The Bigger Picture

Earth is a wonderfully diverse place. As a planet, we are incredibly unique – no other planet (that we are aware of) can claim such environmental diversity. We always come first in the Known Universe Gardening Competition (as judged by the Vogons).

'Second place again?' Image Credit: Apak Studios

                                                           
Our unique atmosphere and range of environments means that different regions are likely to react quite differently to climate change. In the context of river discharge, this makes prediction quite difficult. There are many different natural and anthropogenic forces that may force changes in river discharge, and each may act with a different magnitude. 

Climate change will likely accelerate the hydrological cycle – this could increase discharge (more precipitation), or decrease it (evapotranspiration operating as a counterbalance). Discharge will increase in some areas of the world, decrease in other areas, and not change at all in others. Our blue marble responds in varying ways to the new pressures it has likely never experienced before. 

A map of projected percentage changes in discharge (from the mean) around the world. From Nohara, et al. (2006)

Modelling work using complex Global Circulation Models has generally been consistent in identifying how different regions of the world will be affected. Discharge is projected to decrease in the Mediterranean, Central America and southern Africa. In contrast, discharge is expected to increase in the high latitudes, southern/eastern Asia, and central Africa. These changes will occur because of changes in annual mean precipitation, evaporation and runoff – if precipitation is greater than evaporation, there is more runoff, and greater discharge (and vice versa).

It is clear that this whole idea of changing river discharge and climate change is not a simple exercise. Many have claimed that global river discharge has increased during the 20^th century, but some studies would suggest neither discharge nor precipitation changed much during the last half of the 20^th century. If this is true, viewing the problem of changing discharge though a global lens may be a naïve approach – each region of the world has rivers that will react in very different ways.

This is going to be harder than we thought.