Figure 5-5: The recent net
uptake of carbon on the land is partly due to enhanced CO2 uptake
through plant growth, with a delay before this carbon is returned to the
atmosphere via the decay of plant material and soil organic matter.
Several processes contribute to the enhanced plant growth: changes in land
use and management, fertilizing effects of elevated CO2 and nitrogen,
and some climate changes (such as a longer growing season at high latitudes).
A range of models (identified by their acronyms in the figure) project a
continued increase in the strength of the net carbon uptake on land for
several decades, then a leveling off or decline late in the 21st century
for reasons explained in the text. The model results illustrated here arise
from the IS92a scenario, but similar conclusions are reached using other
withsociety'sphysicalinfrastructure, institutions, and the technologies
embodied within them, and the combined system evolves relatively slowly.
This is obvious, for instance, in relation to the impact of urban design
and infrastructure on energy consumption for heating, cooling, and transport.
Markets sometimes "lock in" to technologies and practices that
are sub-optimal because of the investment in supporting infrastructure,
which block out alternatives. Diffusion of many innovations comes up against
people's traditional preferences and other social and cultural barriers.
Unless advantages are very clear, social or behavioral changes on the part
of technology users may require decades. Energy use and greenhouse gas
are peripheral interests
in most people's everyday lives. Their consumption patterns are driven
not only by demographic, economic and technological change, resource availability,
infrastructure, and time constraints, but also by motivation, habit, need,
compulsion, social structures, and other factors.
WGIII TAR Sections 3.2, 3.8.6,
5.2-3, & 10.3,
SRTT SPM, & SRTT Chapter 4 ES
||Social and economic time scales are not
fixed: They are sensitive to social and economic forces, and could be changed
by policies and the choices made by individuals. Behavioral and technological
changes can occur rapidly under severe economic conditions. For example,
the oil crises of the 1970s triggered societal interest in energy conservation
and alternative sources of energy, and the economy in most Organisation
for Economic Cooperation and Development (OECD) countries deviated strongly
from the traditional tie between energy consumption and economic development
growth rates (see Figure 5-6). Another example
is the observed reduction in CO2 emissions caused by the disruption
of the economy of the Former Soviet Union (FSU) countries in 1988. The response
in both case was very rapid (within a few years). The converse is also apparently
true: In situations where pressure to change is small, inertia is large.
This has implicitly been assumed to be the case in the SRES scenarios, since
they do not consider major stresses, such as economic recession, large-scale
conflict, or collapses in food stocks and associated human suffering, which
are inherently difficult to forecast.
TAR Chapter 2,
WGIII TAR Sections 3.2
10.1.4.3, & WGII SAR Section
||Stabilization of atmospheric CO2
concentration at levels below about 600 ppm is only possible with reductions
in carbon intensity and/or energy intensity greater than have been achieved
historically. This implies shifts toward alternative development
pathways with new social, institutional, and technological configurations
that address environmental constraints. Low historical rates of improvement
in energy intensity (energy use per unit GDP) reflect the relatively low
priority placed on energy efficiency by most producers and users of technology.
By contrast, labor productivity increased at higher rates over the period
1980 to 1992. The historically recorded annual rates of mprovement of global
energy intensity (1 to 1.5% per year) would have to be increased and maintained
over long time frames to achieve stabilization of CO2 concentrations
at about 600 ppm or below (see Figure 5-7).
Carbon intensity (carbon per unit energy produced) reduction rates would
eventually have to change by even more (e.g., up to 1.5% per year (the historical
baseline is 0.3 to 0.4% per year)). In reality, both energy intensity and
carbon intensity are likely to continue to improve, but greenhouse gas stabilization
at levels below 600 ppm requires that at least one of them do so at a rate
much higher than historically achieved. The lower the stabilization target
and the higher the level of baseline emissions, the larger the CO2
divergence from the baseline that is needed, and the earlier it would need
WGI TAR Section 184.108.40.206, WGIII
TAR Section 2.5, & SRES
||Some climate, ecological, and socio-economic
system changes are effectively irreversible over many human lifetimes, and
others are intrinsically irreversible.
||There are two types of apparent irreversibility.
"Effective irreversibility" derives from processes that
have the potential to return to their pre-disturbance state, but take centuries
to millennia to do so. An example is the partial melting of the Greenland
ice sheet. Another is the projected rise in mean sea level, partly as a
result of melting of the cryosphere, but primarily due to thermal expansion
of the oceans. The world is already committed to some sea-level rise as
a consequence of the surface atmospheric warming that has occurred over
the past century. "Intrinsic irreversibility" results from crossing
a threshold beyond which the system no longer spontaneously returns to the
previous state. An example of an intrinsically irreversible change due to
crossing a threshold is the extinction of species, resulting from a combination
of climate change and habitat loss.
WGI TAR Chapter 11, WGII
TAR Chapter 5, & WGII
TAR Sections 16.2.1 & 17.2.5
Figure 5-6: The response of the energy system,
as indicated by the emission of CO2 (expressed as carbon), to
economic changes, indicated by GDP (expressed in Purchasing Power Parity
(PPP) terms). The response can be almost without inertia if the shock
is large. The "oil crisis" -- during which energy prices rose
substantially over a short period of time -- led to an almost immediate
and sustained divergence of the formerly closely linked emissions and GDP
in most developed countries: Japan and United States are shown as examples.
At the breakup of the Former Soviet Union, the two indicators remained closely
linked, leading the emission to drop rapidly in tandem with declining GDP.
WGIII TAR Table 3.1 &
WGII SAR Figure 20-1