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 in the Proceedings of the National Academy of Sciences found that forest age is the primary driver, 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

Voices

Curated perspectives — historical figures and your fellow readers.

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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

1872 June 2025

10 events Latest: June 11th, 2025 · 12 months ago

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June 2025
Study Quantifies Passive vs. Active Forest Carbon
Latest 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
2023
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
October 2022
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
December 2015
Paris Agreement Signed
Policy

Over 100 countries include forest mitigation in their Nationally Determined Contributions, increasing scrutiny of carbon accounting.
December 12th, 2015
December 1997
Kyoto Protocol Adopted
Policy

First international treaty to recognize forest carbon sinks in climate accounting, though excluding tropical forest conservation.
December 11th, 1997
1990
Systematic U.S. Forest Carbon Tracking Begins
Measurement

EPA begins annual greenhouse gas inventory including forest carbon stocks, establishing baseline for future comparisons.
1990
1930
Knutson-Vandenberg Act Enables Reforestation
Policy

Federal law provides funding mechanism for replanting harvested national forest lands, accelerating forest recovery.
1930
1910
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
1905
U.S. Forest Service Established
Policy

Federal agency created partly to address deforestation concerns, beginning era of forest management and conservation.
1905
1872
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

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

1850-1930

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

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.

Then

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

Now

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 this matters now

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.

December 1997 - February 2005

Kyoto Protocol Forest Carbon Debates (1997-2005)

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.

Then

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

Now

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

Why this matters now

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.

January 2023 - December 2023

2023 Global Forest Carbon Sink Collapse

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.

Then

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

Now

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

Why this matters now

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