California’s Senate majority leader and two other legislators have urged the state’s Air Resources Board to review its forest offset program, citing reports from ProPublica and MIT Technology Review that showed it issued tens of millions of carbon credits that may not have provided real climate benefits.

The chief concern in the legislators’ Aug. 6 letter is that the landmark cap-and-trade program, which the board oversees as California’s top climate regulator, isn’t doing enough to drive down emissions as the state strives to meet ambitious climate goals by 2030. Senate Majority Leader Robert Hertzberg, a Van Nuys Democrat, as well as Sens. Josh Becker, a Democrat from Menlo Park, and Bob Wieckowski, a Democrat from Fremont, signed the letter. It was addressed to Liane Randolph, who was appointed chair of the board late last year.

The board projects that the cap-and-trade program will deliver nearly 40% of the climate pollution cuts required by California law this decade. The hope is that the market-based mechanism compels the state’s major polluters to find ways to reduce their emissions, by requiring them to buy or otherwise obtain increasingly scarce permits to emit greenhouse gases.

But the fear is the market price will remain too low to convince polluters to make other, more expensive changes to cut emissions. Among other issues, the industry has banked large numbers of credits during slow economic times, while the forest offsets program has offered a cheap way to obtain the right to emit carbon.

Another concern is that many of the permits produced under that program haven’t achieved the emissions reductions claimed, as ProPublica and MIT Technology Review underscored in a pair of stories in April, citing research by CarbonPlan, a San Francisco nonprofit that analyzes the scientific integrity of carbon removal efforts.

“We’re at risk of undermining the cap-and-trade market by allowing cheap, questionable offsets to substitute for real emissions reductions,” Becker said in an interview. “That keeps the market price artificially low and reduces the incentives for companies to make the change we need to drive down emissions."

This summer, the Air Resources Board began conducting a formal update of its strategies for addressing climate change. In the letter, the senators urged the board to use that process to carefully assess how well cap-and-trade is working and how large of a role the program should play in meeting the state’s climate goals. They also called for a “broader review” of the forest offsets program, suggesting the board had not seriously grappled yet with the criticisms raised.

“We understand that ARB staff reject these findings, but are not convinced that they have addressed or even engaged the substance of the criticisms they received,” the letter said.

In an interview, Wieckowski said the agency has consistently refused to take an honest look at the program or make meaningful changes, despite the flaws that researchers and journalists have highlighted.

“The staff digs in their heels and says, ‘They got it wrong,’” he said. “They’re under the impression that this program is the cat’s meow, even though offsets projects are burning up” in several West Coast wildfires this year.

In a brief written response, Randolph thanked the senators for their letter and said she was “committed to an open, ongoing dialogue with all stakeholders and the Legislature.”

Wieckowski characterized the three-paragraph reply to the senators’ 12-page letter as a “blowoff response.”

Carbon offsets allow companies and individuals to, in theory, “cancel out” some of their CO2 pollution by paying someone else to reduce or draw down emissions. Since 2011, the Air Resources Board has encouraged landowners around the nation to use their forests to generate offsets. If they manage their woodlands in ways that store or absorb more carbon, they can sell the credits to California polluters seeking to meet state climate requirements under cap and trade. Each credit is worth one metric ton of CO2.

The newsrooms’ investigation exposed systemic flaws in the state’s rules for forest offsets. Because the rules gloss over crucial nuances in geography and forest types, some 20% to 38% of the forest credits issued as of last fall are essentially “ghost” credits — offsets with questionable climate benefits. The results also suggest there could be widespread gaming of the market, though the study wasn’t designed to assess the intentions of any individual.

“If this reporting and analysis is correct, then it would appear that a significant share of the carbon offsets program does not meet the offset quality standards codified” in state law, the senators wrote.

Their letter highlighted a separate finding concerning “non-additional” credits generated by forests that were never in danger of being cut down, the subject of the second story by ProPublica and MIT Technology Review. If the offset program provides credits for preserving trees that weren’t in jeopardy, it negates any climate value.

Problems with non-additional credits have led “conservation scientists who have previously worked with and supported California’s forest offsets program to question it,” the senators wrote.

They urged Randolph to “bring an open mind to this discussion and seek additional input” on forest offsets, “including from disinterested parties.”

In an emailed response to questions about the concerns raised in the letter, the Air Resources Board said that Randolph appreciates the feedback from the senators and has “had the opportunity to engage in dialogue” about the conclusions in the reporting.

“She will continue to make herself available to legislators to discuss our climate programs and other air quality programs implemented at CARB as members have questions or raise issues,” the statement said.