SACRAMENTO — Last month, scientists installed monitors on a 2,000-foot television tower in Walnut Grove, near Sacramento, to gauge progress in reduction of emissions. These monitors will not supply all the answers, but respect for data is a good way to proceed in a field often governed by speculation. 

According to news reports, sensors placed on the tower at 100, 300 and 1,600 feet will analyze the amount of carbon dioxide, methane and other gases in California's central valley, center of the state's vast agricultural production. A daily air sample will be sent to a laboratory of the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado. There a computer program known as CarbonTracker will analyze the data. Similar data arrives from monitors in San Francisco, Wisconsin, Maine, Texas, Colorado and Iowa. 

Marc Fisher of the Lawrence Berkeley National Laboratory told reporters, "Climate change is becoming a mainstream issue, and California is taking a leading role. And without an objective measure, it will be difficult to tell if our goals are being met." The goals were laid out in AB 32, by Assembly Speaker Fabian Nunez, passed in 2006 and signed by Governor Schwarzenegger. The measure calls for reductions in greenhouse gas emissions of 25 percent below 1990 levels and reductions of 80 percent by 2050. How California will achieve those goals remains uncertain. The legislation requires the California Air Resources Board (CARB) to develop regulations and mechanisms. CARB recently revealed some good news about air quality.

Prior to October 12, when the "smog season" ended, only a single day in 2007 exceeded the federal limit for smog levels in the San Francisco Bay Area. There were 12 such days in 2006 in the Bay Area and 42 in the Sacramento Area, where this year the count dropped to only 18. Air violation days in the South Coast Air District, part of the Los Angeles air basin, were down from 86 in 2006 to 79 in 2007, and down from 86 to 64 in the San Joaquin Valley air basin. Reports attribute the reductions to lower temperatures, and the state capital of Sacramento, at least, did experience some of the lowest summer temperatures in more than 100 years.

Such low temperatures are not supposed to be happening under the global warming doctrine espoused by former vice-president Al Gore, recent winner of the Nobel Peace Prize. News of that development has obscured revelations of cleaner air in California. But it did emerge, from a British court, that Mr. Gore's Oscar-winning An Inconvenient Truth contains nine errors.

When it comes to issues such as climate change and air quality, one wants to be on solid grounds. Instead of apocalyptic predictions, it is best to report what the data actually say. That is what PRI does in the annual Index of Leading Environmental Indicators. Contrary to Mr. Gore's dark vision, there is considerable room for optimism, and the recent air-quality figures from CARB are encouraging.

Meanwhile, the data collection from the Walnut Grove television tower is a good way to proceed. Scientists should remain open to what this data says, and so should legislators. If the data warrants changes in AB 32, or even a new law, legislators should be open to that possibility. In future, more data collection should come before the legislation, not after.