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Part 2 - The status of the global environment
5. Climate changes due to the increase in
greenhouse gases as predicted by climate models
6. Deforestation and desertification in developing countries
Comments on part 2
5. Climate changes due to the increase in greenhouse gases as predicted by climate models
There is widespread concern that continual man-made emissions of "greenhouse gases" are resulting in global atmospheric warming, local climate changes, and sealevel rise, with the prospect of consequent serious environmental, social, and economic impacts. This paper describes some of the issues involved in modelling the climate system and the model-based predictions currently obtained.
The most comprehensive scientific assessment of climate change was conducted by Working Group I of the Intergovernmental Panel on Climate Change (IPCC), which is organized jointly by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). The results (Houghton et al., 1990, 1992) are the most authoritative and strongly supported statement on climate change that has ever been made by the international scientific community. Most of the points discussed briefly in this paper are addressed more fully in the IPCC reports.
2. The greenhouse effect
Suppose there were no atmosphere on earth, incoming solar radiation would be reflected at the surface and some of it would be absorbed. The earth, in turn, would emit radiation at a longer wavelength, because the surface has a lower temperature than that of the sun, and this would escape to space. For the earth to be in equilibrium in energy, the absorbed solar radiation has to be balanced by the outgoing longwave radiation. At equilibrium the effective radiating temperature of the earth is about 255K (-18°C). This would be the earth's surface temperature in the absence of atmosphere (see fig. 5.1).
Fig. 5.1 A schematic diagram illustrating the greenhouse effect (Source: Houghton et al., 1990)
Actually, the earth has an atmosphere, the major components of which are nitrogen and oxygen. However, it also contains water vapour, CO2, and CH4, although their amounts are very small. The radiative properties of these gases for the shorter solar wavelengths are quite different from those at the infra-red wavelengths. Some of the infrared radiation from the surface is absorbed by these gases within the lower atmosphere and then it is re-emitted, both downwards warming the surface and lower atmosphere - and upwards. To establish an equilibrium state, there must be radiative balance at the top of the atmosphere and energy balance at the surface and within the atmosphere. These gases effectively trap infra-red radiation that would otherwise escape from the earth and are termed "greenhouse gases." The mechanism that influences the temperature of the surface and the atmosphere is called the "greenhouse effect." The effect of the greenhouse gases is to raise the surface temperature by about 33K to 288K (15°C) (see fig. 5.1).
3. Climate models
The earth's climate is determined not only by the atmosphere but also by the ocean, the land surface, the cryosphere, and the biosphere. This entire system is called the "climate system." Figure 5.2 schematically shows the earth's climate system and the complexity of interactions between components of the system. The earth's climate is regulated by interactive dynamic, physical, chemical, and biophysical processes. The characteristic time-scales for each component to recover its equilibrium state after being disturbed differ a great deal, ranging from one to two months for the atmosphere, a few months for the mixed layer of the ocean, several decades for the upper part of the ocean, a few thousand years for the deep ocean, to tens of thousands of years or more for the ice sheet. Mutual interactions produce varying time-scales of climate variation. However, our understanding of those interactions, obtained mainly through numerical experiments with climate models, is still very limited.
Climate models numerically predict or determine the physical states of these components on the basis of physical laws. There are many different modelling approaches, ranging from simple vertically one-dimensional radiative-convective models to the full complexity of three-dimensional coupled atmosphere-ocean general circulation models (AOGCMs). Figure 5.3 shows an example of a grid-box representation of a three-dimensional atmospheric model. The representative physical state of each box is numerically predicted with the use of relevant physical laws.
In order to predict transient, regional climate changes, the models used must allow in some way for the influences of the various components of the climate system and for the significant interactions between them. The type of model that meets this purpose is the one developed from existing three-dimensional global AOGCMs. These provide an effective framework for testing new hypotheses about climate sensitivity and change.
Fig. 5.2 Schematic illustration of the coupled atmosphere-ocean-ice-land climate system (Note: The full arrows are examples of external processes; the open arrows are examples of internal processes in climate change. Source: Houghton et al., 1990)
Fig. 5.3 An example of a grid representation of a three-dimensional atmospheric model. Representative physical states of each box are predicted numerically on the basis of relevant physical laws
4. Climate feedback processes
The earth's climate system is characterized by complicated interactions between the various components of the system. When some change is introduced into the climate system, such as the increase in the atmospheric concentration of CO2, it is fairly easy to evaluate its direct effect through changes in the radiative heating rate. Warming of the tropospheric temperature will follow. However, this warming causes other (indirect) changes in the climate system. After a chain of changes, the initial direct change will be affected again so as to enhance it (positive feedbacks) or to suppress it (negative feedbacks).
As an example of a positive feedback, temperature-water vapour feedback is explained briefly. The maximum amount of water vapour allowed in the air depends on pressure and temperature. Under constant pressure? it increases with the increase in air temperature. Because water vapour is one of the greenhouse gases, the increase in atmospheric water vapour due to the increase in air temperature will enhance the greenhouse effect of water vapour, resulting in a further increase in air temperature.
In discussing global warming due to the increase in greenhouse gases, there are other important positive feedbacks such as snow-albedo feedback and sea ice-albedo feedback. "Albedo" is the total reflectivity of solar radiation. A temperature rise will decrease both the snow-covered area and the sea-ice area. Because snow and sea-ice are good reflectors of solar radiation, the decrease in these areas allows more absorption of solar energy by the earth's surface. This will lead to a further rise in the surface air temperature.
Cloud is recognized as a very important and complicated element in discussing feedbacks. When climate changes, it is not only the amount of cloud that changes but also cloud height and the optical property of clouds. If the cloud amount decreases under global warming, the gain in solar energy at the surface surpasses the loss in long-wave radiation as the albedo of the entire air column decreases except when the surface is covered either by snow or by sea-ice. So this is a positive feedback.
The cloud albedo usually increases either when cloud water increases or when cloud particles change from the ice-phase to the water-phase. These changes are likely to occur under global warming. Because the increase in cloud albedo reduces the increase in surface temperature, this is a negative feedback.
Sensitivity experiments on the treatment of clouds performed so far demonstrate both positive and negative roles depending on the treatment (Senior and Mitchell, 1993). Where a standard, simple cloud prediction scheme based on relative humidity was adopted, the globally averaged surface temperature increase under doubled CO2 was 5.2°C. In another case where cloud radiative properties were allowed to depend on the predicted liquid water content of the clouds, the temperature increase was only 1.9°C. This spread, 1.9°-5.2°C, represents a factor of 2.7 in the response to changing the representation of clouds in the model. The uncertainty about how to formulate clouds poses a formidable obstacle to reliable climate prediction even in the sense of an equilibrium global mean, let alone in the context of transient, regional predictions.
Feedback processes involving biogeochemical or biogeophysical processes have not been clarified yet either. Currently, only the physical aspects of the biomass, i.e. evapotranspiration, albedo, and roughness, are considered in the model. No other processes are considered. Poor understanding in these areas might also introduce uncertainties in climate change prediction.
Table 5.1 Model sensitivities to the effects of internal feedbacks on equilibrium warming in response to a doubling of CO2
|Allow water vapour feedback||1.9°C|
|Allow snow and sea - ice feedback||2.3 - 3.2°C|
|Allow cloud feedback||5.2°C|
Table 5.1 summarizes model sensitivities to the effects of internal feedbacks on equilibrium warming in response to a doubling of CO2.
5. Transient climate response and the ocean
Greenhouse gases are increasing year by year, in the case of CO2 at the rate of 1.8 parts per million by volume (ppmv) per year currently. It takes some time for the earth's climate system to adjust to these changes, because the characteristic or relaxation time of the ocean (the time required for it to return to its former balanced state after being forced to move to an unbalanced state) is not short but several decades for the upper part of the ocean and a thousand years or more for the deep ocean. The total energy required to raise the entire air column by 1°C is equivalent to the energy required to raise 2.5m of sea water by 1°C. As the mean depth of the ocean is about 4,000 m, the thermal inertia of the ocean is about 1,600 times as great as that of the atmosphere based on this simple energy consideration. The ocean, in this sense, is very important in determining the transient climate response. Once the oceanic surface temperature is determined, the atmosphere will respond to it very quickly, say within a month or two.
It is known that gyres exist in the ocean, such as the Kuroshio, the Gulf Stream, the California current, and so on. These currents are closely connected with the surface wind, are directed horizontally, and transport heat in a north-south direction. Another type of circulation in the ocean is thermohaline circulation. The density of sea water is determined by temperature, salinity, and pressure. As a result of differences in temperature and salinity, large-scale density difference is created in the ocean to drive meridional circulations. The full extent of these circulations has not yet been observationally confirmed. However, three-dimensional oceanic circulation models unanimously show very deep thermohaline circulation in the southern hemisphere along the Antarctic continent. Also shown is the very deep circulation in the north-eastern part of the North Atlantic. Deep thermohaline circulation transports energy in a vertical direction, contributing to a delay in the rise in sea surface temperature in those areas through vertical mixing.
Transient response studies by AOGCMs (Stouffer et al., 1989; Cubasch et al., 1992; Manabe et al., 1991; Murphy, 1992; Meehl et al., 1993) show a substantial delay in warming in the southern hemisphere compared with the northern hemisphere (see fig. 5.4), owing to the dominance of the oceanic area in the southern hemisphere. They also show a further delay in warming in the area where deep thermohaline circulations are dominant, i.e. circum-Antarctic ocean and the north-eastern part of the North Atlantic (see fig. 5.4). Manabe et al. (1991) show that the increase in precipitation and thus the dilution of surface salinity in those areas also contribute to delay warming in those areas.
A study of century-scale effects of increased CO2 on the earth's climate by Manabe and Stouffer (1993) shows marked weakening in the thermohaline circulation and thus marked changes in thermal and dynamic structure of the ocean in the quadrupled-CO2 climate, leading to a 7°C increase in the globally averaged surface temperature.
The AOGCM studies have clearly demonstrated the importance of oceanic circulations in determining transient response and local climate changes. However, the current oceanic general circulation models adopted in these AOGCM studies have not been fully verified because of a lack of observed data, especially in deep circulations. Considering the large impact of the ocean on the transient response of the climate system, further observations of the ocean are urgently needed.
6. Possible climate changes predicted by models
Although the predicted climate changes differ from model to model as regards regional details, they agree on many larger-scale aspects. These are summarized as follows:
The troposphere warms, whereas the stratosphere cools.
The global mean surface temperature increase, when CO2 is doubled, is 1.5-4.5°C. The best estimate is about 2.5°C.
The surface warming is greatest in high-latitude winter; least in the tropics.
Fig. 5.4 (a) The time-dependent response of surface temperature (°C) in the coupled ocean-atmosphere model to a 1 per cent per year increase in atmospheric CO2. (b) The equilibrium response of surface air temperature (°C) in the atmosphere-mixed-layer ocean model to a doubling in atmospheric CO2. (c) The ratio of the time-dependent response to the equilibrium response (Source: Manabe et al., 1991)
The hydrological cycle
Global average precipitation and evaporation increase by 3-15 per cent when CO2 is doubled.
Soil moisture increases in high-latitude winter.
Many models predict a decrease in soil moisture in mid-latitude summer.
Cumulus-type precipitation increases, while stratus-type precipitation decreases in the low and middle latitudes, resulting in a decrease in the precipitating area and an increase in high-intensity precipitation (Node and Tokioka, 1989); see fig. 5.5.
Fig. 5.5 Scatter diagram of the precipitation rate (mm/day) versus the ratio of the precipitating area to the global domain (%) for 1-10 January (Note: Ellipses drawn with thick and thin solid lines denote the root mean square scattering for 1xCO2 and 2xCO2, respectively. Dab points for 1xCO2 and 2xCO2 are denoted by crosses and dots, respectively. Source: Noda and Tokioka, 1989)
Temperature increase in the southern hemisphere lags behind that in the northern hemisphere.
Temperature increase lags especially in those areas where deep thermohaline circulations are dominant, such as the circum-Antarctic area and the north-eastern part of the North Atlantic. An increase in precipitation and a melting of sea-ice also help to delay warming in those areas.
When the atmospheric CO2 concentration is increased at the rate of 1 per cent per year, the globally averaged surface temperature increase realized in the model is about 60 per cent of the warming expected in the equilibrium state under the given concentration of CO2 (Stouffer, et al., 1989; Murphy, 1992).
7. Future problems
Uncertainties in predicting possible future climate changes exist in our inadequate understanding and thus inadequate treatment of the following processes (Houghton et al., 1992):
clouds (particularly their feedback effect on warming induced by greenhouse gases, as well as the effect of aerosols on clouds and their radiative properties) and other elements of the atmospheric water budget, including the processes controlling upper-level water vapour;
oceans, which, through their thermal inertia and possible changes in circulation, influence the timing and pattern of climate change; land surface processes and feedbacks, including hydrological and ecological processes that link regional and global climates;
sources and sinks of greenhouse gases and aerosols and their atmospheric concentrations (including their indirect effects on global warming);
polar ice sheets (whose response to climate change also affects predictions of sealevel rise).
Cubasch, U., K. Hasselmann, H. Hock, E. Maier-Reimer, U. Mikolajewicz, B. D. Santer, and R. Sausen. 1992. "Time-dependent greenhouse warming computations with a coupled ocean atmosphere model." Climate Dynamics 8:55 69.
Houghton, I. T., G. J. Jenkins, and J. J. Ephraums (eds.). 1990. Climate Change. The IPCC Scientific Assessment. Cambridge: Cambridge University. Press.
Houghton, J. T., B. A. Callander, and S. K. Varney (eds.). 1992. Climate Change 1992. The Supplementary Report to the IPCC Scientific Assessment. Cambridge: Cambridge University Press.
Manabe, S. and R. J. Stouffer. 1993. "Century-scale effects of increased atmospheric CO2 on the ocean-atmosphere system." Nature 364: 215-218.
Manabe, S., R. J. Stouffer, M. J. Spelman, and K. Bryan. 1991. "Transient responses of a coupled ocean atmosphere model to gradual changes of atmospheric CO2. Part 1: Annual mean." Journal of Climate 4: 785-818.
Meehl, G. A., W M. Washington, and T. R. Karl. 1993. "Low-frequency variability and CO2 transient change. Part 1. Time-averaged difference." Climate Dynamics, 8: 117-133.
Murphy, J. M. 1992. A Prediction of the Transient Response of Climate. Climate Research Technical Note, CRTN32, Hadley Centre, Meteorological Office, London Road, Bracknell, Berkshire RG12 2SY.
Noda, A. and T. Tokioka. 1989. "The effect of doubling the CO2 concentration on convective and non-convective precipitation in a general circulation model coupled with a simple mixed layer ocean model." Journal of the Meteorological Society of Japan 67: 1057-1069.
Senior, C. A. and J. B. F. Mitchell. 1993. "Carbon dioxide climate: The impact of cloud parameterization." Journal of Climate 6: 393-418.
Stouffer, R. J., S. Manabe, and K. Bryan. 1989. "Interhemispheric asymmetry in climate response to a gradual increase of atmospheric carbon dioxide." Nature 342: 660-662.
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