New Methods of Identifying Climate Change in Desert Southwest ‘Unearthed’

LAS CRUCES, NM  - Researchers at The Jornada Experimental Range and New Mexico State University are learning to uncover thousands of years of climate data in environments where no record previously existed. In order to do this, they are digging into the earth itself for a chronology of how vegetation has shifted between grasslands and shrublands in the Desert Southwest. Not only does this give scientists a new method of peering into the climactic record in areas they could not before, but the information they gain will help differentiate between human-caused and natural cycles of climate change.

Every year the monsoon brings much needed rain to the Southwest. This transient moisture transforms the desert and allows the grasslands, that both wildlife and people depend on, to emerge.  When the rains end, the grasses will have spread their seeds, dried up, and will wait for next summer’s rain. Very little here is permanent.

Chihuahuan Desert Grassland Rangeland
Healthy Chihuahuan Desert grassland during monsoon season.

There are no long-lived trees from which scientists can read tree-ring data, or lake sediments to obtain a record of climate change from. The aridity of the environment becomes a barrier to reconstructing a climate record. However, Curtis Monger, LTER Co-Principal Investigator, and his team have been able to bypass this problem by looking to the soil itself. With a combination of chemical analysis, carbon-14 dating, and erosion studies, we can now know if an area of land was covered in grass or shrubs in the past and correlate that with changes in climate.

Does The Soil Remember?

“We have a hypothesis that the landscape remembers climate change,” said Monger who spearheaded the study with funding from the National Science Foundation. “If you go out and get a soil sample in between shrubs where there are no grasses, and using techniques for looking at the carbon isotopes, we can understand if the organic matter in the soil is from grasses that no longer exist or from shrubs that do exist today.”   Monger found that, yes the soil does remember having grasses growing in the past, but what it didn’t tell them was exactly when the grasses grew there. To develop a timeline, they needed a stratigraphic record. They found it in a soil feature that only exists in arid environments.

Chihuahuan Desert Shrubland Erosion
The soil between the shrubs still "remembers" the grasses that once grew there.

Caliche forms in most arid lands when rain carrying calcium carbonate falls onto the soil. Because it is so dry the moisture only sinks down a few feet, where the calcium carbonate begins to accumulate and form an impenetrable layer. Once this layer is formed, new layers form on top of the old, over and over for millennia. Monger hypothesized that they could carbon date these layers and measure the carbon isotopes within each layer. “A record like this,” said Monger pointing to a sample of caliche pulled from an excavation, “that’s less than an inch, can take us back 10,000 years in time.”

The record was so condensed that the millimeter thin slices sampled contained a thousand years of time, too poor of a temporal resolution to be useful. The landscape shifted between grassland and shrubland repeatedly during that time span, so the researchers were unable to create a detailed timeline. The next goal is to use a laser to shave off microns of material to get the resolution down to about 100 years. This will allow for a detailed record to be created.

Caliche Sample Carbon Dating Climate Record
A slice of caliche from beneath the desert soil. yr B.P. is years Before Present.

A third approach helping scientists decode historical climate is found in the layers of eroded sediment. During strong rainstorms, desert shrublands erode much more substantially compared to desert grasslands. By excavating deposited soil downslope and carbon dating sediment layers, scientists can estimate when upslope grasslands transitioned to shrublands.

Soil Erosion Layers
The gray layer at top is the eroded soil from upslope when the grassland transitioned
to shrubland. Notice the distinct boundary between it and the darker layer in the middle.
That boundary was the surface thousands of years ago.


A New Paradigm Shift

Climate change has unique effects on different environments. This new soil memory approach of creating a climate record allows climate scientists to understand natural cycles of climate change as they occur in arid environments.

The Desert Southwest shares a similar arid environment to several other places around the world, including parts of Australia, Asia, the Middle East, and Africa. As global population increases, so does the pressure on those environments to sustain the people living off of it. While the full effects of climate change on those areas are still unknown, scientists now have new tools to find these answers.

“There have been a few times in human history where we have had some major paradigm shifts,” said Monger. Perhaps now we are on the brink of a new paradigm shift, “Can human beings actually change the climate of the earth? Are we a big enough factor globally, that we can actually change the climate?” It is this knowledge that could change how humans view themselves and their role in the global ecosystem.

 

Contacts:
Curtis Monger, NMSU Plant and Environmental Scientist, cmonger@nmsu.edu
Johnny Ramirez, Science Writer, johnny@approachpositive.com @32juanramirez