Note 1: References
1. Extract from Summary for Policymakers, the
Third Assessment Report of Working Group I of the Intergovernmental
Panel on Climate Change (IPCC)
1-1. Model calculations of the concentrations of the non-CO2
greenhouse gases by 2100 vary considerably across the SRES illustrative
emission scenarios, with CH4 changing by between -190 and
+1,970 ppb (present concentration 1,760 ppb), N2O changing
by between +38 and +144 ppb (present concentration 316 ppb), total tropospheric
O3 changing by between -12 and +62%, and a wide range of
changes in concentrations of HFCs, PFCs and SF6, all relative
to the year 2000. In some scenarios, total tropospheric O3
would become as important a radiative forcing agent as CH4
and, over much of the Northern Hemisphere, would threaten the attainment
of current air quality targets.i
1-2. The total amount of O3 in the troposphere
is estimated to have increased by 36% since 1750, due primarily to anthropogenic
emissions of several O3-forming gases.ii This corresponds to a positive
radiative forcing of 0.35 Wm-2. O3 forcing varies considerably by region
and responds much more quickly to changes in emissionsiii than the long-lived
greenhouse gases, such as CO2.
2. Explanation of Terminology
2-1. Radiative Forcing
Radiative forcing is the perturbation to the balance of the energy budget
expressed as the amount of radiation per unit area at the tropopause.
This is caused by factors such as incoming solar radiation, atmospheric
greenhouse gas concentrations, and cloud quantity. The concept of radiative
forcing can be used to quantitatively compare the contribution of these
factors to climate change. Radiative forcing is also used as a gauge
for expressing the extent of the greenhouse effect.
Special Report on Emission Scenarios (from the IPCC)
3. Stratospheric Ozone and Tropospheric Ozone
Approximately 90% of global ozone is found in the stratosphere, and
the remaining 10% exists in the troposphere. Therefore, to prevent the
increased ultraviolet radiation at the Earth's surface resulting from
a 10% reduction in stratospheric ozone, for example, tropospheric ozone
would have to be increased by almost 100% (double) if it were to compensate
for the loss. Doubling tropospheric ozone would have detrimental effects
such as causing further damage to vegetation, adversely impacting human
health, and worsening the greenhouse effect. Therefore, it is unrealistic
to think that an increase in tropospheric ozone will compensate for
stratospheric ozone depletion. The environmental impact of higher tropospheric
ozone levels is expected to become an increasingly serious problem.
i The average hourly value in Japan is 60 ppb.
ii Ozone is formed by photochemical reactions in the atmosphere
of nitrogen oxides, hydrocarbons, and carbon monoxide.
iii The life of ozone is 1 to 2 weeks in summer, and 1 to
2 months in winter.