chapter - Atmospheric and Oceanic Science

12.1. Introduction
Climate scenarios
Climate scenarios are representations about the future, consistent with
assumptions on future greenhouse gases (GHG) emissions and other pollutants and
with the scientific updated knowledge about the effect that the increase in the concentration
of these gases will have on the global climate. Hence, they describe how
given certain future human activities, the composition of the atmosphere and consequently
global climate are expected to change. Therefore, climate scenarios are a
guide on how climate could be in the upcoming decades, according to a set of
assumptions that include: future trends on energy demand, GHG emissions,
changes in the land use and approximations to the physical laws that rule the behavior
of the climatic system over long periods of time. It is important to bear in mind
that the uncertainty that surrounds these assumptions is large and will determine the
range of possible scenarios. With this information, it is intended to estimate how
both natural systems and human activities will be affected.
The election of climate scenarios for the evaluation of impacts is important
since the extreme scenarios may produce extreme impacts while more moderate
scenarios probably show more modest impacts. In consequence, the election of scenarios
can be controversial unless the uncertainties inherent to future projections
are adequately specified in the impact analyses.
The scenarios of emissions, based on assumptions about possible social and
economic evolutions of the world, feed the projections of the GHG concentrations.
They were made by request of the Intergovernmental Panel for Climate Change
(IPCC) and constitute the base upon most scenarios for future climate were made.
12.2. Criteria for the selection of regional climate scenarios
For climate scenarios to be useful for the assessment of impact and decision
making it is advisable that their elaboration comply with five basic criteria:
Consistency with global projections: When the scenarios are not developed
from global scenarios, they must be consistent with the wide range of global warming
projections based on the increase of GHG concentrations. This range varies
between 1.4º and 5.8ºC (IPCC 2001).
Physical plausibility: They should be physically plausible; this means that
they must not violate basic physical laws. Therefore, changes in a region must be
consistent with those in other regions and globally. In addition, the combination of
changes in different variables (which frequently are correlated with one another)
must be physically consistent.
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