Principal Investigator J Perron
Despite the obvious importance of climate in shaping Earth's surface, a quantitative understanding of how climate governs the long-term development of landscapes has been elusive. We are studying several natural experiments in which landscapes have evolved under varying climatic conditions while other factors have remained relatively constant.
(*) Rainfall and erosion across ocean islands -- Volcanic ocean islands are an ideal natural laboratory for studying climate's effects on landscapes: the bedrock is relatively homogeneous, the entire island has experienced approximately the same tectonic history, the initial surface can sometimes be dated, and the interaction of tradewinds with high topography creates dramatic gradients in rainfall. Postdoc Ken Ferrier and graduate student Kim Huppert are exploring how erosional processes and landscape evolution have responded to one of Earth's steepest rainfall gradients, on the Hawaiian island of Kaua'i. In one project, a collaboration with Sujoy Mukhopadhyay (Harvard) and Matt Rosener and Jonathan Stock (USGS) we identified an empirical correlation between rainfall rates and long-term erosion rates, and compared these rates with shorter-term sediment yields to help assess possible threats to coral reefs. In a second project, we focused on bedrock river incision, which drives the development of many landscapes, and showed that the efficiency of river incision depends strongly on rainfall rate, providing rare evidence of a long-suspected climatic influence. This result helps to quantify the influence of climate on one of the main processes that shapes landscapes, and also supports theoretical arguments that rainfall gradients can, through their influence on river incision, alter the form of entire mountain ranges.
We have also been collaborating with several other groups on problems involving the interaction of climate and landscapes on ocean islands, ranging from the response of valley incision after flank collapse to the long-term hydrologic evolution of islands.
(*) Landslides and rainfall extremes in a changing climate -- Rainfall-triggered shallow landslides threaten communities, infrastructure, and ecosystems. The intensity and frequency of extreme rainfall are expected to change under climate warming, but we don’t yet understand how these climatic changes will impact landslide abundance, size, and spatial distribution. Postdoctoral researcher Dino Bellugi is collaborating with climate scientist Paul O’Gorman to assess how future changes in rainfall extremes will affect landslide hazards. Combining a new model for discrete landslides (which predicts the boundaries of individual landslides rather than the average susceptibility of an area) with climate model predictions of rainfall extremes under future warming scenarios, we estimate the changes in landslide volume that are likely to occur in different regions. Surprisingly, we find that the largest relative increase in future landslide hazards may occur in areas that are currently not susceptible to landslides, suggesting that quantitative analyses of Earth’s landscapes should be an important part of future policy response to climate change.
(*) Microclimate and asymmetric topography -- We are studying how microclimates create asymmetric hillslopes. This small-scale natural experiment takes advantage of reliable controls on a landscape's geologic and tectonic history, and demonstrates how Earth's surface energy balance can influence long-term erosion.
(*) Sea level cycles and coral reefs -- The effects of climate variability are not limited to landscapes above sea level. Coral reefs form dramatic landscapes, particularly around ocean islands. Charles Darwin proposed a general model in which reefs progress through a well-defined sequence as an island subsides, beginning as a narrow fringing reef near the coast, then forming a barrier reef surrounding a lagoon, and finally developing into an atoll. But Darwin didn't consider the glacial sea level cycles that have been occurring for the past few million years. How does sea level shape coral reef landscapes? Graduate student Michael Toomey, in collaboration with Andrew Ashton (WHOI), has shown that reefs around many islands (such as the Hawaiian Islands) do not follow Darwin's proposed sequence, and display a wider variety of forms. We developed a simple model for reef growth that illustrates how the rates of reef accretion and island vertical motion control a reef's profile, and we found that glacial sea level variations are essential for reproducing the observed variety of reef forms. Pleistocene coral reefs appear to bear a strong imprint of glaciations.