climate

Climate and landscape evolution

Most landscapes on Earth emerge from a battle between plate tectonics, which deforms Earth’s crust to create mountains, and climate, which provides the rain, wind, snow and ice that grind the mountains back down. It seems like it should be easy to measure how climate affects landscapes – say, how the average rainfall in a given part of the world controls how fast a landscape erodes. Unfortunately it’s not that easy, because there are lots of other things that can influence erosion rates, like plants, the strength of the bedrock, the rate of tectonic deformation, and even humans. This makes it hard to predict how a landscape will respond to a changing climate.

Our approach is to study climate and landscape evolution using natural experiments – landscapes where nature has varied some key aspect of climate while holding other factors mostly constant. Ken Ferrier and Kim Huppert studied bedrock river incision on the Hawaiian island of Kaua’i, which has a very dry side and a very wet side because of the tradewinds, but is made of similar basaltic rock on both sides. They showed that rivers in wetter climates erode more efficiently (which is what we expected, but nobody had been able to measure it before!).

• Ferrier, K.L., K.L. Huppert and J.T. Perron (2013), Climatic control of bedrock river incision. Nature, 496, 206–209, http://doi.org/10.1038/nature11982. 
• Ferrier, K.L., J.T. Perron, S. Mukhopadhyay, M. Rosener, J.D. Stock, K.L. Huppert and M. Slosberg (2013), Covariation of climate and long-term erosion rates across a steep rainfall gradient on the Hawaiian island of Kaua’i. GSA Bulletin, 125, 1146–1163, http://doi.org/10.1130/B30726.1.
• Perron, J.T. (2017). Climate and the pace of erosional landscape evolution. Annual Review of Earth and Planetary Sciences, 45, 561-591, http://doi.org/10.1146/annurev-earth-060614-105405.

Paul Richardson found a similar natural experiment at an even smaller scale: slopes that face the equator get more sun than slopes that face the pole, creating “microclimates” – wetter and drier landscapes on opposite sides of the same hill. He found that this difference in climate can cause the opposing slopes to erode at different rates, eventually making one side of the hill steeper than the other.

• Richardson, P.W., J.T. Perron, S.R. Miller and J.W. Kirchner (2020). Modeling the formation of topographic asymmetry by aspect-dependent erosional processes and lateral channel migration. J. Geophys. Res., 125, e2019JF005377, http://doi.org/10.1029/2019JF005377.
• Richardson, P.W., J.T. Perron, S.R. Miller and J.W. Kirchner (2020). Unraveling the mysteries of asymmetric topography at Gabilan Mesa, California. J. Geophys. Res., 125, e2019JF005378, http://doi.org/10.1029/2019JF005378.

Paul and Naomi Schurr measured the rates at which soil moves downslope (due to both gravity and biological processes) at sites around the world with different climates. Interestingly, they found the strongest relationship between climate and soil transport among relatively dry landscapes, where the addition of a bit of life can have a large impact on soil transport, whereas soil transport rates are more uniform among relatively wet landscapes, perhaps because abundant biological activity both promotes soil transport and stabilizes slopes.

• Richardson, P.W., J.T. Perron and N.D. Schurr (2019). Influences of climate and life on hillslope sediment transport. Geology, 47, 423-426, http://doi.org/10.1130/G45305.1.

Another challenge is that a landscape’s climate might have been different in the past. Over the past few million years, for example, Earth has experienced dozens of ice ages, and this changed the climate even in places where there was no ice. Sam Goldberg is studying how rivers in the Amazon responded to the ice ages. He sees evidence that this caused repeated erosion and sediment aggradation, probably creating the many river terraces that allowed the first humans in South America to disperse throughout the Amazon and eventually develop agriculture.

• Goldberg, S.L., M. Schmidt and J.T. Perron (2021). Fast response of Amazon rivers to Quaternary climate cycles. Journal of Geophysical Research – Earth Surface, 126, e2021JF006416. https://doi.org/10.1029/2021JF006416.

Sam has also discovered how climate can unexpectedly mask the effects of rock strength on erosion. You might expect that rivers erode faster where the bedrock is weaker. That is exactly what most models of bedrock river incision typically assume, but if the river doesn’t have enough flow to carry all the sediment it receives from upstream, that sediment can start to cover the bed and slow down the erosion. Where might rivers tend to have less flow? Drier climates. Sam’s model predicts that rock strength affects erosion and topography less with increasing aridity, a prediction that he is testing by analyzing the topography in landscapes with different climates.

• Goldberg, S.L., M.F. Stokes, and J.T. Perron. Climate modulates the influence of rock type on bedrock river incision. In review.