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1. Options that eliminate or reduce greenhouse gas
2. Options that "offset" emissions by removing greenhouse
gases from the atmosphere, by blocking incident solar radiation, or
by altering the reflection or absorption properties of the earth's
3. Options that help human and ecologic systems adjust or
adapt to new climatic conditions and events.
In this report the first and second types of interventions are
referred to as "mitigation" since they can take effect prior to the
onset of climate change and slow its pace. Mitigation options are
discussed in more detail in Chapter 6 and Part Three. The third
type of intervention is referred to as "adaptation" since its
effects come into play primarily after climate has changed. A
fuller discussion of adaptation appears in Chapter 5 and Part
In comparing mitigation and adaptation, one consideration is
whether a given action will, in addition to providing adaptation or
mitigation benefits, also improve economic efficiency. Even
progressive societies find much of their economic activity falling
short of demonstrated "best practice." New, more efficient
practices are being developed continually, but it takes time for
them to diffuse throughout the economy. There are many obstacles to
more rapid diffusion of better practice, including lack of
information, insufficient supply of components or products,
political interests, inappropriate incentives, and simple human
inertia. In general, however, every society has many opportunities
to improve its overall situation by reducing the gap between
current practice and best practice. Many of the actions taken to
deal with potential greenhouse warming could also improve economic
well-being because they are more efficient than prevailing
practice. These options should be distinguished from another class
of actions: so-called "free-standing" actions, which satisfy other
social or environmental objectives (and may or may not contribute
to economic efficiency as such).
Figure 4.1 compares hypothetical mitigation and adaptation
actions in response to potential greenhouse warming. If climate
change occurs, and no mitigation or adaptation actions are
undertaken, a substantial reduction in real income is likely over
time. Initially, mitigation is likely to reduce real income more
than either doing nothing or taking adaptation measures as climatic
changes emerge. Ultimately, however, mitigation actions could
result in higher real income than waiting and taking adaptation
measures. In this scenario, investing in mitigation reduces
consumption now, but produces advantages in the future.
Expenditures on mitigation options should thus be seen as
investments in the future.
Many combinations of mitigation and adaptation actions are
possible. Choosing the best mix of mitigation and adaptation
strategies depends in part on the discount rate applied to the
investment. The higher the discount rate, the greater the case for
postponement of costly actions. Use of discount rates is one way of
assigning values to future outcomes.