In arguing against the pursuit of global warming policies, Bjorn Lomborg wrote in a Wall Street Journal op-ed that "[c]utting carbon emissions will likely increase water scarcity, because global warming is expected to increase average rainfall levels around the world." In fact, Lomborg's argument ignores that while scientists predict that climate change may increase precipitation globally, they believe some areas will experience more flooding because of the rainfall, whereas others will be disproportionately affected by drought.
Lomborg: Cutting emissions will "increase water scarcity" since climate change will "increase average rainfall levels" globally
From Lomborg's November 9 Wall Street Journal op-ed:
Getting basic sanitation and safe drinking water to the three billion people around the world who do not have it now would cost nearly $4 billion a year. By contrast, cuts in global carbon emissions that aim to limit global temperature increases to less than two degrees Celsius over the next century would cost $40 trillion a year by 2100. These cuts will do nothing to increase the number of people with access to clean drinking water and sanitation. Cutting carbon emissions will likely increase water scarcity, because global warming is expected to increase average rainfall levels around the world.
IPCC: Changes in precipitation are "projected to increase the risks of flooding and drought in many areas"
According to the Intergovernmental Panel on Climate Change's June 2008 report "Climate Change and Water" precipitation will very likely, which the IPCC defines as more than a 90 percent probability, increase in tropical and high-latitude regions and will likely (more than 66 percent probability) decrease in subtropical and low- to mid-latitude regions. The report said that many areas, including the Mediterranean Basin, western United States, southern Africa, and northeastern Brazil, are "projected to suffer a decrease of water resources due to climate change (high confidence)."
From the report's Executive Summary:
Climate model simulations for the 21st century are consistent in projecting precipitation increases in high latitudes (very likely) and parts of the tropics, and decreases in some subtropical and lower mid-latitude regions (likely). Outside these areas, the sign and magnitude of projected changes varies between models, leading to substantial uncertainty in precipitation projections. Thus projections of future precipitation changes are more robust for some regions than for others. Projections become less consistent between models as spatial scales decrease.
By the middle of the 21st century, annual average river runoff and water availability are projected to increase as a result of climate change at high latitudes and in some wet tropical areas, and decrease over some dry regions at mid-latitudes and in the dry tropics. Many semi-arid and arid areas (e.g., the Mediterranean Basin, western USA, southern Africa and northeastern Brazil) are particularly exposed to the impacts of climate change and are projected to suffer a decrease of water resources due to climate change (high confidence).
Increased precipitation intensity and variability are projected to increase the risks of flooding and drought in many areas. The frequency of heavy precipitation events (or proportion of total rainfall from heavy falls) will be very likely to increase over most areas during the 21st century, with consequences for the risk of rain-generated floods. At the same time, the proportion of land surface in extreme drought at any one time is projected to increase (likely), in addition to a tendency for drying in continental interiors during summer, especially in the sub-tropics, low and mid-latitudes.
IPCC: Drought, floods will affect water quality, food security
IPCC: Drought, flooding will "affect water quality and exacerbate many forms of water pollution." The June 2008 report also concluded that, alongside changes in water temperature, extreme flooding and drought will have a negative impact on water quality and food security. From the report:
Higher water temperatures and changes in extremes, including floods and droughts, are projected to affect water quality and exacerbate many forms of water pollution -- from sediments, nutrients, dissolved organic carbon, pathogens, pesticides and salt, as well as thermal pollution, with possible negative impacts on ecosystems, human health, and water system reliability and operating costs (high confidence). In addition, sea-level rise is projected to extend areas of salinisation of groundwater and estuaries, resulting in a decrease of freshwater availability for humans and ecosystems in coastal areas.
Globally, the negative impacts of future climate change on freshwater systems are expected to outweigh the benefits (high confidence). By the 2050s, the area of land subject to increasing water stress due to climate change is projected to be more than double that with decreasing water stress. Areas in which runoff is projected to decline face a clear reduction in the value of the services provided by water resources. Increased annual runoff in some areas is projected to lead to increased total water supply. However, in many regions, this benefit is likely to be counterbalanced by the negative effects of increased precipitation variability and seasonal runoff shifts in water supply, water quality and flood risks (high confidence).
Changes in water quantity and quality due to climate change are expected to affect food availability, stability, access and utilisation. This is expected to lead to decreased food security and increased vulnerability of poor rural farmers, especially in the arid and semi-arid tropics and Asian and African megadeltas.