Climate change

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

By William R. L. Anderegg 5 minutes Lily

When people talk about ways to slow climate change, they often mention trees, and for good reason. Forests occupy a A large quantity carbon dioxide that warms the planet and that people release into the atmosphere when they burn fossil fuels. But will trees keep pace as global temperatures rise? with companies invest more and more in forests as offsetsclaiming that this negates their ongoing greenhouse gas emissions is a multi-billion dollar question.

The results of two studies published in journals Science and Ecology Letters May 12, 2022—one focused on growththe other at death— raise new questions about the extent to which the world can rely on forests to store increasing amounts of carbon in a warming future. Ecologist William Andereggwho participated in both studies, explain why.

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

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

These climatic stresses are increasing much faster as the planet warms than scientists had predicted. We see huge forest fires and forest mortality due to drought much earlier than anyone expected. When those trees die, that carbon goes back into the atmosphere. We also find that the benefits trees derive from higher levels of carbon dioxide in a warming world may be more limited that people realize.

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

Trees and forests do all sorts of other amazing things: they purify air and water, and they provide economic value in terms of timber, tourism, and pollination. So understanding how they will grow is important for many reasons.

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

Two key things affect tree growth: photosynthesiswhich is how trees turn sunlight and carbon dioxide into food, and the cell division process and enlargement.

There has been a long-standing debate about the primary driver of tree growth.

A good metaphor here is a cart with two horses. The cart coming down the road is the growing tree, and there are two horses tied, but we don’t know which one is actually doing the work of pulling the cart. A horse is photosynthesis. That makes a lot of intuitive sense – that’s where all the carbon comes from to build the cells. But we know there’s another horse – to grow more wood, trees have to develop layers of cells, and cells have to expand and divide. This cell growth process is very sensitive to climatic changes and tends to stop when conditions are dry.

Large parts of the western United States have faced severe drought conditions for years. About half of the contiguous United States was in drought in May 2022. [Image: Drought Monitor/UNL/NOAA/USDA]

People assume that photosynthesis is the dominant process almost everywhere. But we found stronger evidence that these drought-sensitive cellular processes do more to stimulate or limit growth.

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

This suggests that droughts, rather than the amount of carbon dioxide in the air, may have the biggest impact on how fast trees grow in the future. were already see more frequent and severe droughts in many regions.

What did you learn about the risk of tree death in the future?

In the other study, we found that reducing global greenhouse gas emissions could have a huge impact to avoid damage forests against forest fires, drought and insects.

We used years of satellite observations, climate data and a network of approximately 450,000 tree plots across the United States where every tree is monitored for climate stress and survival. With this historical data, we built statistical models of the risk American trees face from wildfires, insects, and climate stress, primarily related to drought. Then we looked at what might happen under future climate scenarios, with high carbon emissions, medium emissions, and low emissions. You can explore the results on a interactive map.

The big picture: As the planet warms, wildfires the risk increases noticeably in the current century, especially in the western United States. In a medium emissions scenario, the risk of forest fires is expected to increase fourfold. The risk of drought and insects increases by about 50-80%.

What does this mean for the use of carbon offsets?

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

This has huge implications for the use of forests as carbon offsets.

So far, carbon offset protocols and markets have not really addressed this up-to-date scientific understanding of the risks facing forests of climate change. This tells us that climate policymakers and offset developers need to be very careful about how they rely on forest offsets to generate benefits.

The most encouraging message is that our actions over the next decade matter a lot. If we can slow the speed of climate change and move down a low-emissions path, that does a lot to reduce the risks and increase the benefits. This is not a situation to ‘give up and panic’ – this is our chance to take action that ensures resilient and sustainable forests last for the future.

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

Guillaume RL Anderegg is Associate Professor of Ecology, School of Biological Sciences, University of Utah.