California prides itself on having steadily lowered its greenhouse gas emissions over the past decade, surpassing a major goal for 2020 years early.

But the reality behind that progress is more complicated.

The state does not include every source of gases that contribute to climate change when measuring its progress against the 2020 goal of 431 million metric tons of greenhouse gas emissions — or its even more ambitious 2030 target.

Significantly, California does not factor in emissions from wildfires, even though trees release carbon dioxide when they burn and people often provide the first spark. Out-of-state and international air travel is another area that is excluded, according to the California Air Resources Board.

There are reasons for these omissions: California officials use international standards that track the direct results of humans burning fossil fuels and other such activity within the boundaries of the state.

But the numbers left out of the state’s calculations are staggering. Last year alone, wildfires released 45.5 million metric tons of carbon dioxide into the air, according to state estimates. That’s more than half as much as the state’s industrial sector emits in a typical year.

If California factored wildfire estimates into its final count for 2017, the last year the state tallied all of its greenhouse gas emissions, the state would have seen overall emissions rise, not fall.

State officials insist that’s not the right way to look at the issue. They instead want to focus on how much humans affected those emissions.

But doing so is a tricky endeavor.

“Wildfires, historically, are a natural part of the ecosystem,” said David Edwards, the air board’s assistant division chief for air quality planning and science. “In all likelihood, that carbon would have been emitted anyways in a natural forest cycle.”

The state knows that wildfires have increased in severity in recent years, setting records for their size, destruction and death toll. And it’s clear that human actions and man-made products — whether it be faulty electrical equipment or a person hammering a stake into the ground on a hot day — are frequently the cause of fires. Also, climate change, driven by human pollution, makes the state hotter and drier and generally worsens fire risks, scientists say.

“The fires are much more intense, they’re hotter, they spread faster — the conditions are ripe,” air board spokesman Dave Clegern said. “There’s definitely a (human) contribution. Determining the extent of that in a given situation is difficult.”

Part of the challenge in tabulating emissions from fires is that blazes can break out regardless of human activity. Several that have burned in Northern California this year were caused by lightning, for example. And to gauge their long-term impact on emissions, one must account for the fact that after an area burns, new plants should grow and absorb carbon dioxide, said Ben Houlton, director of UC Davis’ John Muir Institute for the Environment.

Yet the air board said in a January report that its analysis of emissions from the state’s natural lands — including forests — shows they are already “losing more carbon than they are sequestering, with wildfire being the largest cause of carbon loss.”

It may get worse. By the end of the century, cumulative net carbon dioxide emissions from California’s natural lands could reach 518 million metric tons, according to Houlton’s research. And “under the most extreme permanent drought situation,” that number could more than double, he said.

But carbon emissions from fires should not be accounted for the same way as the impacts of burning a gallon of gasoline, said Ross Brown, a principal fiscal and policy analyst at the state Legislative Analyst’s Office who has studied California’s greenhouse gas inventory.

“You have to look at longer time horizons,” Brown said of forest fires’ role in greenhouse gas emissions. “You can argue that it’s not entirely natural at this point, given human interventions in a lot of different ways.” But thinking about fires differently from emissions that are clearly caused by human processes “makes a lot of sense,” he said.

Different fires can emit different pollutants as well. Conflagrations that tear through populated areas, such as the 2017 Tubbs Fire in Wine Country and the 2018 Camp Fire in Butte County, burn not only vegetation but also large amounts of buildings and cars. The pollutants they release differ from blazes such as the 2018 Ranch Fire, which geographically is California’s largest wildland blaze on record and devastated a huge swath of forest north of Clear Lake but incinerated relatively few structures.

Our Climate Challenge coverage Today: A detailed look at California’s emissions. Tuesday: The consequences of rising seas. Wednesday: The quest for electric car chargers. Thursday: How warmer weather affects farming. Friday: Plans to make exurban sprawl more walkable. Saturday: Ways around Washington’s climate gridlock. Sunday: Why cleaning up transportation is so hard. Next Monday: Renewable electricity offers hope.

Read More

“That’s something we are trying to look into to get a better understanding of — what the exposure effects are from when wildfires encroach on urban areas,” said Edwards, of the air board.

Another source of emissions the state does not count when it trumpets its climate achievements is air travel. Flying is one of the most carbon-intensive activities a person can undertake. Planes account for 12% of the country’s transportation emissions, according to the U.S. Environmental Protection Agency. That’s an astonishing figure, given how much more frequently people drive than fly.

But California estimates the greenhouse gas impacts of flights taken only within its boundaries. So a trip from the Bay Area to Southern California is included in the official inventory, but a journey from San Francisco to New York — which would create far more pollution because of the fuel needed for the long journey — is not. Clegern, the air board spokesman, said that’s consistent with the international standards the state follows.

Also, “We can’t regulate airlines,” he said. “That’s a national and international thing.”

So even as California surpasses its 2020 goal and strives to slash emissions far more deeply by 2030, air travel and wildfires won’t figure into the mix — even as their impact continues to grow.

J.D. Morris is a San Francisco Chronicle staff writer. Email: jd.morris@sfchronicle.com Twitter: @thejdmorris