By John P. O’Brien and Brad Marston
An unusually snowless Mount Shasta in June 2021. Photo by Brad Marston.
Note: This article originally appeared in the Spring 2022 issue of the PCT Communicator magazine.
Travelers on the Pacific Crest Trail in Northern California last June may have been surprised to see Mount Shasta with nearly no snow cover. Further north, an unprecedented heatwave enveloped Oregon, Washington, and British Columbia, where parts of the interior reached 120°F—as hot as Death Valley. Then massive wildfires started.
From deserts to glaciated peaks, chaparral to temperate rainforest, the PCT is a tour-de-force of climate-sculpted diversity. But the trail that we have come to know and love is increasingly under threat of rapid change, foreshadowing a future PCT experience unlike any we have known.
Climate Change 101
It is widely accepted within the scientific community that climate is changing and that humans are the dominant driver of that change. Basic laws of physics, understood for over a century through the pioneering work of scientists such as John Tyndall, Eunice Newton Foote, Svante Arrhenius, and Guy Stewart Callendar, are unbendable: Increasing levels of carbon dioxide (CO2) and other greenhouse gases (GHGs) in the air are raising the average temperature and altering the climate in many ways. We know this because basic physics, computer models, and reconstructions of past climates all agree on the same facts. We are now living in the Anthropocene, a new geological period of time marked by a rapidly changing climate driven by our industrialized civilization.
The largest sources of CO2 come from the burning of fossil fuels (coal, oil, and natural gas) and deforestation and land degradation (logging, forest conversion and landscape development). These two primary GHG sources release carbon that had previously been stored away by natural processes on timescales ranging from thousands to millions of years.
As is increasingly becoming recognized, the impacts resulting from climate change are not some hypothetical future yet to be realized; we have been and are experiencing them now. Unprecedented extreme heat, drought, wildfire, sea level rise, ocean acidification, flooding, biodiversity collapse, diminished snowpack, and disappearing glaciers are just a few of the impacts of climate change—which is as diverse as the natural and human systems it affects.
Despite being a global phenomenon, the impacts of climate change are not felt equally everywhere. For example, the Arctic has warmed about three times faster than the global average, and Southern California has warmed about twice as much as Northern California. And while climate change is driving aridification in some regions, other regions are expected to become wetter.
Earth’s Energy Imbalance
The solar radiation that reaches the Earth is measured by scientists in watts per square meter. Some of this radiation is reflected back into space, some is absorbed into the atmosphere, and the remainder is absorbed at the surface. The total global and annual average amount of radiation that reaches the Earth’s surface is about 342 watts per square meter (abbreviated as 342 W/m2).
Radiative forcing is a measure of the influence various climatic factors have on this base number of 342 W/m2. Positive radiative forcing means the Earth receives more solar energy than it reflects into space, which warms the planet. Negative radiative forcing means the opposite: Earth reflects more energy into space than it receives, cooling the planet. If radiative forcing is zero (meaning neither positive nor negative), the Earth’s energy is balanced.
The current radiative forcing (warming) resulting from added greenhouse gases (GHG’s) is about 3.7 Watts for every square meter (W/m2). This may seem small compared to, say, the energy produced by a standard 60-watt light bulb. But spread across the Earth’s surface, 3.7 W/m2 is equivalent to adding the output of, approximately, 2 million large power plants of gigawatt (billion-watt) capacity to the Earth system.
This creates an energy imbalance, and is enough excess energy to drive the heating of the oceans and Earth’s atmosphere and to alter Earth’s hydrologic cycle and large-scale circulation patterns, which control day-to-day weather and long-term climate.
The current CO2 concentration in our atmosphere is about 420 parts per million (ppm), which is approximately 50% higher than the pre-industrial level of 280 ppm. This has resulted in an average global warming of about 1.2 degrees Celsius or equivalently, 2.2 degrees Fahrenheit. Again, this may not seem large, but consider that under purely natural climate forcings, a change of this magnitude would likely take anywhere from 5000-15,000 years. In the Anthropocene we have seen the same happen in about 200 years.
Climate change along the PCT cannot be fully described by one or even a few generalizations due to the many distinct ecosystems it traverses. The impacts from climate change will be as diverse as the PCT is itself. However, let’s focus on two prominent and ubiquitous changes that will affect the hiker experience along the trail: snow loss and temperature rise.
A Low to No-Snow Future for the Sierra Nevada and Cascades
Northbound trips on the PCT ideally start in Campo, California, sometime in April or May to avoid the scorching heat and lack of water in the desert, and also to arrive in the Sierra Nevada when (hopefully) enough winter snow has melted to cross the high mountain passes. However, because water has a non-negotiable freezing point, temperature rise (due to increased GHG radiative forcing) means less precipitation will fall as snow in a warmer future.
By the end of the 21st century, early-season snow along the trail may be nearly absent. Not only will this severely disrupt the ecosystems along the PCT and throughout the Western U.S. that have evolved over thousands of years to depend on these mountain reservoirs of water, but it will fundamentally alter the PCT experience.
The maps below show a median model estimate of average March-July snow water equivalent (SWE), which is the approximate amount of liquid water contained in a certain amount of snow (a general rule of thumb for snow in the Sierra/Cascades is to multiply SWE by 7 to get the snow depth in inches). The PCT is shown in yellow.
The left panel shows a historical representation of snow during the 1960-1980 period. Compared to the “present” period (2000-2020; left-center), there has already been a noticeable reduction in early-season snowpack along the trail. This decrease continues to 2100, when very little snow remains in the Sierra Nevada. To view an animated version of the model below, click here. The animation shows both the long-term decline in snow as well as the year-to-year variability.
(Click to view a larger version of this image.) A median model estimate of average March-July snow water equivalent (SWE). The PCT is shown in yellow. SWE estimates are from a 20-km spatial resolution median model estimate. Figures courtesy of Dr. Alan M. Rhoades (website) of the Berkeley Lab’s Earth & Environmental Sciences Area.
Vanishing Glaciers in the Trinity Alps
Mountain glaciers are visible in many spots along the PCT. Glacier maintenance and growth depends on regular annual accumulation of snow that adds to, or at least protects, glacial ice from summertime melt. In California prior to 2015 there were 22 named glaciers. Now there are only 21.
In Northwest California lies the Trinity Alps, a compact granite range known for its diverse colors and biodiversity. The PCT traverses the Northeast portion of the Trinity Alps Wilderness before veering north into the Russian and Marble Mountain wildernesses. Looking south from the PCT you can see the north aspect of the Alps, which houses the Grizzly glacier—and prior to 2015, the now-extinct Salmon glacier.
A 2013 aerial view of the Salmon and Grizzly glaciers in Northern California’s Trinity Alps. Photo courtesy of Justin Garwood, CA Dept. of Fish and Wildlife.
Both the Grizzly and Salmon glaciers saw dramatic losses in size over the past half-century. From about 1880 to 2015 the Grizzly glacier’s area decreased over 93 percent, leaving only about four acres of ice. Following the prolonged drought of 2011-2015, the nearby Salmon glacier disappeared completely and is now considered extinct.
(Above) Contours showing the historical extent of the now-extinct Salmon glacier, which is estimated to have contained about 60 acres of ice around the turn of the 20th century (circa 1885; black contour), however none remain. Figure courtesy of Justin Garwood, CA Dept. of Fish and Wildlife.
(Above) Contours showing the historical extent of the Grizzly glacier, also estimated to have contained 60 acres of ice around the turn of the 20th century (circa 1885; black contour). Only about one acre of ice remains today (blue shading). Figure courtesy of Justin Garwood, CA Dept. of Fish and Wildlife.
Since 2015, the Grizzly glacier’s loss has slowed considerably, owing largely to the heavy snow winters of 2016-17 and 2018-19. But the past two years of drought and the record high temperatures of 2021 have wreaked havoc on the Grizzly glacier. The most recent estimate of its area indicates that only about an acre of ice remains—meaning a loss of about 99 percent of its preindustrial area. This remnant of the Grizzly glacier now represents the most southwesterly-situated ice in the continental U.S. and will, in all likelihood, vanish in the coming years, bringing the number of California glaciers down to 20.
Glaciers around the world are melting at an alarming and accelerating rate, and the glaciers of the Pacific Northwest and California are no exception. Not only does this have to do with declining snowfall due to warmer winter temperatures, but also accelerated melting due to increasing summertime temperatures. Warming also appears to be amplified with altitude, further increasing snow loss in the mountains.
Hotter Conditions for PCT Travelers
July through October on the PCT can be both extremely hot and extremely dry. A trip-ending wildfire along the trail is becoming more and more likely, and the risk of heat-related illness and dehydration is increasing. Hiking in temperatures exceeding 100 will become exceedingly common by the end of the 21st century—which may make southbound PCT trips impossible to complete due to extreme heat and lack of water in the Southern California desert.
Currently one of the longest stretches along the PCT without reliable water is about 42 miles in California’s Section F from Landers Meadow to Walker Pass (Willow, Yellow Jacket, and McIvers Springs are often dry, and it is dangerous to rely upon water caches). In a drier and hotter climate, where PCT travelers will need even more water to sustain physical needs, that distance could easily double in a future drought as more springs dry up, making an unsupported trip all but impossible.
As air warms it holds more water vapor, accelerating the drying of the land. Indeed, as the climate warms, being able to travel the entire PCT in one trip will become more difficult due to water, fire, smoke, and heat related issues. Some thru-hikers may need to adapt by section hiking, with the choice of section dictated by the time of year to avoid climate extremes.
Extreme rain and summertime lightning fueled by intense deep convection over mountainous areas will mean increasing potential for damage to the trail itself. Atmospheric rivers that are responsible for about half the precipitation received by California are expected to increase in both frequency and strength. The amplified water cycle means more erosion, necessitating the need for increased trail maintenance and funding to do such work. This will make it more important to respect trail switchbacks and avoid the creation of erosional pathways that damage the trail and result in more sediment delivered to the nearby streams, reducing water quality.
A Warming West in Numbers
(Click to view a larger version of this image.) Figure courtesy of John P. O’Brien.
The panels above (left-to-right) show average July-October (the time period when hikers are most likely to experience extreme heat) daily maximum surface temperature for four different past and future time periods. 1960-1980 (left; historical), 2000-2020 (left-center; present), 2040- 2060 (right-center; mid-century), 2080-2100 (right; end-of-century). Temperature data represents the average from 40 independent model simulations as part of the Community Earth System Model Large Ensemble project. More information on the simulations can be found in this journal article (https://doi.org/10.1175/BAMS-D-13-00255.1).
How the PCT Experience Could Change
Taken together, these impacts of climate change lead to several important consequences for long-distance PCT travelers:
- An April-May Campo start may not be desirable in the future due to excessively hot and dry conditions. Southbound trips may become difficult, if not impossible, for the same reasons.
- In the not-too-distant future, there will almost certainly be years when hikers see little to no snow in the Sierras. For example, a 4-year prolonged drought (like the 2012-2016 drought) occurring sometime in the 2080-2100 period would very likely result in a completely snow-free Sierra and northbound travelers may not see any of the white cold stuff until the North Cascades.
- It’s become an increasingly stark reality that more than a century of forest management decisions (overly aggressive fire suppression, discontinuance of indigenous/cultural burning practices, commercial logging) and climate change—which contributes to large-scale forest drying and increased numbers of diseased and dying trees—have drastically altered the behavior of fire in the Western U.S.
- With an expanded fire season it is more and more likely that a long-distance PCT journey will be disrupted by wildfire and smoke. Not only might fire end a trip, it will alter the trail experience through the burned section for decades into the future. For example, the 2021 Dixie Fire, which burned vast swaths of Plumas and Lassen National Forests and Lassen Volcanic National Park, burned across more miles of the PCT than any fire since the establishment of the trail.
A nearly snow-free Mount Shasta on August 25, 2014. This once rare occurrence will become an increasingly frequent sight for
hikers along the PCT even early in the summer. Photo courtesy of Andrew Calvert, United States Geological Survey.
Making the Right Choices for a Better Future
It is important to keep in mind that the future is still unwritten, and actions we take today will dictate what we experience tomorrow. For example, a recent study found that a low-emissions pathway (called SSP1-2.6) with ~3.6º warming by 2100 reduces the single-year drought risk in Southwestern North America by nearly 20% compared to the high-emissions, business-as-usual pathway (SSP3-7.0) with ~8º warming by 2100.
We can tackle climate change, and there’s plenty of space for hope for two critical reasons: first, we know exactly what is causing climate change; and second, we know exactly how to slow and ultimately stop it. And while we also know there is uncertainty in future climate projections stemming from model differences and natural variability, it’s clear that the largest source of uncertainty comes from the choices we make as a species.
This certain understanding—and the overwhelming scientific consensus on climate change—uniquely distinguishes it from other scientific, health, and technological challenges we face. Start acting today for a better tomorrow. We can all take steps as individuals for a better future by reducing our GHG footprints. For example, PCT hikers can avoid shipping resupply boxes by buying locally in trail towns, and use public transportation to reach trailheads (the PCTA maintains an excellent webpage with information about public transportation).
Learn more about climate change solutions…
Founded in 2014, Project Drawdown is a nonprofit organization that has become a leading source for information on climate change solutions. The main principles of the project are to: 1. Reduce sources by bringing emissions to zero and stopping pollution; 2. Support carbon sinks and uplift nature’s carbon cycle; 3. Improve society by fostering equality for all. The project website’s solutions page provides updated information on nearly 100 solutions to mitigate climate change and move us closer to climate drawdown—the point in the future where greenhouse gases stop climbing and start to steadily decline.
The largest and most necessary steps toward mitigating climate change must come from coordinated action. So vote—and call and write your elected officials and hold them accountable for their decisions and the legislation they introduce. Connect with community members, form action groups, and demand bold action on climate. And finally, be willing to engage in the often difficult conversations surrounding climate change by focusing on finding common ground and shared values with those who may be skeptical. Stewardship of our natural environments spans political differences.
Many of the necessary changes may also be attractive for other reasons. Solar and wind-generated electricity is becoming less expensive than energy supplied by large regional utilities. Freedom from imported oil is also appealing and stewardship of our natural environments cuts across political divisions.
We owe it to nature herself—which supports our very existence—and to future generations to do the work to mitigate climate change as quickly as possible so future generations have the same opportunity we have had to experience the PCT in all its grandeur, glory, and life affirming challenges.
John P. O’Brien is a climate scientist at the National Center for Atmospheric Research. As a Northern California, Trinity County native, he studies anthropogenic forcing on the large-scale atmospheric dynamics that control west coast climate variability.
Brad Marston is a climate and quantum physicist at Brown University. He has been section hiking the PCT since 2013 and can be found on Twitter @Brad_Marston.