Heat

Sectoral Impacts
could lead to greater tensions and conflicts over claims to water
sources and priority of water uses.
However, the exact spatial patterns of change in water stress
remain uncertain, mainly because of the persistent shortcomings
of global climate models in simulating future precipitation patterns.
This is particularly relevant in the Indian monsoon domain,
where a large share of the world’s population depends highly
on natural water resources, which are already under significant
stress today, while up to now no robust statement can be made
about the future response of monsoon rainfall to climate change.
Moreover, while this climate model uncertainty is apparent from
the studies discussed here, it should also be noted that each of
these studies only uses a single hydrological model. As hydrological
models have many structural differences, systematic comparison
of different models is necessary to quantify the associated uncertainty,
but has hardly been carried out, particularly for scenarios
near 4°C warming.
The above studies also highlight the difficulty of assessing
on-the-ground water stress or scarcity on a global scale. Locations
around the world differ greatly in water management practices,
water-use efficiency of agriculture and other water users, and adaptation
options to changing water availability, among other factors.
Moreover, looking only at long-term averages of seasonal-mean
water availability neglects the importance of subseasonal processes.
Climate change is expected to alter the seasonal distribution of
runoff and soil water availability, likely increasing the number of
such extreme events as floods and droughts, both of which can
have devastating effects, even if annual mean numbers remain
unchanged. In order to better estimate climate change impacts
on water resources at potentially vulnerable locations, future
water resources research will thus increasingly have to consider
finer spatial and temporal scales. Besides changes in runoff and
soil moisture, there are many other physical processes that are
important for a comprehensive assessment of water related climate
change impacts, including groundwater extraction and recharge,
salination of aquifers and estuaries, melting glaciers, water temperatures,
sediment fluxes, and the ability of existing hydrological
features—both natural (for example, river beds) and artificial (for
example, dams and reservoirs)—to handle changed water flows.
Glacial runoff, for example, is critical in the dry season in India,
China, and South America. Global-scale studies of these factors
are rare, let alone for temperatures at or above 4°C.
Finally, one major outcome of the above studies is that it is
primarily the combination of climate change, population change,
and changes in patterns of demand for water resources that will
determine future water stress around the world, rather than climate
change alone. This will be further shaped by levels of adaptive
capacity. In many countries, particularly in the developing world,
the adverse impacts of decreasing runoff and total water availability
would probably be greatly exacerbated by high rates of
population growth and by the fact that many of these countries
are already water scarce and thus have little capacity to satisfy the
growing demand for water resources. Conversely, positive impacts
of climate change are expected to occur primarily in countries that
have higher adaptive capacities and lower population growth rates.
In the context of a 4°C world, the strong dependence of water
stress on population also means that the timing of the warming
is important. Depending on the scenario, world population is
projected to grow until the second half of this century, but this
trend is expected to reverse towards the year 2100 and beyond,
shrinking the world population. Thus, in a rapidly warming world,
the most adverse impacts on water availability associated with a
4°C world may coincide with maximum water demand as world
population peaks (Fung et al. 2011).
Ecosystems and Biodiversity
Ecosystems and their species provide a range of important goods
and services for human society. These include water, food, cultural
and other values. In the AR4 an assessment of climate change
effects on ecosystems and their services found the following:
• If greenhouse gas emissions and other stresses continue at or
above current rates, the resilience of many ecosystems is likely
to be exceeded by an unprecedented combination of change
in climate, associated disturbances (for example, flooding,
drought, wildfire, insects, and ocean acidification) and other
stressors (global change drivers) including land use change,
pollution and over-exploitation of resources.
• Approximately 20 to 30 percent of plant and animal species
assessed so far are likely to be at increased risk of extinction, if
increases in global average temperature exceed of 2–3° above
preindustrial levels.
• For increases in global average temperature exceeding 2 to 3°
above preindustrial levels and in concomitant atmospheric
CO 2
concentrations, major changes are projected in ecosystem
structure and function, species’ ecological interactions and
shifts in species’ geographical ranges, with predominantly
negative consequences for biodiversity and ecosystem goods
and services, such as water and food supply.
It is known that past large-scale losses of global ecosystems
and species extinctions have been associated with rapid climate
change combined with other ecological stressors. Loss and/or
degradation of ecosystems, and rates of extinction because of
human pressures over the last century or more, which have intensified
in recent decades, have contributed to a very high rate of
extinction by geological standards. It is well established that loss
or degradation of ecosystem services occurs as a consequence of
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