1.2 Natural Climate Variations
1.2.1 Natural Forcing of the Climate System
The Sun and the global energy balance
The ultimate source of energy that drives the climate system is radiation from
the Sun. About half of the radiation is in the visible short-wave part of the
electromagnetic spectrum. The other half is mostly in the near-infrared part,
with some in the ultraviolet part of the spectrum. Each square metre of the
Earth’s spherical surface outside the atmosphere receives an average throughout
the year of 342 Watts of solar radiation, 31% of which is immediately reflected
back into space by clouds, by the atmosphere, and by the Earth’s surface.
The remaining 235 Wm-2 is partly absorbed by the atmosphere but most
(168 Wm-2) warms the Earth’s surface: the land and the ocean.
The Earth’s surface returns that heat to the atmosphere, partly as infrared
radiation, partly as sensible heat and as water vapour which releases its heat
when it condenses higher up in the atmosphere. This exchange of energy between
surface and atmosphere maintains under present conditions a global mean temperature
near the surface of 14°C, decreasing rapidly with height and reaching a
mean temperature of –58°C at the top of the troposphere.
For a stable climate, a balance is required between incoming solar radiation
and the outgoing radiation emitted by the climate system. Therefore the climate
system itself must radiate on average 235 Wm-2 back into space. Details
of this energy balance can be seen in Figure 1.2, which shows
on the left hand side what happens with the incoming solar radiation, and on
the right hand side how the atmosphere emits the outgoing infrared radiation.
Any physical object radiates energy of an amount and at wavelengths typical
for the temperature of the object: at higher temperatures more energy is radiated
at shorter wavelengths. For the Earth to radiate 235 Wm–2, it
should radiate at an effective emission temperature of -19°C with typical
wavelengths in the infrared part of the spectrum. This is 33°C lower than
the average temperature of 14°C at the Earth’s surface. To understand
why this is so, one must take into account the radiative properties of the atmosphere
in the infrared part of the spectrum.
Figure 1.2: The Earth’s annual and global mean energy balance.
Of the incoming solar radiation, 49% (168 Wm-2) is absorbed by
the surface. That heat is returned to the atmosphere as sensible heat, as
evapotranspiration (latent heat) and as thermal infrared radiation. Most
of this radiation is absorbed by the atmosphere, which in turn emits radiation
both up and down. The radiation lost to space comes from cloud tops and
atmospheric regions much colder than the surface. This causes a greenhouse
effect. Source: Kiehl and Trenberth, 1997: Earth’s Annual Global Mean
Energy Budget, Bull. Am. Met. Soc. 78, 197-208. |
The natural greenhouse effect
The atmosphere contains several trace gases which absorb and emit infrared radiation.
These so-called greenhouse gases absorb infrared radiation, emitted by the Earth’s
surface, the atmosphere and clouds, except in a transparent part of the spectrum
called the “atmospheric window”, as shown in Figure
1.2. They emit in turn infrared radiation in all directions including downward
to the Earth’s surface. Thus greenhouse gases trap heat within the atmosphere.
This mechanism is called the natural greenhouse effect. The net result is an
upward transfer of infrared radiation from warmer levels near the Earth’s
surface to colder levels at higher altitudes. The infrared radiation is effectively
radiated back into space from an altitude with a temperature of, on average,
-19°C, in balance with the incoming radiation, whereas the Earth’s
surface is kept at a much higher temperature of on average 14°C. This effective
emission temperature of -19°C corresponds in mid-latitudes with a height
of approximately 5 km. Note that it is essential for the greenhouse effect that
the temperature of the lower atmosphere is not constant (isothermal) but decreases
with height. The natural greenhouse effect is part of the energy balance of
the Earth, as can be seen schematically in Figure 1.2.
Clouds also play an important role in the Earth’s energy balance and in
particular in the natural greenhouse effect. Clouds absorb and emit infrared
radiation and thus contribute to warming the Earth’s surface, just like
the greenhouse gases. On the other hand, most clouds are bright reflectors of
solar radiation and tend to cool the climate system. The net average effect
of the Earth’s cloud cover in the present climate is a slight cooling:
the reflection of radiation more than compensates for the greenhouse effect
of clouds. However this effect is highly variable, depending on height, type
and optical properties of clouds.
This introduction to the global energy balance and the natural greenhouse effect
is entirely in terms of the global mean and in radiative terms. However, for
a full understanding of the greenhouse effect and of its impact on the climate
system, dynamical feedbacks and energy transfer processes should also be taken
into account. Chapter 7 presents a more detailed analysis
and assessment.
Radiative forcing and forcing variability
In an equilibrium climate state the average net radiation at the top of the
atmosphere is zero. A change in either the solar radiation or the infrared radiation
changes the net radiation. The corresponding imbalance is called “radiative
forcing”. In practice, for this purpose, the top of the troposphere (the
tropopause) is taken as the top of the atmosphere, because the stratosphere
adjusts in a matter of months to changes in the radiative balance, whereas the
surface-troposphere system adjusts much more slowly, owing principally to the
large thermal inertia of the oceans. The radiative forcing of the surface troposphere
system is then the change in net irradiance at the tropopause after allowing
for stratospheric temperatures to re-adjust to radiative equilibrium, but with
surface and tropospheric temperatures and state held fixed at the unperturbed
values. A detailed explanation and discussion of the radiative forcing concept
may be found in Appendix 6.1 to Chapter
6.
External forcings, such as the solar radiation or the large amounts of aerosols
ejected by volcanic eruption into the atmosphere, may vary on widely different
time-scales, causing natural variations in the radiative forcing. These variations
may be negative or positive. In either case the climate system must react to
restore the balance. A positive radiative forcing tends to warm the surface
on average, whereas a negative radiative forcing tends to cool it. Internal
climate processes and feedbacks may also cause variations in the radiative balance
by their impact on the reflected solar radiation or emitted infrared radiation,
but such variations are not considered part of radiative forcing. Chapter
6 assesses the present knowledge of radiative forcing and its variations,
including the anthropogenic change of the atmospheric composition.
