Environmental Engineering Reference
In-Depth Information
and subgrid scale processes that are not rigorously derived
from physical principles (although the overall form of the
parameterization might be derived from physical consid-
erations), but are derived from observed correlations. An
example of a parameterization would be to compute the
percentage of cloud cover in a given atmospheric grid box
based on the grid-averaged relative humidity and whether
the vertical motion is upward or downward. However,
because parameterizations are not rigorously derived, the
observed correlationmight not be applicable if the under-
lying conditions change - that is, as the climate changes.
such as changes in the amount and vertical distribution of
water vapour in the atmosphere, changes in the extent
of seasonal ice and snow, changes in the rate of variation
of temperature with height in the atmosphere (the so-
called lapse rate) and changes in the amount, location and
properties of clouds (Harvey, 2000: Chapter 3; Randall
et al ., 2007: Section 8.6). These characteristics of the
climate system all respond to changes in temperature
over a period of days to months. However, other changes
would occur over a period of decades to centuries, which
would lead to further changes in the radiative balance.
These include changes in the distribution and extent of
different plant types, changes in the extent of ice caps,
and changes in the concentration of CO 2 itself. These
are referred to as slow feedbacks. Most prominent are
a variety of positive feedbacks between climate and the
carbon cycle, including:
9.2.1 Radiative forcing, radiative feedbacksand
climatesensitivity
The climate of the Earth automatically adjusts itself so
as to achieve a balance (in the global average) between
the absorption of solar radiation and the emission of
infrared radiation to space. An increase in atmospheric
CO 2 concentration, or the addition of aerosols to the
atmosphere, changes climate by upsetting this balance;
any change in this balance is called the radiative forcing
[and has units of Wm 2 - the change in net radiation
(absorbed solar minus emitted infrared) averaged over
the entire area of the Earth]. If there is a radiation surplus,
the climate begins to warm, but as the climate warms the
emission of infrared radiation to space increases, reducing
the surplus. However, as soon as the climate begins to
warm, other things that affect the radiative balance also
change. Any process that adds to an initial imbalance is a
positive feedback, as a larger temperature is then required
in order to restore radiative balance (an example of a
positive feedback would be the increase in atmospheric
water vapour - a greenhouse gas - or the decrease in the
area of highly reflective snow and sea ice as the climate
warms). Conversely, any process that subtracts from the
initial imbalance is a negative feedback, as it results in
a smaller temperature change in response to the initial
radiative forcing (an example of a negative feedbackwould
be a hypothetical increase in the amount of low-level cloud
as the climate warms). The ratio of the long-term change
in global mean temperature to the radiative forcing is
called the climate sensitivity. However, the term is now
used by climatologists to refer to the eventual global
mean warming specifically for a fixed doubling in the
concentration of atmospheric CO 2 . The consensus for
the past 30 years is that the climate sensitivity is very likely
to fall between 1.5 C and 4.5 C.
The feedbacks that determine the climate sensitivity
are referred to as fast feedbacks. These involve things
1. The likely transition of the terrestrial biosphere from
a net sink (absorber) to a net source (emitter) of
CO 2 due to eventual adverse effects on increasing
temperature on photosynthesis and an increase in
the rate of respiration with increasing temperature
(Matthews et al ., 2007).
2. The catastrophic transition of carbon-rich biomes
(such as the Amazon rainforest) to grasslands due
to the transition to a dryer climate (Friedlingstein
et al ., 2006).
3. Substantial releases of CO 2 and CH 4 from thawing
permafrost (Khvorostyanov et al ., 2008; Schuur et al .,
2009; Dorrepaal et al ., 2009).
Thus, the long-term climate response to a doubling of
the CO 2 concentration caused by human emissions could
be substantially greater than the climate sensitivity based
on fast feedbacks alone because the CO 2 concentration
would not be limited to the initial CO 2 doubling (that is,
the assumption of a fixed CO 2 increase that underlies the
concept of climate sensitivity would be violated).
9.2.2 Earlycoupledatmosphere-oceangeneral
circulationmodels (AOGCMs)
The most detailed and realistic climate models are cou-
pled three-dimensional atmospheric and oceanic general
circulation models (AOGCMs). These models divide the
atmosphere and ocean into a horizontal grid with a typ-
ical resolution of 2-4 latitude by 2-4 longitude in the
latest models, with typically 10 to 20 layers in the vertical
in both the atmosphere and ocean. They directly simu-
late winds, ocean currents, and many other features and
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