Climate Change and the European Water Dimension - Agri ...

tropospheric ozone are photochemical destruction in the atmosphere and deposition
to the surface (mainly to vegetation). Ozone concentrations can be strongly
influenced by anthropogenic emissions, but it is not straightforward to construct a
historical record of atmospheric ozone concentrations. Based on evaluation of ozone
surface measurements from the 19 th and early 20 th century ozone concentrations in
Northern Hemisphere have approximately doubled since the pre-industrial era. The
estimated radiative forcing due to tropospheric ozone is 0.35 Wm -2 . In addition to its
direct effect, ozone has an indirect effect on some greenhouse gases (particularly
methane) because it influences the tropospheric OH radical concentration and thus
the oxidation rates of many gases.
Stratospheric ozone can influence climate both by absorbing incoming shortwavelength
radiation and by absorbing the outgoing infrared radiation. The estimated
overall radiative forcing caused by the depletion of the stratospheric ozone layer that
has taken place is –0.15 Wm -2 .
I.H. Global warming and the hydrological cycle
Feedbacks between the atmospheric hydrological cycle and global warming have
been extensively studied and discussed within the scientific community. A basic
reason for expecting an impact of global warming on water in the atmosphere is that
a warmer atmosphere can contain more water vapour. However, global warming may
in many ways influence the evaporation, transport and precipitation of water, e.g. via
atmospheric and oceanic processes. Feedbacks can be established, because water
is itself an important greenhouse gas and also clouds have an influence on the
radiative balance of the Earth. Current models predict that the water vapour feedback
approximately doubles the global warming compared to what it would be for fixed
water vapour concentrations.
Trends in water vapour concentrations in the lower troposphere have been
investigated based on analysis of surface measurements and observations from
weather balloons and satellites. The IPCC TAR concludes that water vapour
concentrations in the Northern Hemisphere have likely increased several per cent per
decade since the early 1970’s, while changes in the Southern Hemisphere have not
yet been assessed. Also the cloud cover over mid- and high latitude continental
areas in the Northern Hemisphere appears to have increased by about 2% since the
beginning of the 20 th century.
Greenhouse gases exercise radiative forcing much more efficiently at the low
temperatures in the upper troposphere than at the close-to-surface temperatures in
the boundary layer, and, in fact, models indicate that increases in water vapour in the
region above the boundary layer (i.e. in the free troposphere) is the main reason for
strong feedback effects. However, the water vapour feedback in the upper
troposphere is more complex than in the boundary layer, where relative humidity
tends to remain fixed, and thus water vapour concentrations increase with
temperature. In the upper troposphere, a variety of dynamical and microphysical
processes influence water vapour. Observational evidence for this region is
ambiguous. Satellite observations have shown a statistically significant increasing
trend of water vapour in the upper troposphere in a zone around equator (10 0 N to
10 0 S) for the period 1980 to 1997. Positive as well as negative trends were found for
other latitudes but none of them were statistically significant.
Clouds have a cooling effect on the surface of the Earth by reflecting incoming short
wavelength radiation (albedo effect), but they also absorb outgoing long-wavelength
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