The significance of environmental change

The Earth is a dynamic, non-static and constantly changing system, in which all components (the atmosphere, geosphere, cryo-sphere, hydrosphere, and biosphere including mankind) interact. Research on global environmental change tries to understand how these complex systems interact, and to identify the nature of linkages between them. This research may ultimately provide the basis for predicting future global environmental changes and their human consequences. To be able to forecast future environmental changes, an understanding of past environmental changes is essential. Such research concentrates on the time evolution of processes operating on time scales from decades to millennia and their interactions through time. Hopefully this will enable assessments of causes and effects in this complicated dynamic system. Palaeoenviron-mental studies also show how rapidly Earth systems may respond to forcing factors, which is important in planning future environmental change. This research provides a database of environmental conditions in the past which can be used for testing numerical models of atmospheric, terrestrial and marine processes.

For the time prior to instrumental records, evidence of environmental change comes from 'natural archives' or proxy records (Table 1.1), providing information about palaeoenvironmental conditions including past atmospheric composition, tropospheric aerosol loads, explosive volcanic eruptions, air and sea temperatures, wind and precipitation patterns, ocean chemistry and productivity, sea-level changes, ice-sheet dimensions, and variations in solar activity. Of crucial importance, however, is the ability to date the different records accurately in order to determine whether events occurred simultaneously, or whether events led or lagged behind others.

Glaciers and ice sheets are some of the best archives of past environmental change, as demonstrated by ice cores obtained from the Antarctic and Greenland ice sheets (see Chapter 3) and through the history of glacier

Table I. I Characteristics of natural archives (adapted from Bradley and Eddy, 1991)

Archive

Temporal resolution

Temporal range (yr)

Information

Historical records

Day/hour

1000

T H B V M L S

Tree rings

Season'year

10,000

THQBVMS

Lake sediments

1 to 20 years

10,000-1.000,000

T H C„ B V M

Ice core5

1 year

100,000

T H Ca B V M S

Pollen

100 years

100,000

T H B

Loess

1 00 years

1.000,000

H B M

Ocean cores

100-1000 years

10,000.000

TC.BM

Corals

I year

100,000

CWL

Palaeosols

100 years

100,000

THQV

Geomorphic features

100 years

10.000.000

T H V L

Sedimentary rocks

1 year

10,000.000

H H C, V M

T: temperature; H: humidity or precipitation; C: chemical composition of air (Ca), water (Cw), or soil (Cs); B: biomass and vegetation patterns; V; volcanic eruptions; M: geomagnetic field variations; L: sea-level; S: solar activity.

T: temperature; H: humidity or precipitation; C: chemical composition of air (Ca), water (Cw), or soil (Cs); B: biomass and vegetation patterns; V; volcanic eruptions; M: geomagnetic field variations; L: sea-level; S: solar activity.

fluctuations obtained from the glaciated regions of the world (see Chapter 5). Records of glacier fluctuations contribute important information about the range of natural variability and rates of change with respect to energy fluxes at the Earth's surface over long time-scales. Reconstructed Holocene and historical glacier fluctuations indicate that the glacier extent in many mountain ranges has varied considerably during recent millennia and centuries, exemplified by the Little Ice Age and late-twentieth century weather extremes. The general shrinkage of Alpine glaciers during the twentieth century is a major reflection of rapid change in the energy balance at the Earth's surface. An annual loss of a few decimetres of glacier ice depth is largely consistent with the estimated anthropogenic greenhouse forcing (a few W/m2). The rapid glacier retreat in the first half of the twentieth century was probably little affected by emissions of greenhouse gases. The later general retreat may, however, include an increasing component of human influence. Recent glacier shrinkage may now coincide with increased human-induced radiative forcing. Glacier mass balance measurements therefore become one of the key indicators for evaluating possible future trends.

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