Dec 4 - McClatchy-Tribune Business News Formerly Knight Ridder/Tribune Business News - Edie Lau The Sacramento Bee, Calif.

As California carries out a new mandate to cut global warming pollution, its progress will be measured against its greenhouse gas output in 1990 -- the target point the state must reach by 2020.

That may sound simple enough. But, like California itself, figures for that benchmark year rest on ever-shifting ground.

Over the past decade, five state reports attempting to quantify 1990 emissions give five different numbers -- ranging from 425 million metric tons to 468 million metric tons of greenhouse gases.

Gerry Bemis, a California Energy Commission civil engineer who compiled the most recent inventory of greenhouse gas emissions, said the numbers will always be at least a bit squishy.

"As information and ... estimating techniques improve over time, estimate emissions for a selected year can and will change," he wrote. " ... The 'exact' value for 1990 will always remain unknown."

That fact illustrates the challenge ahead as the California Air Resources Board figures out how to extract meaningful reductions of climate-changing gases from industry.

"They have a great deal of work to do," said Energy Commissioner Jeffrey Byron, whose agency until now was in charge of tracking state greenhouse gas sources.

Passage this year of the landmark Assembly Bill 32 gave that job to the air board as part of a far-reaching effort to limit California greenhouse gases.

In adopting AB 32, the Legislature made California the first state in the nation to tackle climate change by mandate, although many countries around the world -- particularly signatories to the Kyoto Protocol -- are well under way with such efforts.

Greenhouse gases such as carbon dioxide are so called because, like the windows of a greenhouse, they trap heat in the atmosphere.

Produced naturally as well as by human activities such as burning fossil fuels, these gases, as they accumulate in the atmosphere, raise the planet's overall temperatures. That increase has the potential to cause significant changes in climate, including more frequent episodes of severe weather such as heat waves, droughts and hurricanes.

California risks losing coastline to rising sea levels caused by melting glaciers and the expansion of warming waters. It also stands to lose snowpack and gain stronger and longer heat waves, climate models suggest.

The new state limits on greenhouse gases are expected to directly affect electric power utilities, oil and gas producers, landfill operators and cement manufacturers, among others.

While individuals aren't apt to be asked to clamp down on their own emissions, to the extent that they are consumers served by the affected industries, the new law will touch everyone.

How much each industry, and individual members of each industry, must cut back has yet to be determined. The law requires the state as a whole to scale back to 1990 emissions levels by 2020, but does not specify how.

(An earlier target set by Gov. Arnold Schwarzenegger by executive order in 2005, to reduce state global warming pollution to 2000 levels by 2010, was not included in the law and will not be pursued by the ARB.)

Technically, the law doesn't kick in until, but the Air Resources Board isn't waiting. Agency staff held a public workshop Friday to discuss how to establish the 1990 emissions level as the limit for 2020, a job it has to accomplish within the next year.

To industry, coming up with a credible figure for 1990 is critical.

The five different figures that the five state reports attempted to quantify over the years have a disparity of 10 percent.

"Without an accurate, comprehensive picture ... ," said Steve Brink, vice president of public resources at the California Forestry Association, "the whole process will be in question."

Beemis, the Energy Commission civil engineer who has produced several greenhouse gas inventories for the state, said reasons behind the changing numbers are many and varied.

Sometimes it's just a matter of simple error.

Beemis said that in an earlier inventory, he mistakenly counted some emissions twice.

"It was an error that was worth two or three million metric tons, if I remember right," he said. "It was a subcategory that I didn't realize was a subcategory, and I counted the total and the subtotal."

Other times, recalculations are required as scientists get a better handle on data. For example, understanding of the "warming potential" of particular gases has changed over time.

In the world of greenhouse gas accounting, emissions of all gases are reported in "carbon dioxide equivalents." At one time, one molecule of methane was thought to have the same heat-trapping power as 11 molecules of carbon dioxide.

In his most recent inventory, a draft of which was released in late October, Beemis said he used an updated factor of 21 for methane -- that is, one molecule of methane equals 21 molecules of carbon dioxide.

Ironically, the uncertainty of climate science is what has prevented many governments, including the United States, from imposing involuntary measures to cut greenhouse gas pollution.

Alluding to the reluctance to act in the face of uncertainty, Beemis said, "We shouldn't ignore our responsibilities just because there are uncertainties. ...

"Policymaking can go on and does go on in the real world, rather than the perfect world," he said. "Policymakers need to make decisions based on the best information we can provide to them."

Deciding how to handle the shifting metrics of greenhouse gas accounting will be a policy matter for the Air Resources Board to decide, said Peggy Taricco, chief of the board's emission inventory branch.

"Because we'll be in a world of trading and markets, we may want a lot more certainty," Taricco said, referring to markets for buying and selling carbon emissions -- one method of enabling businesses to meet their reduction goals. "We may not want to be tweaking constantly."

Amid the slippery numbers of the past, one trend is clear: California greenhouse gas output continues to rise. Bemis found emissions increased 14 percent between 1990 and 2004, reaching 492 million metric tons in 2004.

The Golden State's emissions are higher than those of every state but Texas. Worldwide, California ranks among the top 20 producers.