The Forgotten Role of Forest Disturbances in Our Carbon Budget

In the fight against climate change, forests play a starring role. Northern forests, in particular, act as one of the planet’s largest carbon sinks, absorbing significant amounts of carbon dioxide from the atmosphere. But how much carbon do these forests absorb? And why do estimates of this vital carbon sink vary so widely?

Our research, published in Nature Communications Earth & Environment, addresses this puzzle. We’ve uncovered key discrepancies between two methods used to measure carbon sinks: dynamic global vegetation models (DGVMs) and atmospheric inversions. These two approaches provide estimates that differ by a staggering 50%. Our study explains why this gap exists and how we can bridge it by focusing on something often overlooked: forest disturbances and regrowth.

Why Are Forests So Important?

Forests are like giant sponges, soaking up carbon dioxide (CO2), a major driver of global warming. The forests of the northern hemisphere—spanning North America, Europe, Russia, and China—play a particularly crucial role. They store carbon in their trunks, roots, and soils, helping to offset emissions from human activities such as burning fossil fuels. However, the ability of forests to act as carbon sinks is not static. It changes as forests age and respond to disturbances like wildfires, logging, or even storms.

Understanding how much carbon these forests can absorb is critical for creating accurate global carbon budgets. These budgets are essential for policymakers and scientists alike, as they provide the data needed to track our progress in tackling climate change and to predict future climate scenarios. But what happens when different scientific methods give us different answers?

The Discrepancy: DGVMs vs. Atmospheric Inversions

There are two main methods for estimating the amount of carbon absorbed by forests. DGVMs simulate how forests grow and absorb carbon over time, while atmospheric inversions track CO2 levels in the atmosphere and estimate where carbon is being absorbed or released.

For northern forests, DGVMs estimate a carbon sink of around 4 petagrams of carbon dioxide per year (that’s 4 billion tons). Atmospheric inversions, on the other hand, suggest the sink is much larger—about 8.1 petagrams per year. That’s a huge difference. So, who’s right?

Our research reveals that DGVMs likely underestimate the northern carbon sink because they don't account well for two key factors: forest disturbances and the age of trees. DGVMs tend to overestimate carbon losses from events like wildfires, while failing to capture the significant carbon uptake that occurs as forests regrow after disturbances.

The Importance of Forest Age and Regrowth

One of the critical oversights in many models is how they treat forest age. Forests are not static entities. After a disturbance, such as a fire or logging, forests begin to regrow. Young trees grow rapidly, absorbing large amounts of carbon as they mature. However, most DGVMs do not include detailed information about forest age and therefore miss this rapid regrowth phase. Instead, they simulate forests as if they are all mature, old-growth forests, which absorb carbon at a slower rate.

By integrating satellite data on forest age and biomass, we were able to estimate the amount of carbon absorbed during regrowth. Our results show that regrowth alone accounts for 4 petagrams of carbon dioxide per year—about half of the entire northern forest carbon sink. This new understanding helps reconcile the difference between DGVMs and atmospheric inversions.

Why Does This Matter for Climate Change?

The future of our carbon budget depends heavily on how well we can protect and manage forests. Our research highlights that the current carbon sink is largely due to young, fast-growing forests regrowing after past disturbances. This regrowth phase won’t last forever. As forests mature, their ability to absorb carbon will slow down, and if these forests are not properly managed or protected, they could become carbon sources instead. Many countries are now experiencing declines in land carbon sinks due to factors such as climate change, pest outbreaks, and forest aging. This is a major challenge for reaching carbon neutrality, as many nations are relying on forests to offset emissions from other sectors.

The urgency to protect and restore forests has never been greater. Our findings emphasise the need for sustainable forest management practices that promote forest regrowth and resilience to disturbances. Climate change is expected to increase the frequency of extreme events like wildfires and storms, which could further threaten forest carbon sinks. By better understanding the role of disturbances and regrowth, we can improve our climate models and make more informed decisions about managing our natural resources.

Looking Ahead: A Call for Better Models

Our study underscores the importance of advancing DGVMs to better capture forest disturbances and age-related dynamics. These DGVMs are the land components of Earth system models (ESMs) that make climate projections to inform IPCC reports and guide global climate policy. If we misattribute the carbon sink and fail to include crucial processes like forest regrowth, we risk undermining these projections by not accurately simulating carbon-climate feedbacks. Improving these models will provide a more reliable picture of how forests respond to environmental changes, helping us predict future carbon budgets more accurately.

Forests are not only carbon sinks; they are also biodiversity hotspots, water reservoirs, and providers of livelihoods for millions of people. Protecting these ecosystems will not only mitigate climate change but also ensure they continue to support life on Earth.