The Climate System

Although it is common to consider climate as simply a function of the atmospheric circulation over a period of time, to do so overlooks the complexity of factors that determine the climate of a particular region. Climate is the end-product of a multitude of interactions between several different subsystems — the atmosphere, oceans, biosphere, land surface, and cryosphere — which collectively make up the climate system. Each subsystem is coupled in some way to the others (Fig. 2.3) such that

Changes of Solar Radiation

SPACE

ATMOSPHERE

Changes of Atmospheric Composition

Changes In Biosphere

ATMOSPHERE

Changes of Land Features, Orography, Albedo, etc.

EARTH

Changes of Land Features, Orography, Albedo, etc.

EARTH

Changes of Ocean Thermohaline Circulation

FIGURE 2.3 Schematic diagram of major components of the climatic system. Feedbacks between various components play an important role in climate variations.

changes in one subsystem may give rise to changes elsewhere (see Section 2.3). Of the five principal subsystems, the atmosphere is the most variable; it has a relatively low heat capacity (low specific heat) and responds most rapidly to external influences (on the order of 1 month or less). It is coupled to other components of the climate system through energy exchanges at the surface (the atmospheric boundary layer) as well as through chemical interactions that may affect atmospheric composition (Junge, 1972; Jaenicke, 1981; Bolin, 1981). Only recently has it been possible to assess variations in atmospheric composition and turbidity through time (Raynaud et al., 1993; Zielinski, 1995). Such variations are of particular importance because they may be a fundamental cause of past climatic variations.

The oceans are a much more sluggish component of the climate system than the atmosphere. Surface layers of the ocean respond to external influences on a timescale of months to years, whereas changes in the deep oceans are much slower; it may take centuries for significant changes to occur at depth. Because water has a much higher heat capacity than air, the oceans store very large quantities of energy, and act as a buffer against large seasonal changes of temperature. On a large scale, this is reflected in the differences between seasonal temperature ranges of the Northern and Southern Hemispheres (Table 2.1). On a smaller scale, proximity to the ocean is a major factor affecting the climate of a region. Indeed, it is probably the single most important factor, after latitude and elevation.

At the present time, the oceans cover 71% (361 X 106 km2) of the Earth's surface and hence play an enormously important role in the energy balance of the Earth (see Section 2.4). The oceans are most extensive in the Southern Hemisphere, between 30 and 70° S, and least extensive in the zone 50-70° N and poleward of 70° S (Fig. 2.4). This distribution of land and sea is of great significance; it is largely

■■ TABLE 2.1 Mean Temperatures (°C) and Temperature Differences

Extreme months Year

(a) Surface

Northern Hemisphere

8.0 (January)

21.6 (July)

15.0

Southern Hemisphere

10.6 (July)

16.5 (January)

13.4

Entire globe

12.3 (January)

16.1 (July)

14.2

(b) Middle Troposphere (300-700-mb layer)

Mean temperatures

Equator

-8.6

-8.6

-8.6

North Pole

-41.5 (January)

-25.9 (July)

-35.9

South Pole

-52.7 (July)

-38.3 (January)

-47.7

Temperature differences

Equator-North Pole

32.9 (January)

17.3 (July)

27.3

Equator-South Pole

29.7 (January)

44.1 (July)

39.1

After Flohn, 1978 and Van Loon et al„ 1972.

After Flohn, 1978 and Van Loon et al„ 1972.

responsible for the differences in atmospheric circulation between the two hemispheres, and has important implications for glaciation of the Earth (Flohn, 1978). On a global scale, the relative proportions of land and sea have changed little during the Quaternary, in spite of sea-level changes due to the growth and decay of continental ice sheets. When sea level was 100 m below current levels ocean area decreased by only 3% (though this is equivalent to a 10% increase in land-surface area). Such changes undoubtedly had regional significance; in particular, sea-level changes may

FIGURE 2.4 Percentage distribution of land and ocean by 5° latitude band. Land area shaded. Upper figures give percentage of hemispheric surface area equatorward of latitudes shown. Arrows indicate mean latitudinal ranges of seasonal snow cover (see Table 2.3).

have had important effects on oceanic circulation and certainly must have influenced the degree of continentality of some areas (e.g., Barry, 1982; Nix and Kalma, 1972).

The oceans play a critical role in the chemical balance of the atmospheric system, particularly with respect to atmospheric carbon dioxide levels. Because the oceans contain very large quantities of C02 in solution, even a small change in the oceanic C02 balance may have profound consequences for the radiation balance of the atmosphere, and hence climate (Sundquist, 1985). The role of the oceans in global C02 exchanges is of particular importance, not only for an understanding of past climatic variations but also for insight into future C02 trends in the atmosphere (Baes, 1982; Bolin, 1992).

The land surface of the Earth interacts with other components of the climate system on all timescales. Over very long periods of time, continental plate movements (in relation to the Earth's rotational axis) have had major effects on world climate (Tarling, 1978; Frakes et al., 1992). It is no coincidence that the frequency of continental glaciation increased as the plates moved to increasingly polar positions. Similarly, mountain-building episodes (orogenies) have had major effects on world climate. Apart from the dynamic effects on atmospheric circulation (Yoshino, 1981; Ruddiman and Kutzbach, 1989) the presence of elevated surfaces at relatively high latitudes, where snow can persist throughout the year, may be a prerequisite for the development of continental ice sheets (Ives et al., 1975).

The latitudinal distribution of land and sea is of fundamental significance for both regional and global climate. In particular, the presence of highly reflective snow- and ice-covered regions at high latitudes strongly affects Equator-Pole temperature gradients (Table 2.1b). In the Southern Hemisphere, the presence of the high elevation Antarctic plateau south of -75° S (Fig. 2.4) causes there to be a much stronger Equator-Pole temperature gradient than in the Northern Hemisphere. As a result, an intense westerly circulation pattern develops above the surface layers (60% stronger, on average, than westerlies in the Northern Hemisphere [Peixoto and Oort, 1992]). The stronger temperature gradient also results in the subtropical high pressure belt of the Southern Hemisphere being located closer to the Equator than in the Northern Hemisphere (29-35° S as compared with 33-41° N; Fig. 2.5). This difference, stemming primarily from the polar location of Antarctica and its associated low temperatures, gives rise to a basic asymmetry in the position of climatic zones in both hemispheres (Korff and Flohn, 1969; Flohn, 1978).

The cryosphere consists of mountain glaciers and continental ice sheets, seasonal snow and ice cover on land, and sea ice. Its importance in the climate system stems from the high albedo of snow- and ice-covered regions, which greatly affects global energy receipts (Kukla, 1978). At present, about 8% of the Earth's surface is permanently covered by snow and ice (Table 2.2) but seasonal expansion of the cryosphere causes this figure to double (Table 2.3). The hemispheric differences are particularly profound. In the Northern Hemisphere, 4% of the total area is permanently ice covered (mainly the Arctic Ocean [~3%] and Greenland). In winter months, sea-ice formation and snowfall on the continents results in a 6-fold increase in snow and ice cover. By midwinter, 24% of the Northern Hemisphere is generally covered by snow and ice. In the Southern Hemisphere, most of the permanent ice cover is land-based on the Antarctic continent, and seasonal changes are due almost

O 30

O N. HEMISPHERE X S. HEMISPHERE @® ANNUAL MEAN

X 00 Xo ex

A7TC)

FIGURE 2.5 Relationship between latitude of main axis of subtropical anticyclones and hemispheric (Equator-Pole) temperature gradient in preceding month (after Korff and Flohn, 1969).

TABLE 2.2 Present Extent of Permanent Snow and Ice (Glaciers, Ice Caps, and Sea lce)°

Area Volume Sea-level

Northern Hemisphere

Greenland 1.73 3.0 7.5

Other locations 0.5 0.12 0.3

Total land-based snow and ice 2.23

Sea ice 8.87

Total for Northern Hemisphere 11.0 Southern Hemisphere

Antarctica 13.0 29.4 73.5

Other locations 0.032 <0.01 <0.02

Total land-based snow and ice 13.032

Sea ice 4.2

Total for Southern Hemisphere 17.23 Entire Globe

Total land-based snow and ice -15.3

Total for entire globe -28.3

" From Kukla (1978), Hughes et al. (1981), and Hollin and Schilling (1981).

TABLE 2.3 Seasonal Changes in Snow- and Ice-cover Area (x 106 km2); Snow and Ice Extent Based on the Period 1967-74

Maximum extent

Minimum extent

Percentage

Percentage

Month

Area

(%)

Month

Area

(%)

Northern Hemisphere

February

60.1

24"

August

11.0

4"

Southern Hemisphere

October

34.0

13"

February

17.2

7"

Entire globe

December

79.1

16b

August

42.3

8*

From Kukla (1978). " Percentage of area of hemisphere. b Percentage of area of entire globe.

From Kukla (1978). " Percentage of area of hemisphere. b Percentage of area of entire globe.

entirely to an increase in sea-ice formation (Fig. 2.6). By midwinter, 13% of the Southern Hemisphere is generally covered by snow and ice. It is of particular interest that the cryosphere, considered on a global scale, doubles in area over a relatively short period — from August to December, on average. Given the variability in seasonal timing of snow- and ice-cover changes in both hemispheres, it is quite probable that very large area increases may occur over an even shorter period, and this has important implications for theories of climatic change (Kukla, 1975). Clearly, part of the cryosphere undergoes extremely large seasonal variations and hence has a very short response time. Glaciers and ice sheets, on the other hand, respond very slowly to external changes, on the timescale of decades to centuries; for large ice sheets, adjustment times may be measured in millennia.

The final component of the climate system is the biosphere, consisting of the plant and animal worlds, though vegetation cover and type are mainly of significance for climate. Vegetation not only affects the albedo, roughness, and évapotranspiration characteristics of a surface, but also influences atmospheric composition through the removal of carbon dioxide and the production of aerosols and oxygen. Absence of vegetation may result in significant increases in particulate loading of the atmosphere, at least locally, and this may of itself be a significant factor in altering climate (Charney et al., 1975; Overpeck et al., 1996). Vegetation type varies greatly from one region to another (Table 2.4). Forests and woodlands cover 34% of the continents and play a major role in the removal of atmospheric C02 (Woodwell et al., 1978; Potter et al., 1993; Ciais et al., 1995). Deserts and desert scrublands occupy -13% of the continents, and are the major sources of wind-blown dust (though cultivated lands are increasingly susceptible to wind erosion also). The response time of the biosphere varies widely, on the order of years for individual elements of the biosphere to centuries for entire vegetation communities. Carbon sequestration in terrestrial ecosystems has varied over glacial-interglacial cycles because of large-scale changes in the area of different ecosystem types. Thus, the area of forests during the Last Glacial Maximum (LGM) was reduced to less than one-third of the forest cover today, with a corrresponding reduction in carbon storage in forest ecosystems (Van Campo et al., 1993; Peng et al., 1998). Overall, carbon storage on land was 30% lower during the LGM than it is today.

SNOWi PACK ICE

ICE CONCENTRATION;

MOH | MEDIUM I LOW

FIGURE 2.6 Extent of snow and ice at four intervals during the year. Note the maximum global ice extent in November, minimum in August, see Table 2.3 (from Kukla, 1978).

November

FIGURE 2.6 Extent of snow and ice at four intervals during the year. Note the maximum global ice extent in November, minimum in August, see Table 2.3 (from Kukla, 1978).

November

SNOWi PACK ICE

ICE CONCENTRATION;

MOH | MEDIUM I LOW

TABLE 2.4 Areas of Major Ecosystems of the World and Their Estimated Carbon Content and Albedo — Today and at the Last Glacial Maximum

Modem Modern LGM

area carbon Albedo" LGM area carbon

TABLE 2.4 Areas of Major Ecosystems of the World and Their Estimated Carbon Content and Albedo — Today and at the Last Glacial Maximum

Modem Modern LGM

area carbon Albedo" LGM area carbon

Ecosystem

(10' km2)

storage (Pg)

(%)

(I04 km1)

storage (Pg)

Boreal forest

11.8

310.2

7-15

2.3

63.5

Temperate forest

13.0

343.9

13-17

3.9

109.2

Tropical forest

14.3

399.9

7-15

6.1

159.4

Xerophytic woodlands

11.3

147.0

15-20

19.0

249.2

All forests and

woodlands

50.4

1201.0

31.3

581.3

Arctic and Alpine tundra

10.7

204.4

10-15

14.7

281.9

Steppes and mountain

shrublands

30.8

337.9

15-20

41

444.7

All steppes and tundras

41.5

542.3

55.7

726.6

Cool and polar deserts

4.0

27.4

10-20

15.8

64.0

Hot deserts

14.5

21.8

25-44

19.7

29.6

All deserts

18.5

49.2

35.5

93.6

Cultivated lands

14.1

195.0

8-20

Bogs

0.7

128.1

TOTAL

125.2

2115.6

122.5

1401.5

From Van Campo et al. (1993) " From Lieth (1975).

From Van Campo et al. (1993) " From Lieth (1975).

Human beings are, of course, part of the biosphere and human activities play an increasingly important role in the climate system. Increases in atmospheric C02 concentration, changes in natural vegetation, increases in particulate loading of the lower troposphere, and reductions in atmospheric ozone concentrations in the stratosphere may all be attributed to man's worldwide activities (see Chapter 4 in MacCracken et al., 1990; Schimel et al., 1996). The rate of such changes is rapid and the extent to which the climate system can adjust to them without drastic changes in climate or climatic variability remains uncertain. The only certainty is that mankind has become exceedingly vulnerable to any unexpected perturbations of climate. Common sense argues for action to limit those activities that may contribute to global-scale climatic effects (see Chapters 3 and 4 in Abrahamson, 1990).

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