Geomorphological Signatures of Varying Climate

A LONGSTANDING question in geomorphology(1,2) is whether the topography of a particular landscape is in balance with current climate-driven processes, or contains relict signatures of past climates. For the glaciated landscapes of the Northern Hemisphere, the latter obviously applies but the situation is far from clear in regions where climate-driven processes have changed only in intensity, rather than character. We have addressed this question using a mathematical model of landscape evolution(3,4) and find that both cases-contemporary balance and relict features-are possible. For a sinusoidal climate fluctuation, we find that all climate states (wet and dry) leave geomorphological signatures only when there is no active uplift. With active uplift and the associated increase in erosion, the topography tracks the current climate and any relict features are likely to reflect only the wettest conditions previously experienced by the landscape. In both cases, the temporal evolution of the landscape in response to cyclic climate forcing is complex, and leads to the unexpected result that valley density is largest during periods dominated by slow downslope movement of sediment, rather than during times of strong fluvial incision, as would be anticipated from steady-state models(5-7).