As was mentioned in the previous section on precipitation predictions from GCMs, there are some important implications for water supplies. In this section, we will take a look at how the model results might impact surface water in the future.
Surface water is of great importance since it is the primary source of water for agriculture. It is estimated that 69% of worldwide water use is for irrigation, and of this, about 62% comes from surface water which includes streams, lakes, and reservoirs.
When rain falls on the surface, most of it is absorbed by the soil, and then from there, it slowly migrates to streams, and from there into lakes and reservoirs, but it ultimately leaves a region, through stream flow and evaporation, to return to the oceans or the atmosphere — this is just a part of the large global water cycle. The total amount of water flowing in the streams of a region provides a useful measure of how much water is available for agriculture. We'll discuss this in depth in Module 9.
It takes around 3,000 liters of water to produce enough food to satisfy one person's daily dietary needs. This is a considerable amount when compared to that required for drinking, which is just 2-5 liters. To produce food for over 7 billion people who inhabit the planet today requires the water that would fill a canal ten meters deep, 100 meters wide and 7.1 million kilometers long – that's enough to circle the globe 180 times!
As we have seen, a GCM will predict the amount of rainfall over the surface of the Earth, and if we combine that with a model of the topography of the land areas (which is included in the GCMs), we can figure out how much water will flow as surface water through different regions. This has been done by taking the average precipitation from 20 GCMs operating under the SRES A1B scenario and then calculating how that surface water flow compares to the long-term average from 1900 to 1970. The resulting data provide us with a very good idea of what to expect in the future if we follow the A1B scenario.
The results of the surface water predictions can be seen here, but we will focus in on 3 snapshots from this history in a series of maps. We begin with a view of the predictions for the year 2020.
This image is a 30-year average, centered on the year 2020. The blue areas will see an increase in streamflow, and the red areas will see a decrease. The map includes contour lines that separate the values according to the labeled tick marks on the map scale. As we might expect, the changes are relatively small at this point, but the Mediterranean region has noticeably less streamflow.
As we move forward in time, to 2050, the changes become more dramatic.
And as we continue into the future, the picture in 2080 looks like this:
By the year 2080, we see that there are some fairly stark differences in streamflow. A large swath around the Mediterranean, including much of Europe, North Africa, and the Middle East all will have significant reductions in streamflow, which will add stress to an agricultural system that is already operating at close to its limit. Consider also that the world by this time will certainly have a minimum of 10 billion people. Much of the Southwestern US (including California) and Central America will also experience a reduction in streamflow. This is clearly bad news for a place like California, which is already going to great lengths to bring surface water to its cities and major agricultural production areas via a system of canals. And for Central America, continuing drought will lead to even more mass migration to the US southern border. We will discuss this in much more detail in Module 9.
On the other hand, consider where most of the Earth's population is — China and India. Both of these regions, according to the model results, should experience an increase in surface water availability, which is good news.
Another big trend is that the high latitudes of the northern hemisphere experience a very large increase in surface runoff. This could be important if population patterns shift northward in a warming world.
We now consider how these changes add up over the whole globe — is surface runoff increasing or decreasing as we go into the future? The 3 maps shown above have been averaged along lines of latitude to give a simpler sense of the change, and then these latitudinal averages are summed and weighted according to the different areas each latitudinal band represents to give a global sum.
As can be seen, the tropics and the high latitude regions tend to get wetter through time, while the mid-latitudes tend to become drier, and on a global scale, there is slightly more surface water runoff as we move into the future — though a 3.3% change is not too large. Nevertheless, remember the map pattern of the change — this is the more important aspect of the model data.
So, in summary, as with precipitation, the future seems to hold a mixture of more and less surface water runoff, and this will have some important implications for where we will produce our food in the future and which places may be better suited for human habitation in the future. We will discuss the implications of the projections for regional drought in places such as the south-central US and Australia in more detail in Modules 9.