Corals unlock African climate history

29-05-2012   Changing rainfall in Madagascar appears modulated by a natural 50-70 year climate cycle originating from the Pacific Ocean. In his PhD thesis Craig Grove argues that it is important to go back in time to determine past natural climate variability, in order to understand the human impact on modern climate. Grove will defend his PhD thesis 'Madagascar’s Climate History Unlocked by Giant Corals' at the VU University Amsterdam on June 4, 2012.

Corals record vital climate information while their skeletons slowly grow for hundreds of years. The coral-based climate project entailed the drilling of coral cores to reconstruct past environmental conditions for various reefs across Madagascar. Massive corals grow about 1 cm per year, archiving specific chemical signals in their skeleton related to climate. Grove introduced a novel scanning technique to measure the amount of fluorescent soil material that rivers flush into the sea where the coral builds its skeleton. This allows for reconstructing past changes in river flow from the amount of soil in the coral skeleton, and identify rainfall patterns for the past 300 years. Soil erosion, land degradation and modern climate change all pose a mounting threat to coastal nations in the western Indian Ocean. However, vital long-term observational data are missing to properly assess and clarify the impact of a changing climate and land-use.

Combining coral luminescence with geochemistry, coral records revealed periods of high soil erosion in the mid-20th century that were linked to extensive periods of deforestation in Madagascar, rather than periods of high rainfall. After separating the local deforestation signal from the regional rainfall induced soil erosion signal, Grove identified a far-field cause, linked to the Pacific Ocean thousands of kilometres away. Indeed, the corals provide the first evidence that fluctuating ocean temperatures in the Pacific are driving rainfall over the western Indian Ocean. When colder than usual sea temperatures develop in the north-central Pacific, a complex ocean-atmosphere connection triggers increased Indian Ocean temperatures and rainfall across eastern Madagascar. Furthermore, rainfall over southern Africa and eastern Australia declines in response to the same cycle in the Pacific Ocean. These results have important implications for assessing future rainfall patterns across Africa where water resource management is increasingly important under the warming climate.

Craig Grove (1982) was born in Scotland. He got a Masters degree in Oceanography form the University of Southampton in 2004. Since 2007 he worked at NIOZ Texel at his PhD research.

Grove will continue this work at the NIOZ, branching out into different areas across eastern and southern Africa to further investigate how African rainfall is being controlled.

PhD Thesis
Craig A. Grove, Madagascar's climate history unlocked by giant corals, pp 217.