Reading the (Mormon) tea leaves at the bottom of packrat nests tells the story of Late Quaternary climate change in Western North America

Lay summary authored by Robert Harbert. Their full paper can be read in Open Quaternary:

During a 1980 episode of Cosmos Carl Sagan said “You have to know the past to understand the present.”2 For those of us who study geographic distributions of plants it is clear that the plant fossil record can tell us a lot about how ecosystems have changed through Earth’s history. By studying the shifts in where certain plants occur during different climatic periods we can begin to understand how present-day ecosystems may respond to anthropogenic climate change.

In this study we take this reasoning and invert it. The present can inform our understanding of the past. By taking data on the distribution of plant species today from the Global Biodiversity Information Facility (GBIF) and combining those with current climate models we can build a predictive model for climate based on local plant communities. We call this modeling protocol CRACLE (Climate Reconstruction Analysis using Coexistence Likelihood Estimation).

Here we apply CRACLE to the estimation of climate from Western North America over the past ~50,000 years. We are able to do this due to a combination of the fact that packrats (Neotoma spp.) are compulsive hoarders of plant twigs, fruit, leaves, and seeds and the arid climate preserves plant material in packrat nests (also called ‘middens’) for thousands of years. By analyzing samples of these plants from a range of time throughout this region we are able to piece together the climate history of Western North America as recorded through the changing distributions of plants.


A 14,600 year old fossilized packrat (Neotoma spp.) midden from Northern Baja California, MX. Numerous plant fragments, packrat dung, and small stones visible embedded in the matrix. Visible twigs, leaves, and seeds likely include Juniperus californica, Agave, Artemisia, Ephedra, and various cacti, composites, and grasses. Credit: R. Harbert

Our climate record shows that Western North America was 4-6°C cooler during the Last Glacial Maximum (LGM — 26-19ka) with the most pronounced departure from modern climatological averages coming during the warmer months. We also calculate that the rapidly warming climate of the period just after the LGM shows warming at a rate that is approximately 10 times slower than what is predicted for the next 100 years. During past warming periods, as glaciers melted, species migrated north in step with the changing climate, but species today must contend with climate change occurring at a rate not seen in at least the last 50 millenia. Whether or not migration will happen fast enough to prevent mass extinctions in our future remains to be seen as we find no analog for this rate of change in the packrat midden fossil record.

This record fills in a substantial gap in the terrestrial paleoclimate record of Western North America and represents a novel source of data. Many thousands more samples of fossilized packrat middens exist and are being studied. As we know more about where plants lived in the past we will be able to use CRACLE modeling to build more detailed and robust reconstructions of paleoclimate for this region.

Title: ”Mormon Tea” is a common name for the plant genus Ephedra, a common component of packrat midden macrofossil assemblages.
2Sagan, C. E. (author and presenter). (1980) Episode 2: One Voice in the Cosmic Fugue [Television series episode]. In Adrian Malone (Producer), Cosmos: A Personal Voyage. Arlington, VA: Public Broadcasting Service.

Read the full paper:

Harbert, R.S. and Nixon, K.C., 2018. Quantitative Late Quaternary Climate Reconstruction from Plant Macrofossil Communities in Western North America. Open Quaternary, 4(1), p.8. DOI:

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