Research Interests (This is out of date; see Publication and Group )

• Climate Dynamics
• The Quasi-equilibrium Tropical Circulation Model (QTCM)
• Biosphere-land surface-atmosphere Interaction
• The Simple-Land (SLand) model
• Amazon Deforestation
• Vegetation-climate interaction in Africa
.............. Sample media coverage of the Science article
• Carbon cycle-climate interaction

Description

My general research interests are in the field of climate change and climate variability on time scales ranging from seasonal-interannual to glacial-interglacial cycles. My approach is to study the Earth system as a whole, focusing on the interactions among various components, in particular, the atmosphere, the hydrosphere and the biosphere. Currently my research covers two different but inter-connected areas: carbon cycle-climate interaction and the modeling of atmosphere-land-vegetation-ocean system. I characterize myself as a modeler and theoretician, but often I also find myself eagerly involved in observation and data analysis that provide constant inspiration for, and needless to say, the final judgement of model and theory.

• Climate variability; Amazon deforestation; Water budget study; Interannual and interdecadal variability in the African and Asian monsoon regions

  Low frequency atmospheric variabilities on seasonal to interdecadal time scales are partly due to slow changes in the atmospheric boundary conditions such as Sea Surface Temperature (SST), soil moisture and vegetation, which are in turn influenced by the atmosphere. The longer time scales inherent in these components are thus useful in predicting climate beyond the chaos-limited weather predictability of a few days.

  My research uses the information of these boundary conditions both offline or in an interactive manner to explore the extent to which climate can be predicted. For instance, in a recent paper published in the journal Science , I used a coupled atmosphere-land-vegetation model to study the interannual and interdecadal climate variability in the Sahel where it had undergone a long drought from late 1960s to the 1980s. This work demonstrated the importance of a dynamic vegetation and soil moisture in enhancing this multi-decadal variation initiated by a change in the global SST pattern. I am also concerned about human impact on the climate and ecosystem such as deforestation and desertification. I believe these effects can not be well understood without a deep understanding of the natural variability at first place. I thus try to study both natural and anthropogenic changes. For this purpose, an extensive use of models and comparison with observational data have become essential.

• Carbon cycle and climate change--interaction of the physical and biochemical world

  As a major greenhouse gas, carbon dioxide has played a major role in the changing climate throughout Earth's history. During the last 400 thousand years, the atmospheric CO2 concentration has gone up and down, often in synchrony with temperature changes, as recorded in the air bubbles trapped in the Antarctic ice sheet. On this background of relatively slow CO2 variation, is the rapid CO2 increase caused by human activities, namely fossil fuel burning and land use changes.

  Many mysteries remain in the global carbon cycle, such as what caused the cycles in the glacial-interglacial CO2 variation, and the `missing carbon sink' which is yet to be accounted for in order to close the present-day carbon budget. My current research aims to contribute to our understanding of these questions. A less emphasized issue is the role of terrestrial biosphere which exhibits high heterogeneity in space from individual plant to continental scales, and multiple time scales from minutes for leaf-level transpiration to many thousands of years for soil carbon accumulation. I have developed a model called VEGAS (Vegetation Global Atmosphere and Soil) to address vegetation dynamics and biospheric carbon cycle. This is has been coupled to the HAMburg Ocean Carbon Cycle model (HAMOCC) to address both the glacial-interglacial and present-day carbon-climate interaction.

• Earth system modeling: coupled atmosphere-land-ocean-carbon modeling, with applications to past and future climate change

  Computer model provides a powerful tool for studying the complex Earth system. I have been using models with hierarchical complexity, ranging from simple analytical models to comprehensive GCMs (General Circulation Models). But my main tools are a suite of models slightly less complex than the state-of-the-art GCMs, but including the major physical and biogeochemical processes involved.

  Models developed in house include:

  VEGAS (VEgetation Global Atmosphere and Soil): vegetation dynamics and terrestrial carbon cycle; coupled to SLand.

  SLand (Simple-Land): Land surface energy and water budget, flux exchange with lower atmosphere.

  QTCM (Quasi-equilibrium Tropical Circulation Model): a simple GCM, developed in collaboration with D. Neelin at UCLA; in collaboration with R. Iacono (ENEA/Italy), a global version of this model has been developed.

  In addition, these have been coupled to the HAMburg Ocean Carbon Cycle model (HAMOCC) (developed at the Max-Planck Inst. for Meteorology) and a simple physical ocean model.

  We have an Earth system model that simulates the important geophysical and biogeochemical processes, yet fast to run and easy to analyze. In the past, scientists have been somewhat limited to either highly complex models such as GCMs, or extremely simple models such as energy balance models (EBMs). Thanks to the improved understanding of the climate system, and to the fast increase in computing power, it has become possible to have a reasonably sophisticated model with balanced complexity among the individual components, yet much faster than GCMs (For instance, QTCM is about 200 times faster than CCM3 at similar resolution; yet it performs well in simulating climate variations such as El Nino influence on global precipitation.). Indeed, most of the models described above have been developed/run on a laptop computer runing linux. In addition, we also use semi-empirical models as well as GCMs, depending on the problem in hand. These models are enabling us to tackle some long-standing issues in paleoclimatology and climate change.