Klimamodelle geben Antworten auf alle denkbaren Zukunftfragen. Politische Entscheider nehmen diese Informationen dankbar auf, da sie ihre Planungen entsprechend einrichten können. Aber stimmen die aus den Modellen entstammenden Prognosen überhaupt? Ein Forscherteam um Timm Hoffman hat nun anhand von historischen Fotos die Modellprojektionen mit der realen Vegetationsentwicklung im südlichen Afrika verglichen. Das ernüchternde Ergebnis: Die Natur hat sich meist ganz anders entwickelt wie von den Modellen angenommen. Im Gegensatz zu den Modellannahmen konnte kein signifikanter Langzeittrend bei den Niederschlägen festgestellt werden. Vergetationsgürtel, die schrumpfen sollten, wuchsen an. Das Vertrauen in die Modelle ist entsprechend gering. Eine politische Planung oder gar CO2-Schadensberechnung auf Basis der Simulationen ist nicht möglich. Hier der Abstract der Arbeit, die im Fachblatt Anthropocene im März 2019 erschien:
Rethinking catastrophe? Historical trajectories and modelled future vegetation change in southern Africa
Most projections of climate change for southern Africa describe a hotter and drier future with catastrophic consequences for the environment and socio-ecological sustainability of the region. This study investigated whether evidence of the projections for the climate and vegetation of the subcontinent is already evident. Analysis of the climate record indicate that the historical trend of increasing temperature is consistent with future projections for the region. Rainfall, however, apparently has not changed significantly. Results from analysis of 1321 repeat historical photographs indicate broad trends in vegetation trajectories in the major biomes of southern Africa. The Savanna biome has experienced a rapid increase in woody plant at rates un-anticipated by the models. Contrary to early projections for the Succulent Karoo biome, biomass and cover have increased, largely in response to changes in land-use practices. Cover in the fire-adapted Fynbos biome has remained stable or increased over time with unanticipated expansion of forest species, particularly in localities protected from fire for long periods. The shrub-dominated Nama-karoo biome has increased in grass cover. Rather than contracting, as suggested in the early models, the Grassland biome has apparently expanded westwards into former Nama-karoo biome sites. The Savanna biome has experienced a rapid increase in woody plants at rates not anticipated by the models. The broad trends in historical trajectories illustrate how land-use management has influenced vegetation change in the past. They also provide a useful context for evaluating future changes and developing mitigation strategies for some of the worst impacts of climate change in the future.