Present climate and biological change - Climate Change: Biological and Human Aspects - page 239

remote-controlled aircraft to monitor the energy flux in aerosol brown clouds over
the Indian Ocean caused by biomass burning and fossil fuel consumption. By flying
the craft in formation, they were able to get a mass of data on aerosol concentration,
amounts of soot and solar fluxes. They flew 18 missions in March during the dry sea-
son when air flow brings pollution from Asia across the Indian ocean. The IPCC, up
to and including its 2007 report, largely considered aerosols to have a cooling effect:
sulphate aerosols reflect, there is a global dimming component and there is indirect
cooling forcing (aerosols can seed moist air and so facilitate the forming of clouds,
which are reflective). But some components of aerosol clouds (black carbon - soot -
from combustion for example) can absorb solar radiation and so heat the atmospheric
layer in which the aerosol resides (1-3 km high). What they found was that there was
a much stronger warming factor than previously thought and that this would warm
the climate of the region as much as the recent increases in greenhouse gases. Indeed,
they propose that this has further spurred the observed retreat of Himalayan glaciers.
(See also a review article by Peter Pilewskie, 2007.)
But the Indian Ocean aerosols may have other environmental consequences. In
2011 Amato Evan, James Kossin and colleagues reported on the increase in the
intensity of pre-monsoon Arabian Sea tropical cyclones (what would be called hur-
ricanes in the Atlantic) in the northern Indian Ocean during the period 1979-2010.
They suggested that these could be a consequence of a simultaneous upward trend
in anthropogenic black carbon and sulphate emissions. They used a combination of
observation over more than 30 years, and model data to demonstrate that the anomal-
ous circulation, which is radiatively forced by these anthropogenic aerosols, reduces
the basin-wide vertical wind shear, so creating an environment that allows tropical
cyclones to become more intense. This is not a trivial concern. In 1998, a major
cyclone resulted in more than 1100 deaths in western India, and Cyclone Gonu in
2007 caused more than US$4 billion in collective damage to Oman, the United Arab
Emirates and Iran (Sriver, 2011). The problem is that a longer time series than 30
years is required and so, as often is common with research, this work will need to
continue.
In 2011 United Nations Environment Programme (UNEP) and the World Meteoro-
logical Organization published the report Integrated Assessment of Black Carbon and
Tropospheric Ozone . It highlighted that soot (black carbon) aerosols act to enhance
short-term warming forcing of the climate. It estimated that controlling emissions of
black carbon aerosols could reduce short-term climate forcing by around 0.2 ◦ Cin
addition to reducing human health impacts and damage to crops.
Overall, various phenomena and factors, including those we may not even yet
appreciate, should make us pause and realise that there are limits to certainty in the
IPCC assessments. This is something the IPCC themselves appreciate.
Successive IPCC reports have provided improved appraisals of our scientific under-
standing of the way global and regional climates are changing. Nonetheless, there
is much we do not know and, for instance with regards to both remote sensing and
computer modelling, we are constantly pushing at the advancing edge of technical
capability. Yet we are still left with the overall question of how robust are the IPCC's
conclusions? As noted, the IPCC arrive at their decisions by committee, which in turn
bases its conclusions on its members' own interpretations of a variety of evidence,
both palaeoclimatic and palaeoclimatic combined with computer models. Even so,