by Judith Curry

Increasing CO2 may actually help relieve the water stress associated with increasing global population.

The importance of population, climate change and CO2 plant physiological forcing in determining future global water stress

Andrew Wiltshire, Jemma Gornall, Ben Booth, Emily Dennis, Pete Falloon, Gillian Kay, Doug McNeall, Carol McSweeney, Richard Betts

Future levels of water stress depend on changes in several key factors including population, climate- change driven water availability, and a carbon dioxide physiological-forcing effect on evaporation and run-off. In this study we use an ensemble of the HadCM3 climate model forced with a range of future emissions scenarios combined with a simple water scarcity index to assess the contribution of each of these factors to the projected population living in water stress over the 21st century. Population change only scenarios increase the number of people living in water stress such that at peak global population 65% of people experience some level of water stress. Globally, the climate model ensemble projects an increase in water availability which partially offsets some of the impacts of population growth. The result is 1 billion fewer people living in water stress by the 2080s under the high end emissions scenarios than if population increased in the absence of climate change. This study highlights the important role plant-physiological forcing has on future water resources. The effect of rising CO2 is to increase available water and to reduce the number of people living in high water stress by around 200 million compared to climate only projections. This effect is of a similar order of magnitude to climate change.

Citation: 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.

From the UK Met Office press release:

Higher carbon dioxide (CO 2 ) 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.

) 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.

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 (CO 2 ) on 21st century water resources. This study found an overall increase in the levels of water stress over the 21st Century.

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 CO 2 in the atmosphere is one of the main contributing factors leading to climate change. However, CO 2 also affects plants directly. One of the side effects of increasing atmospheric CO 2 is to reduce the amount of water plants need to grow. This ‘CO 2 effect’ increases the amount of water available for human consumption.



When you include the CO 2 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 ‘CO 2 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 CO 2 rather than other greenhouse gases (GHGs). If CO 2 is mitigated at the expense of other GHGs it may be that the benefits of CO 2 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.

JC comments: This paper illustrates the importance of considering multiple factors when assessing climate change impacts (particularly AGW). In my 2010 Congressional Testimony, I alluded to the water resource issue in terms of climate change winners and losers:

A view of the climate change problem as irreducibly global fails to recognize that some regions may actually benefit from a warmer and/or wetter climate. Areas of the world that currently cannot adequately support populations and agricultural efforts may become more desirable in future climate regimes.

Arguably the biggest global concern regarding climate change impacts is concerns over water resources. This concern is exacerbated in regions where population is rapidly increasing and water resources are already thinly stretched. China and South Asia (notably India, Pakistan, and Bangladesh) are facing a looming water crisis arising from burgeoning population and increasing demand for water for irrigated farming and industry.

“The consensus of AR4 models . . . indicates an increase in annual precipitation in most of Asia during this century; the relative increase being largest and most consistent between models in North and East Asia. The sub-continental mean winter precipitation will very likely increase in northern Asia and the Tibetan Plateau and likely increase in West, Central, South-East and East Asia. Summer precipitation will likely increase in North, South, South-East and East Asia but decrease in West and Central Asia.” [From the IPCC AR4]



Based on the IPCC’s simulations of 21st century climate, it seems that rainfall will increase overall in the region (including wintertime snowfall in Tibet), and the IPCC AR4 WGII does not discuss the impact of temperature and evapotranspiration on fresh water resources in this region.

It is good to see integrated studies such as the Wiltshire et al. paper; it remains to be seen whether the IPCC AR5 assessments of such impacts will be an improvement over the AR4 assessment, and whether the possibility of winners as well as losers will be acknowledged.

