Quantifying the U.S. forest carbon sink

Overview

American forests have stored more carbon over the past two decades than at any point in the last century. A new study published in the Proceedings of the National Academy of Sciences identifies what's driving this: forest age accounts for the largest share, locking in 89 million metric tons of carbon annually as trees reach peak growth stages. Climate factors—temperature, precipitation, and elevated carbon dioxide—add another 66 million metric tons per year.

The research matters because international climate agreements require countries to account for their carbon sinks. Until now, no study had systematically separated what portion of forest carbon storage results from deliberate human action versus natural processes. That distinction determines whether carbon credits are genuine climate progress or accounting fiction—a question worth billions of dollars in the voluntary carbon market.

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Key Indicators

89M

Metric tons/year from forest age

Carbon locked in annually due to forests reaching peak growth stages

66M

Metric tons/year from climate

Additional carbon sequestration from temperature, precipitation, and CO2 changes

31M

Metric tons/year lost to deforestation

Carbon released annually from human-caused forest clearing

11.9%

U.S. emissions offset by forests

Share of annual U.S. carbon emissions captured by forest carbon sink

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People Involved

Brent Sohngen

Lead researcher on passive vs. active forest carbon study

David J. Lewis

Co-author of PNAS forest carbon study

Eric C. Davis

Co-author of PNAS forest carbon study

Organizations Involved

The Ohio State University

Research University

Lead institution on forest carbon study

A major public research university whose Department of Agricultural, Environmental and Development Economics led the forest carbon attribution study.

U.S. Forest Service

Federal Agency

Primary U.S. forest carbon data source

The federal agency responsible for managing national forests and providing forest carbon data through its Forest Inventory and Analysis program.

U.S. Environmental Protection Agency (EPA)

Federal Agency

Publishes annual U.S. greenhouse gas inventory including forest carbon

Develops the annual Inventory of U.S. Greenhouse Gas Emissions and Sinks tracking emissions and forest carbon absorption since 1990.

Timeline

Study Quantifies Passive vs. Active Forest Carbon
Research

Ohio State-led study in PNAS separates natural and human contributions to U.S. forest carbon storage for the first time.
June 11th, 2025
Global Forest Carbon Sink Hits 20-Year Low
Development

Wildfires in Canada and Amazon drought cause forests to absorb only a quarter of typical annual CO2, raising concerns about sink stability.
2023
Researchers Quantify CO2 Fertilization Effect
Research

Sohngen, Lewis, and colleagues publish study in Nature Communications showing elevated CO2 has strong positive effect on U.S. forest wood volume.
October 2022
Paris Agreement Signed
Policy

Over 100 countries include forest mitigation in their Nationally Determined Contributions, increasing scrutiny of carbon accounting.
December 12th, 2015
Kyoto Protocol Adopted
Policy

First international treaty to recognize forest carbon sinks in climate accounting, though excluding tropical forest conservation.
December 11th, 1997
Systematic U.S. Forest Carbon Tracking Begins
Measurement

EPA begins annual greenhouse gas inventory including forest carbon stocks, establishing baseline for future comparisons.
1990
Knutson-Vandenberg Act Enables Reforestation
Policy

Federal law provides funding mechanism for replanting harvested national forest lands, accelerating forest recovery.
1930
U.S. Forest Area Stabilizes
Historical

After declining from 46% to 34% of total U.S. land since 1630, forest area remains roughly constant despite population growth.
1910
U.S. Forest Service Established
Policy

Federal agency created partly to address deforestation concerns, beginning era of forest management and conservation.
1905
Eastern U.S. Forest Cover Reaches Historic Low
Historical

Forest cover in the Eastern United States falls to about 48% of pre-settlement levels after decades of clearing for agriculture and timber.
1872

Scenarios

Predict which scenario wins. Contrarian picks score more — points lock in when the scenario resolves.

Predictions leaderboard

Carbon Credit Protocols Adopt New Methodology

The study's framework for separating passive and active carbon could reshape how carbon credit registries assess additionality. If protocols require projects to demonstrate carbon storage beyond what would occur naturally from forest aging and climate factors, many existing forestry credits could lose value. Registries would need to recalculate baselines, potentially invalidating credits worth billions.

Discussed by: Carbon market analysts at Sylvera, Clean Air Task Force, and Environmental Defense Fund

Consensus —

U.S. Updates Climate Pledges with Refined Forest Accounting

The methodology could enable the United States to report more granular forest carbon data in its Nationally Determined Contributions under the Paris Agreement. Rather than claiming credit for all forest carbon uptake, the U.S. could specify which portions result from active management policies versus passive natural processes, potentially setting a new transparency standard.

Discussed by: Climate policy researchers, UNFCCC reporting analysts

Consensus —

Forest Carbon Sink Weakens as Trees Age Past Peak Growth

If the dominant driver of current carbon storage is forest age composition—trees reaching peak growth stages—the sink could weaken as forests mature past that window. Models suggest U.S. forest carbon storage may equilibrate by 2040 as recovery from historical clearing tapers off. Without active management or new planting, the annual sequestration rate would decline.

Discussed by: Forest Service researchers, IPCC contributors, climate modelers

Consensus —

Research Spurs Investment in Active Forest Management

By demonstrating that only 23 million metric tons annually comes from deliberate reforestation while 31 million is lost to deforestation, the study could justify expanded federal investment in forest management. Current funding sits at $640 million annually; the research suggests increased investment could boost the net carbon sink by 12-17% by 2050.

Discussed by: USDA economists, forest landowner associations, carbon investment firms

Consensus —

Historical Context

19th Century U.S. Deforestation and 20th Century Recovery

1850-1930

What Happened

From 1850 to 1900, an average of 13 square miles of forest was cleared every day for agriculture and timber. Eastern forest cover fell to 48% of pre-settlement levels by 1872. The pulse of carbon emissions from this clearing was followed by regrowth as marginal farmland was abandoned and conservation policies took hold.

Outcome

Short Term

Forest area stabilized after 1910 at about 34% of U.S. land. Reforestation programs began planting millions of trees.

Long Term

Temperate U.S. forests became a net carbon sink by mid-20th century. By 2022, forests stored 59.7 billion metric tons of carbon, up 12% since 1990.

Why It's Relevant Today

The current study shows that structural changes from this historical recovery—forests reaching peak growth stages—account for the largest share of present carbon storage, at 89 million metric tons annually.

Kyoto Protocol Forest Carbon Debates (1997-2005)

December 1997 - February 2005

What Happened

Negotiators spent eight years debating how to account for forest carbon in international climate commitments. Countries with large forest sinks wanted full credit; others argued this would let nations claim reductions without changing industrial emissions. The final rules allowed afforestation and reforestation but excluded tropical forest conservation.

Outcome

Short Term

The Clean Development Mechanism funded tree-planting projects but created ongoing disputes about baseline accuracy and additionality.

Long Term

The Paris Agreement expanded forest accounting but left methodology questions unresolved, contributing to current carbon credit quality concerns.

Why It's Relevant Today

The debate over what forest carbon is 'natural' versus 'additional' from human action remains central to climate policy. This study provides the first systematic framework for making that distinction.

2023 Global Forest Carbon Sink Collapse

January 2023 - December 2023

What Happened

Canadian wildfires released greenhouse gases equivalent to adding a third of China's annual emissions. Amazon drought prevented forests from absorbing typical amounts of CO2. Combined, global forests absorbed only a quarter of their average annual carbon dioxide uptake.

Outcome

Short Term

The event demonstrated that passive carbon sinks are vulnerable to climate disruption and cannot be assumed stable.

Long Term

Researchers warned that narrowing margins between forest emissions and removals could tip forests from sink to source.

Why It's Relevant Today

The instability of passive carbon sinks underscores why distinguishing natural from managed forest carbon matters—active management may provide more reliable climate benefits.