Research

West African Climate Variability - Past 1 million years

Sub-Saharan Africa is home to millions of people who rely on rain-fed agriculture. It is also prone to sudden, prolonged droughts which have devastating human consequences. The well-known "Sahel Drought" began suddenly and unexpectedly in the late 1960's, resulting in widespread famine, mass emigration, and political instability. While the drought is well documented in instrumental records, the underlying causes are not completely understood. The relative brevity of the instrumental period limits our capacity to understand long-lived droughts; paleoclimatic records in the region a sparse, but offer an opportunity to enhance our understanding of drought in the region.

My research focuses on examining a globally unique climate archive, sediment from Lake Bosumtwi, a 1.07 million-year-old crater impact lake in Ghana, West Africa.

The sediment cores from Lake Bosumtwi were collected during a 2004 ICDP coring mission. A total of nearly 2 km of sediment cores were collected, including the entire 300-m-long sequence that spans the entire history of sedimentation at the lake. This sediment record is among the longest, and highest-resolution terrestrial records ever collected, and it allows me and my colleagues to address some critical questions about West African climate history.

Analyses of Lake Bosumtwi sediments have already yielded important results. Former DGESL student Tim Shanahan led a study that used a multiproxy approach to reconstruct hydrologic variability from the lake sediments over the past 2660 years, and found that the region is prone to prolonged, multidecadal to century-long droughts that are more severe than the "Sahel Drought" of the late 20th century. They also found evidence that the West African Monsoon and drought in the region is impacted by variability in Atlantic sea surface temperatures. Their findings were published in Science in April 2009.

Scanning x-ray fluorescense (XRF) can provide a semi-quantitative measure of elemental abundance in lake sediments, and was one of the techinques used by Shanahan et al. (2009), to reconstruct past hydrologic variability. One of my primary research efforts is to extend the Lake Bosumtwi record of climate variability back through the complete million-year history of the lake, and to evaluate the utility of XRF to reconstruct other climatic information, specifically, changes in wind speed and direction, and the position of the ITCZ.

Late Holocene Climate and Glacier Variability in the Arctic

The Arctic is a particularly interesting, and important, player in global climate, and understanding the dramatic changes that have occurred there in the past has long been research interest for geologists and paleoclimatologists. On longer timescales, changes in the size of ice caps mediates many of the primary climatic changes of ice ages, and on shorter timescales, positive feedbacks make the region particularly sensitive to changes in global climate. This is true of anthropogenic warming as well; the Arctic has experienced many of the most severe impacts of warming in the 20th century, and is projected to warm the most in the 21st century.

I have had the opportunity to study arctic paleoclimate and glacier-lake systems for several years now. In 2004, I was a member of the Svalbard REU, cutting my teeth in Arctic fieldwork, and delving into the mechanics of modern glaciolacustrine sedimentation dynamics at Lake Linné.

More recently, I studied the climate histories recorded by the lake sediments and glacier forefields of two lakes (Hallet Lake and Greyling Lake) in the Chugach Mountains of south-central Alaska. Both glacier-lake systems document a gradual cooling, and expansion of glaciers over the past ca. 4000 years, until the past 100 years, when temperatures warmed. and the glaciers retreated rapidly. The results of this multiproxy, paired-lake study were published in a special issue of the Journal of Paleolimnology, and can be found online here.

In Hallet Lake sediments, changes in biogenic silica content (BSi), a biogeochemical proxy of whole lake production, proved to be a sensitive recorder of summer temperatures in the region. This allowed to develop a relationship between the lake sediments and climate, and to reconstruct summer temperatures in the region for the past 2000 years at decadal resolution. The record shows a gradual cooling over the past 2000 years, with warmer than average periods from 100 to 500 AD, and 1300 to 1500 AD, and cooler periods from 600 to 1200 AD and 1700 1900 AD. The warming from 1870 AD to the present is both the most rapid, and largest warming over the past 2000 years. The details of our BSi-temperature calibration, and the summer temperature record, are published here.

Most recently, I collaborated in a project to synthesize decadal-scale climate variability in the Arctic over the past 2000 years. We compiled 23 well-resolved paleoclimate records from lakes, ice cores, and tree rings from around the Arctic, and found that they tracked temperature well through the instrumental record, and share a common primary signal over the past 2kyr - a gradual cooling forced by decreasing summer solar insolation driven by orbital precession. Despite the continued reduction of insolation due to precession, this cooling was reversed in the 20th century. While our study does not address the cause of the reversal, an abundance of research, well summarized by the 2007 IPCC , makes it clear that increasing concenrations of greenhouse gases were primarily responsible for the reversal of the trend. Another important result of this work, was that the sensitivity of the synthesized temperatures to insolation was virtually identical to those modeled by NCAR's CCSM climate model, improving our confidence that the model is realistically simulating these effects in the arctic. Our results were published in Science in September 2009. Also, I was interviewed on the BBC about our results, and you may hear that interview here