Tuesday, September 22, 2015

Fire and Ice in the Sierra Nevada

Fig.1. A view of Half Dome in the Sierra Nevada (Credit: Dimitry B. via Flickr, 2013)

Earlier this year, California Gov. Jerry Brown issued water restrictions for first time ever in the state’s history, mandating that urban water suppliers cut their use by 25 percent. This measure addressed California’s ongoing drought, which has entered its fourth year and contributed to deadly wildfires and billions lost in agriculture. A new study published in Nature Climate Change suggests that this record-breaking streak may stretch back far more than a few years ago.

Springtime measurements high in the Sierra Nevada put snow levels at only 5 percent of the annual average. This alpine snowpack can provide up to 30 percent of California’s annual water but suffered due to low winter precipitation and warm weather. Such low levels sparked the interest of Valerie Trouet and Soumaya Belmecheri, researchers at the University of Arizona Tree-Ring Research Laboratory.

Fig.2. Tree rings can tell a tree’s age and the climate of past years (Credit: Cobalt via Flickr, 2006)

Evaluating a Hidden Reservoir

With the help of colleagues from the University of Arkansas and the National Oceanic and Atmospheric Administration (NOAA), the scientists combed through tree ring (growth ring) measurements from two previous studies and reconstructed annual snowpack levels stretching back to 1405. What they found was startling: the Sierra Nevada snowpack has reached its lowest levels in 500 years.

Belmecheri et al. turned to the precipitation-sensitive blue-oak trees (Quercus douglasii) in central California to map drought history in the state. These trees act as good climate proxies given their proximity to the Sierra Nevada snowpack and their distinct, larger rings in wetter years as opposed to those in drier years. Tree ring data from 1,505 blue oak trees showed precipitation for the past 610 years, and another set of tree rings allowed them to measure estimated annual spring temperatures.

The researchers found that the mountains suffered from high summer temperatures and low winter precipitation, melting an already low snowpack. With snow levels the lowest they have been in half a millennium, a major reservoir for the parched state is nearly gone.

Fig.3. Satellite images of the Sierra Nevada snowpack in January 2013 (left) and January 2014 (right) show very different snow coverage amounts (Credit: NASA, 2014)

Climate Change in the Trees

A low snowpack is not the only sign that California suffers from extremes in the climate. A recent study published in Ecosphere used written records from the past 105 years to prove that wildfires in the Sierra Nevada are found increasingly in high-elevation forests, past the 8,000-foot line. Although this was a rare occurrence in the past, lead investigator Mark Schwartz with the John Muir Institute of the Environment at UC-Davis believes that climate change is to blame.

As temperatures increase, tree density high in the subalpine forests can also increase. This change is driven by movement of lower-elevation plants up the slopes in a warmer climate. A drop in precipitation means plenty of fuel for a fire. With this drought - the most severe in the last 1,200 years - the chance for fire is high. Schwartz and his colleagues worry that the shift in forest composition will only be exacerbated as fire destroys recent growth and new species start to fill up the high-elevation forests.

From snowpacks to wildfires, trees offer a useful climate proxy to track changes over the millennia. The precipitation and temperature data stored in tree rings have been used to accurately date volcanic eruptions and understand how drought harms future plant growth. Now they are needed to protect California’s water supply and manage its forests, as well as offer a glimpse of what might happen as the severity of North American droughts increases due to global warming.Through concerted efforts, scientists and policymakers can address climate change with information etched in the tree trunks.

Belmecheri, S., Babst, F., Wahl, E. R., Sahle, D. W., & Trouet, V. (2015). Multi-century evaluation of Sierra Nevada snowpack. Nature Climate Change. doi: 10.1038/nclimate2809

Griffin, D., & Anchukaitis, K. J. (2015). How unusual is the 2012-2014 California drought? Geophysical Research Letters, vol. 41: 9017-9023. doi: 10.1002/2014GL062433

Schwartz, M. W., et al. (2015). Increasing elevation of fire in the Sierra Nevada and implications for forest change. Ecosphere, vol. 6 (7). doi: 10.1890/ES15-00003.1


A mass of snow formed from layers of snow accumulated in particular geographic regions and high altitudes where the climate is cold for extended periods of time
tree rings (growth rings)
Can be seen in a horizontal cross section cut through a tree trunk. They are the result of new growth (generally one ring per year) and can show pronounced, wide rings in wetter weather and thinner rings during droughts.

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