Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 40

Europe

1.0

0.5

0.0

1900 1950

Year

2000

Models using only natural forcings

Models using both natural and anthropogenic forcings

Figure 3.2 Temperature change relative to the 1901-50 mean for the period 1906-
2005. The black line indicates observed values, the coloured bands give the modelled
data covered by 90% of the recent model simulations. (Modified from Climate Change
2007: The Physical Science Basis. Contribution of Working Group I to the Fourth
Assessment Report of the Intergovernmental Panel on Climate Change (eds S. Solomon,
D. Qin, M. Manning, et al .). Cambridge University Press, Cambridge and New York.)

increased in northern parts, while precipitation has decreased in the Mediterranean
(IPCC 2007). These tendencies may be associated with changes in the North
Atlantic Oscillation (NAO), a north-south dipole in sea-level pressure across the
Atlantic (e.g. Hurrell et al . 2003), which has its strongest signature in winter.
The prevalence of more positive winter NAO values from the 1970s to the 1990s
reflects the enhanced westerly air flow across the North Atlantic, moving warm
moist air over much of Europe in winter and resulting in wet conditions in
Northern Europe and dry conditions in the south. However, topography may
generate fine spatial scales in climate, and observed changes in air temperature or
precipitation may thus differ from the average picture.
The IPCC Fourth Assessment Report (2007) notes that the increase in air
temperature since the middle of the last century is very likely (i.e. >90% prob-
ability) to be due to the increase in anthropogenic greenhouse gas concentrations.
The observed changes can be accurately simulated only if climate models include
greenhouse gas forcing (Fig. 3.2).
What can we expect for a future climate? General Circulation Models (GCMs)
give a temperature increase of about 0.2°C per decade over the next two decades
for a range of emission scenarios (IPCC 2007). Further warming will be caused
by the continued emission of greenhouse gases at or above current rates. The
projected changes in the climate system during this century are very likely to be