect. We propose a simple climate
model with a small number of parameters. We gather data from a large number of weather
stations around the world for the period 1959–2002. We then estimate the parameters using
dynamic panel data methods, and quantify the parameter uncertainty. Next, we decompose
the estimated temperature change of 0.73 ◦C into a greenhouse effect of 1.87 ◦C, a radiation
effect of −1.09 ◦C, and a small remainder term. We also use our method to find the level of
greenhouse gases consistent with a temperature increase of 2 ◦C, for different levels of solar
radiation. Finally, we subject our findings to extensive sensitivity analyses.

Global warming, local dimming, and the impact of carbon dioxide.
This is joint work with Jan Magnus and Bertrand Melenberg. It was submitted for publication
in Oktober 2009.
This paper builds on the results of Global warming and local dimming: the statistical evidence.
Disentangling the greenhouse and the radiation effect is crucial for judging policy measures
aimed at restricting future temperature increases. There is, however, much controversy about
the response of temperature to these two effects, mainly because of the complex role played
by aerosols in the climate system. Here we disentangle the two effects directly on the basis of
data, using information from weather stations around the world for the period 1959 to 2002.
We find that the radiation effect has obscured the greenhouse effect by more than 50%,
implying that a doubling of the atmospheric equivalent carbon dioxide concentration would
lead to a temperature increase of 5.3 degrees Celsius. In case the recent brightening trend
persists, we find that carbon dioxide emissions need to be cut by 150 million tons per year
for the period 2010-2050 in order to avoid ’dangerous’ climate change, that is, a temperature
increase of more than 2 degrees Celsius since 1850. Our data-based results will be helpful in
the development and calibration of general circulation models.