In a warming world, trees are not a panacea for climate change

By William RL Anderegg 5 Minutes Read

When people talk about ways to slow down climate change, they often mention trees, and for good reason. Forests absorb a large amount of the planet-warming carbon dioxide that humans put into the atmosphere when they burn fossil fuels. But will trees maintain that pace as global temperatures rise? With companies increasingly investing in forests as compensation, saying it will wipe out their ongoing greenhouse gas emissions, it’s a matter of billions of dollars.

The results of two studies published May 12, 2022 in the journals Science and Ecology Letters — one focusing on growth, the other on death — raise new questions about how much the world can rely on forests to store increasing amounts of carbon. beat in a global warming. future. Ecologist William Anderegg, who was involved in both studies, explains why.

What does the new research tell us about trees and their ability to store carbon?

The future of forests is at the cutting edge, with a tug-of-war between two very important forces: the benefits trees derive from increasing carbon dioxide levels and the stresses they face from the climate, such as heat, drought, fires, pests, and pathogens.

That climate stress is increasing much faster as the planet warms than scientists had expected. We’re seeing massive wildfires and drought-induced forest deaths much sooner than anyone expected. When those trees die, that carbon goes back into the atmosphere. We’re also seeing evidence that the benefits trees get from higher levels of carbon dioxide in a warming world may be more limited than people realize.

This tells us it’s probably not a good idea to rely on forests for a widespread carbon sink in the 21st century, especially if societies don’t cut their emissions.

Trees and forests do all kinds of other amazing things – they purify the air and water, and they provide economic value in terms of timber and tourism and pollination. So it is important for many reasons to understand how they will grow.

There’s an argument that, with more carbon dioxide in the atmosphere, trees will simply grow more and trap that carbon. What did your study think?

Two main things affect tree growth: photosynthesis, which is how trees convert sunlight and carbon dioxide into food, and the process of cell division and expansion.

There has long been a debate about what is the biggest engine for tree growth.

A good metaphor here is a cart with two horses. The cart that moves across the road is the tree that grows, and two horses are attached to it, but we don’t know which one does the work to pull the cart. A horse is photosynthesis. That’s very intuitive – it’s where all the carbon comes from for building cells. But we know there’s another horse — to grow more wood, trees have to grow cell layers, and the cells have to expand and divide. That cell growth process is very sensitive to climate change and tends to grind to a halt when conditions are dry.

Large parts of the western US have been dealing with severe drought for years. About half of the contiguous US was in drought in May 2022. [Image: Drought Monitor/UNL/NOAA/USDA]Almost everywhere, people assume that photosynthesis is the dominant process. But we found stronger evidence that these cellular processes that are sensitive to drought actually do more to stimulate or limit growth.

We used tree ring data from thousands of trees in the US and Europe and photosynthesis measurements from towers in nearby forests to check whether tree growth and photosynthesis were correlated over time. If they followed the same pattern, increasing or decreasing in the same years, that would have suggested that photosynthesis was the horse that pulled the cart. Instead, we found no correlation.

That suggests that drought, rather than the amount of carbon dioxide in the air, may have the biggest impact on how quickly trees grow in the future. We are already seeing more frequent and severe droughts in many regions.

What have you learned about the risk of tree death in the future?

In the other study, we found that lowering global greenhouse gas emissions can have a huge impact on preventing damage to forests from wildfires, droughts and insects.

We used years of satellite observations, climate data and a network of about 450,000 tree plots in the US, where each tree is monitored for climate stress and survival. With that historical data, we built statistical models of the risk American trees face from wildfires, insects and climate stress, mainly related to drought. We then looked at what could happen in future climate scenarios, with high carbon emissions, medium emissions and low emissions. You can explore the results on an interactive map.

The big picture: As the planet warms, the risk of wildfires has increased significantly over the past century, especially in the western US. In a medium emissions scenario, the risk of wildfires is expected to increase by a factor of four. Drought and insect risks increase by about 50% to 80%.

What does this mean for the use of CO2 offsets?

Together, these studies suggest that the benefits of carbon dioxide for growth will not be nearly as great as people thought, and that the risk of climate stress, especially wildfires, drought and insects, will be much greater than people expect.

This has enormous consequences for the use of forests as carbon offsets.

So far, carbon offsetting protocols and markets haven’t really struggled with this updated scientific understanding of the risks forests face from climate change. This tells us that climate policy makers and offset developers need to be very careful about how they rely on forest offsets to reap benefits.

The more hopeful message is that our actions in the next decade matter immensely. If we can curb the speed of climate change and move down a path of lower emissions, that will go a long way to lowering the risk and increasing the benefits. This is not a situation where we roll up our sleeves, but it is our chance to take steps to ensure that resilient and sustainable forests continue to exist in the future.

What we do with our own emissions and efforts to slow climate change is hugely important for the future of forests.

William RL Anderegg is an associate professor of ecology, School of Biological Sciences, University of Utah.


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