Feedbacks in the climate system

A Saharan dust storm originating in Mali blew off the west coast of Africa on 6 June 2006. Although partially hidden by the dust storm, the differences of the underlying landscape are still apparent as the sands of the Sahara give way to vegetation of the south. The Sahel is particularly vulnerable to desertification - land degradation from climate change and/or human activity that transforms a region to a desert.

increase, the actual rise in global average temperature was likely to be more than doubled to about 3.0°C. This section lists the most important of these feedbacks.

Water vapour feedback

This is the most important.12 With a warmer atmosphere more evaporation occurs from the ocean and from wet land surfaces. On average, therefore, a warmer atmosphere will be a wetter one; it will possess a higher water vapour

Bilder Als Vollbild Darstellen
Figure 5.13 Schematic of the climate system.

content. Since water vapour is a powerful greenhouse gas, its potential feedback has been very thoroughly studied. It is found to provide on average a positive feedback of a magnitude that models estimate to approximately double the increase in the global average temperature that would arise with fixed water vapour.13

Cloud-radiation feedback

This is more complicated as several processes are involved. Clouds interfere with the transfer of radiation in the atmosphere in two ways (Figure 5.14). Firstly, they reflect a certain proportion of solar radiation back to space, so reducing the total energy available to the system. Secondly, they act as blankets to thermal radiation from the Earth's surface in a similar way to greenhouse gases. By absorbing thermal radiation emitted by the Earth's surface below, and by themselves emitting thermal radiation, they act to reduce the heat loss to space by the surface.

The effect that dominates for any particular cloud depends on the cloud temperature (and hence on the cloud height) and on its detailed optical properties (those properties which determine its reflectivity to solar radiation and its interaction with thermal radiation). The latter depend on whether the cloud is of water or ice, on its liquid or solid water content (how thick or thin it is) and on the average size of the cloud particles. In general for low clouds the reflectivity effect wins so they tend to cool the Earth-atmosphere system; for high clouds, by contrast, the blanketing effect is dominant and they tend to warm the system. The overall feedback effect of clouds, therefore, can be either positive or negative (see box below).

Climate is very sensitive to possible changes in cloud amount or structure, as can be seen from the results of models discussed in later chapters. To illustrate this, Table 5.1 shows that the hypothetical effect on the climate of a small percentage change in cloud cover is comparable with the expected changes due to a doubling of the carbon dioxide concentration.

Ocean-circulation feedback

The oceans play a large part in determining the existing climate of the Earth; they are likely therefore to have an important influence on climate change due to human activities.

The oceans act on the climate in four important ways. Firstly, there are close interactions between the ocean and the atmosphere; they behave as a strongly coupled system. As we have already noted, evaporation from the oceans provides the main source of atmospheric water vapour which, through its latent heat of condensation in clouds, provides the largest single heat source for the atmosphere. The atmosphere in its turn acts through wind stress on the ocean surface as the main driver of the ocean circulation.

Secondly, they possess a large heat capacity compared with the atmosphere; in other words a large quantity of heat is needed to raise the temperature of the oceans only slightly. In comparison, the entire heat capacity of the atmosphere is equivalent to less than 3 m depth of water. That means that in a world

Blanketing of thermal radiation

Precipitation

Blanketing of thermal radiation

Precipitation

' ' i ' '< i r t i llllflfl/lflfl/lflf '///'/'/.V'/'///'/' /// / /// / /// /

Evaporation

Boundary layer

Figure 5.14 Schematic of the physical processes associated with clouds.

Cloud radiative forcing

A concept helpful in distinguishing between the two effects of clouds mentioned in the text is that of cloud radiative forcing (CRF). Take the radiation leaving the top of the atmosphere above a cloud; suppose it has a value R. Now imagine the cloud to be removed, leaving everything else the same; suppose the radiation leaving the top of the atmosphere is now R'. The difference R' - R is the cloud radiative forcing. It can be separated into solar radiation and thermal radiation components that generally act in opposite senses, each typically of magnitude between 50 and 100 W m- 2. On average, it is found that clouds tend slightly to cool the Earth-atmosphere system.

A map of cloud radiative forcing (Figure 5.15a) deduced from satellite observations illustrates the large variability in CRF over the globe with both positive and negative values. It is also helpful to study separately the shortwave and longwave components of the atmosphere's radiation budget (Figure 5.15b), the variations of which are dominated by variations in cloud cover and type. Model simulations are able to capture the overall pattern of these variations; the big challenge is to simulate the changing pattern with adequate detail and accuracy (see Question 8). It is through careful comparisons with observations that progress in the understanding of cloud feedback will be achieved.

Global Warming
Net cloud radiative forcing (W m-2)

Figure 5.15 (a) Annual mean net cloud radiative forcing (CRF) for the period March 2000 to February 2001 as observed by the CERES instrument on the NASA Terra satellite. (b) Comparison of the observed longwave (pink/red), shortwave (orange) and net radiation at the top of the atmosphere for the tropics (20° N-20° S) as deviation from the mean for 1985-90; data from the ERBE instrument on the ERBS satellite and the CERES instrument on the TRMM satellite. Note the influence of the eruption of Pinatubo volcano.

Table 5.1 Estimates of global average temperature changes under different assumptions about changes in greenhouse gases and clouds

Greenhouse gases

Clouds

Change (in °C) from current average global surface temperature of 15 °C

As now

As now

0

None

As now

-32

None

None

-21

As now

None

4

As now

As now but +3% high cloud

0.3

As now

As now but +3% low cloud

-1.0

Doubled CO2 concentration otherwise as now

As now (no additional cloud feedback)

1.2

Doubled CO2 concentration + best estimate of feedbacks

Cloud feedback included

3

that is warming, the oceans warm much more slowly than the atmosphere. We experience this effect of the oceans as they tend to reduce the extremes of atmospheric temperature. For instance, the range of temperature change both during the day and seasonally is much less at places near the coast than at places far inland. The oceans therefore exert a dominant control on the rate at which atmospheric changes occur.

Thirdly, through their internal circulation the oceans redistribute heat throughout the climate system. The total amount of heat transported from the equator to the polar regions by the oceans is similar to that transported by the atmosphere. However, the regional distribution of that transport is very different (Figure 5.16). Even small changes in the regional heat transport by the oceans could have large implications for climate change. For instance, the amount of heat transported by the north Atlantic Ocean is over 1000 terawatts (1 terawatt = 1 million million watts = 1012 watts). To give an idea of how large this is, we can note that a large power station puts out about 1000 million (109) watts and the total amount of commercial energy produced globally is about 12 terawatts. To put it further in context, considering the region of the north Atlantic Ocean between northwest Europe and Iceland, the heat input (Figure 5.16) carried by the ocean circulation is of similar magnitude to that reaching the ocean surface there from the incident solar radiation. Any accurate simulation of likely climate change, therefore, especially of its regional variations, must include a description of ocean structure and dynamics.

Ice-albedo feedback

An ice or snow surface is a powerful reflector of solar radiation (the albedo is a measure of its reflectivity). As some ice melts, therefore, at the warmer surface, solar radiation which had previously been reflected back to space by the ice or snow is absorbed, leading to further increased warming. This is another positive feedback which on its own would increase the global average temperature rise due to doubled carbon dioxide by about 20%.

In addition to the basic temperature feedback, four feedbacks have been identified, all of which play a large part in the determination of climate, especially its regional distribution. It is therefore necessary to introduce them into climate models. Because the global models allow for regional variation and also include the important non-linear processes in their formulation, they are able in principle to provide a full description of the effect of these feedbacks (see Figure 5.17). They are, in fact, the only tools available with this potential capability. It is to a description of climate prediction models that we now turn.

Figure 5.16 Estimates of transport of heat by the oceans. Units are terawatts (1012 W or 1 million million watts). Note the linkages between the oceans and that some of the heat transported by the North Atlantic originates in the Pacific.

Figure 5.16 Estimates of transport of heat by the oceans. Units are terawatts (1012 W or 1 million million watts). Note the linkages between the oceans and that some of the heat transported by the North Atlantic originates in the Pacific.

Climate feedback comparisons

Climate feedbacks affect the sensitivity of the climate in terms of the temperature change A7s at the surface that occurs for a given change AQ in the amount of net radiation at the top of the troposphere (known as the radiative forcing14). AQ and ATs are related by a feedback parameter f (units Wm-2K-1) according to

If nothing changes other than the temperature (see fig 2.8), f is just the basic temperature feedback parameter f0 = 3.2 W m-2 K-1 (i.e the change in radiation at the top of the troposphere that leads to a 1 °C change at the surface).

However, as we have seen other changes occur that result in feedbacks. The total feedback parameter f allows all the feedbacks to be added together f=fo + fl +f2 + f 3 + ...

where f1, f2, f3 etc. are the feedback parameters describing water vapour, cloud, ice-albedo feedbacks, etc.

The amplification a of the temperature change A7s that occurs with a total feedback parameter f compared with the basic temperature feedback f0 is a = fo/f

Estimates of the feedback parameters for the main feedbacks from different climate models are:15

Water vapour (including lapse rate feedback - see Note 12) - 1.2 ± 0.5

Total feedback parameter (sum of f0 and the three above16) - 1.1 ± 0.5

Note that with this total feedback parameter the amplification factor is about 2.9 and the resulting climate sensitivity to doubled carbon dioxide a little over 3 °C.

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