14.1.2.2. Future: Climate Scenarios
Climate modeling has proven to be extremely useful in building projections
for climate change and scenarios of future climate under different forcings.
General circulation models have demonstrated their ability to simulate realistically
the large-scale features of observed climate; hence, they are widely used to
assess the impact that increased loading of the atmosphere with greenhouse and
other gases might have on the climate system. Although there are differences
among models with regard to the way they represent the climate system processes,
all of them yield comparable results on a global basis. However, they have difficulty
in reproducing regional climate patterns, and large discrepancies exist among
models. In several regions of the world, distributions of surface variables
such as temperature and rainfall often are influenced by the local effects of
topography and other thermal contrasts, and the coarse spatial resolution of
GCMs cannot resolve these effects. Consequently, large-scale GCM scenarios should
not be used directly for impact studies, especially at the regional and local
levels (von Storch, 1994); downscaling techniques are required (see TAR
WGI Chapters 10 and 13).
At the large scale, rates of mean annual temperature changes in the Latin American
region for the next century are projected to be 0.2-2°C (Carter and
Hulme, 2000) under the low-emissions scenario (B1) produced as part of the IPCC
Special Report on Emission Scenarios (SRES). The warming rate could range
between 2 and 6°C for the higher emissions case (A2). Most GCMs produce
similar projections for temperature changes on a global basis; projected changes
in precipitation remain highly uncertain.
For impact studies, it is crucial to have a projection of concurrent changes
of temperature and precipitation at the regional scale. Various scenarios of
climate change for Latin America have been put forward on the basis of GCM projections
under the IS92a scenario. Most of these regional scenarios are based on GCM
experiments that are downscaled through statistical techniques. Derived climate
change scenarios for Mexico suggest that climate in Mexico will be drier and
warmer (Perez, 1997). Several hydrological regions in Mexico are highly vulnerable
to decreased precipitation and higher temperatures (Mendoza et al., 1997).
A regional climate change scenario for central Argentina in response to CO2
doubling under the IS92 scenario for the year 2050, also obtained through a
statistical downscaling approach, shows a smaller increase in minimum temperature
as compared to the maximum and larger increases for summer than for winter months,
which generates enhanced temperature amplitudes (Solman and Nuñez, 1999).
In addition, a decrease in precipitation is projected over the region, which
is larger for summer (12%) than for winter months (5%). This result highlights
an important consequence in the rainfall regime over the region: A large decrease
in rainfall projected for the rainy season will seriously affect soil moisture,
hence agricultural production in the region.
Several climate change scenarios for other parts of Latin America rely on linear
interpolation of GCM output to estimate increases in surface temperature and
precipitation (Mata, 1996; Carril et al., 1997; Hofstadter and Bidegain,
1997, Paz Rada et al., 1997; Centella et al., 1998; MARENA, 2000).
In the case of Costa Rica (MINAE-IMN, 2000), under the IS92a scenario for the
year 2100, the results show a small increase in precipitation for the southeastern
Caribbean region and an important decrease—close to 25%—in the northwestern
Pacific region. This latter region already experiences water problems as a result
of El Niño and an increasing demand from infrastructure for tourism and
irrigation. Under the same climate scenario, mean temperature in Costa Rica
is expected to rise by more than 3°C by 2100, and tendencies in actual climate
series (1957-1997) show already an increase of 0.4°C every 10 years
for the more continental Central Valley areas. This last estimation may reflect
signals other than the one related to climate change.
Results from climate scenarios for Nicaragua imply an additional pressure on
productivity sectors and human activities. Under the IS92a emissions scenario,
mean temperature for the Pacific watershed would be expected to rise, ranging
from 0.9 for the year 2010 to 3.7°C for the year 2100, and precipitation
would decrease by 8.4% for the year 2010 and 36.6% for the year 2100. For the
Caribbean watershed, mean temperature would increase, ranging from 0.8°C
for the year 2010 to 3.3°C for the year 2100, and precipitation would decrease
in a range between 8.2% for the year 2010 and 35.7% for the year 2100 (MARENA,
2000).
Potential effects of climate change in Brazil suggest changes of 4-4.5°C
in surface temperature as a result of increased CO2 concentrations
(de Siqueira et al., 1994, 1999). Central and south-central Brazil may
experience increases of 10-15% in autumn rainfall; reductions could appear
during December, with high risk of drought during summer, affecting crops (see
Table 14-1).
Analysis of climate variations during the instrumental period and evidence
suggested by paleoclimatic and other proxy climate information suggests that
climate variations and change have been found in several regions in Latin America.
Most climate records cover the past century; at this time scale, there have
been indications of multidecadal and interannual variability, some linked to
extremes of the Southern Oscillation. The lack of continuous and long-term records
from the past does not allow one to identify climate patterns with a high degree
of confidence to determine whether these climates were similar to or much different
from that of present times—particularly with respect to the frequency and
intensity of extreme events such as drought, floods, freezes, heat waves, and
especially hurricanes and tropical storms. However, multidecadal variations
have been identified in rainfall and streamflow records in the region, although
no clear unidirectional trend indicators of climate change have been identified.
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