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REPORTS - ASSESSMENT REPORTS |
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Climate Change 2001: Synthesis Report |
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| Question 5
What is known about the inertia and time scales
associated with the changes in the climate system, ecological systems,
and socio-economic sectors and their interactions?
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| Inertia is a widespread inherent characteristic of the
interacting climate, ecological, and socio-economic systems. Thus some
impacts of anthropogenic climate change may be slow to become apparent,
and some could be irreversible if climate change is not limited in both
rate and magnitude before associated thresholds, whose positions may be
poorly known, are crossed. |
B5.1-4, B5.8,
B5.10-12 & B5.14-17 |
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| Inertia
in Climate Systems |
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Stabilization of CO2 emissions
at near-current levels will not lead to stabilization of CO2
atmospheric concentration, whereas stabilization of emissions of shorter
lived greenhouse gases such as CH4 leads, within decades, to
stabilization of their atmospheric concentrations. Stabilization
of CO2 concentrations at any level requires eventual reduction
of global CO2 net emissions to a small fraction of the current
emission level. The lower the chosen level for stabilization, the sooner
the decline in global net CO2 emissions needs to begin (see
Figure SPM-5). |
Q5.3 & Q5.5 |
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| After stabilization of the atmospheric concentration
of CO2 and other greenhouse gases, surface air temperature is
projected to continue to rise by a few tenths of a degree per century for
a century or more, while sea level is projected to continue to rise for
many centuries (see Figure SPM-5).
The slow transport of heat into the oceans and slow response of ice sheets
means that long periods are required to reach a new climate system equilibrium. |
Q5.4 |
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| Some changes in the climate system, plausible
beyond the 21st century, would be effectively irreversible. For
example, major melting of the ice sheets (see Question
4) and fundamental changes in the ocean circulation pattern (see Question
4) could not be reversed over a period of many human generations.
The threshold for fundamental changes in the ocean circulation may be
reached at a lower degree of warming if the warming is rapid rather than
gradual.
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Q5.4 & Q5.14-16
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 Figure
SPM-5: After CO2 emissions are reduced and atmospheric concentrations
stabilize, surface air temperature continues to rise slowly for a century
or more. Thermal expansion of the ocean continues long after CO2
emissions have been reduced, and melting of ice sheets continues to contribute
to sea-level rise for many centuries. This figure is a generic illustration
for stabilization at any level between 450 and 1,000 ppm, and therefore
has no units on the response axis. Responses to stabilization trajectories
in this range show broadly similar time courses, but the impacts become
progressively larger at higher concentrations of CO2. |
Q5
Figure 5-2 |
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| Inertia in Ecological
Systems |
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| Some ecosystems show the effects of climate change
quickly, while others do so more slowly. For example, coral bleaching
can occur in a single exceptionally warm season, while long-lived organisms
such as trees may be able to persist for decades under a changed climate,
but be unable to regenerate. When subjected to climate change, including
changes in the frequency of extreme events, cosystems may be disrupted as
a consequence of differences in response times of species. |
Q5.8 & Q3
Table 3-2 |
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| Some carbon cycle models project the global terrestrial
carbon net uptake peaks during the 21st century, then levels off or declines.
The recent global net uptake of CO2 by terrestrial ecosystems
is partly the result of time lags between enhanced plant growth and plant
death and decay. Current enhanced plant growth is partly due to fertilization
effects of elevated CO2 and nitrogen deposition, and changes
in climate and land-use practices. The uptake will decline as forests reach
maturity, fertilization effects saturate, and decomposition catches up with
growth. Climate change is likely to further reduce net terrestrial carbon
uptake globally. Although warming reduces the uptake of CO2 by
the ocean, the oceanic carbon sink is projected to persist under rising
atmospheric CO2, at least for the 21st century. Movement of carbon
from the surface to the deep ocean takes centuries, and its equilibration
there with ocean sediments takes millennia. |
Q5.6-7 |
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| Inertia in Socio-Economic
Systems |
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| Unlike the climate and ecological systems, inertia
in human systems is not fixed; it can be changed by policies and the choices
made by individuals. The capacity for implementing climate change
policies depends on the interaction between social and economic structures
and values, institutions, technologies, and established infrastructure.
The combined system generally evolves relatively slowly. It can respond
quickly under pressure, although sometimes at high cost (e.g., if capital
equipment is prematurely retired). If change is slower, there may be lower
costs due to technological advancement or because capital equipment value
is fully depreciated. There is typically a delay of years to decades between
perceiving a need to respond to a major challenge, planning, researching
and developing a solution, and implementing it. Anticipatory action, based
on informed judgment, can improve the chance that appropriate technology
is available when needed. |
Q5.10-13 |
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| The development and adoption of new technologies can
be accelerated by technology transfer and supportive fiscal and research
policies. Technology replacement
can be delayed by "locked-in" systems that have market advantages
arising from existing institutions, services, infrastructure, and available
resources. Early deployment of rapidly improving technologies allows learning-curve
cost reductions. |
Q5.10 & Q5.22 |
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| Policy
Implications of Inertia |
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| Inertia and uncertainty in the climate, ecological,
and socio-economic systems imply that safety margins should be considered
in setting strategies, targets, and time tables for avoiding dangerous
levels of interference in the climate system. Stabilization target
levels of, for instance, atmospheric CO2 concentration, temperature,
or sea level may be affected by:
- The inertia of the climate system, which will cause climate change
to continue for a period after mitigation actions are implemented
- Uncertainty regarding the location of possible thresholds of irreversible
change and the behavior of the system in their vicinity
- The time lags between adoption of mitigation goals and their achievement.
Similarly, adaptation is affected by the time lags involved in identifying
climate change impacts, developing effective adaptation strategies, and
implementing adaptive measures. |
Q5.18-20 & Q5.23 |
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| Inertia in the climate, ecological, and socio-economic
systems makes adaptation inevitable and already necessary in some cases,
and inertia affects the optimal mix of adaptation and mitigation strategies.
Inertia has different consequences for adaptation than for mitigation -- with
adaptation being primarily oriented to address localized impacts of climate
change, while mitigation aims to address the impacts on the climate system.
These consequences have bearing on the most cost-effective and equitable
mix of policy options. Hedging strategies and sequential decision making
(iterative action, assessment, and revised action) may be appropriate responses
to the combination of inertia and uncertainty. In the presence of inertia,
well-founded actions to adapt to or mitigate climate change are more effective,
and in some circumstances may be cheaper, if taken earlier rather than later. |
Q5.18-21 |
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| The pervasiveness of inertia and the possibility of
irreversibility in the interacting climate, ecological, and socio-economic
systems are major reasons why anticipatory adaptation and mitigation actions
are beneficial. A number of opportunities
to exercise adaptation and mitigation options may be lost if action is delayed. |
Q5.24 |
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