Research projects

Current projects

Reconstructing past hydroclimate variability in Southeast Asia

The Asian Monsoon system is an important component of the global climate system that plays a major role in the transport of heat and moisture from the tropics to higher latitudes.  Even small variations in the strength and/or timing of seasonal rainfall can have significant impacts on the billions of people living within the Asian monsoon domain, yet climate model projections of future monsoon changes still remain uncertain.  While paleoclimate records have significantly advanced our understanding of summer monsoon variability in some regions, we still know very little about the range and mechanisms of monsoon variability in Mainland Southeast Asia (MSEA). To address this need, we have been developing high-resolution multi-proxy records of MSEA paleoclimate over the Late Pleistocene to Holocene utilizing speleothems from Laos and Vietnam since 2010.  We are working in close collaboration with partners including the Middle Mekong Archaeology Project and Phong Nha Ke Bang National Park to conduct this work.

Through integrating paleoclimate proxy data, instrumental climate data, cave monitoring, and climate model analyses,  our research addresses three main questions:  (1) How have Southeast Asian monsoon intensity and regional precipitation patterns varied in response to orbital forcing, millennial-scale abrupt climate events, and interannual to multi-decadal climate modes?; (2) How are climatic and environmental signals recorded within the geochemistry of speleothem calcite?  What are the sources of uncertainty?; and (3) How do the MSEA speleothem records relate to broader spatial and temporal patterns of past climate variability in the tropical Indo-Pacific, the Asian monsoon region, and high latitudes?  This work aims to improve understanding of the dynamics underlying precipitation variability in MSEA with a broad goal of providing important constraints on climate model projections.

This work has been supported by National Science Foundation awards AGS-1405472, AGS-1603056, and AGS-2103129

Selected references:

Griffiths, M.L., Johnson, K.R., Pausata, F.S.R., White, J.C., Henderson, G.M., Wood, C.T.*, Yang,    H.*, Ersek, V., Conrad, C., Sekhon, N.* (2020). End of Green Sahara amplified mid-to late Holocene megadroughts in mainland Southeast Asia. Nature Communications, 11(1). (Featured in Nature Communicatons Editor’s Highlights: https://www.nature.com/collections/eihfbddfac)  Contributed equally to this work. https://doi.org/10.1038/s41467-020-17927-6

Wang, J.K.*, Yu, J.Y., Johnson, K.R. (2020). Pacific and Atlantic controls of the relationship between Mainland Southeast Asian and East China interannual precipitation variability. Climate Dynamics, 1-14. https://doi.org/10.1007/s00382-020-05227-0

Wang, J. K.*, Johnson, K. R., Borsato,A., Amaya, D.J., Griffiths, M. L., Henderson, G. M., Frisia, S., Mason, A .(2019).  Hydroclimatic variability in Southeast Asia over the past two millennia.  Earth and Planetary Science Letters, 525, 115737. https://doi.org/10.1016/j.epsl.2019.115737

Yang, H*., Johnson, K. R., Griffiths, M., Yoshimura, K. (2016). Interannual controls on oxygen isotope variability in Asian monsoon precipitation and implications for paleoclimate reconstructions. Journal of Geophysical Research: Atmospheres, 121(14), 8410–8428. https://doi.org/10.1002/2015JD024683

Media highlights:

Ancient megadrought may explain civilization’s ‘missing millennia’ in Southeast Asia

End of ‘Green Sahara’ May Have Spurred a Megadrought in Southeast Asia

UCI and international institutions link Southeast Asia megadrought to drying in Africa


Climate investigations with Mexican Archives (CIMA)

The majority of climate models project enhanced aridity throughout Mexico as a result of anthropogenic warming.  However, instrumental data, existing paleoclimate proxy records, and climate model simulations demonstrate that precipitation in the region exhibits substantial interannual to decadal scale variability in response to naturally recurring internal climate modes.  A more complete understanding of the impacts of natural variability and forced anthropogenic climate change on water resources in Mexico is critically important given the potentially serious impacts of increased drought frequency and magnitude on the Mexican economy, agriculture, and public water supply. While detailed paleoclimate records spanning the Late Holocene have provided insight into the mechanisms of interannual to multidecadal hydroclimate variations in tropical Mesoamerica, few such records exist in Northern Mexico.  Furthermore, only a handful of records are long enough to investigate millennial to orbital scale variability, and none are located in NE Mexico.  This project uses precisely-dated speleothems to develop the first records of NE Mexico hydroclimate  spanning the last glacial-interglacial cycle. The overarching goal of this work is to explore the dynamic response of regional precipitation to changing external boundary conditions (insolation, ice sheets, carbon dioxide) and internal factors, such as Atlantic Meridional Overturning Circulation (AMOC) and Atlantic and Pacific sea surface temperature variability. The specific objectives are to: (1) Determine the response of NE Mexican hydroclimate to orbital and millennial-scale forcing through construction of a centennial resolution 128,000 year multi-proxy speleothem record, (2) Determine the response of NE Mexican hydroclimate to decadal to multidecadal scale sea surface temperature variability through construction of a decadal resolution record of the Common Era, and (3) Synthesize this data with other available records and analyze spatiotemporal patterns in the hydroclimate record of Mexico, which combined with paleoclimate model analyses will enable testing of specific hypotheses about the dynamic mechanisms underlying Mexican hydroclimate variability.   To learn more about the CIMA project and our partners, visit our website here (Spanish version here).


Previous Projects

American Indian Summer Institute in Earth System Science (AISESS): A residential summer institute for American Indian high school students

Native Americans remain severely underrepresented in the geosciences, despite a clear need for qualified geoscience professionals within Tribal communities.  To address this need, we have developed the American Summer Institute in Earth System Science (AISIESS),  a two-week residential program for high school students. During the AISESS program, students spend one week camping on the La Jolla Band of Luiseño Indians reservation and one week on the UC Irvine campus.  During both weeks, students learn about Earth System Science through interactive discussions and hands-on laboratory and field exercises.  Each week also incorporates Native studies courses, visits from native STEM professionals, evening talking circles, and other cultural activities. The program culminates with a “mini-conference” wherein students present research projects on a geoscience issue relevant to their own tribal communities.  Previous research on learning indicates the most successful Earth Science curricula for Native American students share three common features (Riggs, 2007): (1) a strong emphasis on “place-based”, experiential, outdoor learning, (2) inclusion of relevant indigenous knowledge, and (3) involvement of native community members, elders, and educators.   Our program addresses these requirements through immediate immersion in field-projects on tribal lands, frequent interaction with the tribal community and tribal environmental professionals, inclusion of native educators, and completion of a capstone project on a specific earth or environmental science issue that is particularly relevant to their own tribal communities.

This project is funded by NSF award #1108524.

Media highlights:

https://ps.uci.edu/news/2398

https://www.earthmagazine.org/article/down-earth-cave-scientist-and-paleoclimatologist-kathleen-johnson

Testing Climatic Controls on Speleothem Radiocarbon

Reconstructing past atmospheric radiocarbon (14C) concentration is essential for improving the calibration of the radiocarbon timescale, investigating past variations in solar activity and Earth’s paleomagnetic field, and investigating the redistribution of carbon between terrestrial, atmospheric, and marine reservoirs that occurred over the past 50,000 years.  Uncertainty in the radiocarbon calibration curve during the deglaciation and last glacial period, however, limits our understanding of these key issues.  Previous work suggests that U-Th dated speleothems hold great potential for improving the record of atmospheric 14C, but may be complicated by changes in dead carbon fraction (DCF) derived from bedrock.  To fully realize the potential of speleothems for 14C calibration, therefore, there is a need for detailed process-based cave studies to determine the modern controls on speleothem DCF and the potential impact of past climate variability on DCF.  Towards this end, we are conducting a detailed study of C cycling at Heshang Cave, Hubei Province, China (30º27’N, 110º25’E; 294 m), the site of ongoing, extensive modern calibration and paleoclimate reconstruction efforts, through collection and analysis of the radiocarbon content of soil CO2, dripwater DIC, modern calcite, and fossil speleothem samples to investigate the controls on speleothem radiocarbon and to determine how these might change with climate.  This project was supported by NSF award # 0903101.

Timing and mechanisms of past hydrologic variability from Sierra Nevada speleothems

 Precipitation in the southwestern United States is highly seasonal and exhibits inter-annual to inter-decadal variability which is linked to naturally recurring large scale atmospheric circulation patterns associated with sea surface temperature (SST) anomalies such as the Pacific Decadal Oscillation (PDO) and the El Nino-Southern Oscillation (ENSO).  Proxy data from tree rings and lake sediments from the region indicate there were episodes of significantly decreased precipitation (~900-1250 AD) that were of greater magnitude and longer duration than any episode seen in the 20th century, yet the mechanisms behind these “mega-droughts” are still not well understood.  Cave calcite deposits (speleothems) represent one of the best terrestrial archives of past climate variability and hold great potential to improve our understanding of past hydrologic variability magnitude and mechanisms, but to this date have been widely underutilized in the western US.  In collaboration with the National Park Service, we are developing well-calibrated speleothem records of past hydrologic variability from Crystal Cave in Sequoia National Park.  We have been conducting a detailed modern calibration study since 2007 and are  currently completing a ~1,000 year, annual resolution speleothem record from this site.  This project has been supported by a Newkirk Center for Science and Society faculty seed grant.


References:

McCabe-Glynn, S.*, Johnson, K. R., Strong, C., Zou, Y., Yu, J.-Y., Sellars, S., Welker, J. M. (2016). Isotopic signature of extreme precipitation events in the western US and associated phases of Arctic and tropical climate modes. Journal of Geophysical Research: Atmospheres, 121(15), 8913–8924. https://doi.org/10.1038/ngeo1862

McCabe-Glynn, S.*, Johnson, K. R., Strong, C., Berkelhammer, M., Sinha, A., Cheng, H., Edwards, R. L. (2013). Variable North Pacific influence on drought in southwestern North America since AD 854. Nature Geoscience, 6, 617 – 621. https://doi.org/10.1038/ngeo1862