Radiative forcing and climate response
The main area of my research is determining the climate response to radiative forcing. Essentially, if you push the climate, how does it push back? There are lots of ways to push the climate: changes in solar irradiance, changes in aerosols, changes in greenhouse gases, and so on. This question is fundamentally at the core of understanding climate change, in that humanity is adding a very specific set of radiative forcing agents to the climate system.
This question has a great deal of overlap with other areas of my research. The studies of geoengineering, volcanic eruptions, and aerosols all deal with the climate response to specific types of radiative forcing. I've also been using tools from control theory to better understand how one might be able to diagnose climate response, even though definitions of radiative forcing are not always clear, nor is radiative forcing always easy to calculate.
Quick and dirty fixes for climate change
Geoengineering, also called climate engineering, describes how mankind might deliberately modify the climate to reduce some of the effects of global warming. I am primarily interested in climate model simulations of solar geoengineering, which describes a set of technologies that reduce the amount of solar radiation that reaches the surface. Some examples include putting giant mirrors in space, putting sulfate particles in the stratosphere, or brightening marine low clouds.
A great deal can be learned about the climate system by simulating solar geoengineering in climate models. These sorts of investigations are proving to provide new ways of understanding the dynamical behavior of the climate system, particularly related to climate feedbacks and the hydrological cycle.
One of my main projects is the Geoengineering Model Intercomparison Project (GeoMIP), a worldwide collaboration of climate modeling centers. GeoMIP was designed to get as many climate models as possible to do the same computer simulations of geoengineering so we can determine what the robust climate effects might be.
Of course, climate model simulations are very different from the myriad scientific, social, political, and ethical issues that would come with real-world experimentation or deployment. These issues are quite complex and cannot be answered by only considering physical science. But it is these very issues that have opened new doors toward interdisciplinary research, and as a result, I have many colleagues in a variety of disciplines with whom I regularly interact.
I am not involved with any real-world tests of geoengineering, nor do I have any current plans to become involved with any such proposed or ongoing activities.
Stratospheric sulfate particles
When volcanoes erupt, they put out a bunch of different things, including ash, water, CO2, and sulfur dioxide. The latter gas gets transformed into sulfate aerosols, which are highly reflective microscopic particles. If these particles make it up into the stratosphere, which can happen with very large, explosive volcanic eruptions, the amount of sunlight reflected back to space can be enough to cool the planet.
One of my research interests is figuring out the climate effects of large volcanic eruptions. I have compared climate model simulations of stratospheric sulfate aerosols with observations for multiple volcanic eruptions. I also ascertained that for high latitude eruptions, the time of year of the eruption may be more important than how big the eruption is.
Particle interactions with sunlight and clouds
Pollution is a byproduct of many human activities. These particles are released from cars, power plants, and cooking stoves. These particles, also called aerosols, have masked anywhere between 0 and 50% of the amount of global warming we have experienced. That's a really large range, and the climate sensitivity to CO2 depends on where we actually fall within that range.
I am interested in aerosol effects on climate. What is the magnitude of aerosol radiative forcing? How do these aerosols interact with clouds? What are the future trajectories of global warming based on how sensitive the climate is to aerosol radiative forcing? Can changes in aerosol emissions in part explain changes climate variability?
Control theory and system identification
The climate system is incredibly complex and dynamic, so I am interested in tools that were developed for dynamical systems.
Control theory is essentially the study of feedback loops. My colleagues and I are among the first people to use tools from control theory to deliberately introduce feedback loops into general circulation models. Through these efforts, we have discovered new methods of simulating geoengineering, understanding climate system feedbacks, and comparing climate forcing agents. We have also revealed some of the fundamental roles feedback can play in managing uncertainty.
Chemical signatures of ecosystem stress
Plants are the source of approximately 90% of the global emissions of volatile organic compounds (VOCs). There is evidence to suggest that plants increase their VOC emissions when they are stressed (e.g., wounding, drought, heat, etc.). My colleagues and I are attempting to use VOC emission measurements as sensitive indicators of ecosystem stress. The implications of these investigations are profound: What are the human health effects of increased VOC emissions due to climate change? What are the climate effects of the secondary organic aerosols that form from VOCs? Can VOC emissions be used to quantify drought? Can VOC emission measurements be used in precision agriculture or biofuel production? (For example, such-and-such plant needs more water but cooler temperatures, so it should be irrigated and planted in such-and-such location.)
The effects of CO2 on coral reefs
Coral reefs are known to be sensitive to climate change, both due to increased heat stress and ocean acidification from increasing amounts of dissolved CO2 in the ocean. My colleagues and I are engaged in providing deeper understanding of the carbonate chemistry system on ecosystem scales. We are currently characterizing hystereses due to diurnal and nocturnal cycles between reef ecosystem net photosynthesis and calcification. We are also characterizing measurements of different carbonate chemistry parameters based on how much error each quantity introduces into the total measurements of dissolved carbon.