NORTH BAY – As government policy evolves on all levels to cut human-caused emissions of what are considered climate-changing gases, state and Bay Area air-quality regulators recently released draft guidelines aimed at helping planning officials and the building industry gauge how much would be considered too much from a given project per environmental-protection laws.

The guidance has been requested since the passage of Senate Bill 97, which required analysis of such emissions as an environmental impact under the California Environmental Quality Act, or CEQA, and subsequent legal action by the state Attorney General’s office, environmental groups and project opponents for failure to address this impact.

State government is still formulating specific greenhouse-gas “thresholds of significance,” which trigger the level of environmental-impact review required for a project, but the Governor’s Office of Planning and Research in mid-April released draft emissions-mitigation guidelines. However, the state Air Resources Board, which in December released draft rules, continues to work on revised thresholds after a flurry of construction industry comments challenging the metrics.

“For a while, we hoped we would get thresholds we can use,” said Kristine Gaspar, senior environmental planner for Santa Rosa-based Winzler & Kelly.

Meanwhile, the Bay Area Air Quality Management District in late April released a draft update of 1999 CEQA thresholds for various pollutants and for the first time included proposals for greenhouse-gas thresholds.

In the air district’s proposal, prepared by Sacramento-based EDAW, the district recommended local agencies in its jurisdiction follow its guidelines until greenhouse-gas emissions controls in Assembly Bill 32, also called the Global Warming Solutions Act of 2006, and Senate Bill 375, which seeks to link those controls to regional transportation planning, take effect. The district covers the nine Bay Area counties, except for northeastern Solano and northwestern Sonoma counties, although the Northern Sonoma district uses the Bay Area district’s thresholds.

“GHG CEQA significance thresholds evaluated herein are intended to serve as interim levels during the implementation of the AB 32 Scoping Plan and SB 375, which will occur over a few years’ time,” wrote EDAW associate principal Honey Walters in the draft.

If the Bay Area air district’s proposed thresholds are adopted, they could be the default for the lead agency weighing the climate-change impact of a project, according to Pat Collins, who leads Winzler & Kelley’s environmental group in Santa Rosa.

“Once the air district or ARB comes up with guidelines, people like us have to follow them, else we have to provide substantial evidence as to why we did not follow them,” she said.

In its proposed greenhouse-gas thresholds, the air district would estimate emissions attributable to land use from 2010 through 2020, reductions in emissions from AB 32 regulations and new projects expected during that time period.

Land use-related emissions of such gases include passenger and heavy-duty vehicle travel, which account for half amount produced annually, in-state and imported electricity production, 26 percent, natural gas used in homes and businesses, 24 percent, and wastewater treatment, 1 percent.

Gov. Arnold Schwarzenegger’s 2005 executive order called for the state to reduce projected 2020 emissions to 1990 levels, or by 23.9 percent. AB 32 regulations are estimated to account for 21.1 percent of that reduction, so greenhouse-gas curtailment under CEQA would need to reduce emissions over the next decade by 2.8 percent, or the equivalent of 2 million metric tons of carbon dioxide, according to the air district’s proposal.

The district has suggested three alternatives to the state Air Resources Board’s draft greenhouse-gas thresholds.

A “bright line” threshold would require projects with emissions above a certain amount per year to take measures to bring the total down to the threshold. The district is considering thresholds ranging from 1,175 to 3,175 annual metric tons per project, or an amount equal to 65 to 176 typical single-family homes. The lower line would require 58 percent of projects to mitigate calculated emissions by up to 35 percent to reach the regional 2 million-ton reduction by 2020. The upper limit would apply to only 10 percent of projects and would leave the region with 400,000 metric tons to deal with through other means in the next decade.

The second proposed threshold option would require all projects to have an emissions reduction of at least 24 percent to be considered having “less than significant” environmental impact. That “could prove very difficult for the smallest of projects to implement sufficient mitigation measures” and require a lot of guidance for project applicants and lead agencies, according to the draft.

The third threshold option was to blend the previous two options, with all projects required to reduce emissions by 5 percent, the equivalent of a project’s being near public transit or accommodating bicycles. Thresholds could range from 1,725 to 2,475 annual metric tons, equivalent to 95 to 135 homes, with projects with higher emissions have to reduce them by 25 percent to 35 percent, respectively.

Regional transportation planning agencies are working on a third approach to greenhouse-gas emissions limits. As part of the implementation of SB 375, they are looking at regional limits based on vehicle miles traveled, or VMTs. One approach to regional allocation could be an annual limit of 1.5 million metric tons, or about 400 projects with 3,750 tons each.

Ms. Collins and others in the public planning and the building industry have expressed concerns about looming litigation as emissions standards solidify.

“The more specific the guidelines and thresholds are the more potential there is to do things wrong,” she said.

The district plans to revise its proposed CEQA thresholds this month and hold more workshops stretching into June.