What does the Fourth National Climate Assessment spell for the Sierra Nevada?

by Brian Bahouth

Carson City – Climate change used to be an abstract concept for many Americans, something that happens to polar bears and ice sheets, people and things far away, but the 4th annual National Climate Assessment released last Friday graphically brings the many and devastating effects of a warming climate to the front door of every American.

Aggregating and synthesizing the data of a warming global climate and its many effects has proceeded at the painstaking pace of scientific research, and the 2018 climate assessment builds on many decades of inquiry and data collection and describes an atmosphere and associated ecosystems in crisis and near crisis as the atmosphere continues to warm.

There have been and will continue to be algal blooms in the Great Lakes.  More frequent super-sized hurricanes have and will continue to devastate areas on the Atlantic seaboard and Gulf Coast.  Crippling drought and resultant declines in food production are projected for California and mid-western farmers, but for the inter-mountain southwest, to include Nevada, the two leading impacts of climate change can be seen in water resources and forest health, especially the dramatic rise in the occurrence and severity of wildfires.

This figure shows the annual wildfire area burned in the United States (red) and the annual federal wildfire suppression expenditures (black), scaled to constant 2016 U.S. dollars (Consumer Price Index deflated). Trends for both area burned and wildfire suppression costs indicate about a fourfold increase over a 30-year period. Source: U.S. Forest Service.

In recent days Nevada has had front-row seats to one of the most destructive wildfires in the nation’s history.  Climate assessment authors acknowledge that forests vary widely in their susceptibility to a warming climate, but in the western US warmer temperatures have made for better conditions for reproduction of various species of bark beetle.  As a result, millions of trees have died in recent years leaving highly flammable carcasses behind, especially in the Sierra Nevada.  In the context of the recent “Camp” fire in northern California, the assessment’s specific mention of severe tree mortality in the Sierra Nevada may have an eerie ring for communities up and down the mountain range.

From the 2018 National Climate Assessment:

Five years of consecutive drought ended in California in 2017, with 2015 being the hottest and driest year in the historical record (since the late 1800s). The drought weakened trees and enabled extensive bark beetle outbreaks, which killed 40 million trees across 7.7 million acres of Sierra Nevada forests through 2015. Annual tree mortality increased by an order of magnitude to thousands of dead trees per square mile during this period. The winters 2015–2016 and 2016–2017 brought significant precipitation to much of California, but drought stress remained high in many areas. An additional 62 million trees died in 2016, and 27 million trees died in 2017, bringing the total to at least 129 million trees since 2010. Mortality was most severe at lower elevations, on southwest- and west-facing slopes, and in areas with shallow soils.
This level of tree mortality in the Sierra Nevada is unprecedented in recorded history. In some of the most heavily impacted areas, 70% of trees died in a single year. Much of this mortality was attributed to the western pine beetle colonizing ponderosa pine, but other tree and shrub species were also affected. Some forests once dominated by ponderosa pine are now dominated by incense cedar. This change in stand structure and composition has increased the likelihood of high-intensity surface fires and large wildfires. In general, widespread tree mortality can alter local hydrology (with more water availability but also higher peak flows) and negatively affect ecosystem services (for example, decreased timber supply and decreased recreation opportunities), effects that will persist for many years.
A five-year drought in California (2011–2016) led to western pine beetle outbreaks, which contributed to the mortality of 129 million trees. As a result, the structure and function of these forests are changing rapidly. Prolonged droughts are expected to become more common as the climate continues to warm, increasing stress on lower-elevation tree species. Photo credit: Marc Meyer, U.S. Forest Service.

Despite the conclusions of 13 federal agencies as expressed in the 2018 National Climate Assessment, President Trump continues to disregard a preponderance of scientific evidence and question whether humans are causing the atmosphere to warm, but according to the 2018 assessment, research scientists have added to the ongoing documentation that shows human activity is partly the cause of more frequent drought, tree mortality, and wildfire frequency. 

The cumulative forest area burned by wildfires has greatly increased between 1984 and 2015, with analyses estimating that the area burned by wildfire across the western United States over that period was twice what would have burned had climate change not occurred. Source: the 4th annual Climate Assessment, adapted from Abatzoglou and Williams 2016.

According to the assessment, more frequent extreme weather events will drive the magnitude and severity of “ecological disturbances,” which will over the coming months and years essentially change forest “structure and function across large landscapes.”


According to the 2018 climate assessment, human-caused climate change is partly responsible for intensifying droughts and occasional large floods in the southwestern US.  Higher than average annual temperatures across Nevada and the southwest, “have significantly altered the water cycle in the Southwest region. These changes include decreases in snowpack and its water content, earlier peak of snow-fed streamflow, and increases in the proportion of rain to snow.  These changes, attributed mainly to climate change exacerbate hydrological drought.”

Temperatures increased across almost all of the Southwest region from 1901 to 2016, with the greatest increases in southern California and western Colorado. This map shows the difference between 1986–2016 average temperature and 1901–1960 average temperature. Source: adapted from Vose et al. 2017.

For communities on both sides of the Sierra Nevada, the mountains are a massive water bank that releases water as snow melts, but a changing climate means a change in the timing and amount of water that comes out of the mountains for agricultural, industrial and domestic uses.  With growing numbers of people relocating to the sunny southwest and less water being stored as snow, the assessment poignantly highlights the need for adaptive water management techniques that balance declining supplies with increasing demands.  The following graphic from the 2018 climate assessment illustrates the changing nature of water stored as snow over the past 63 years.

Trends in April Snowpack in the Western United States, 1955–2018

The assessment points out that efforts are being made to reduce C02 emissions, and as evidence Nevada voters approved a ballot measure to increase the state’s Renewable Portfolio Standard to 50 percent by 2030.  The assessment also notes that “gains in water-use efficiency over the last 30 years have resulted in total U.S. water consumption staying relatively constant,” but the 2108 assessment projects that if global C02 emissions continue at current levels, models project substantial reductions in snowpack, less snow and more rain, shorter snowfall seasons, earlier runoff, and warmer late-season stream temperatures.  Fewer days with precipitation would lead to increased year-to-year variability.”

The Colorado River

In 2018 it is old environmental news in Nevada that the water level of Lake Mead on the Colorado River is as low as it has ever been.  According to the 2018 climate assessment, Lake Mead has dropped 130 feet since 2000, which accounts for 60 percent of the lake’s volume.  Assessment authors attribute continued drought and water withdrawals for agricultural and domestic use as primary causes of a dying river.

The 2018 climate assessment looks to the future of the Colorado River and Lake Mead.

The reduction of Lake Mead increases the risk of water shortages across much of the Southwest and reduces energy generation at the Hoover Dam hydroelectric plant at the reservoir outlet. Local water utilities, the governments of seven U.S. states, and the federal governments of the United States and Mexico have voluntarily developed and implemented solutions to minimize the possibility of water shortages for cities, farms, and ecosystems. The parties have taken four key actions:

The information in the above writing is but a tiny bit of the nearly 1,700 page 2018 National Climate Assessment.  Despite it’s monstrous size, the source document is well organized and readable, and we encourage readers to have a look at the report itself …


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