Geoscience Reference
In-Depth Information
equilibrium climate sensitivity of 0.75 implies
that this radiative forcing, if maintained, will
eventually yield about 1.2
o
C of warming. Over the
instrumental record, the global mean temperature
has risen by about 0.7
o
C, implying another 0.5
o
C
remaining after the ocean sufficiently heats up.
How much has the heat content of the ocean
already increased? Based on available hydro-
graphic data from 1955-1998, the world ocean
between the surface and 3000m depth gained ~1.6
×
ing changes in globally averaged temperature, it
must be stressed that this distinction may change
if adopting a different framework, such as the
evaluation of regional climate variability and
change. For example, due to loss of its sea ice
cover, rises in surface air temperature are expected
to be especially pronounced over the Arctic Ocean.
In the framework of human-induced global
climate change (see
Table 13.2
), this may be viewed
as part of the feedback process that amplifies the
global average temperature response to increased
greenhouse gas concentrations. However, if one
were to conduct a regional study of the Arctic, one
could legitimately view the sea ice loss as a forcing
on Arctic temperature change. Similarly, global
climate change may be attended by shifts in
patterns of atmospheric circulation, precipitation
and cloud cover. While on the global scale these
would be viewed as feedbacks, investigations of
regional impacts could view them as forcings.
Another framework issue regards how one
views transitions between glacial and interglacial
conditions. While changes in ice sheet area and
greenhouse gas concentrations during these
transitions are appropriately viewed as slow feed-
backs, if one considers full glacial and interglacial
conditions as two equilibrium states, these slow
feedbacks may instead be thought of as climate
forcings. With estimates of the global tempera-
ture change between the equilibrium states
and the forcings, one then has another way to
estimate equilibrium climate sensitivity. Numbers
obtained from this approach turn out to agree
fairly well with those coming from global climate
models. Hence, in summary, depending on the
chosen framework, one person's feedback may be
another person's forcing.
10
22
J. Compared with atmospheric kinetic
energy (p. 70), this is a very large number.
An obvious shortcoming of the concept of
equilibrium climate sensitivity is that radiative
forcing is always changing. Consider explosive
volcanic eruptions. While the global radiative
forcing from a single eruption can be very
significant (2-3W m
-2
at peak), the forcing is
short-lived (several years) such that the system
can never come into equilibrium with it (while the
global surface temperature can be temporarily
reduced by several tenths of a degree, this is much
smaller than the calculated temperature change in
equilibrium with the peak forcing). Similarly, the
system could never be in equilibrium with solar
variability associated with the 11-year sunspot
cycle. If we were to somehow freeze the current
radiative forcing from human activities at its
present value, the climate system would eventually
approach a new temperature in equilibrium
with it (assuming no complications like multiple
volcanic eruptions). However, radiative forcing
from human activities has grown over the past
century and will continue to grow in the future,
meaning that the equilibrium temperature value
has changed and will continue to change. Put
differently, the picture over the past 100 years and
into the future is a climate system constantly
trying to catch up with a growing radiative forcing
but always lagging behind it.
C
THE CLIMATIC RECORD
1 The geological record
Understanding the significance of climatic trends
over the past 100 years requires that they be
viewed against the backdrop of earlier conditions.
4
The importance of framework
While the distinction between climate forcing and
feedback is fairly straightforward when consider-
