19.8.2. What does Each Reason for Concern Indicate?
Looking across these different reasons for concern, what can we conclude about
what change in global average temperature is "dangerous"? A few general
caveats apply:
- In spite of many studies on climate change impacts, there is still 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 economic growth and
development of new technologies, that could reduce 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—or perhaps even appropriate—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 over the 21st century for each reason for concern and
draw some preliminary conclusions about what change may be dangerous for each
reason for concern. Note that the following findings do not incorporate the
costs of limiting climate change to these levels. Also note that there is substantial
uncertainty regarding the temperatures mentioned below. These magnitudes of
change in global mean temperature should be taken as an approximate indicator
of when various categories of impacts might happen; they are not intended to
define absolute thresholds.
For simplification, we group different levels of global mean temperature increase
into "small," "medium," and "large." "Small"
denotes a global mean temperature increase of up to approximately 2°C;4
"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. In addition, changes in global mean temperature
do not describe all relevant aspects of climate-change impacts, such as rates
and patterns of change and changes in precipitation, extreme climate events,
or lagged (or latent) effects such as rising sea levels.
19.8.2.1. Unique and Threatened Systems
Tropical glaciers, coral reefs, mangroves, biodiversity "hot spots,"
and ecotones 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 many of these unique and threatened 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 become adversely affected.
19.8.2.2. Distributional Impacts
The impact of climate change will not be evenly distributed among the peoples
of the world. There is high confidence that developing countries tend to 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 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. In addition,
impacts are likely to vary between and within countries. Thus, not all developing
or developed countries will necessarily have benefits or damages in unison.
19.8.2.3. Aggregate Impacts
Figure 19-7: Impacts of or risks from climate change, by reason for
concern. Each row corresponds to a reason for concern; shades correspond
to severity of impact or risk. White means no or virtually neutral impact
or risk, light gray means somewhat negative impacts or low risks, and dark
gray means more negative impacts or higher risks. Global average temperatures
in the 20th century increased by 0.6°C and led to some impacts. Impacts
are plotted against increases in global mean temperature after 1990. This
figure addresses only how impacts or risks change as thresholds of increase
in global mean temperature are crossed, not how impacts or risks change
at different rates of change in climate. Temperatures should be taken as
approximate indications of impacts, not as absolute thresholds. |
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 GDP; there is low confidence that aggregate nonmarket impacts would
be negative. Some studies find a potential for small net positive market impacts
under a small to medium temperature increase. However, given the uncertainties
about aggregate estimates, the possibility of negative effects cannot be excluded.
In addition, 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 regarding distributional analysis 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 cancelling out losses for others and because weights
that are used to aggregate over individuals are necessarily subjective.
19.8.2.4. 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).
19.8.2.5. Large-Scale Singularities
Large-scale singularities in the response of the climate system to external
forcing, such as shutdown of the North Atlantic THC or collapse of the WAIS,
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.
Figure 19-7 sums up the reasons for concern regarding
impacts relative to change in temperature. Each row corresponds to a reason
for concern, and the shades correspond to the severity of impact or risk. White
means no or virtually neutral impact or risk, light gray means somewhat negative
impacts or low risks, and dark gray means more negative impacts or higher risks.
The period 1850-1990 warmed by 0.6°C and led to some impacts. Unique
and threatened systems were affected, and the magnitude and frequency of some
extreme events have changed. Future impacts are plotted against increases in
global mean temperature after 1990.
Adverse impacts are estimated to occur in three reasons for concern even at
a small increase in temperature: unique and threatened systems, extreme weather
events, and distributional impacts. For the other two reasons for concern—adverse
impacts and large-scale discontinuities—adverse impacts begin at the medium
level of temperature increase for the former and a large temperature increase
for the latter.
