CONCLUSION

Our principal conclusions are these:

It may become necessary to try to limit atmospheric concentration to a level which
is probably less than 800 and may be as low as 500 ppmv.

(2) If atmospheric concentration is to be kept below 500-300 ppmv, the use of fossil
fuels should peak and start to decline within the next several decades, possibly as early as 2010
and probably by 2040 (Fig. 5). If this does not happen anyway, for reasons unrelated to C02,
then explicit actions to retard fossilvfuel use would need to become effective within the next
few decades (Fig. 8).

(3) In addition, if future global energy demand lies in the range de?ned by the IIASA Low
and High Scenarios, the contribution of non-fossil energy sources would have to increase
rapidly within the next 35-50 yr, reaching levels comparable both to present total energy
consumption and to future maximum fossil-fuel use. The transition from fossil to non-fossil
sources must be well under way within this period (Fig. 5).

(4) The lower the C02 limit, the earlier must be the time to begin to reduce fossil fuel use.
To achieve a concentration limit of 500 ppmv, vigorous action would have to be effective within
the next 10-20 yr; for a limit of 600 ppmv, less strenuous action would need to become effective
in 20?30 yr. If growth in energy demand proves to be much less than the IIASA Low Scenario,
then the urgency for actions either esplicitiy to restrict fossil-fuel use or to promote the
introduction of non-fossil sources would be substantially reduced.

These conclusions lead us to believe that US energy policy should not be neutral with
respect to the C03 issue. We see the need at least for the following:

(I) An aggressive research program to improve our understanding of C03 effects and reduce
uncertainty.

(2) International cooperation to develop a world energy strategy, including understandings
as to what actions might be taken to limit C02.

(3) Encourage development and improvement of non-fossil energy technologies which could
contribute in a major way to satisfy total world energy needs. These are principally solar and
nuclear systems; we believe that their present well?documented dif?culties can be much
alleviated. If that comes to pass, a combination of both is likely to be needed.

(4) Encourage more ef?cient use of energy sources world wide, which will provide more
time to understand the C03 problem and/or ease the severity of the needed response to curtail
fossil fuels.