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Research Overview

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Changes in atmospheric composition and chemistry can be powerful drivers of the Earth's climate, as we are witnessing today with the human-caused increase in greenhouse gases and the resulting gradual warming of our planet.

 There is a great need for an improved understanding of the Earth's climate system and how it responds to various forcings. For example, we know with certainty that the anthropogenic increase in greenhouse gas concentrations is causing the Earth to warm, and will continue to do so. But there is uncertainty in how much and how fast the planet would warm for a given increase in carbon dioxide concentrations. The Earth's climate system is vast and relatively slow-responding, so it's not feasible to study it by direct experimentation (as you would with a smaller system in a lab setting). One of the best ways we can gain understanding about the Earth's climate system is to examine the geologic record of past climate. In this way we can improve our understanding of how the Earth's environmental conditions (e.g., temperature) respond to changing climate forcings (e.g., carbon dioxide concentration).

 Most of my research uses records from ancient glacial ice to answer questions about the Earth's climate system. The oldest ice on Earth is contained in the Greenland (over 100,000 years old) and Antarctic (over 800,000 years old) ice caps. As this ice accumulated slowly (from fallen snow in places where it's too cold to ever melt) year after year, it recorded environmental conditions such as temperature (in oxygen and hydrogen isotopes) and windiness (in the amount and grain size of dust particles). One very special attribute of glacial ice as a geologic record is that it also records past atmospheric composition. When snow gets transformed to ice in a glacier (via slow recrystallization under pressure of overlying snow), some of the air present in the snow gets trapped in bubbles in the ice. Because of this trapped ancient air, we now know how greenhouse gases have varied in the atmosphere for the last 800,000 years.

 One main direction of my research is using carbon-14 of ancient atmospheric CH4 to understand past changes in the global CH4 budget. Carbon-14 is an excellent tracer for distinguishing between biospheric (e.g., wetlands, animals) and geologic (methane hydrates, natural gas seeps) CH4 emissions. Our work using ancient ice samples from Antarctica is showing that natural wetlands respond to climate warming with increased methane emissions. This work is also showing that large reservoirs of ancient carbon (such as methane hydrates and permafrost) do not seem to release significant amounts of methane into the atmosphere in response to warming, which is good news for the future. Other work using more recent (preindustrial age) ice from Greenland is also showing that emissions of fossil methane from natural gas seeps are very small, and that current estimates of anthropogenic fossil methane emissions are too low.

 Another direction of current research is reconstructing past carbon monoxide (CO) concentrations and stable isotopic composition. CO is a highly reactive gas and is a major sink of atmospheric hydroxyl radicals (OH), which serve as the "cleansing agent" of the atmosphere. Through its impact on OH, CO concentration in the atmosphere can have a profound influence on the lifetimes of other gases and on atmospheric chemistry in general. For example, increasing CO may cause a drop in OH. OH is what removes methane (CH4) from the atmosphere, so increasing CO may cause an accompanying increase in atmospheric CH4, which is a powerful greenhouse gas. CO stable isotopes help with understanding the changes in the individual sources and sinks of CO.

We are also working on new projects that involve the use of carbon-14 of carbon monoxide as a more direct tracer for atmospheric OH, both in the past (for the preindustrial period) and in the modern atmosphere.

 I am also interested in cosmogenic production of carbon-14 in glacial ice. Carbon-14 is produced in glacial ice by secondary cosmic ray particles (e.g., fast neutrons, muons) from oxygen-16. Cosmogenic carbon-14 content in glacial ice can tell us about ice flow history and ablation rate, and may contain information about past variations in solar activity.

Research Interests

• Understanding natural and anthropogenic climate and environmental change, particularly from the perspective of atmospheric composition and chemistry.