Viewpoints

Action needed to avoid severe impacts on European water cycle

water-cycle

According to a new study authored by climate change researchers, substantial emissions reductions are required to avoid severe impacts on the water cycle in Europe.

Our current emissions trajectory is beyond a +3°C warming, and we’d need to mitigate global warming to 1.5°C to make a difference on water impacts in Europe.

In a warmer world, heavy rainfall becomes heavier, highest river flows further increase, and lowest flows decrease. In addition, these changes affect wider areas. Impacts of climate change at 1.5, 2, and 3°C mean global warming above pre-industrial level have been assessed and compared for precipitation, snowpack, runoff, and discharge in Europe. This was done by using the results of a number of climate models as input for hydrological models. The hydrological impacts in a warmer Europe were compared with those from the reference period of 1971-2000.

Precipitation

Generally, changes in precipitation and evapotranspiration are higher for higher levels of global warming. Where precipitation is projected to decrease with increased warming, these decreases become larger, where precipitation is projected to increase, it increases more.

Regionally, there are robust increases in total annual precipitation in most of central, western, and northern Europe, for all levels of warming. The decreases in precipitation projected around the Iberian coast become larger and more widespread with increasing warming. Changes in precipitation are negligible or uncertain in the rest of southern Europe and in the UK.

As expected, the snowpack decreases in most parts of Europe.

Evapotranspiration

At 3°C warming mean, annual evapotranspiration is clearly higher and changes are more robust than at 1.5°C in most of Europe except the south. Evapotranspiration is projected to decrease on the Iberian Peninsula because there is simply not enough moisture available to increase actual evapotranspiration due to the decreases in precipitation.

Runoff

Mean annual runoff is indicative of available water resources for agriculture, water supply, navigation, etc. Mean annual maximum runoff is indicative of recurring high flows, and mean annual low runoff is indicative of 
dry conditions.

In parts of Europe where runoff is affected by climate change, there is a distinct increase in the changes to mean, low, and high runoff at 2°C compared to 1.5°C. Between 2 and 3°C, the changes in low and high runoff levels continue to increase, but the changes to mean runoff are less clear.

For all levels of warming, the changes to runoff are strongest in winter, with large increases in runoff seen in Scandinavia and the Alpine regions. Robust increases in runoff affect the Scandinavian mountains at 1.5°C, and extend over most of Norway, Sweden, and northern Poland at 3°C. Decreases in mean annual runoff are seen only in Portugal at 1.5°C warming, but at 3°C warming, decreases to runoff are seen around the entire Iberian coast, the Balkan Coast, and parts of the French coast.

Discharge of Europe’s largest rivers

As a result of the increase in mean runoff for large parts of northern Europe, annual mean discharge of rivers in this area increase as warming level increases. In a similar way, decreases in river discharge with increasing warming level are consistent with runoff decreases in southern Europe.

In other parts of Europe, changes in mean annual discharge are less clear. This was illustrated for the rivers Rhine and Danube, and points at contrasting changes in winter to summer: in winter, discharges increase due to higher winter precipitation and earlier snowmelt, while summer discharge decreases due to lower snowmelt runoff from the Alps and increased evapotranspiration.

The article first appeared on Climate Change Post and is a digest of a scientific paper by Donnelly et al., 2017. Climatic Change 143: 13-26

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