Cloud Over CO2 Storage in Trees

New Research Raises Questions

A series of recent scientific studies is casting a shadow over assumptions about the long-term reliability of forests as carbon sinks, raising questions that have significant implications for international climate policy and for the credibility of forest-based carbon offset mechanisms. The research suggests that forests may be less stable carbon stores than previously assumed, with the ability of trees to continue absorbing atmospheric CO2 potentially compromised by the same climate changes that forest conservation is meant to help prevent.

Scientists studying forest carbon dynamics in tropical and temperate forests have documented growing evidence that drought, temperature stress, and insect outbreaks are reducing net carbon uptake in forests that were previously considered reliable sinks. Measurements from long-term forest monitoring plots show that tree growth rates are declining in some regions under climate stress, while rates of tree mortality are increasing in others.

The timing of these findings is particularly significant. They come as governments are negotiating the REDD+ mechanism to fund forest conservation as a climate mitigation strategy, and as the voluntary carbon market for forest offsets is growing rapidly. The reliability of forest carbon stores as a long-term climate solution depends critically on the assumption that the carbon stored in protected forests remains sequestered — an assumption these findings complicate.

Forests Under Climate Stress

The evidence for climate stress in forests comes from multiple sources and multiple biomes. In the Amazon, researchers have documented years in which severe drought turned the world's largest tropical forest from a net carbon sink into a net carbon source. In the western United States, bark beetle outbreaks facilitated by warmer temperatures have killed millions of hectares of conifer forest, releasing large quantities of carbon as the dead trees decompose.

In Australia, prolonged drought has contributed to widespread mortality of eucalyptus trees in inland forests, a phenomenon described by researchers as climate-driven dieback. In Mediterranean Europe, extreme heat and drought events have caused increased tree mortality and shifted the carbon balance of forests in Spain, France, and Portugal. Even the Amazon, long thought to be relatively resilient to climate variability, has shown signs of weakening carbon uptake under warming and drying conditions.

The mechanisms behind these changes are varied but interrelated. Higher temperatures increase the rate at which soil microbes decompose organic matter, releasing CO2 even as photosynthesis may be boosted by higher CO2 concentrations. Water stress reduces photosynthesis more than it reduces respiration in many tree species, creating a carbon deficit during drought years. And disturbances such as fire and insect outbreaks, which are intensified by climate change, can release decades of accumulated carbon stocks in a matter of days or weeks.

Implications for REDD+ and Carbon Accounting

For policymakers developing REDD+ and other forest carbon mechanisms, these findings raise important questions about how to ensure that claimed emission reductions are permanent. If forests that are protected from deforestation today may lose significant carbon stocks to climate-driven processes in the future, the long-term climate benefit of forest conservation commitments made today is uncertain.

The research does not undermine the case for forest conservation as a climate strategy — protecting forests from deforestation remains among the most cost-effective mitigation options available, and intact forests are likely to be more resilient to climate stress than degraded ones. But it does suggest that climate policy frameworks must explicitly account for permanence risk and that the assumption of indefinitely stable forest carbon stocks used in some accounting frameworks may need to be revised.

Advocates for robust REDD+ design argue that the findings reinforce the importance of buffer reserves, conservative crediting approaches, and long-term monitoring commitments in forest carbon projects. If a significant portion of the credits from forest conservation must be set aside to insure against future climate-driven losses, this will reduce the financial returns available to conservation projects — potentially affecting the viability of some programmes and the overall scale of investment in forest conservation.

The Scientific Debate

While the findings of increased climate stress in forests are well-documented, the scientific community continues to debate the magnitude and trajectory of these changes and their implications for long-term forest carbon storage. Some researchers emphasise that elevated CO2 itself can stimulate tree growth through the fertilisation effect, potentially offsetting some of the negative impacts of temperature and drought stress. Others argue that nutrient limitations will prevent forests from fully realising the growth benefits of higher CO2.

The balance of evidence suggests that the net effect of climate change on forest carbon storage will vary considerably by region, forest type, and the specific trajectories of temperature and precipitation change under different emissions scenarios. In some regions, forest carbon uptake may increase for some decades before declining; in others, the negative effects may dominate from the outset. The uncertainty in these projections reflects both the complexity of forest ecosystems and the range of possible climate futures.

What is clear is that the simple narrative of forests as reliable long-term carbon sinks — in which protecting a hectare of forest today permanently removes a fixed quantity of carbon from the climate problem — is an oversimplification. A more nuanced accounting of forest carbon dynamics under climate change is needed to ensure that forest conservation investments deliver the climate benefits they are intended to provide.

Frequently Asked Questions

Can forests become carbon sources rather than carbon sinks?+

Under certain conditions, forests can indeed transition from acting as net carbon sinks — absorbing more CO2 from the atmosphere than they release — to acting as net carbon sources that add to atmospheric carbon concentrations. This transition can occur as a result of disturbances such as fire, drought, insect outbreaks, or logging, as well as from the progressive effects of climate change on forest physiology and ecosystem processes.

Research in the Amazon has documented periods where drought stress caused widespread tree mortality and reduced photosynthesis, leading to years in which the forest released more carbon than it absorbed. The severe droughts of 2005 and 2010 each caused the Amazon basin to shift from a net carbon sink to a net carbon source for extended periods. Similar dynamics have been observed in boreal forests subjected to beetle outbreaks and fire under warmer, drier conditions.

The physiological mechanisms are complex. Higher temperatures increase the rate of respiration in trees and soil microbes, releasing CO2 at a faster rate. If temperature increases outpace the photosynthetic gains from higher CO2 concentrations and longer growing seasons, the net carbon balance of a forest can shift toward emission. Droughts reduce photosynthesis more than respiration in many species, creating a carbon deficit that can persist for months or years.

The implications for climate policy are significant. If forests lose some of their carbon sink capacity as temperatures rise, the contribution of land-use sector carbon uptake to climate stabilisation scenarios may be smaller than assumed in many models. This would increase the burden on energy system decarbonisation and could affect the reliability of forest-based carbon offsets as a climate mitigation strategy.

How does drought affect forest carbon storage?+

Drought is one of the most significant threats to forest carbon storage because it affects trees through multiple simultaneous mechanisms. Water stress reduces photosynthesis as stomata close to prevent water loss, reducing the rate at which trees absorb CO2 from the atmosphere. At the same time, drought increases the risk of fire, which can release enormous quantities of carbon stored in biomass and soil organic matter within hours or days.

Severe or prolonged drought can cause widespread tree mortality through hydraulic failure — the collapse of the water-conducting system that transports moisture from roots to leaves. Mass tree death events, sometimes called "die-offs" or "forest dieback," have been documented in the Amazon, in Australian eucalypt forests, in North American pine and juniper woodlands, and in various European forest types during extreme drought years. The carbon released from dying and decomposing trees can be substantial.

Climate projections for many tropical and subtropical forest regions indicate increased frequency and severity of drought events under higher emissions scenarios. This means that the forests most important for global carbon storage — particularly the Amazon basin and Southeast Asian tropical forests — may face increasing periods of climate stress that reduce their carbon sink strength or even reverse it. The timing and magnitude of these changes remain uncertain, but they represent a significant risk to carbon accounting frameworks that assume stable forest carbon stocks.

In the context of REDD+ and forest carbon offsets, drought-driven carbon losses raise important questions about how to account for natural variability in forest carbon stocks. Methodological frameworks must distinguish between losses caused by policy failures — where deforestation continues despite conservation commitments — and losses caused by climate events beyond anyone's control. Getting this distinction right is essential for maintaining the integrity of results-based payment systems.

What does research on forest carbon permanence mean for climate policy?+

The permanence of forest carbon storage is a fundamental issue in climate policy because carbon credits for avoided deforestation or forest restoration are only climatically meaningful if the carbon remains sequestered for a time period comparable to the atmospheric lifetime of CO2. If forests are subsequently cleared, degraded, or die back due to climate stress, the carbon benefits claimed are reversed, potentially undermining the climate contribution of the credits that were issued.

This concern has led to the development of various accounting approaches designed to address impermanence risk. Temporary credits, which expire after a fixed period and must be renewed or replaced, were the approach adopted under the CDM for afforestation and reforestation projects. Buffer reserves, in which a portion of project credits are set aside in a risk pool to compensate for future losses, are used in voluntary carbon market standards.

Research on forest carbon dynamics has helped quantify the magnitudes of different permanence risks. Large-scale disturbances including fire, drought, and insect outbreaks have been found to cause carbon losses of 10 to 50 percent of above-ground biomass in affected forests, depending on disturbance severity and forest type. Accounting for these risks requires building appropriate risk buffers into project-level and jurisdictional carbon accounting frameworks.

Climate change itself introduces a new and poorly quantified source of permanence risk. As temperatures rise and drought frequency increases, forests that are currently stable carbon stores may face increasing pressure. The carbon accounting frameworks developed for REDD+ and voluntary markets were largely based on historical climate conditions; adapting these frameworks to account for climate-driven changes in forest dynamics is a significant methodological challenge that the scientific and policy communities are actively working to address.

Are there differences between tropical and temperate forests in their carbon stability?+

Tropical, temperate, and boreal forests differ significantly in their carbon dynamics and in the stability of their carbon stores under climate change. Tropical forests hold the highest carbon densities and have historically been relatively stable carbon sinks, but they are increasingly vulnerable to drought stress as temperatures rise and precipitation patterns shift. The Amazon basin, Congo basin, and Southeast Asian tropical forests are among the most important but also most vulnerable carbon stores globally.

Boreal forests — the vast coniferous forests that stretch across Canada, Russia, and Scandinavia — are warming faster than any other forest biome and are experiencing dramatic changes in fire regimes, insect outbreaks, and permafrost dynamics that are affecting their carbon balance. The thawing of permafrost soils in high-latitude regions is releasing carbon that has been locked in frozen ground for thousands of years, potentially turning these regions from net carbon sinks to sources.

Temperate forests in Europe and North America have been significant carbon sinks in recent decades, partly due to the regrowth of forests on formerly agricultural land and partly due to the fertilising effect of elevated CO2 on forest growth. However, research suggests that this sink may be weakening as forests age, as drought events become more frequent, and as nitrogen availability limits the carbon uptake response to elevated CO2.

The scientific picture that emerges is one in which the forest carbon sink that has been absorbing a significant fraction of human CO2 emissions may be less reliable in the future than it has been in the past. This does not undermine the case for forest conservation, but it reinforces the imperative for parallel and immediate reductions in fossil fuel emissions. Relying on forests to absorb ongoing high emissions is not a durable strategy in a warming world.

How should REDD+ programs account for climate-driven carbon losses?+

The challenge of accounting for climate-driven carbon losses in REDD+ programs is both technically and politically complex. Technically, distinguishing between carbon losses caused by deforestation — which is what REDD+ is designed to reduce — and losses caused by drought, fire, or other climate events that would have occurred regardless of conservation policy requires sophisticated monitoring systems and careful attribution analysis.

Most established REDD+ accounting frameworks use a combination of approaches to address this challenge. Reference emission levels are typically established using historical deforestation data and are intended to represent the counterfactual trajectory in the absence of REDD+ policies. Carbon stock changes caused by natural disturbances are generally excluded from the accounting if they can be demonstrated to be temporary and not caused by policy failures.

Buffer pools — reserves of credits held back from the market to compensate for potential future losses — are the most common mechanism for managing permanence risk in project-level accounting. The size of the buffer required for any given project is determined by an assessment of the risk factors including fire risk, drought risk, political stability, and the strength of land tenure protections. High-risk projects must set aside a larger proportion of their credits in the buffer, reducing the volume available for sale.

At the jurisdictional or national level, the accounting approach used under the UNFCCC REDD+ framework provides more flexibility. Countries report their total forest carbon stocks and deforestation rates over time, and emissions from natural disturbances can in some cases be factored out of the accounting. This approach recognises that national governments have limited control over large-scale natural disturbances while maintaining accountability for policy-driven changes in deforestation rates.