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Can Redwoods Survive a Hotter Planet?

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Community Voices is a series in which guest writers share their perspectives on redwood forests.


If we humans do our part, we can build a future in which we and redwoods continue to flourish.

“When we try to pick out anything by itself, we find it hitched to everything else in the Universe,” John Muir wrote in My First Summer in the Sierra. I was reading a lot of Muir during my first summer in the Sierra, the summer I fell in love with giant sequoia trees.

I had spent most of the past 10 years traveling around the world to research a book about humanity’s environmental future. When I returned to the United States to write what became Earth Odyssey, I lived in a log cabin deep in the forest of the Sierra Nevada. A mile past the end of the paved road that led back to civilization, the cabin had electricity and running water, but no telephone, much less Internet access. But a 40-minute hike over the ridge lay the North Grove of Calaveras Big Trees State Park, home to some of the biggest and oldest trees—indeed, some of the biggest and oldest living things—on Earth.

“Climb the mountains and get their good tidings,” Muir urged, and I eagerly complied. I visited the North Grove countless times, in all four seasons. Even at the height of springtime’s exuberance, when Douglas squirrels squawked a counterpoint to the insistent tapping of white-headed woodpeckers, and dogwood blossoms splashed color among the awakening undergrowth, the grove was pervaded by a deep calm. The giant sequoia towered skyward, massive yet graceful, dominating their environment without overwhelming it. From the forest floor arose the rich aroma of pine needles and duff warmed in the morning sun, while the creek gurgled with the new life imparted by the melting snows of the High Sierra. The giant sequoia seemed at once inseparable from—yet indifferent to—the swirl of nature’s elements; they had held this ground forever. As moment followed moment, time dissolved into the eternal now and the splendor of life on this Earth was revealed as a never-ending miracle.

The largest giant sequoia in Calaveras Big Trees State Park
The largest giant sequoia in Calaveras Big Trees State Park is doing its share to defend itself, other redwoods, and even the homo sapiens who caused global warming, from the gathering crisis. If we humans do our part, we can build a future in which humans and redwoods alike continue to flourish. Photo by Wayne Hsieh, Flickr Creative Commons, © 2019.
Muir’s observation that every single thing on Earth is “hitched” to everything else is now recognized as a fundamental ecological truth. Having spent much of my life reporting and writing about ecological matters, I can think of no more urgent example of this insight than global warming. Burning coal, driving gas-guzzlers, leveling forests—even when these actions happen thousands of miles away, they imperil redwoods as surely as loggers’ saws and axes did in Muir’s time. As global temperatures keep rising, the redwoods that Muir revered and did so much to protect need fresh champions—and fresh thinking and policies, including the Green New Deal idea that is gaining traction in Washington, D.C., and around the world.

Humanity must act at unprecedented speed and scale to avoid catastrophic heat waves, storms, droughts, and sea level rise that could kill hundreds of millions of people, warned the United Nations Intergovernmental Panel on Climate Change (IPCC) in its October 2018 report.

The report’s title, Global Warming of 1.5°C, alludes to a milestone scientific finding: While a temperature increase of 2°C above pre-industrial levels used to be regarded as the threshold of dangerous impacts, the latest science—coupled with observations of the record hurricanes, wildfires, and Arctic ice melt already happening after “only” 1°C of warming—says that 1.5°C is the new limit for avoiding unparalleled climate breakdown.

The good news, the IPCC adds, is that 1.5°C is achievablethere are no barriers in science or technology; rather, it’s a matter of political will and economic transformation. Humanity must stop burning fossil fuels (unless we capture their carbon emissions); we must stop cutting down so many trees; and we must stop raising so many cattle, among a host of other necessities. We must transition to solar and other forms of zero carbon energy, shift to regenerative agriculture and forestry, eat less meat, and embrace other climate-friendly alternatives to the status quo. And we must do all this very quickly. Humanity must achieve “carbon neutrality”—meaning, our societies must emit no more carbon dioxide than they extract from the atmosphere—by 2050.

If that sounds impossible, here’s another piece of good climate news, one with direct relevance to redwoods: California, the world’s fifth largest economy and home to the vast majority of Earth’s redwoods, is on track to make the transformations needed to hit the 1.5°C target. Bipartisan leadership from former Republican Governor Arnold Schwarzenegger and then Democrat Jerry Brown helped the state cut its greenhouse gas emissions to 1990 levels by 2018. In 2018, Brown signed into law Senate Bill 100, which requires 100 percent non-carbon electricity in California by 2045. Brown also signed an executive order requiring the entire state economy (not just the electricity sector) to be carbon-neutral by 2045. Disproving claims that protecting the climate puts people out of work and companies out of business, California has done all this even as its economy grew and created jobs faster than the United States as a whole did.

“When you put [the world’s fifth largest economy] on this path, it sends a huge message to the rest of the United States and the world that this is possible,” said Kevin de León, the former state senate president pro-tempore and the author of Senate Bill 100.

Happily, growing trees turns out to be a kind of miracle weapon against climate destabilization.

Through photosynthesis, trees “inhale” carbon dioxide and store it in their leaves, branches, and roots, rather than letting it remain in the atmosphere, trapping heat. This is why scientists call forests a carbon “sink,” whereas trees that are cut down or burned are a carbon “source.” Growing trees thus offsets some of the warming that humans’ emissions would otherwise cause.

Nevertheless, rising temperatures will test all human societies and natural ecosystems, including redwoods, in the years ahead. In the entire history of civilization, global temperatures have never risen as fast as they are rising today. And the physical inertia of the climate system—the fact that carbon dioxide remains in the atmosphere for many decades after being emitted—guarantees that temperatures will keep rising for some time. Hence, climate action must now embrace a twin imperative known as “avoiding the unmanageable and managing the unavoidable.” Humanity must avoid an unmanageable amount of temperature rise—by limiting global warming to 1.5°C, the IPCC says—even as we manage the temperature rise that’s already locked in due to past emissions.

Fog provides coast redwoods with much of the moisture they need
The outlook for coast redwoods is better than it is for giant sequoia, and water is the main reason. Fog provides coast redwoods with much of the moisture they need, augmented by the rains that fall from the lowest of the clouds. Photo by Stephen Sillett.
Our research underscores that climate change ranks as the top threat to redwoods,” said Anthony Ambrose, a plant ecologist at the University of California, Berkeley. Ambrose was a core collaborator on the Redwoods and Climate Change Initiative (RCCI), which Save the Redwoods League launched in 2009 to increase scientific understanding of how giant sequoia and coast redwoods will be affected by climate change. “It’s not only higher temperatures that concern scientists,” Ambrose told me; it’s also the knock-on effects of higher temperatures. “The main worries are water availability, fire patterns, and increased potential stress from bugs, fungi, or other pathogens,” he added.

The outlook for coast redwoods is better than it is for giant sequoia, and water is the main reason. Weather generally moves west to east in California, from the Pacific across the Central Valley to the Sierra. The coast is frequently engulfed in fog as moist ocean air meets land. This pattern is projected to continue as temperatures rise; the coast could even become foggier if higher inland temperatures pull still more ocean air eastward (though climate models are uncertain on this point). Fog provides coast redwoods with much of the moisture they need, augmented by the rains that fall from the lowest of the clouds. (Even during the record drought California endured from 2011 to 2017, the northern coast received significant rainfall.) This more favorable water situation may help explain why the temperature increases registered to date do not seem to have bothered old-growth coast redwoods. As temperatures continue rising in the decades ahead, the most important projected effect for coast redwoods is that their range will shift northward as redwoods in the Big Sur area face greater stresses.

Giant sequoia face a tougher outlook, Ambrose said, because their water supply is “highly dependent on the Sierra Nevada snowpack, and that snowpack is changing rapidly” as global warming intensifies. The snowpack’s plight was already evident in 2009, when I interviewed Greg Stock, a geologist for the National Park Service. “The way things are going, Yosemite’s glaciers will be entirely gone within a few decades,” Stock told me. “The melting is already having an effect on the visuals available to park visitors. We don’t see as many snow-covered peaks as before. As the melting continues, there will be much more troubling impacts, because glacial melt is the source of some of the streams and rivers the flora and fauna of Yosemite rely on.”

And melting snow isn’t the only way global warming threatens giant sequoia. Higher temperatures cause more precipitation to fall as rain rather than snow. Such rain ends up in rivers that carry it to the valleys below. By contrast, snowmelt sinks slowly into the soil and replenishes the groundwater below. This lack of replenished soil moisture poses an exceptional threat to giant sequoia, said Ambrose. “One giant sequoia tree can consume up to 4,000 liters of water a day in the summer because sequoia have so much leaf area that transpirates moisture, [passes moisture through the leaves’ pores],” he explained. “When there’s not enough snowmelt to provide that water, the trees compensate by pulling more water out of the soil. That leads to a drier immediate ecosystem, which increases the threats from wildfires as well as from pests and pathogens.”

Giant sequoia are famously impervious to most forest fires. But Ambrose warned that even giant sequoia could die in exceptionally hot and lasting wildfires fueled by other trees that have perished in severe, long-lasting droughts. The record drought from 2011 to 2017 also spurred a proliferation of bark beetles that “are now attacking some giant sequoia in Sequoia and Kings Canyon National Parks,” Ambrose added. “This is the first time we’ve seen that, which is pretty concerning.”

“Redwood trees are incredibly resilient, but when do they reach a tipping point?” Ambrose asked. “I don’t think the species will disappear; there are redwoods in various parts of the planet now. But the existing redwood groves here in California? Those might disappear. That’s my concern.”

Redwoods are far from shrinking violets: Given a fair chance, they might well survive the climate turbulence that lies ahead, just as they have survived countless natural threats for millennia.

Giving redwoods a fair chance means, first and foremost, limiting global warming to the absolute minimum—science is clear on that. Traditional threats to redwoods, notably the leveling of trees and the construction in their place of roads, houses, and shopping malls, remain a danger as well. But every such battle could be won, and yet the larger cause may be lost if global warming is not contained, and soon.

Hence, an irony: Saving redwoods alone can no longer protect the redwoods. If humans do not contain global warming, redwoods will likely wither and perish, at least in California. Thus a recalibration of strategies and tactics seems necessary. For example, such long-standing practices as purchasing private land containing redwoods and putting it into public trust must now take into account that global warming will increasingly shift the geographical ranges where redwoods can thrive. As part of its vision for the next 100 years, Save the Redwoods League is making restoration of coast redwood forests a strategic priority. 

But how much global warming, with its attendant droughts and wildfires, can redwoods endure? Can redwoods survive a future in which global temperatures increase 2°C? Or do they, like human societies, need global warming to halt at 1.5°C? How would redwoods fare if, god forbid, global temperatures increase by 3.5°C, our civilization’s current trajectory?

No one can say. I would urge Save the Redwoods League to address this key question in their next phase of RCCI research. A given forest restoration plan may make perfect sense in the face of today’s 1°C increase, but fail utterly if temperatures instead rise by 1.5°C or more. The twin imperative of avoiding the unmanageable while managing the unavoidable requires that the best available science be translated into far-reaching public policy.

“We’ve got to build resilience into our ecosystems, especially our forests, if we’re going to deal with what climate change will throw at us,” said Robert Wilkinson, a professor of environmental studies at the University of California, Santa Barbara. “

Clear-cutting leaves exposed soils, which dry out more quickly than those with forest cover, and erode more readily during precipitation events. We need our forests, especially up in the mountains, to be like sponges,” Wilkinson added, “so when there are big storms or other high-precipitation events, the trees and soil can soak up that water like a sponge. In the short term, that means less flooding downstream. In the long term, it means that water is stored underground and available to use later. Then, if the climate system swerves in the other direction, and suddenly there isn’t enough water coming off the mountains, we can rely on that naturally stored water, both to supply our own needs and to keep the forests less vulnerable to wildfires.”

As for limiting temperature rise, bear in mind the good news mentioned above: We already have in hand most of the technologies and practices we need. A scientifically rigorous yet nontechnical compilation of our options is found in the book Drawdown, edited by Paul Hawken. Because most of these reforms are both job creators and moneymakers, the happy fact is that stabilizing the climate could become the biggest economic enterprise of our time, a huge source of employment, profits, and poverty alleviation. This happy fact—saving the climate can also save our economy—is the foundation of the Green New Deal that has gotten so much attention recently.

Rep. Alexandria Ocasio-Cortez, the rock star new congresswoman from New York, and her supporters in the Sunrise Movement began pushing the incoming Democratic majority in the US House of Representatives to back a Green New Deal. The idea fast went on to gain endorsements by 2020 presidential prospects Bernie Sanders, Kamala Harris, and Elizabeth Warren. The new prime minster of Spain, Pedro Sanchez, has also backed the idea, telling elites at the World Economic Forum in Davos they should not fear it because it will create, not destroy, jobs.

As I explained 20 years ago in Earth Odyssey when proposing a Global Green Deal—essentially, a global version of a Green New Deal—this program would rely on government leadership to subsidize activities that are valuable to society but not yet supported by market forces, while also halting subsidies to such climate-destroying activities as exploring for more oil.

Governments would establish rules of the road so that market prices reflect the social costs of overheating the atmosphere and clear-cutting forests. Thus, a key component of a Global Green Deal would be to set a rising price on carbon that would drive companies and consumers to choose, say, a solar electric car rather than a gas-guzzling SUV.

There’s one last essential in the toolbox of change. “People need to get close to nature,” Ambrose urged. “If you get out and experience redwoods, you’ll want to do something about global warming to protect them.” Not long ago, I visited the South Grove of Calaveras Big Trees State Park, which contains one of the RCCI research plots. To get there, one crosses the north fork of the Stanislaus River. Over the preceding five years, as California endured the worst drought in its recorded history, the Sierra snowpack had dwindled. Luckily, the latest winter had brought lots of precipitation, and I could see the results when crossing the Stanislaus. A river that had been a shrunken, pitiful thing the previous summer was now a frothy, roaring torrent. Trees and foliage were vibrant as well.

With Muir’s advice to “climb the mountains and get their good tidings” flashing in my brain, I followed the trail past one giant sequoia after another until I reached the end. There, beside Big Trees Creek, resided the largest tree in the park. Estimated to be 2,000 years old, the tree is by no means the loveliest of giant sequoia, nor is it the tallest. Its extraordinary 25-foot diameter, however, recalls the encouraging fact that old trees store more carbon than young ones do, and that redwoods are three times more effective at carbon storage than the average tree species.

Muir had it right: Everything is connected to everything else. This massive tree was doing its share to defend itself, other redwoods, and even the homo sapiens who caused global warming, from the gathering crisis. If we humans do our part, we can build a future in which humans and redwoods alike continue to flourish, reveling in the mystery and wonder of life on the only planet we have.

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About Mark Hertsgaard

Mark Hertsgaard

Mark Hertsgaard is the environment correspondent for The Nation magazine and the author of seven books that have been translated into 17 languages, including Earth Odyssey and HOT.



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