Areas of research

Coring the sediments of Prince Pond, Rhode Island.

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

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. Our group uses organic molecules preserved in lake sediments to reconstruct ancient environments and to understand why they changed. We 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.

Our tools include the hydrogen and carbon isotopic composition of plant wax compounds. We rely on our work with modern water isotopes (see below) to better interpret plant wax δ²H.

Active projects include (click links for award abstracts & more info):

• Exciting new project to reconstruct 50,000 years of Indian Summer Monsoon history and investigate monsoon dynamics using sediments from Loktak Lake, India (recently funded by the National Science Foundation) 
• Late glacial to late Holocene paleoclimate in the equatorial Andes from Lake Tota, Colombia (recently funded by the National Science Foundation)
• Late Holocene hydroclimate history of the eastern Amazon (funded by the David and Lucile Packard Foundation) 
• Late Holocene climate and environmental conditions of the Peruvian high Andes. This project is funded by the National Science Foundation, the National Geographic Society and the  International Center for Energy, Environment and Sustainability (InCEES) at Washington University
• Forward modeling 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.

   

Simulated precipitation in the Indo-Pacific Warm Pool during the Last Glacial Maximum (21,000 years ago).

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. In addition to generating new data and synthesizing other datasets, our group uses climate models to test how past climate states influenced important features of tropical atmospheric-oceanic circulation. 

Some of our group’s favorite tools include:

• The PAGES Iso2k Database, which contains hundreds of paleo-water isotope proxy records covering the past two millennia.
• The water isotope-enabled version of the Community Earth System Model and other climate models, which track water isotopologues and their fractionation throughout the water cycle.

We are actively using data synthesis and isotope-enabled climate model simulations to:

• Evaluate water cycle/temperature relationships at the global scale during the Common Era (paper just accepted at Nature Geoscience– coming soon!) (funded by NSF and the David and Lucile Packard Foundation)
• Reconstruct the behavior of the Pacific Walker Circulation during the past millennium (paper just published in Nature) (funded by NSF)
• Investigate behavior of climate modes in the North Atlantic region during the past several centuries (funded by the David and Lucile Packard Foundation)
• Evaluate changes in Central American hydroclimate during the Last Glacial Maximum and the Holocene using isotope-enabled models and “tagged water” simulations. We are using these simulations to help interpret 400,000 years of data from drill cores from Lake Petén Itzá (funded by NSF and the David and Lucile Packard Foundation) 

These projects are “past” in terms of funding, but investigations are ongoing: 

• Influence of glacial boundary conditions on moisture sources to the deep tropics (link)
• Migrations of the tropical rain belt during the Holocene (link)

One of our precipitation collectors in the Okavango Delta, Botswana.

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

The δ¹⁸O and δ²H of meteoric waters are powerful tracers of tropical precipitation, evaporation, and circulation. We 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.

Active projects include:

• Storm- to seasonal-scale analysis of triple oxygen isotopes in meteoric waters from Uganda, the United States, and elsewhere (funded by the David and Lucile Packard Foundation and the National Science Foundation)
• Optimization of laser-based techniques for triple oxygen isotope analysis (check out our recent paper in Atmospheric Measurement Techniques) (funded by the David and Lucile Packard Foundation)

Projects that are “past” in terms of funding, but investigations are ongoing:

• Impacts of a changing climate on water resources in the Okavango Delta, Botswana. This work was funded by the National Geographic Society.
• Characterizing the influence of Congo rainforest moisture on precipitation in East Africa. This project, a collaboration with physical and human geographers, was funded by the National Science Foundation.