| Fig. 2.1.4. A Changes in temperature and C02, methane and dust concentration in the last 160,000 years. B C02 concentration over the last 1000 years and the consumption of fossil fuels since 1850. The C02 concentration was determined from ice cores. Direct measurements of C02 concentration have been made on Mount Mauna Loa in Hawaii since 1960. C Change in methane concentration in the atmosphere since 1400 and the increase in the world population since approximately 1600 (WBGU 1997)

(55 million years ago) is characterised by a transient increase of 5-7 K in the mean global temperature. It has been shown that this increase in temperature was triggered by a landslide on the continental shelf of the Atlantic releasing 10002000 Gt carbon as methane from methane hydrate. This release is similar to the current release of C02 from fossil fuels (6 Gt/yearx 100 years corresponds to 600 Gt). This event in the earth's history may be used as a model to evaluate the consequences of the present human activity.

It cannot be concluded from the parallel course of C02 and temperature curves that C02 alone causes the climate change (see, e.g., Veizer et al. 2000). There are various factors which increase the trend of climatic change or decrease it. These are in particular (Mitchell 1989; IPCC 1996):

• Position of the earth relative to the sun (Mi-lankovitch effect),

• Changes in outgoing radiation from vegetation and snow whilst cooling,

• Changes in absorption of glaciers with increasing dust cover,

• Repeated release of methane hydrates from ocean sediments,

• Complex interactions of various trace gases from aerosols, which may have an increasing as well as decreasing effect (IPCC 1996).

• The temperature of the earth is an equilibrium between short wavelength incident radiation and long wavelength emissions.

• The incident radiation reaching the earth's surface is in a narrow "incident radiation window" in the visible light range. The incident radiation is reduced as the cloud cover increases (low clouds with higher reflective properties) and with the amount of aerosols in the atmosphere. Aerosols can be of natural as well as anthropogenic origin.

• Emissions result from a relatively narrow "emission window", in which water vapour has the lowest emission minimum. The emission window is getting increasingly

0 0

Post a comment