Climate Change 2001 : Impacts, Adaptation,and Vulnerability - Contribution of working group II to the IPCC third assesement. IPCC (International Panel for Climate Change). Genêve, 2001.

Chapters :
1.4.3.4 Climate Variability and Extrem Events ; 4.3.2 Precipitation ; 4.3.6.1 Trends in Observed Streamflows ; 4.3.8 Changes in Flood Frequency ; 4.3.11 Glaciers and Small Ice Caps ; 13.1.3 Recent Climate Variability in Europe including Recent Warming ; 13.2.1.1.1 Changes in hydrological cycle ; 13.2.1.4 Moutains and Subarctic Environments.

(Each chapter is based on a wide range of scientific reports, details of the bibliography can be find in each chapter bibliography.)

General increase in Northern Hemisphere mid and high latitudes (particularly in autumn and winter) in the precipitation trends. Precipitation over northern Europe has increased by 10-40% in the 20th century, whereas some parts of southern Europe have dried by as much as 20%.

Most of Europe has experienced increases in surface air temperature during the 20th century that, averaged across the continent, amount to about 0.8°C in annual temperature. Warming in annual mean european temperature has occurred preferentially as a result of nighttime rather than daytime temperature increases.

The globally averaged surface temperature is projected to increase by 1.4 to 5.8°C over the period 1990 to 2100. These results are for the full range of 35 SRES scenarios, based on a number of climate models. The projected rate of warming is much larger than the observed changes during the 20th century and is very likely to be without precedent during at least the last 10,000 years, based on palaeoclimate data. (IPCC 2001- SP)

Current climate models simulate a climate change-induced increase in annual precipitation in high and mid-latitudes. Global climate models currently cannot simulate with accuracy short-duration, high-intensity, localized heavy rainfall, and a change in mean monthly rainfall may not be representative of a change in short-duration rainfall.

Global climate change is likely to bring changes in climate variability and extreme events. Potential changes in intense rainfall frequency are difficult to infer from global climate models, largely because of coarse spatial resolution. However, there are indications that the frequency of heavy rainfall events generally is likely to increase with global warming.

More frequent heat waves, less frequent cold spells, greater intensity of heavy rainfall events, more frequent midcontinental summer drought.

Runoff tends to increase where precipitation has increased and decrease where it has fallen over the past few years.

Variations in flow from year to year have been found to be much more strongly related to precipitation changes than to temperature changes. In large parts of eastern Europe, European Russia, central Canada, and California, a major (and unprecedented) shift in streamflow from spring to winter has been associated not only with a change in precipitation totals but more particularly with a rise in temperature: Precipitation has fallen as rain, rather than snow, and therefore has reached rivers more rapidly than before.

Most, but not all, valley glaciers and small ice caps have been in general retreat since the end of the Little Ice Age.

At the global scale, there is a simulation of a general decline in valley glacier mass (and consequent rise in sea level), indicating that the effects of higher temperatures generally are more significant than those of additional winter accumulation. Model studies of individual glaciers have shown general retreat with global warming; with a simulated retreat in Alpine glaciers with higher temperatures and changes in winter accumulation.

Calculations at the European scale indicate that under most climate change scenarios, northern Europe would see an increase in annual average streamflow, but southern Europe would experience a reduction in streamflow. In much of mid-latitude Europe, annual runoff would decrease or increase by about 10% by the 2050s, but the change resulting from climate may be smaller than “natural” multidecadal variability in runoff.

Low-flow frequency generally will increase across most of Europe, although in some areas where the minimum occurs during winter the absolute magnitude of low flows may increase because winter runoff increases: The season of lowest flow shifts toward summer. An implication of simulated changes in streamflow is that riverine flood risk generally would increase across much of Europe and that in some areas, the time of greatest risk would move from spring to winter.

Glacier retreat has implications for downstream river flows. In rivers fed by glaciers, summer flows are supported by glacier melt (with the glacier contribution depending on the size of the glacier relative to basin area, as well as the rate of annual melt). If the glacier is in equilibrium, the amount of precipitation stored in winter is matched by melt during summer.

However, as the glacier melts as a result of global warming, flows would be expected to increase during summer—as water is released from long-term storage—which may compensate for a reduction in precipitation. As the glacier gets smaller and the volume of melt reduces, summer flows will no longer be supported and will decline to below present levels.

The duration of the period of increased flows will depend on glacier size and the rate at which the glacier melts; the smaller the glacier, the shorter lived the increase in flows and the sooner the onset of the reduction in summer flows.

Floods and droughts are likely to become more frequent in mountainous area. Indirect impacts include changes in erosion and sedimentation patterns.

In most temperate mountain regions, the snowpack is close to its melting point, so it is very sensitive to changes in temperature. As warming progresses in the future, current regions of snow precipitation increasingly will experience precipitation in the form of rain. For every 1°C increase in temperature, the snowline rises by about 150 m; as a result, less snow will accumulate at low elevations than today, although there could be greater snow accumulation above the freezing level because of increased precipitation in some regions. The timing of streamflow therefore alters significantly. At higher altitudes, most of the precipitation continues to fall as snow, so the distribution of flow through the year is altered little.In areas where snowfall currently is marginal, snow may cease to occur.

Changes in storminess over the northeast Atlantic have been analyzed and show that although storminess has increased in recent decades, storm intensities are no higher than they were early in the 20th century.

Impacts of climatic change on physical systems will affect water, snow, and ice, and shifts in extremes will lead to changes in the frequency and intensity of natural hazards. Permafrost also could be significantly perturbed by warming, leading to a reduction of slope stability and a consequent increase in the frequency and severity of rock and mudslides.