This chapter synthesizes the results of Work Group II of the Third Assessment
Report (TAR) and assesses the state of knowledge concerning Article 2 of the
United Nations Framework Convention on Climate Change (UNFCCC). The TAR's
task is to define what is known about the effects of climate change: how sensitive
systems are, what adaptive capacity they have, and what their vulnerability
is. It is not the goal of this assessment to determine whether these effects
are tolerable or are considered dangerous.
The goal of this chapter is to synthesize information on climate change impacts
in a manner that will enable readers to evaluate the relationship between increases
in global mean temperature and impacts. The chapter focuses on certain "reasons
for concern" that may aid readers in making their own determination about
what is a "dangerous" climate change. Each reason for concern is consistent
with a paradigm that can be used by itself or in combination with other paradigms
to help determine what level of climate change is dangerous. The reasons for
- The relationship between global mean temperature increase and damage to
or irreparable loss of unique and threatened systems
- The relationship between global mean temperature increase and the distribution
- The relationship between global mean temperature increase and global aggregate
- The relationship between global mean temperature increase and the probability
of extreme weather events
- The relationship between global mean temperature increase and the probability
of large-scale singular events such as the breakup of the West Antarctic Ice
Sheet or the collapse of the North Atlantic thermohaline circulation.
In addition, we examine what observed effects of climate change tell us with
regard to Article 2 of the UNFCCC. Increase in global mean temperature since
1900 (i.e., mean global warming) is used as the common metric against which
impacts are measured. This metric is closely related to greenhouse gas (GHG)
concentrations but is more relevant for impact assessments.
Some general caveats apply to all of the reasons for concern:
- In spite of many studies on climate change impacts, there still is substantial
uncertainty about how effective adaptation will be (and could be) in ameliorating
negative effects of climate change and taking advantage of positive effects.
- The effect of changes in baseline conditions, such as population and
economic growth and development of new technologies that could change vulnerability,
has not been adequately considered in most impact studies.
- Most impact studies assess the effects of a stable climate, so our understanding
of what rates of change may be dangerous is limited.
It does not appear to be possible to combine the different reasons for concern
into a unified reason for concern that has meaning and is credible. However,
we can review the relationship between impacts and temperature for each reason
for concern and draw some preliminary conclusions about the potential severity
and risk of impacts for the individual reasons for concern. Note that the following
findings do not incorporate the costs of limiting GHG emissions to levels that
are sufficient to avoid changes that may be considered dangerous. Also note
that there is substantial uncertainty regarding the impacts of climate change
at the temperatures mentioned. These temperatures should be taken as approximate
indications of impacts, not as absolute thresholds. In addition, change in global
mean temperature does not describe all relevant aspects of climate change impacts,
such as rate and pattern of change and changes in precipitation, extreme climate
events, or lagged (or latent) effects such as rising sea levels. For simplification,
we group different levels of temperature increase into "small," "medium,"
and "large." "Small" denotes a global mean temperature increase
of as much as approximately 2°C; "medium" denotes a global mean
temperature increase of approximately 2-3°C; and "large"
denotes a global mean temperature increase of more than approximately 3°C.
Based on a review of the literature of observations of climate change impacts,
as reflected in other chapters in the TAR, we conclude the following:
- Observations: Statistically significant associations between trends
in regional climate and impacts have been documented in ~100 physical processes
and ~450 biological species or communities in terrestrial and polar environments.
Although the presence of multiple factors (e.g., land-use change, pollution,
biotic invasion) makes attribution of observed impacts to regional climate change
difficult, more than 90% (~99% physical, ~80% biophysical) of the changes documented
worldwide are consistent with how physical and biological processes are known
to respond to climate. Based on expert judgment, we have high confidence that
the overall patterns and processes of observations reveal a widespread and coherent
impact of 20th-century climate changes on many physical and biological systems.
Signals of regional climate change impacts may be clearer in physical and biological
systems than in socioeconomic systems, which also are simultaneously undergoing
many complex changes that are not related to climate change, such as population
growth and urbanization. Socioeconomic systems have complex and varying mechanisms
for adapting to climate change. There are preliminary indications that some
social and economic systems have been affected in part by 20th-century regional
climate changes (e.g., increased damages from flooding and droughts in some
locations). It generally is difficult to separate climate change effects from
coincident or alternative explanations for such observed regional impacts.
- Unique and Threatened Systems: Tropical glaciers, coral reefs, mangroves,
ecotones, and biodiversity "hot spots" are examples of unique and
threatened entities that are confined to narrow geographical ranges and are
very sensitive to climate change. However, their degradation or loss could affect
regions outside their range. There is medium confidence that several of these
systems will be affected by a small temperature increase; for example, coral
reefs will bleach and glaciers will recede. At higher magnitudes of temperature
increase, other and more numerous unique and threatened systems would be adversely
- Distribution of Impacts: The impacts of climate change will not be evenly
distributed among the peoples of the world. There is high confidence that developing
countries will be more vulnerable to climate change than developed countries,
and there is medium confidence that climate change would exacerbate income inequalities
between and within countries. There also is medium confidence that a small temperature
increase would have net negative impacts on market sectors in many developing
countries and net positive impacts on market sectors in many developed countries.
However, there is high confidence that with medium to high increases in temperature,
net positive impacts would start to decline and eventually would turn negative,
and negative impacts would be exacerbated. Estimates of distributional effects
are uncertain because of aggregation and comparison methods, assumptions about
climate variability, adaptation, levels of development, and other factors.
- Aggregate Impacts: With a small temperature increase, there is medium
confidence that aggregate market sector impacts would amount to plus or minus
a few percent of world gross domestic product (GDP), and there is low confidence
that aggregate nonmarket impacts would be negative. Most people in the world
would be negatively affected by a small to medium temperature increase. Most
studies of aggregate impacts find that there are net damages at the global scale
beyond a medium temperature increase and that damages increase from there with
further temperature increases. The important qualifications raised with regard
to distributional analysis (previous bullet item) also apply to aggregate analysis.
By its nature, aggregate analysis masks potentially serious equity differences.
Estimates of aggregate impacts are controversial because they treat gains for
some as canceling out losses for others and because the weights that are used
to aggregate over individuals are necessarily subjective.
- Extreme Climate Effects: The frequency and magnitude of many extreme
climate events increase even with a small temperature increase and will become
greater at higher temperatures (high confidence). Extreme events include, for
example, floods, soil moisture deficits, tropical and other storms, anomalous
temperatures, and fires. The impacts of extreme events often are large locally
and could strongly affect specific sectors and regions. Increases in extreme
events can cause critical design or natural thresholds to be exceeded, beyond
which the magnitude of impacts increases rapidly (high confidence).
- Large-Scale Singularities: Large-scale singularities in the response
of the climate system to external forcing, such as shutdown of the North Atlantic
thermohaline circulation or collapse of the West Antarctic ice sheet, have occurred
in the past as a result of complex forcings. Similar events in the future could
have substantial impacts on natural and socioeconomic systems, but the implications
have not been well studied. Determining the timing and probability of occurrence
of large-scale singularities is difficult because these events are triggered
by complex interactions between components of the climate system. The actual
impact could lag the climate change cause (involving the magnitude and the rate
of climate change) by decades to millenia. There is low to medium confidence
that rapid and large temperature increases would exceed thresholds that would
lead to large-scale singularities in the climate system.