increase in response to global warming and further enhance it.
We calculate with confidence that:
E some gases are potentially more effective than others at changing climate,
and their relative effectiveness can be estimated. Carbon dioxide has been
responsible for over half the enhanced greenhouse effect in the past, and is
likely to remain so in the future.
E atmospheric concentrations of the long-lived gases (carbon dioxide, nitrous
oxide and the CFCs) adjust only slowly to changes in emissions. Continued
emissions of these gases at present rates would commit us to increased
concentrations for centuries ahead. The longer emissions continue to
increase at present day rates, the greater reductions would have to be for
concentrations to stabilize at a given level.
E the long-lived gases would require immediate reductions in emissions from
human activities of over 60% to stabilize their concentrations at today’s
levels; methane would require a 15—20% reduction.
Based on current model results, we predict:
E under the IPCC Business-as-Usual (Scenario A) emissions of greenhouse
gases, a rate of increase of global mean temperature during the next century
of about 0.3 °C per decade (with an uncertainty range of 0.2 °C to 0.5 °C per
decade); this is greater than that seen over the past 10 000 years. This will
result in a likely increase in global mean temperature of about 1 °C above the
present value by 2025 and 3 °C before the end of the next century. The rise
will not be steady because of the influence of other factors.
Later sections of the summary addressed the scientific uncertainties and the
question of the degree to which anthropogenic climate change had been observed
in the climate record.
Over the period of the preparation of the IPCC report, a significant change
occurred in the attitudes of the scientists involved. To begin with there was a
strong feeling, particularly amongst some scientists, that the scientific uncertainty
was too large for any useful statement to be made regarding future climate
change. However, gradually we all realized our responsibility to articulate
carefully and honestly the knowledge which is available, distinguishing clearly
between what could be said with a good degree of certainty and the areas where
the uncertainty is large. After all, there were many not expert in the science who
felt few inhibitions about making forecasts of future climate change — often of an
extreme kind. Also we increasingly recognized that there was enough certainty in
the science to provide meaningful information regarding the likely future,
provided that the uncertainty was also fully explained.
4 Ownership by Scientists and by Governments
Many of the world’s leading scientists involved with the understanding of climate
and climate change contributed to the report. Inevitably they came mostly from
J. Houghton
4
We calculate with confidence that:
E some gases are potentially more effective than others at changing climate,
and their relative effectiveness can be estimated. Carbon dioxide has been
responsible for over half the enhanced greenhouse effect in the past, and is
likely to remain so in the future.
E atmospheric concentrations of the long-lived gases (carbon dioxide, nitrous
oxide and the CFCs) adjust only slowly to changes in emissions. Continued
emissions of these gases at present rates would commit us to increased
concentrations for centuries ahead. The longer emissions continue to
increase at present day rates, the greater reductions would have to be for
concentrations to stabilize at a given level.
E the long-lived gases would require immediate reductions in emissions from
human activities of over 60% to stabilize their concentrations at today’s
levels; methane would require a 15—20% reduction.
Based on current model results, we predict:
E under the IPCC Business-as-Usual (Scenario A) emissions of greenhouse
gases, a rate of increase of global mean temperature during the next century
of about 0.3 °C per decade (with an uncertainty range of 0.2 °C to 0.5 °C per
decade); this is greater than that seen over the past 10 000 years. This will
result in a likely increase in global mean temperature of about 1 °C above the
present value by 2025 and 3 °C before the end of the next century. The rise
will not be steady because of the influence of other factors.
Later sections of the summary addressed the scientific uncertainties and the
question of the degree to which anthropogenic climate change had been observed
in the climate record.
Over the period of the preparation of the IPCC report, a significant change
occurred in the attitudes of the scientists involved. To begin with there was a
strong feeling, particularly amongst some scientists, that the scientific uncertainty
was too large for any useful statement to be made regarding future climate
change. However, gradually we all realized our responsibility to articulate
carefully and honestly the knowledge which is available, distinguishing clearly
between what could be said with a good degree of certainty and the areas where
the uncertainty is large. After all, there were many not expert in the science who
felt few inhibitions about making forecasts of future climate change — often of an
extreme kind. Also we increasingly recognized that there was enough certainty in
the science to provide meaningful information regarding the likely future,
provided that the uncertainty was also fully explained.
4 Ownership by Scientists and by Governments
Many of the world’s leading scientists involved with the understanding of climate
and climate change contributed to the report. Inevitably they came mostly from
J. Houghton
4