Conservation and Sustainable Use of the Biosphere - WBGU

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Conservation and Sustainable Use of the Biosphere - WBGU

240 F The biosphere in the Earth System

example of this phenomenon in the climate context is

certainly the idea of the ‘runaway greenhouse effect’

that can be explained using the example of the greenhouse

gases methane, water vapour and carbon dioxide.

Greenhouse gases in the atmosphere absorb the

long-wave reflective radiance from the Earth’s surface

and radiate in accordance with their temperature

part of that back to the Earth’s surface. The

warming associated with that process can initiate a

series of feedbacks if organic (but also inorganic)

sinks for greenhouse gases turn into sources because

of higher temperatures, and additional greenhouse

gases are released. The overall effect is certainly disputed.

However, the frequent assumption is that the

effect of (for example man-made) released carbon

dioxide could be increased by a factor of 1.2–3.6 (Nisbet,

1994). This range shows the large degree of

uncertainty that exists in evaluating reinforcing feedback

effects. An extreme image would certainly be

the nightmarish concept of warming releasing the

entire complement of methane enclosed in the Arctic

permafrost soils. The reinforcement factor would

be immense. For this scenario there are estimates

(Lashof, 1991) that indicate that the seasonality of

weather conditions and the temperature-dependent

activities of the methane-forming bacteria play a

dominant role in the reinforcement of feedback

effects.

The carbon dioxide-climate-biosphere interaction,

by contrast, demonstrates a time-dependent

feedback strength. Palecki (1991) assumes an overall

positive feedback for periods under ten years. Only

when CO 2

constitutes the short-term limiting factor

for increased plant growth can it be absorbed to a

certain extent. Climatic changes should however as a

result of reduced biospheric activity lead to an

increase in atmospheric carbon.Warmer years resulting

from that process cause water stress and could

increase the effects further. The key role here is linkage

to the hydrological cycle. Higher temperatures

tend to lead to higher precipitation levels. The characteristic

effect is still unclear for the time scale. For

longer periods (over ten years), however, a negative

feedback seems to dominate since the biomass

increase further reduces the amount of carbon in the

atmosphere. This effect is also cited as a possible

explanation for the current stability of the global climate

(Nisbet, 1994). Growth of large forest stands in

the last 10,000 years could have had a stabilizing

effect on the basis of negative feedback.

F 5.2

Physiological and metabolic importance of the

biosphere

The existence and growth of boreal forests has

another important function. The temperature is

increased through lower albedo of the forests. Deforestation

would as a result of lower temperatures not

necessarily lead to the emission of methane, but possibly

set in motion a ‘runaway cooling chain’. The

snow-covered areas would increase and the temperature

would drop further because of their strong

reflective capacity. This is not a switch in the sense

described above since it is triggered anthropogenically;

but the effect indicates a possible critical element

within the Earth System. In Table F 5.2-1 certain

biomes are presented with respect to their physiological

and metabolic significance for the Earth

System.

F 5.2.1

Amazon Basin

The Amazon Basin is a crucial source of warmth and

humidity for the global climate system on the basis of

its size and its proximity to the equator (Pielke et al,

Table F 5.2-1

Qualitative assessment of the functional significance of biomes for aspects of global biogeochemical and energy cycles.

Source: Nisbet et al, 1994

Forest Grassland Desert Marine biosphere

tropical moderate boreal savannah tundra

Reflectance low low very low moderate high very high low

Intensity of very high high low low uncertain very low low

hydrological cycle

Net primary very high very high high moderate low very low low

production

Long-term very high high high moderate high low very high

carbon storage

Nutrient transmission low uncertain low high low very high uncertain

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