Wednesday, 16 January 2002: 2:29 PM
Does landsurface matter in weather and climate?
The IGBP (International Geosphere-Biosphere Programme) core project BAHC (Biospheric Aspects of the Hydrological Cycle) has reviewed our knowledge on "Vegetation, Water, Humans and the Climate: A New Per-spective on an Interactive System" (a forthcoming BAHC synthesis). Here, I summarise the most important aspects of the first chapter devoted to landsurface-climate interaction. These aspects include: Global weather forecasts have substantially improved in part because of more realistic representations of near-surface processes. Changes in albedo in snow-cov-ered areas during spring and changes in soil moisture have been shown to be particularly important. Theoretical work demonstrates that the atmosphere-vegetation system is an intransi-tive system, i.e., the system yields multiple equilibria depending on the initial conditions from which the system is started. For example, several models show that under present-day insolation and ocean temperatures, two solutions are found for North Africa and Central Asia: a dry climate and a more humid climate. Moreover the atmosphere-vegetation system can react to external forcing in a disproportional way whichs leads to abrupt climate and vegetation change in some regions. Interpretation of palaeoclimatic and palaeobotanic records suggest that atmosphere-vegetation interaction in the past has amplified climate changes which are triggered by orbital forcing, i.e., changes in the Earth's orbit around the sun. For example, the syner-gism between the sea-ice albedo feedback in the Arctic and the so-called taiga-tundra feed-back leads to warmer winters than today in the mid-Holocene (some 6000 years ago) despite of less insolation at this time (this effect is referred to as the biome paradox in the literature). This synergism also contributed to the last glacial inception some 115.000 years ago in a significant way. (Most models cannot explain the last glacial incep-tion without taking into consideration biogeophysical effects.) Likewise, atmosphere-veg-etation interaction is the major amplification factor for the greening of North Africa during the early and mid-Holocene. The non-linearity of this feedback helps to explain the rather abrupt aridification of North Africa some 5.500 years ago. Humankind has altered the Earth's surface; it is estimated that over 45% of the land surface is directly affected by human-induced land cover change. Over the last few centuries, the intensity and scale of these modifications has increased significantly, and model exper-iments suggest that this land cover change has an impact not only on regional climate, but also on the global scale. Biogeochemical feedbacks directly affect the chemical composition of the atmosphere, mainly the carbon cycle and thereby, the greenhouse effect. Biogeochemical feedbacks act globally because of rapid (usually within one or two years) global mixing. Without consid-eration of biogeochemical feedbacks, projections of climate change in the upcoming decades would be incomplete.