Heat

Sectoral Impacts
stunting indicate substantial increases, particularly in severe
stunting in Sub-Saharan Africa (23 percent) and South Asia
(62 percent) (Lloyd, Kovats, and Chalabi 2011).
Water Resources
It is well established that climate change will bring about substantial
changes in precipitation patterns, as well as in surface
temperature and other quantities that govern evapotranspiration
(see for example, Meehl, Stocker, and Collins 2007). The associated
changes in the terrestrial water cycle are likely to affect the nature
and availability of natural water resources and, consequently,
human societies that rely on them. As agriculture is the primary
water consumer globally, potential future water scarcity would put
at risk many societies’ capacity to feed their growing populations.
However, other domestic and industrial water uses, including cooling
requirements, for example, for thermal power plants, as well as the
functioning of natural ecosystems also depend on the availability
of water. The magnitude and timing of surface water availability
is projected to be substantially altered in a warmer world. It is
very likely that many countries that already face water shortages
today will suffer from increased water stress in a 4°C world and
that major investments in water management infrastructure would
be needed in many places to alleviate the adverse impacts, and
tap the potential benefits, of changes in water availability. In the
following, recent model predictions are referenced in order to
provide an outline of the nature and direction of change expected
for warming of 4°C and beyond.
Changes to Levels of Precipitation and
Water Stress in a 2°C World and in a 4°C+
World
Fung et al. (2011) explicitly investigate the difference between
a 4°C world and a 2°C world, using the MacPDM hydrological
model, which is driven by a large perturbed-physics climate model
ensemble based on the HadCM3L climate model. Because they
define the 1961–1990 average temperature as their baseline, their
4°C world is actually about 4.4°C warmer than the preindustrial one.
The bottom line of this study is that globally changes in
annual runoff are expected to be amplified once warming has
reached 4°C compared to one in which it has reached 2°C; that
is, on a large scale, the hydrological response to global warming
appears rather linear. Regions experiencing drier conditions
—namely, generating less runoff—under 2°C warming are projected
to become even drier under 4°C (and vice versa). Drier
conditions are projected for southern Europe, Africa (except
some areas in the northeast), large parts of North America and
South America, and Australia, among others. Wetter conditions
are projected for the northern high latitudes, that is, northern
North America, northern Europe, and Siberia. In the ensemble
average, mean annual runoff decreases in a 2°C world by around
30, 20, 40, and 20 percent in the Danube, Mississippi, Amazon,
and Murray Darling river basins, respectively, while it increases
by around 20 percent in both the Nile and the Ganges basins,
compared to the 1961–190 baseline period. Thus, according to
Fung et al. (2011), all these changes are approximately doubled
in magnitude in a 4°C world.
Fung et al. (2011) also look at a simple water stress index, using
the ratio of annual mean runoff to population in a given basin as
a measure of water resources per capita. The SRES A1B emissions
scenario, from which the 2°C and 4°C climate projections are
derived, is set in relation to a scenario of future population growth
based on a medium UN population projection. In a 2°C world,
relatively small runoff changes combined with large population
growth over the next few decades mean that changes in water stress
would mostly be dominated by population changes, not climate
changes. Increasing water demand would exacerbate water stress
in most regions, regardless of the direction of change in runoff.
However, in a 4°C world, climate changes would become large
enough to dominate changes in water stress in many cases. Again,
water stress is expected to increase in southern Europe, the United
States, most parts of South America, Africa, and Australia, while
it is expected to decrease in high latitude regions. A fragmented
picture emerges for South and East Asia, where increased runoff
from monsoon rainfall in some areas competes with populationdriven
increases in demand (while other areas may see reduced
monsoon runoff).
There are complexities beyond this large-scale, annual mean
picture. In five of the six major river basins studied in detail by
Fung et al. (2011), the seasonality of runoff increases along with
global warming, that is, wet seasons become wetter and dry seasons
become drier. This means that while an increase in annual
mean runoff, for example, in the Nile or the Ganges basin may
appear beneficial at first sight, it is likely to be distributed unevenly
across the seasons, possibly leading to increased flooding in the
high-flow season, while hardly improving water stress in the
low-flow season. This would have severe adverse consequences
for affected populations, especially if the seasonality of runoff
change would be out of phase with that of demand, such as for
crop growing or the cooling of thermal power plants. Major investments
in storage facilities would be required in such cases in order
to control water availability across the year and actually reap the
local benefits of any increases in runoff. For such basins as the
Ganges, another reason to strengthen water management capacities
is that hydrological projections for the Indian monsoon region
are particularly uncertain because of the inability of most climate
models to simulate accurately the Indian monsoon. Quantitative
results for this region based on a single climate model (as used by
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