This doesn't build my confidence that scientists can calculate future climatic conditions

Science Centric | News | How much water runs down the rivers

Global river discharge normally amounts to 35,000 to 40,000 cubic kilometres annually. According to the CRU climate data it increased by 7.7 percent during the last century, the researchers report. Approximately 95,000 to 110,000 cubic kilometres of precipitation fall on land surface each year. However, because regional amounts of precipitation and trends vary between different data sets, and other data do not indicate a clear global trend, it remains unclear whether there is actually an increase in global river discharge.

Following precipitation, land use had the largest impact on river discharge. During the last century, humans have increased global discharge by 1.7 percent, especially by deforestation. While irrigation caused significant regional decreases in discharge, its global effect on river discharge was negligible.

Over the last century, global warming decreased river discharge by 0.9 percent. This trend, due basically to increased evapotranspiration, was strongest at high latitudes and in parts of Central Asia. The global temperature signature has become increasingly evident in recent decades, the researchers write. Calculations based on three IPCC scenarios indicate that this trend will continue and that global warming alone could reduce global river discharge by six percent by the end of the 21st century.

Theoretically, the rise in the concentration of CO2 could reinforce this trend. The greenhouse gas could have a fertilising effect that leads to an expansion of vegetation cover. On a regional level, more plants would take more water from the soil and thus decrease river discharge. Globally, however, the fertilisation effect is negligible. Another direct effect of increased CO2 concentration has increased discharge by more than one percent between 1901 and 2002: under a higher concentration of CO2 plants need to open their stomata less in order to take up enough CO2 for growth. Therefore, evapotranspiration decreases and they take less water from the ground.

'The net effect of the rising CO2 concentration in the atmosphere could increase global river discharge by a further five percent by 2100,' says Dieter Gerten. Globally, this would probably compensate the negative impacts of temperature. However, temperature and CO2 effects would not necessarily affect the same regions. The researchers are planning further studies to investigate possible developments of future water availability and demand worldwide.