Climate change may result in a reduction in the amount of fresh water (water yield) that the US National Forests and Grasslands contribute to the national water supply, according to a study published in Scientific Reports this week.

Forested land covers approximately 29% of the US, supplying over half of the country’s fresh water and offsetting between 10-20% of current US fossil fuel emissions. The National Forests account for 9% of the total land area and provide 14% of the national water supply.

Using climate projections derived from 20 global climate models, Ge Sun and colleagues created a model to investigate the potential impacts of climate change on the water supply and carbon sequestration by the 170 National Forests and Grasslands in the US. The authors found that ecosystem productivity - the rate of generation of biomass - is projected to increase by up to 24%, while water yield is projected to decrease by 4%-7% by 2100. They suggest that although precipitation levels are likely to increase during this period, they will be offset by increasing levels of evaporation and transpiration owing to increases in air temperature.

The authors warn that an increase in carbon sequestration and ecosystem productivity may result in increased forest growth which, when coupled with a drying trend, could lead to an increased risk of wildfire. They also argue that water supplies and aquatic habitats could be threatened by a reduction in water yield.