Research

My research is guided by one overarching question: How does a changing climate affect rainfall and ecosystems in the Earth’s tropical regions?

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Sampling spray from the Zambezi River at Victoria Falls, Zimbabwe

To address this question, I integrate stable isotope hydrology of modern environments, sedimentary organic geochemistry of past environments, and earth system model experiments that connect the two.

I focus dually on ancient and modern environments in order to provide a geologic perspective on present-day climate change, while using present-day climate change to inform the geologic past.

 

THE MODERN TROPICAL WATER CYCLE

How does global warming and cooling influence how water moves between tropical land surfaces, the atmosphere, and the ocean? How does it influence rainfall?

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Our rain gauge and  rainwater collector in Sulawesi.

The δ18O and δ2H of meteoric water are excellent tracers of tropical precipitation, evaporation, and circulation. I use modern in situ and satellite measurements of water isotopes to decode the physical processes – such as monsoonal moisture sources, transport paths, and moisture recycling – that drive the tropical water cycle.

I then measure water isotope variations at poorly-studied sites in Africa, Southeast Asia, and the tropical Pacific to better understand the processes that influence moisture availability. For example, in Spring 2016 I established a climatic and isotopic monitoring program in and around the Okavango Delta, southern Africa, a World Heritage Site that is changing rapidly due to rising temperatures. This work was funded by the National Geographic Society.

ANCIENT CLIMATES AND ENVIRONMENTS

What drives long-term variations in tropical monsoons, and how do variations impact – or feed back with – terrestrial land surfaces and ecosystems?

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Coring the sediments of Prince Pond, Rhode Island.

High-quality, instrumental climate records only date back ~150 years. Paleoclimate archives help us understand what caused natural climate variability and climate change before that, and how those processes relate to what we’re seeing today. I use organic molecules preserved in the lake sediments to reconstruct ancient environments and to understand why they changed. I focus on hydroclimate during geologically “recent” time periods, e.g. the past millennium and the late Pleistocene/Holocene (the past ~60,000 years), because they’re highly relevant to human time scales.

My tools include the hydrogen and carbon isotopic composition of plant wax compounds. I rely on my work with modern water isotopes (see above) to better interpret plant wax δ2H. I am also building a forward model of plant wax δ2H using measurements of modern plants and the NCAR Community Earth System Model. Forward models allow us to make “apples to apples” comparisons among different paleoclimate proxies, and to directly compare results with climate models (see below).

EARTH SYSTEM MODELING AND DATA SYNTHESIS

How do individual climate forcings, such as greenhouse gases and changes in the Earth’s orbit, affect rainfall and water cycling in the tropics?

Measurements can only tell us what happened. Climate models tell us why. I use the water isotope-enabled version of the NCAR Community Earth System Model to test how climate forcings altered tropical climate and ecosystems. I then compare these results to proxy reconstructions and to modern water isotope observations.

I am currently working on modeling projects for three different time periods: the Last Glacial Maximum, the past millennium, and the Holocene (just funded by NSF!).

In addition, I coordinate the PAGES Iso2k Project. Iso2k is a community-driven effort to develop a global database of paleo-water isotopes covering the past two millennia. The database will be used to investigate decadal to centennial-scale variability in atmospheric circulation and relationships to temperature change, from both data and model perspectives.

For more info, check out my current Publications.