Small Shifts, Large Impacts: Global Climate Change and Regional Hydrology
Enno Schefuß, University of Bremen, Germany

Future climate change is projected to not only result in global warming due to continued greenhouse gas emissions but also to have profound impact on regional continental hydrology affecting quantity and quality of water resources, crop yields, fire occurrence, flooding and soil erosion. As regional hydrologic responses differ strongly regionally depending on land surface characteristics and interactions between atmosphere, ocean and land, climate models so far diverge not only in magnitude but also in sign of predicted changes. While some of the discrepancies may arise from yet not well understood biogeochemical feedbacks, like vegetation response, and/or anthropogenic effects, such as increased land-use, others may result from an incomplete understanding of atmospheric responses to changed boundary conditions, such as ice cover and the strength of the ocean heat conveyor, i.e. the so-called thermohaline circulation.
Nature provided a few ‘disturbance experiments’ during the past as ‘benchmark cases’ for atmospheric responses to changes in circulation and global temperatures. These continental hydrologic changes are recorded in sedimentary archives on land and offshore marine settings. While lakes record specifically local signals marine archives can receive their terrestrial signals from a large-scale integrated catchment and also record oceanic changes, such as shifts in surface temperatures.

To asses paleo-rainfall changes parameters are applied as proxies providing insights into past continental hydrologic changes, which, in an ideal case, can also be parameterized in general circulation models for evaluation of climate model performance. The isotopic composition of rainfall is determined by external factors like global ice volume and transport pathway of moisture as well as atmospheric processes such as condensation temperature and rainfall intensity. In the tropics, the latter, called the ‘amount effect’ is dominating the signal.

Compound-specific isotope analyses of terrestrial plant lipids in sedimentary archives are a novel tool to address paleo-rainfall changes. Despite uncertainties regarding the significance of minor factors, such as relative humidity changes, hydrogen isotope compositions of plant lipids are well-suited to monitor past changes in continental rainfall which can directly be compared to model results. Results of such analyses reveal distinct patterns of regional rainfall changes in response to sudden past climate changes, which will enable refined predictions for future scenarios.