My research focuses on studying past climate change (paleoclimate) in order to understand better what lies in our future, as greenhouse gas emissions rise. I use climate model output, statistical approaches, and paleoclimate data created here in our lab to piece together the history of Earth’s climate. My group specializes in the generation of organic geochemical records of past climate, derived from fossil molecules (called "biomarkers") preserved in sediments and rocks. Biomarkers produced by plants and microbes are particularly useful tools because environmental conditions like temperature and aridity are encoded in their molecular structure. We can calibrate these relationships using modern-day samples, and then apply these biomarker "proxies” to reconstruct climate deep into the Earth’s geologic past. We have also pioneered the use of statistical methods for reconstructing climate, developing Bayesian modeling techniques and paleoclimate data assimilation in order to generate spatially complete views of past climate and the dynamics behind it.

Research highlights

Past climates inform our future. Together with an international team of paleoclimate scientists, I led a review paper in Science on how studying past climates is critical for understanding future climate change. We argue that paleoclimate can provide critical constraints on Earth processes like climate sensitivity, and that climate models should be tested against ancient climates in order to assess their reliability for projecting climate changes under higher greenhouse gases.

J. E. Tierney, C. J. Poulsen, I. P. Montañez, T. Bhattacharya, R. Feng, H. L. Ford, B. Hönisch, G. N. Inglis, S. V. Petersen, N. Sagoo, C. R. Tabor, K. Thirumalai, J. Zhu, N. J. Burls, Y. Goddéris, G. L. Foster, B. T. Huber, L. C. Ivany, S. K. Turner, D. J. Lunt, J. C. McElwain, B. J. W. Mills, B. L. Otto-Bliesner, A. Ridgwell, Y. G. Zhang (2020). Past climates inform our future. Science, 370, eaay3701. Read here or contact me if you do not have access.
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How cold was the last Ice Age? Using a large collection of sea-surface temperature data from the Last Glacial Maximum (21,000 years ago) and paleoclimate data assimilation, we created spatially-complete maps of Ice Age climate. Our new reconstruction revises our estimates of cooling during the last ice. We found it was about 6˚C (11˚F) colder than pre-industrial times. We also used the Ice Age climate to estimate Earth's climate sensitivity, and found it to be 3.4˚C per doubling of CO2.

J. E. Tierney, J. Zhu, J. King, S. B. Malevich, G. J. Hakim, C. Poulsen (2020). Glacial cooling and climate sensitivity revisited. Nature, 584, 569--573. Read here or contact me if you do not have access.
Unraveling Indo-Pacific water isotope signatures during the last Ice Age. Graduate student Grace Windler used leaf wax isotopes and isotope-enabled climate model simulations to figure out what happened to the water cycle in the deep tropics during the last Ice Age. We found that the eastern Indian ocean was extremely dry, but that convection persisted in the western Pacific due to shift in the rising branch of the Walker circulation cell.

G. Windler, J. E. Tierney, J. Zhu, C. J. Poulsen (2020). Unraveling glacial hydroclimate in the Indo-Pacific Warm Pool: perspectives from water isotopes. Paleoceanography and Paleoclimatology, 35, e2020PA003985. Read here or contact me if you do not have access.
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