Geoscience Reference
In-Depth Information
projected through the twenty-first century, the
effects of plate tectonics (operating on timescales
of millions of years) and Milankovich forcings
(operating on timescales of tens of thousands
of years) are irrelevant. Note also that while
Milankovich forcings are associated with very
significant impacts on the seasonal and spatial
distribution of solar radiation incident on the
surface, impacts on incident radiation when
globally averaged through the annual cycle are
quite small. For example, while a decrease in
obliquity means less summer radiation in the
Northern Hemisphere summer, it means more in
the Southern Hemisphere winter, with these
seasonal effects largely canceling out.
Milankovich forcings hence contrast funda-
mentally with the effects of changing solar
irradiance, volcanic eruptions, or human-induced
changes in atmospheric greenhouse gas concen-
trations and surface albedo, all of which, con-
sidered in terms of their immediate effect, have a
globally and annually averaged impact on the
radiation balance at the top of the atmosphere.
Because of this property, they are termed
radiative
forcings
. For example, an increase in solar output
will lead to more radiation incident to the top of
the earth's atmosphere, irrespective of latitude
or season. The immediate effect will be a globally
averaged radiation imbalance at the top of the
atmosphere (more energy coming in than going
out), leading to a rise in temperature that would
eventually bring the earth/atmosphere system
into a new radiative equilibrium. Similarly, the
immediate response to increasing the concentra-
tion of greenhouse gases will be a globally
averaged decrease in longwave emission to space,
a radiation imbalance promoting warming, also
eventually leading to a new radiative balance
(provided that the forcing remains constant).
Global climate change (change due to human
influences by our adopted conventions) is best
viewed in the context of global radiative forcing.
In the IPCC framework adopted here, radiative
forcing specifically refers to the amount by
which a factor alters the globally and annually
averaged radiation balance at the top of the
atmosphere, expressed in units of W m
-2
,
evaluated as forcing relative to the year 1750, the
start of the Industrial Revolution. In 2005, there
was an estimated radiative forcing from human
activities of 1.6W m
-2
.
2 Climate feedbacks
Building on the framework of radiative forcing,
consider further the change in global average
surface temperature resulting from increasing
the atmospheric concentration of carbon dioxide.
As just discussed, because of the imposed per-
turbation, more of the longwave radiation emitted
upward from the surface is absorbed by the
atmosphere, and directed back towards the
surface. The result is a radiation imbalance at the
top of the atmosphere - net solar radiation
entering the top of the atmosphere exceeds the
longwave loss to space. The climate forcing from
adding carbon dioxide is hence positive. Now
consider the feedbacks. The most important of
these is the water vapor feedback. Warming results
in more evaporation, and a warmer atmosphere
can carry more water vapor. However, water
vapor is also a greenhouse gas, so it causes further
warming. Some of the earth's snow cover and sea
ice will melt, reducing the earth's surface albedo,
also causing further warming. These are examples
of positive feedbacks, as they amplify the global
surface temperature change induced by the
climate forcing. If the carbon dioxide concentra-
tion in the atmosphere were lowered, thereby
imposing a negative climate forcing, the positive
feedbacks would foster further cooling.
A fascinating aspect of the global climate
system is that positive feedbacks dominate. For
example, one of the responses to increasing
greenhouse gases could be an increase in cloud
cover, which through increasing the planetary
albedo would represent a negative feedback.
However, this and other potential negative feed-
backs would only appear to be capable of slowing
the rate of warming, not reversing it.
