At-Risk Forests

At-Risk Forests

Global analysis identifies at-risk forests.

Forests are engaged in a delicate, deadly dance with climate change, hosting abundant biodiversity and sucking carbon dioxide out of the air with billions of leafy straws. They can be a part of the climate solution as long as global warming, with its droughts, wildfires and ecosystem shifts, doesn’t kill them first.

In a study published in Science, William Anderegg, the inaugural director of the University of Utah’s Wilkes Center for Climate Science and Policy, and colleagues quantify the risk to forests from climate change along three dimensions: carbon storage, biodiversity and forest loss from disturbance, such as fire or drought. The results show forests in some regions experiencing clear and consistent risks. In other regions, the risk profile is less clear, because different approaches that account for disparate aspects of climate risk yield diverging answers.


William Anderegg

“Large uncertainty in most regions highlights that there's a lot more scientific study that's urgently needed.”


An international team

Anderegg assembled a team including researchers from the United Kingdom, Germany, Portugal and Sweden.

“I had met some of these folks before,” he says, “and had read many of their papers. In undertaking a large, synthetic analysis like this, I contacted them to ask if they wanted to be involved in a global analysis and provide their expertise and data.”

Their task was formidable –assess climate risks to the world’s forests, which span continents and climes and host tremendous biodiversity while storing an immense amount of carbon. Researchers had previously attempted to quantify risks to forests using vegetation models, relationships between climate and forest attributes and climate effects on forest loss.

“These approaches have different inherent strengths and weaknesses,” the team writes, “but a synthesis of approaches at a global scale is lacking.” Each of the previous approaches investigated one dimension of climate risk: carbon storage, biodiversity, and risk of forest loss. For their new analysis, the team went after all three.

Three dimensions of risk

“These dimensions of risk are all important and, in many cases, complementary. They capture different aspects of forests resilience or vulnerability,” Anderegg says.

  • Carbon storage: Forests absorb about a quarter of the carbon dioxide that’s emitted into the atmosphere, so they play a critically important role in buffering the planet from the effects of rising atmospheric carbon dioxide. The team leveraged output from dozens of different climate models and vegetation models simulating how different plant and tree types respond to different climates. They then compared the recent past climate (1995-2014) with the end of the 21st century (2081-2100) in scenarios of both high and low carbon emissions. On average, the models showed global gains in carbon storage by the end of the century, although with large disagreements and uncertainty across the different climate-vegetation models. But zooming in to regional forests and taking into account models that forecast carbon loss and changes in vegetation, the researchers found higher risk of carbon loss in southern boreal (just south of the Arctic) forests and the drier regions of the Amazon and African tropics.
  • Biodiversity: Unsurprisingly, the researchers found that the highest risk of ecosystems shifting from one “life zone” to another due to climate change could be found at the current boundaries of biomes – at the current transition between temperate and boreal forests, for example. The models the researchers worked from described changes in ecosystems as a whole and not species individually, but the results suggested that forests of the boreal regions and western North America faced the greatest risk of biodiversity loss.
  • Disturbance: Finally, the authors looked at the risk of “stand-replacing disturbances,” or events like drought, fire or insect damage that could wipe out swaths of forest. Using satellite data and observations of stand-replacing disturbances between 2002 and 2014, the researchers then forecast into the future using projected future temperatures and precipitation to see how much more frequent these events might become. The boreal forests, again, face high risk under these conditions, as well as the tropics.

“Forests store an immense amount of carbon and slow the pace of climate change,” Anderegg says. “They harbor the vast majority of Earth's biodiversity. And they can be quite vulnerable to disturbances like severe fire or drought. Thus, it's important to consider each of these aspects and dimensions when thinking about the future of Earth's forests in a rapidly changing climate.”

Future needs

Anderegg was surprised that the spatial patterns of high risk didn’t overlap more across the different dimensions.

“They capture different aspects of forests' responses,” he says, “so they wouldn't likely be identical, but I did expect some similar patterns and correlations.”

Models can only be as good as the basis of scientific understanding and data on which they’re built and this study, the researchers write, exposes significant understanding and data gaps that may contribute to the inconsistent results. Global models of biodiversity, for example, don’t incorporate dynamics of growth and mortality or include the effects of rising CO2 directly on species. And models of forest disturbance don’t include regrowth or species turnover.

“If forests are tapped to play an important role in climate mitigation,” the authors write, “an enormous scientific effort is needed to better shed light on when and where forests will be resilient to climate change in the 21st century.”

Key next steps, Anderegg says, are improving models of forest disturbance, studying the resilience of forests after disturbance, and improving large-scale ecosystem models.

The recently-launched Wilkes Center for Climate Science and Policy at the University of Utah aims to provide cutting-edge science and tools for decision-makers in the US and across the globe. For this study, the authors built a visualization tool of the results for stakeholders and decision-makers.

Despite uncertainty in the results, western North America seems to have a consistently high risk to forests. Preserving these forests, he says, requires action.

“First we have to realize that the quicker we tackle climate change, the lower the risks in the West will be,” Anderegg says. “Second, we can start to plan for increasing risk and manage forests to reduce risk, like fires.”

Find the full study here.


by Paul Gabrielsen, first published in @theU.

Interactive Forest Maps

Wildfire, Drought & Insects

Dying forests in the western U.S.

Threats impacting forests are increasing nationwide.

Planting a tree seems like a generally good thing to do for the environment. Trees, after all, take in carbon dioxide, offsetting some of the emissions that contribute to climate change.

But all of that carbon in trees and forests worldwide could be thrown back into the atmosphere again if the trees burn up in a forest fire. Trees also stop scrubbing carbon dioxide from the air if they die due to drought or insect damage.

The likelihood of those threats impacting forests is increasing nationwide, according to new research in Ecology Letters, making relying on forests to soak up carbon emissions a much riskier prospect.

“U.S. forests could look dramatically different by the end of the century,” says William Anderegg, study lead author and associate professor in the University of Utah School of Biological Sciences. “More severe and frequent fires and disturbances have huge impacts on our landscapes. We are likely to lose forests from some areas in the Western U.S. due to these disturbances, but much of this depends on how quickly we tackle climate change.”


William Anderegg

"We’ve seen devastating fire seasons with increasing severity in the past several years. Generally, we expect the western U.S. to be hit hardest."


The researchers modeled the risk of tree death from fire, climate stress (heat and/or drought) and insect damage for forests throughout the United States, projecting how those risks might increase over the course of the 21st century.

See their findings in an interactive map at

By 2099, the models found, that United States forest fire risks may increase by between four and 14 times, depending on different carbon emissions scenarios. The risks of climate stress-related tree death and insect mortality may roughly double over the same time.

But in those same models, human actions to tackle climate change mattered enormously—reducing the severity of climate change dramatically reduced the fire, drought and insect-driven forest die-off.

“Climate change is going to supercharge these three big disturbances in the U.S.,” Anderegg says. “We’ve seen devastating fire seasons with increasing severity in the past several years. Generally, we expect the western U.S. to be hit hardest by all three of these. And they’re somewhat interconnected too. Really hot and dry years, driven by climate change, tend to drive lots of fires, climate-driven tree mortality and insect outbreaks. But we have an opportunity here too. Addressing climate change quickly can help keep our forests and landscapes healthy.”

The study is published in Ecology Letters and was supported by the National Science Foundation, U.S. Department of Agriculture, David and Lucille Packard Foundation and Microsoft’s AI for Earth.

Find the full study at Ecology Letters.


by Paul Gabrielsen, first published at @TheU.


How Trees Grow

How Trees Grow

William Anderegg

What we’re still learning about how trees grow.

What will happen to the world’s forests in a warming world? Will increased atmospheric carbon dioxide help trees grow? Or will extremes in temperature and precipitation hold growth back? That all depends on whether tree growth is more limited by the amount of photosynthesis or by the environmental conditions that affect tree cell growth—a fundamental question in tree biology, and one for which the answer wasn’t well understood, until now.

A study led by University of Utah researchers, with an international team of collaborators, finds that tree growth does not seem to be generally limited by photosynthesis but rather by cell growth. This suggests that we need to rethink the way we forecast forest growth in a changing climate and that forests in the future may not be able to absorb as much carbon from the atmosphere as we thought.

“A tree growing is like a horse and cart system moving forward down the road,” says William Anderegg, an associate professor in the U’s School of Biological Sciences and principal investigator of the study. “But we basically don’t know if photosynthesis is the horse most often or if it’s cell expansion and division. This has been a longstanding and difficult question in the field. And it matters immensely for understanding how trees will respond to climate change.”

The study is published in Science and is funded by the U.S. Department of Agriculture, the David and Lucille Packard Foundation, the National Science Foundation, the U.S. Department of Energy and the Arctic Challenge for Sustainability II.

Growth rings - oldest growth is at the top.

Source vs. sink

We learned the basics in elementary school—trees produce their own food through photosynthesis, taking sunlight, carbon dioxide and water and turning it into leaves and wood.

There’s more to the story, though. Converting carbon gained from photosynthesis into wood requires wood cells to expand and divide.

So trees get carbon from the atmosphere through photosynthesis. This is the trees’ carbon source. They then spend that carbon to build new wood cells—the tree’s carbon sink.

If the trees’ growth is source-limited, then it’s limited only by how much photosynthesis the tree can carry out and tree growth would be relatively easy to predict in a mathematical model. So rising carbon dioxide in the atmosphere should ease that limitation and let trees grow more, right?

But if instead the trees’ growth is sink-limited, then the tree can only grow as fast as its cells can divide. Lots of factors can directly affect both photosynthesis and cell growth rate, including temperature and the availability of water or nutrients. So if trees are sink-limited, simulating their growth has to include the sink response to these factors.

The researchers tested that question by comparing the trees’ source and sink rates at sites in North America, Europe, Japan and Australia. Measuring carbon sink rates was relatively easy—the researchers just collected samples from trees that contained records of growth. “Extracting wood cores from tree stems and measuring the width of each ring on these cores essentially lets us reconstruct past tree growth,” says Antoine Cabon, a postdoctoral scholar in the School of Biological Sciences and lead author of the study.

Measuring carbon sources is tougher, but doable. Source data was measured with 78 eddy covariance towers, 30 feet tall or more, that measure carbon dioxide concentrations and wind speeds in three dimensions at the top of forest canopies, Cabon says. “Based on these measurements and some other calculations,” he says, “we can estimate the total forest photosynthesis of a forest stand.”


The researchers analyzed the data they collected, looking for evidence that tree growth and photosynthesis were processes that are linked, or coupled. They didn’t find it. When photosynthesis increased or decreased, there was not a parallel increase or decrease in tree growth.

“Strong coupling between photosynthesis and tree growth would be expected in the case where tree growth is source limited,” Cabon says. “The fact that we mostly observe a decoupling is our principal argument to conclude that tree growth is not source-limited.”

Surprisingly, the decoupling was seen in environments across the globe. Cabon says they did expect to see some decoupling in some places, but “we did not expect to see such a widespread pattern.”

The strength of coupling or decoupling between two processes can lie on a spectrum, so the researchers were interested in what conditions led to stronger or weaker decoupling. Fruit-bearing and flowering trees, for example, exhibited different source-sink relationships than conifers. More diversity in a forest increased coupling. Dense, covered leaf canopies decreased it.

Finally, coupling between photosynthesis and growth increased in warm and wet conditions, with the opposite also true: that in cold and dry conditions, trees are more limited by cell growth.

Cabon says that this last finding suggests that the source vs. sink issue depends on the tree’s environment and climate. “This means that climate change may reshape the distribution of source and sink limitations of the world forests,” he says.

A new way to look forward

The key takeaway is that vegetation models, which use mathematical equations and plant characteristics to estimate future forest growth, may need to be updated. “Virtually all these models assume that tree growth is source limited,” Cabon says.

For example, he says, current vegetation models predict that forests will thrive with higher atmospheric carbon dioxide. “The fact that tree growth is often sink limited means that for many forests this may not actually happen.”

That has additional implications: forests currently absorb and store about a quarter of our current carbon dioxide emissions. If forest growth slows down, so do forests’ ability to take in carbon, and their ability to slow climate change.

Find the full study @

Other authors of the study include Steven A. Kannenberg, University of Utah; Altaf Arain and Shawn McKenzie, McMaster University; Flurin Babst, Soumaya Belmecheri and David J. Moore, University of Arizona; Dennis Baldocchi, University of California, Berkeley; Nicolas Delpierre, Université Paris-Saclay; Rossella Guerrieri, University of Bologna; Justin T. Maxwell, Indiana University Bloomington; Frederick C. Meinzer and David Woodruff, USDA Forest Service, Pacific Northwest Research Station; Christoforos Pappas, Université du Québec à Montréal; Adrian V. Rocha, University of Notre Dame; Paul Szejner, National Autonomous University of Mexico; Masahito Ueyama, Osaka Prefecture University; Danielle Ulrich, Montana State University; Caroline Vincke, Université Catholique de Louvain; Steven L. Voelker, Michigan Technological University and Jingshu Wei, Polish Academy of Sciences.


- by Paul Gabrielsen, first published in @theU


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Allergy Season

Climate Change & Allergies

William Anderegg

With spring around the corner, here's some bad news for allergy sufferers: Human-caused climate change has both worsened and lengthened pollen seasons across the U.S. and Canada, a study Monday reports.

The new research shows that pollen seasons start 20 days earlier, are 10 days longer and feature 21% more pollen than they did in 1990.

“The strong link between warmer weather and pollen seasons provides a crystal-clear example of how climate change is already affecting people's health across the U.S.,” said study lead author William Anderegg, a biologist at the University of Utah.

"Climate change is making pollen seasons worse across the U.S., and that has major implications for asthma, allergies and other respiratory health problems," he told USA TODAY.

Climate change, aka global warming, is caused by the burning of fossil fuels such as oil, gas and coal, which release greenhouse gases such as carbon dioxide and methane into the atmosphere.

Allergies to airborne pollen can be more than just a seasonal nuisance to many. Allergies are tied to respiratory health and have implications for viral infections, emergency room visits and even children’s school performance, according to a statement from the University of Utah. More pollen, hanging around for a longer season, makes those impacts worse.

Climate change has two broad effects, according to the study. First, it shifts pollen seasons earlier and lengthens their duration. Second, it increases the pollen concentrations in the air so pollen seasons are, on average, worse.

Anderegg's research team looked at measurements from 1990 to 2018 from 60 pollen count stations across the U.S. and Canada, maintained by the National Allergy Bureau.

Although nationwide pollen amounts increased by around 21% over the study period, the greatest increases were recorded in Texas and the Midwest, and more among tree pollen than among other plants.

"Our findings are consistent with a broad body of research on pollen seasons, respiratory health and climate change," Anderegg said. "Other studies have also found increasing pollen loads in many regions and, in controlled greenhouse settings, that warmer temperatures and higher carbon-dioxide concentrations increase plant pollen production."

The researchers also found that the contribution of climate change to increasing pollen amounts is accelerating.

“Climate change isn’t something far away and in the future," Anderegg concluded. "It’s already here in every spring breath we take and increasing human misery. The biggest question is – are we up to the challenge of tackling it?”

The study was published in the Proceedings of the National Academy of Sciences, a peer-reviewed journal.


First published @ usatoday

Forest Futures

Forest Futures

Know the risks of investing in forests.

Given the tremendous ability of forests to absorb carbon dioxide from the atmosphere, some governments are counting on planted forests as offsets for greenhouse gas emissions—a sort of climate investment. But as with any investment, it’s important to understand the risks. If a forest goes bust, researchers say, much of that stored carbon could go up in smoke.

In a paper published in Science, University of Utah biologist William Anderegg and his colleagues say that forests can be best deployed in the fight against climate change with a proper understanding of the risks to that forest that climate change itself imposes. “As long as this is done wisely and based on the best available science, that’s fantastic,” Anderegg says. “But there hasn’t been adequate attention to the risks of climate change to forests right now.”

Meeting of Minds

William Anderegg

In 2019, Anderegg, a recipient of the Packard Fellowship for Science and Engineering from the David and Lucile Packard Foundation, convened a workshop in Salt Lake City to gather some of the foremost experts on climate change risks to forests. The diverse group represented various disciplines: law, economics, science and public policy, among others. “This was designed to bring some of the people who had thought about this the most together and to start talking and come up with a roadmap,” Anderegg says.

This paper, part of that roadmap, calls attention to the risks forests face from myriad consequences of rising global temperatures, including fire, drought, insect damage and human disturbance—a call to action, Anderegg says, to bridge the divide between the data and models produced by scientists and the actions taken by policymakers.

Accumulating Risk

Forests absorb a significant amount of the carbon dioxide that’s emitted into the atmosphere—just under a third, Anderegg says. “And this sponge for CO2 is incredibly valuable to us.”

Because of this, governments in many countries are looking to “forest-based natural climate solutions” that include preventing deforestation, managing natural forests and reforesting. Forests could be some of the more cost-effective climate mitigation strategies, with co-benefits for biodiversity, conservation and local communities.

But built into this strategy is the idea that forests are able to store carbon relatively “permanently”, or on the time scales of 50 to 100 years—or longer. Such permanence is not always a given. “There’s a very real chance that many of those forest projects could go up in flames or to bugs or drought stress or hurricanes in the coming decades,” Anderegg says.

Forests have long been vulnerable to all of those factors, and have been able to recover from them when they are episodic or come one at a time. But the risks connected with climate change, including drought and fire, increase over time. Multiple threats at once, or insufficient time for forests to recover from those threats, can kill the trees, release the carbon, and undermine the entire premise of forest-based natural climate solutions.

“Without good science to tell us what those risks are,” Anderegg says, “we’re flying blind and not making the best policy decisions.”

Mitigating Risk

In the paper, Anderegg and his colleagues encourage scientists to focus increased attention on assessing forest climate risks and share the best of their data and predictive models with policymakers so that climate strategies including forests can have the best long-term impact. For example, he says, the climate risk computer models scientists use are detailed and cutting-edge, but aren’t widely used outside the scientific community. So, policy decisions can rely on science that may be decades old.

“There are at least two key things you can do with this information,” Anderegg says. The first is to optimize investment in forests and minimize risks. “Science can guide and inform where we ought to be investing to achieve different climate aims and avoid risks.”

The second, he says, is to mitigate risks through forest management. “If we’re worried about fire as a major risk in a certain area, we can start to think about what are the management tools that make a forest more resilient to that disturbance.” More research, he says, is needed in this field, and he and his colleagues plan to work toward answering those questions.

“We view this paper as an urgent call to both policymakers and the scientific community,” Anderegg says, “to study this more, and improve in sharing tools and information across different groups.” Read the full paper @



by Paul Gabrielsen first published in @theU