Mike Evans

Dr. Evans is located at the Dept. of Geology & ESSIC, University of Maryland, CSS 3239, College Park, MD 20742.

Dr. Evans is interested in the mechanisms by which tropical processes both influence and respond to global change on seasonal to centennial timescales. The role of the hydrological cycle is especially important to uncover, because of the multiple, intricate, and linked roles of all three phases of water in the climate system, and the importance of water resources for society. Are new mechanisms required, or are the observations consistent with the activity of known patterns of climate variability? Validation of mechanistic studies using paleoproxy data are limited, because proxy paleoclimate data tend to be sparsely and unevenly distributed, multivariate in nature, and contain systematic errors. Dr. Evans addresses these questions and problems using a combination of proxy data development, analysis, and modeling.

1. Development and calibration of chronology and rainfall estimates from trees from the terrestrial tropics, using intraseasonal oxygen isotope measurements. A continuous flow stable isotope ratio mass spectrometry facility has been established, and a new, economical, rapid and precise method for making the necessary oxygen isotopic measurements has been developed. Pilot studies in Costa Rica, Peru, Indonesia and Brazil established the viability of the approach. Projects are underway to develop long paleo-raingauge records and error estimates from sparsely-observed ENSO and monsoon influenced regions worldwide.
2. Forward modeling of proxy observations to better understand underlying nonlinear, multivariate and frequency dependencies. The Vaganov-Shashkin (VS) forward model of the environmental and biological controls on conifer tree growth was validated for almost 200 sites in North America and Russia. An observed and modeled mid-1970s shift in southeastern US tree growth dependencies was associated with the regional dynamical effects of anthropogenic climate change. Future projects include identification of data biases, study of forest growth under future and past climate change scenarios, and model development.
3. Inverse modeling of paleoclimates based on sparse observational networks and modified objective analyses. The Pacific sea surface temperature (SST) field was reconstructed for up to the past four centuries from coral and tree-ring data. Results are available from the World Data Center-A for Paleoclimatology. Future projects include development of more robust calibration and reconstruction methodologies, joint precipitation-temperature-drought field reconstructions, and improved reconstructions of the Hadley Circulation and of Pacific decadal variability.

Use of physical climate models and data analyses to investigate mechanisms of tropically mediated climate change on time scales relevant to the greenhouse warming debate. Based on 2-4 centuries of reconstructed Pacific Basin sea surface temperature fields, it was suggested that Pacific decadal SST variability may result from decade-to-decade changes in the strength and frequency of ENSO, an idea consistent with recent modeling and observational studies. Future research will continue to explore the viability of this hypothesis.