Because we had limited funding for NARCCAP, we decided to focus on
the uncertainty across different AOGCMs and RCMs. Hence, we chose to
use only one emissions scenario, which is used for all simulations.
We chose the A2 emissions scenario since it was one of the 'marker'
scenarios developed through the IPCC and was a common one used at the
time NARCCAP was being planned. The scenario is one described in
Nakicenvoic et al. (2000) in the Special
Report on Emissions Scenarios (SRES) that was commissioned by
the IPCC. Some of the scenarios described in that volume have been
used in the climate model simulations assessed in both the IPCC 2001
and 2007 reports.
The A2 scenario is at the higher end of the SRES emissions
scenarios (but not the highest), and this was preferred because, from
an impacts and adaptation point of view, if one can adapt to a larger
climate change, then the smaller climate changes of the lower end
scenarios can also be adapted to. A low emissions scenario potentially
gives less information from an impacts and adaptation point of view.
In addition, the current actual trajectory of emissions (1990 to
present) corresponds to a relatively high emissions scenario.
We present here a brief summary of the major characteristics of the
scenario. The SRES scenarios were developed by considering various
possible futures of world development in the 21st century, including
such factors as economic development, technological development,
energy use, population change, and land-use change. Four major story
lines or about world development were developed, which were quantified
into four scenarios families, and a total of 40 different scenarios
across the four story lines/families were constructed. The authors of
the SRES scenarios consider all scenarios to be equally plausible and
they did not assign any probabilities to them.
The A2 story line is characterized by heterogeneity. Self reliance
and local identities are emphasized, and population increases
continuously. Population reaches over 10 billion by 2050. Economic
development is regionally oriented and economic and technological
development is relatively slow, compared to the other story lines.
From these major factors, and using Integrated Assessment Models
(IAMs), emissions of the major greenhouse gases were developed for the
21st century. Cumulative CO2 emissions by the middle and
end of the 21st century are projected to be about 600 and 1850 GtC
respectively, and expected CO2 concentrations (in parts per
million, ppm) for the middle and end of the 21st century in this
scenario are about 575 and 870 ppm, respectively. The current
concentration of CO2 is about 380 ppm. Methane and nitrous
oxide increases grow rapidly in the 21st century. Sulfur dioxide
increases to a maximum value just before 2050 (105 MtS/yr) and then
decreases in the second half of the century (60 MtS/yr by 2100).
Nakicenovic et al., 2000. Special Report on Emissions Scenarios.
A Special Report of Working Group III of the Intergovernmental Panel
on Climate Change. Cambridge University Press: Cambridge. 599