California's federal lands are hemorrhaging carbon dioxide. Wildfires are largely to blame

LOS ANGELES — The ecosystems on the American Southwest’s federal lands are hemorrhaging carbon dioxide into the atmosphere faster than any other region in the U.S., according to a recent study from the U.S. Geological Survey.

While federal land ecosystems in most states are sequestering carbon dioxide on average, California’s lost six times more than any other state during the 17-year period from 2005 to 2021 that the study analyzed.

“In California, it’s primarily a story of fire,” said Benjamin Sleeter, a research geographer with the USGS who led the ecosystem analysis in the new study.

While scientists typically expect the movement of carbon in and out of ecosystems to cancel out in the long run, human intervention and climate change have destabilized the delicate balance. It’s made the daunting task of modeling carbon flowing between ecosystems and the atmosphere, which has challenged scientists for decades, even harder.

“On long-term timescales, the terrestrial biosphere would be carbon neutral because there would only be so much carbon to go around,” said Anna Michalak, a carbon cycle researcher with Carnegie Science, a nonprofit research institute. But it’s not so simple, she said, because “we’re digging up carbon that hasn’t been in circulation for millions of years, and we’re injecting that carbon into the atmosphere.”

Wildfire, logging and land development dominate in the top five states where federal lands act as the greatest carbon sources. Plant growth governs the top five where the lands act as the greatest carbon sinks.

Generally, the rich networks of plant life on Earth — from California’s coastal scrublands and marshes to the Amazon rainforest to the Sahara Desert — sequester carbon away from the atmosphere in their limbs, trunks and leaves. They exchange this carbon with the atmosphere through many pathways, including sucking it up to grow and releasing it when they decay or burn in a wildfire.

Many lands act as carbon sinks, and in many cases — including in most states in the eastern United States — climate change can increase how much ecosystems absorb, since there’s more carbon in the air for plants to suck up.

“That carbon is not only warming the planet but also being used as additional food for [plants],” said Michalak. It “literally means that things are growing faster than they are dying.”

But what the ecosystems can taketh, they can giveth back. Humans continue to cut down plants and trees to develop residential communities and industrial sites. Research shows humans are igniting large wildfires more frequently and climate change is exacerbating such fires in the Southwest, including by making explosive fire growth about 25% more frequent in California.

California’s carbon picture varies widely from year to year. Drought can slow plant growth. Logging and development projects are at the whim of market pressures and politics. Fire seasons are erratic.

In five of the 17 years the USGS team analyzed, California’s federal lands acted as a carbon sink — not a source. For example, in 2019, the state experienced well over two times its average precipitation in many regions, boosting plant growth and, despite the devastating Kincade fire, had a relatively mild fire season.

But just a year later, the state’s federal lands released more carbon than any other year in the study period due to a record-setting fire season that burned over 4 million acres and, according to the USGS study, emitted over 90 million tons of carbon dioxide on federal land alone.

Scientists say this “ climate whiplash” — from intense rainy years with fast plant growth followed by grueling dry years that wither the vegetation and make it more flammable — is fueling the state’s wildfire problem. Climate change is making the whiplash even more extreme.

On average, each acre of California’s federal lands lost roughly three-quarters of a ton of carbon dioxide every year from 2005 to 2021, the USGS study found.

The land’s biological processes are generally balanced: Plant growth and decay result in roughly 4 tons of carbon dioxide absorbed per acre every year, while microorganisms eating plant matter in the soil release 4 tons. However, fire releases an extra half ton, while logging and development projects remove another fifth of a ton. (Though not all of that ends up in the atmosphere, Sleeter said — timber, for example, is used in construction and so retains its carbon.)

Some scientists who have compared simulations to the real-world aftermath of wildfires argue that models like those used in the USGS study tend to overestimate how many trees are burned in blazes, and consequently how much carbon they release. Other scientists have pointed out that the USGS model seems to underestimate the carbon lost due to logging and thinning projects.

It’s symptomatic of significant uncertainties in carbon modeling that have troubled scientists for decades. Across the field, “the uncertainty in these carbon sinks … A, is uncomfortably large and B, has not really been shrinking over time,” said Michalak.

In a monumental 2018 assessment of the state of carbon modeling for North America, scientists from across the continent assessed top models in the field. Using a similar approach to the USGS team, their estimate for the net amount of carbon the continent absorbed every year was still a large range, between 0.2 billion and 1.2 billion tons.

Part of the problem is that, unlike weather forecasters who can wait a day to check their predictions, carbon cycle researchers have little ability to directly measure carbon flow to test their models, Michalak said.

As the USGS team continues to refine its models and analysis — including extending the work to all land, not just land owned by the federal government — it hopes the results can help inform leaders and policymakers.

In a follow-up study based on the data, Sleeter and his colleagues found land conservation, restoration and management could flip California from a carbon source to a carbon sink by midcentury, if pursued aggressively.