The impact of climate, population, and carbon dioxide on water resources

August 2013 - This new study combines the effects of changes in carbon dioxide, climate, and population on projections of future global water resources.

  • Higher carbon dioxide (CO2) concentrations in the future will change the way plants use water, and might help relieve the water stress caused by a larger global population and climate change. 
  • Population growth is projected to be the main driver of an increase in the number of people experiencing water stress in 21st Century.
  • Climate change is projected to be less important, and may actually decrease the global number of people in water stress - although regionally there will be winners and losers.

Projections suggest that the population of the world will increase substantially in the 21st century, peaking at near 9 billion in the 2050s, before the growth levels off later in the century.
 
This large number of extra people will need water; for their agriculture, industry, and domestic consumption. Changes in weather due to climate change will affect the water resources available to this expanding population.
 
A new modeling study led by Met Office scientist Andy Wiltshire attempts to link changing population with the impact of climate change and carbon dioxide (CO2) on 21st century water resources. This study found an overall increase in the levels of water stress over the 21st Century, as illustrated in the figure above.

This increase is mainly the result of population growth, with around 65% of the population projected to be exposed to some level of water stress at the time of peak population. By the end of the century it is estimated that around 4 billion people will live in regions of high water stress due to population change. 
 
Climate change will affect both rainfall and evaporation - both important factors influencing available water. Overall, this study found up to 1 billion less people living in high water stress by the end of century than would have been the case without climate change. However, there are winners and losers in this study as climate change acts to increase water availability in some regions and decrease it in others.
 
The increase in CO2 in the atmosphere is one of the main contributing factors leading to climate change. However, CO2 also affects plants directly. One of the side effects of increasing atmospheric CO2 is to reduce the amount of water plants need to grow. This 'CO2 effect' increases the amount of water available for human consumption. 
 
When you include the CO2 effect, the number of people living in a state of high water stress is reduced by around 200 million compared to population growth and climate change alone. Importantly, the 'CO2 effect' leads to fewer people living in water stress for most of the regions of the world, unlike the effects of climate change which lead to large regional differences in water availability.
 
This research shows that there are potentially important implications from the particular mix of greenhouse gases and the future level of warming on the impacts of climate change. There may be some benefits of climate change in the face of rapid population growth, most strongly felt if the climate change is driven by increasing atmospheric CO2 rather than other greenhouse gases (GHGs).  If CO2 is mitigated at the expense of other GHGs it may be that the benefits of CO2 on water resources are reduced but the impacts of climate change are still felt. This has implications for global GHG emissions targets that aim to limit climate change according to temperature targets alone.
 
Importantly, this study only deals with one aspect of climate impacts on water resources. Other studies, entirely consistent with the results presented here, have shown that climate change affects other parts of the water system, enhancing the risks of flooding and drought.

References

Wiltshire, A. J., Gornall, J., Booth, B. B. B., Dennis, E., Falloon, P., Kay, G., McSweeney, C., McNeall, D., and Betts, R. A.: The importance of population, climate change and CO2 plant physiological forcing in determining future global water stress., Global Environmental Change 2013.

Last updated: 14 April 2014