3.4. Environmental Scenarios
Observations during the 20th century have demonstrated clearly the multifaceted
nature of anthropogenic environmental changes. Therefore, it is reasonable to
expect that changes in climate anticipated for the future will occur in combination
with other changes in the environment. Some of these changes will occur independently
of climate change (e.g., groundwater depletion, acidification); others are a
cause of climate change (e.g., changing atmospheric CO2 concentration);
and still others are a direct consequence of climate change (e.g., sea-level
rise). All of these could have a role in modifying the impacts of future climate
change. Hence, realistic scenarios of nonclimatic environmental factors are
required to facilitate analysis of these combined effects and quantify them
in impact assessments.
This section introduces environmental changes that are of importance at scales
from subcontinental to global and describes how scenarios commonly are constructed
to represent them. Requirements for environmental scenarios are highly application-
and region-specific. For example, scenarios of CO2 concentration
may be important in considering future vegetation productivity under a changing
climate but are unlikely to be required for assessment of human health impacts.
Most of the scenarios treated here relate to atmospheric composition: CO2,
SO2, sulfur and nitrogen deposition, tropospheric O3,
and surface UV-B radiation. Scenarios of water resources and marine pollution
also are examined. Changes in the terrestrial environment are addressed in Section
3.3, and changes in sea level are addressed in Section
3.4.2. CO2 Scenarios
126.96.36.199. Reference Conditions
Aside from its dominant role as a greenhouse gas, atmospheric CO2
also has an important direct effect on many organisms, stimulating photosynthetic
productivity and affecting water-use efficiency in many terrestrial plants.
In 1999, the concentration of CO2 in the surface layer of the atmosphere
(denoted as [CO2]) was about 367 ppm (see Table 3-2),
compared with a concentration of approximately 280 ppm in preindustrial times
(see TAR WGI Chapter 3). CO2
is well mixed in the atmosphere, and, although concentrations vary somewhat
by region and season (related to seasonal uptake by vegetation), projections
of global mean annual concentrations usually suffice for most impact applications.
Reference levels of [CO2] between 300 and 360 ppm have been widely
adopted in CO2-enrichment experiments (Cure and Acock, 1986; Poorter, 1993;
see Table 3-2) and in model-based impact studies.
[CO2] has increased rapidly during the 20th century, and plant growth
response could be significant for responsive plants, although the evidence for
this from long-term observations of plants is unclear because of the confounding
effects of other factors such as nitrogen deposition and soil fertility changes
(Kirschbaum et al., 1996).
188.8.131.52. Development and Application of [CO2]
Projections of [CO2] are obtained in two stages: first, the rate
of emissions from different sources is evaluated; second, concentrations are
evaluated from projected emissions and sequestration of carbon. Because CO2
is a major greenhouse gas, CO2 emissions have been projected in successive
IPCC scenarios (Scenarios A-DShine et al., 1990; IS92 scenariosLeggett
et al., 1992; SRES scenariosNakicenovic et al., 2000). To
obtain scenarios of future [CO2] from those of emissions, global
models of the carbon cycle are required (e.g., Schimel et al., 1995).
Some estimates of [CO2] for the SRES emissions scenarios are given
in Table 3-2.
In recent years, there has been growing interest in emissions scenarios that
lead to [CO2] stabilization (see Section 3.8.4).
Typically, levels of [CO2] stabilized between 350 and 1000 ppm have
been examined; these levels usually are achieved during the 22nd or 23rd century,
except under the most stringent emissions targets (Schimel et al., 1997a).
Work to develop storylines for a set of stabilization scenarios is reported
in Chapter 2 of WGIII. Whatever scenarios emerge, it is
likely to be some time before a set of derivative CO2-stabilization
impact and adaptation assessments are completed, although a few exploratory
studies already have been conducted (UK-DETR, 1999).
Experimental CO2-enrichment studies conventionally compare responses
of an organism for a control concentration representing current [CO2]
with responses for a fixed concentration assumed for the future. In early studies
this was most commonly a doubling (Cure and Acock, 1986), to coincide with equilibrium
climate model experiments (see Section 3.5). However,
more recent transient treatment of future changes, along with the many uncertainties
surrounding estimates of future [CO2] and future climate, present
an infinite number of plausible combinations of future conditions. For example,
Table 3-2 illustrates the range of [CO2]
projected for 2050 and 2100 under the SRES emissions scenarios, using simple
models. To cover these possibilities, although doubled [CO2] experiments
are still common, alternative concentrations also are investigated (Olesen,
1999)often in combination with a range of climatic conditions, by using
devices such as temperature gradient tunnels (Wheeler et al., 1996).