Over the last two centuries, human civilization has become a geological force. We are inducing planetary environmental conditions like those that Earth has not experienced for millions of years. Without mitigation of emissions, we may generate greenhouse gas concentrations and global temperatures more akin to those of the early Paleogene, over forty million years ago, than those of the current geological period, the Neogene. Recognizing human society’s new power, my group’s research has two major themes: (1) understanding different past states of the Earth system and the transitions between them, in order to test models of future global change, and (2) quantifying human impact on the global climate and assessing technology and policy solutions for limiting this impact. Some examples of our research are highlighted below.

Because humans are creating climatic forcings unprecedented in human experience, the records of Earth’s ancient past provide one of our best guides to paths the Earth system may take in the future. My research group seeks to understand these records through two different approaches: statistically synthesizing multi-proxy data sets to construct and test global-scale hypotheses about ancient climate and biogeochemistry, and developing magnetic techniques for studying the sedimentary record of the biogeochemical iron cycle and interpreting its environmental implications.

Our work on past sea level change is motivated by the fact that a changing climate is causing global sea level (GSL) to rise, partly as a consequence of melting continental ice sheets. A critical source of information about the future susceptibility of ice sheets to collapse, and thus about what constitutes ‘dangerous anthropogenic interference’ with Earth’s climate system, is the record of past sea level changes. During the Pleistocene (the last 2.6 million years), GSL retreated to as low as about 130 meters lower than today during peak glaciations. During the Last Interglacial stage (about 125 thousand years), global temperatures were likely slightly warmer than today – by an amount similar to that expected under low-end future scenarios – and the elevations of fossil reefs and geomorphological markers suggest GSL may have been a few meters higher than today. If it was, then two urgent follow-up questions are: what ice sheet melted to produce those extra meters of water, and how long did it take to melt?

Kopp et al. (2009) took a first crack at these questions by compiling a new database of Last Interglacial sea level proxies that quantifies their uncertainties in a consistent manner, using a geophysical model of sea level to estimate the uncertainties of and covariances in the relationship between ice sheet melt and regional sea level change, and developing a novel statistical model for combining these uncertainties to produce a probability distribution for local and global sea level through the Last Interglacial.

Together with colleagues at Harvard University, my research group is now focusing on extending this statistical model from sea level to the volumes of individual ice sheets, examining multiple interglacial periods, and assessing the relative importance of external forcing and internal dynamics in explaining the differences between these interglacials.

Because iron, the fourth-most abundant element in Earth’s crust, is redox-active under surface conditions, it is a critical element in Earth surface processes: both a key player in many abiotic reactions and an essential nutrient for living organisms. The bio-availability of iron is a product of environmental conditions; through its effects on biological productivity, it is also a feedback. Changes in the sedimentary record of iron cycling therefore reflect broader biogeochemical and climatic changes.

Our current research in this area focuses on understanding the paleoenvironmental implications of a radical change in sedimentary iron biogeochemistry in the mid-Atlantic U.S. during the Paleocene-Eocene Thermal Maximum (PETM), a severe global warming event that occurred 55 million years ago. Kopp et al. (2007) and Schumann et al. (2008) found that a clay layer deposited during the PETM in the Salisbury Embayment (which stretches from New Jersey to Virginia) recorded the unusually rich growth of magnetotactic bacteria and of other unique and presumptively eukaryotic iron biomineralizing organisms. Combined with knowledge about the ecological distribution of modern magnetotactic bacteria, this finding suggests that biogeochemical changes during the PETM led to the development of enlarged suboxic zones in the sediments of the Atlantic Coastal Plain.

Because meter-scale suboxic zones occur today within the mobile mud belts of tropical river-dominated continental shelves, such as the Amazon Shelf, we hypothesize that sedimentological and hydrological changes during the PETM fostered the development of analogous conditions on the Eastern seaboard of North America. By mapping the distribution of magnetofossils and the PETM clay, Kopp et al. (2009) found support for an Appalachian-fed, Amazon-like river system located around the modern Potomac or Susquehanna rivers. The development of this river system may be linked to changes in temperature, precipitation, rainwater acidity, and storminess.

Key next steps in this research area focus on testing the “Appalachian Amazon” hypothesis by examining Paleogene geochemical and sedimentological changes across the Salisbury Embayment, expanding the magnetofossil database to other regions and other times and on better understanding the preservation of magnetofossils in modern analog environments.

Limiting climate changes requires a global clean energy revolution, replacing the carbon-intensive energy technologies of the nineteenth and twentieth centuries with more sustainable, low-carbon alternatives. Achieving this revolution in a manner that minimizes the costs and maximizes the benefits to society requires an integrated perspective that ties together human and natural systems and attempts to comprehend the uncertainties within them. My research group seeks to bring the global, long-term perspective of geobiology and Earth system science to this pressing policy challenge. Our key tools in this research area are integrated assessment models (IAMs), which combine representations of human systems (such as the global economy) and the Earth system.

A variety of governmental regulations, including energy efficiency regulations, have the primary benefit or secondary co-benefit of reducing greenhouse gas emissions. Full cost-benefit analysis of such regulations must incorporate the benefit to society of reduced greenhouse gas emissions. This objective can be accomplished through the use of the “social cost of carbon” (SCC), an economic measure of the increase in social welfare associated with reduced carbon dioxide emissions. The U.S. government’s first official SCC estimates were published in March 2010 and have since been employed in a range of energy efficiency and air pollution regulatory impact assessments. The current US government estimates suffer from significant limitations, however (see Kopp & Mignone, 2011, for a review). My research group is currently exploring alternative ways of estimating the SCC, with focuses on improving the incorporation of scientific and economic uncertainty and risk aversion, and on improving reduced-form IAMs of the sort used in calculating the SCC.

One major omission from the current SCC estimates is the effects of potential Earth system “tipping points,” such as West Antarctic or Greenland ice sheet collapse, shutdown of the Atlantic Meridional Overturning Circulation, disruption of the Indian Summer Monsoon, permanent El Niño, or sustained drought in the Amazon rainforest. Incorporating such tipping points into policy analysis requires better understanding of their socio-economic consequences and of the susceptibility of the Earth system to crossing them. To explore the former, my research group is employing climate change scenarios in which tipping points are crossed to force the energy system and land use in a higher-complexity IAM. Our work on the geological record of past Earth system changes can help explore the latter.

Among the prominent consequences of global warming is sea level rise. Sea level rise is generally considered in terms of changes in mean global sea level, but from a human perspective this is not the most relevant quantity; nobody lives on the mean globe. Of greater importance for adaptation is regional sea level rise. Two major categories of effects cause large differences between regional and global sea level: dynamic effects, related to ocean-atmosphere circulation and steric inhomogenities, and “static equilibrium” effects, related to changes in the Earth’s gravitational field, the deformation of the lithosphere, and the orientation of Earth’s spin axis caused by melting ice sheets. These two categories of effects are studied by different sets of researchers: the former is the realm of climate modelers, while the latter is primarily the domain of geophysicists who study post-glacial rebound. Kopp et al. (2010) was the first global study to examine projections of the relative magnitude of these effects in the next two centuries. This study used an idealized melt scenario and assume that no feedbacks occur between dynamic and static equilibrium effects. My research group’s future efforts will involve more realistic scenarios and test the potential for feedbacks. One of our goals is to provide a tool for regional policymakers that integrates all the relevant factors contributing to local sea level rise in order to help guide infrastructure planning.

Since the publication of an editorial by Paul Crutzen in 2006, there has been an increasing amount of discussion among scientists about deliberate attempts to modify Earth’s climate through techniques such as the injection of sulfate aerosols into the stratosphere. While full-scale implementation of such geoengineering techniques is not likely in the near term, a number of prominent voices have called for scientific and engineering research into these technologies, including “small-scale” field trials. For a field trial to be useful, however, it must produce signals detectable above background climatic variability; as a consequence, such trials may also significantly impact humans and ecosystems. I therefore believe that such experiments should, like biomedical experiments on humans and animals, take place only within a well-established and throughly considered ethical and policy framework. Morrow et al. (2009) developed candidate ethical guidelines based upon those governing human and animal research; we are now examining institutional frameworks for implementing these or similar guidelines.