7.2.2. Aggregate Impacts
With a small temperature increase, aggregate market-sector impacts could amount
to plus or minus a few percent of world GDP (medium confidence); aggregate nonmarket
impacts could be negative (low confidence). The small net impacts are mainly
the result of the fact that developed economies, many of which could have positive
impacts, contribute the majority of global production. Applying more weight
to impacts in poorer countries to reflect equity concerns, however, can result
in net aggregate impacts that are negative even at medium warming. It also is
possible that a majority of people will be negatively affected by climate change
scenarios in this range, even if the net aggregate monetary impact is positive.
With medium to higher temperature increases, benefits tend to decrease and damages
increase, so the net change in global economic welfare becomes negative -- and
increasingly negative with greater warming (medium confidence). Some sectors,
such as coastal and water resources, could have negative impacts in developed
and developing countries. Other sectors, such as agriculture and human health,
could have net positive impacts in some countries and net negative impacts in
other countries. [19.5]
Results are sensitive to assumptions about changes in regional climate, levels
of development, adaptive capacity, rates of change, valuation of impacts, and
methods used for aggregating losses and gains, including the choice of discount
rate. In addition, these studies do not consider potentially important factors
such as changes in extreme events, advantageous and complementary responses
to the threat of non-climate-driven extreme events, rapid change in regional
climate (e.g., resulting from changes in ocean circulation), compounding effects
of multiple stresses, or conflicting or complementary reaction to those stresses.
Because these factors have yet to be accounted for in estimates of aggregate
impacts and estimates do not include all possible categories of impacts, particularly
nonmarket impacts, estimates of aggregate economic welfare impacts of climate
change are considered to be incomplete. Given the uncertainties about aggregate
estimates, the possibility of negative effects at a small increase in temperature
cannot be excluded. [19.5]
7.2.3. Distribution of Impacts
Developing countries tend to be more vulnerable to climate change than developed
countries (high confidence). Developing countries are expected to suffer more
adverse impacts than developed countries (medium confidence). A small temperature
increase would have net negative impacts on market sectors in many developing
countries (medium confidence) and net positive impacts on market sectors in
many developed countries (medium confidence). The different results are attributable
partly to differences in exposures and sensitivities (e.g., present temperatures
are below optimal in mid- and high latitudes for many crops but at or above
optimal in low latitudes) and partly to lesser adaptive capacity in developing
countries relative to developed countries. At a medium temperature increase,
net positive impacts would start to turn negative and negative impacts would
be exacerbated (high confidence). The results of these studies do not fully
take into account nonmarket impacts of climate change such as impacts on natural
systems, which may be sensitive to small amounts of warming. Particularly vulnerable
regions include deltaic regions, low-lying small island states, and many arid
regions where droughts and water availability are problematic even without climate
change. Within regions or countries, impacts are expected to fall most heavily,
in relative terms, on impoverished persons. The poorest members of society can
be inferred to be most vulnerable to climate change because of their lack of
resources with which to cope and adapt to impacts, but few studies have explicitly
examined the distribution of impacts on the poor relative to other segments
of society. [19.4]
Impacts on unmanaged systems are likely to increase in severity with time,
but impacts on managed systems could increase or decrease through the 21st century.
The distribution of impacts over the 21st century is influenced by several factors.
As GHG concentrations increase, the magnitude of exposure to change in climate
stimuli also would increase. Nonclimate pressures on natural and social systems,
which increase the vulnerability of systems, also may grow through time as a
result of population growth and increased demands for land, water, public infrastructure,
and other resources. Increased population, incomes, and wealth also mean that
more people and human-made resources potentially would be exposed to climate
change, which would tend to increase market-sector damages in absolute dollar
terms; this has been the case historically. Counteracting these tendencies are
factors such as increased wealth and technology and improved institutions, which
can raise adaptive capacity and reduce vulnerability to climate change. [8,
19.4]
Whether impacts and vulnerability increase or decrease with time is likely
to depend in part on the rates of climate change and development and may differ
for managed and unmanaged systems. The more rapid the rate of climate change,
the greater would be future exposure to potentially adverse changes and the
greater the potential for exceeding system thresholds. The more rapid the rate
of development, the more resources would be exposed to climate change in the
future - but so too would the adaptive capacity of future societies. The benefits
of increased adaptive capacity are likely to be greater for intensively managed
systems than for systems that presently are unmanaged or lightly managed. For
this reason, and because of the possibility that nonclimate pressures on natural
systems may increase in the future, the vulnerability of natural systems is
expected to increase with time (medium confidence). [19.4.2,
19.4.3]
Future development paths, sustainable or otherwise, will shape future vulnerability
to climate change, and climate change impacts may affect prospects for sustainable
development in different parts of the world. Climate change is one of many stresses
that confront human and natural systems. The severity of many of these stresses
will be determined in part by the development paths followed by human societies;
paths that generate lesser stresses are expected to lessen the vulnerability
of human and natural systems to climate change. Development also can influence
future vulnerability by enhancing adaptive capacity through accumulation of
wealth, technology, information, skills, and appropriate infrastructure; development
of effective institutions; and advancement of equity. Climate change impacts
could affect prospects for sustainable development by changing the capacity
to produce food and fiber, the supply and quality of water, and human health
and by diverting financial and human resources to adaptation. [18]
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