The fate of Sierra Nevada glaciers
“Spoiler alert,” is how Yosemite National Park Geologist Dr. Greg Stock began his Sierra Nevada Aquatic Research Laboratory (SNARL) lecture on Tuesday: “if you thought [Sierra Nevada glaciers] were doing well, they’re not.”
Dr. Stock spoke to the largest ever audience of SNARL lecture attendees, beginning with the history of glaciers in the Sierra Nevada, and concluding with their bleak future.
What is a glacier? “A persistent body of dense ice that moves under its own weight,” Dr. Stock said. That movement is key to the identification of glaciers. Glaciers move in two ways: through “deformation,” meaning the slow movement of the glacier because of its own weight, like honey pooling down a tipping plate, and “sliding,” meaning the water that melts at to the bottom of the glacier allows the whole glacier mass to slide as a single unit.
The Sierra Nevada range has been glaciated many times, with a peak in glacial activity about 20,000 years ago. At that time, during what’s called the Tioga glaciation, a two thousand foot-thick ice sheet rested on Tuolumne.
Glaciers would expand and retreat according to warmer and colder periods, switching back and forth roughly every 3,000 years; these warmer and colder periods were the result of how much energy the earth received from the sun at that time.
But no one even knew for sure the Sierra Nevada had glaciers until John Muir took it upon himself to prove it, Dr. Stock said.
“He was a true believer in glaciers sculpting the Sierra landscape, but a very distinguished geologist, Josiah Whitney, disagreed with him,” Dr. Stock said. “Whitney thought earthquakes were responsible. He said to Muir, find me a glacier.”
So in 1871, Muir set off to the Clark Range to do just that. He found a glacier there, but “Poignantly, I can tell you that glacier is gone,” said Dr. Stock; “it melted away in the 1970s.”
We now know there are about 1,700 ice features in the Sierra Nevada, and about 122 true glaciers. Most of these glaciers are in north or northeast-facing cirques.
Dr. Stock focused his talk in particular on the Lyell and Maclure Glaciers, which sit side by side in Yosemite’s high country, and are both the headwaters of the Tuolumne River.
The study of Sierra Nevada glaciers, particularly these two, has been ongoing since Muir’s time. Dr. Stock said that study is indispensible because “Glaciers are really important indicators of a changing climate.”
Muir was the first to study the Maclure Glacier, using it to prove to Whitney that some Sierra Nevada ice bodies were moving. In 1872, he hiked up to Maclure with Galen Clark and Joseph LeConte, and planted seven foot-tall whitebark pine stakes in the glacier, then came back 46 days later to check their movement. What he found was that the stakes had moved about an inch a day.
Dr. Stock was careful to note that these glaciers are not remnants of the last glaciation, but rather of the “Little Ice Age” that occurred about 700 years ago. That Little Ice Age ended in 1850, at the beginning of the Industrial Revolution.
The National Park service took over the study of Lyell and Maclure Glaciers in 1931. Those annual surveys ended in 1975, and few researchers visited Lyell and Maclure in the intervening years. Dr. Stock resumed their study in 2006, this time with the help of new technology like airborne LiDAR detection courtesy of the Airborn Snow Observatory.
“What we found was that the Maclure Glacier had lost 70 percent of its surface area since Muir’s study,” said Dr. Stock.
Deformation of the glacier has decreased because the thickness of the ice has diminished so dramatically, but sliding has increased. Any movement of the Maclure Glacier is now no longer a sign of that glacier’s health; in fact, it’s more likely the result of accelerated melting.
The Lyell Glacier is doing even worse than Maclure, Dr. Stock said. “Lyell has stagnated … it’s not moving anymore,” he said. That’s likely because Lyell has lost 100 feet of vertical ice, thinning to the point where it can no longer move through deformation.
“This means Lyell is at the end,” he said. Dr. Stock said that the drought, coupled with increased levels of carbon dioxide from the burning of fossil fuels, will likely make this the biggest year of glacier decline yet in the Sierra Nevada.
While CO2 levels rose during inter-glacial periods in the geologic record, Dr. Stock said those levels have almost doubled in the past 60 years, and are now at the highest levels in 650,000 years.
Were we not adding carbon dioxide to the earth’s atmosphere at such an alarming rate, we might be able to sit back and merely wait for the next glacial period to recover the glaciers.
“Without the big addition of greenhouse gasses, we might be heading into another glacial period in the next several thousand years,” Dr. Stock said. “But I don’t think that will happen.”
Assuming these current conditions persist, he said Lyell Glacier may be lost in as little as five years. While an El Niño year might help (and one is rumored), it would likely only push glacier persistence out by another 10-15 years, he said.
“The Sierra Nevada has been ice free before, and it almost certainly is going to be ice free again, but human fingerprints are on what’s happening to our ice in the Sierra Nevada and around the world,” Dr. Stock said.
The implication of the loss of these Sierra Nevada glaciers is that first, “We’ll lose a really important source of water,” Dr. Stock said. He anticipated decreased or nonexistent late-summer stream flows, and what water remains at that time of year will likely be much warmer, which will affect aquatic and riparian ecosystems.
Dr. Stock said we’ll also lose an important archive of information on climate change and the natural history of the area.
And finally, “We’re going to lost the opportunity to visit these alpine environments in the park, and lose the history of the glaciers,” he said. “We’re looking at the loss of most if not all Sierra Nevada glaciers in the next few decades.
“The work we’re doing [now] is probably the last of its kind for a long time.”
