Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 45

The Emission Scenario of the Special Report on Emission Scenarios (SRES)
A1. The A1 storyline and scenario family describes a future world of very rapid economic growth, global
population that peaks in mid-century and declines thereafter and the rapid introduction of new and more efficient
technologies. Major underlying themes are convergence among regions, capacity building and increased cultural
and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario
family develops into three groups that describe alternative directions of technological change in the energy
system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil
energy sources (A1T), or a balance across all sources (A1B) (where balanced is defined as not relying too heavily
on one particular energy source, on the assumption that similar improvement rates apply to all energy supply and
end-use technologies).
A2. The A2 storyline and scenario family describes a very heterogeneous world. The underlying theme is self-
reliance and preservation of local identities. Fertility patterns across regions converge very slowly, which results
in continuously increasing population. Economic development is primarily regionally oriented and per capita
economic growth and technological change more fragmented and slower than other storylines.
B1. The B1 storyline and scenario family describes a convergent world with the same global population that
peaks in mid-century and declines thereafter, as in the A1 storyline, but with rapid change in economic structures
toward a service and information economy, with reductions in material intensity and the introduction of clean and
resource-efficient technologies. The emphasis is on global solutions to economic, social and environmental
sustainability, including improved equity, but without additional climate initiatives.
B2. The B2 storyline and scenario family describes a world in which the emphasis is on local solutions to
economic, social and environmental sustainability. It is a world with continuously increasing global population, at
a rate lower than A2, intermediate levels of economic development, and less rapid and more diverse technological
change than in the A1 and B1 storylines. While the scenario is also oriented towards environmental protection and
social equity, it focuses on local and regional levels.
An illustrative scenario was chosen for each of the six scenario groups A1B, A1FI, A1T, A2, B1 and B2. All should
be considered equally sound.
The SRES scenarios do not include additional climate initiatives, which means that no scenarios are included that
explicitly assume implementation of the United Nations Framework Convention on Climate Change or the
emission targets of the Kyoto protocol.
Figure 3.6 Description of the SRES emission scenarios ( Climate Change 2007: The
Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report
of the Intergovernmental Panel on Climate Change (eds S. Solomon, D. Qin, M. Manning,
et al .). Cambridge University Press, Cambridge and New York.)
There is now much evidence for these observed trends and potential future
changes to freshwaters as a result of climate change. Here, we present a series of
examples and case studies based on the analysis of long-term data series, field
experiments and physical modelling.
Thermal regimes of lakes and streams
The thermal regime of water bodies is mainly determined by the local weather.
The net heat exchange across the air-water interface is given by the sum of energy
fluxes related to radiation, latent and sensible heat (Edinger et al . 1968; Imboden &
Wüest 1995). A shift in climate variables such as air temperature, radiation, cloud
cover, wind or humidity will influence these heat fluxes and thus alter the heat
balance of lakes and rivers. Model studies predict that lake temperatures, especially
in the epilimnion, will increase with increasing air temperature, so that temperature
profiles, thermal stability and mixing patterns are expected to change as a result
of climate change (e.g. Hondzo & Stefan 1993; Stefan et al . 1998).