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showed that this effect was markedly greater than estimates used up to that time
by IPCC climatologists. Aeroplane condensation trails also cause significant global
dimming and this was demonstrated in a rare experiment following the attack on the
World Trade Center in New York on 11 September 2001, when for 3 days virtually
all of the US civilian jet air fleet was grounded (Travis et al., 2002). Furthermore, the
IPCC estimates for aerosol forcing for their first three assessments (1990, 1995 and
2001) were based on satellite measurements only over the ocean. Subsequently, with
improved orbital remote sensing, it was possible to make estimates of aerosol forcing
over land. This indicated that aerosol forcing was in fact at the high end of the earlier
range the IPCC used. These results suggest that present-day direct radiative cooling
forcing due to aerosols is stronger than that represented in the IPCC assessments up to
and including their third report. The implication is therefore that future atmospheric
warming due to an increase in anthropogenic greenhouse gases will be greater than
the IPCC had predicted (Bellouin et al., 2005).
As for the longer-term trends in global dimming, 2005 saw the publication of
the summaries of two sets of data. One was from a set of hundreds of terrestrial
sunlight measurements gathered by an international team led by Martin Wild, an
atmospheric scientist at the Swiss Federal Institute of Technology in Zurich. The
other study looked at satellite data and was led by Rachel Pinker from the University
of Maryland. Both found a similar trend. It would appear that since the 1950s up
to the early 1990s global dimming had been taking place. However, since the early
1990s there had been some recovery, with 'global brightening'. This is thought to
be due to the decline in the Soviet Block's dirty industries as well as the increased
use of improved cleaner technology by Western developed nations. Currently work is
underway to estimate how much global dimming has offset late-20th-century global
warming. The IPCC's 2007 assessment notes that global dimming does not seem to
have continued after 1990.
If global dimming is something the IPCC have (prior to 2007) not fully taken into
account then this could, at least in part, explain why high-latitude warming has actually
been greater than their models predicted. It could be that the cleaner air of the poles
(protected by circumpolar air currents) was not as globally dimmed. This is but one
possibility. In this instance much of the academic research relating to global dimming
and brightening was published after 2000. The IPCC only considered science research
published prior to its 2001 report, so the IPCC's first three assessments could not take
this dimension into account. The IPCC's report for 2007 began to include such
considerations. It noted that 'global dimming' is not global in extent and it has not
continued as a significant effect after 1990. Decreases in solar radiation at the Earth's
surface from 1970 to 1990 had an urban bias. Further, there have been increases in
solar radiation received at the Earth's surface since about 1990. An increasing aerosol
load due to human activities decreases regional air quality and the amount of solar
radiation reaching the Earth's surface. In some areas, such as Eastern Europe, recent
observations of a reversal in global dimming to that of brightening link changes in
solar radiation to air quality improvements.
A fresh twist to the aerosol climate forcing story came in 2007 when Veerabhad-
ran Ramanathan, Muvva Raman, Gregory Roberts and a few colleagues mainly
from the Scripps Institution of Oceanography in the USA used three lightweight,
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