SAN FRANCISCO -- New probes bolted onto a giant antenna tower are sniffing for greenhouse gases, part of an experiment to measure how much this region is generating.

Last month, a crew installed the monitoring probes at heights of 760 feet and 400 feet on Sutro Tower, a huge red and white structure in the heart of San Francisco mostly used to transmit radio and TV signals. There also is a probe 1,500 feet high on the Richland Tower in Walnut Grove.

A group of university and government scientists is trying to lay the groundwork for understanding whether the state's aggressive new anti-global warming laws will work.

The California Greenhouse Gas Emissions Project, known as CALGEM, is the brainchild of scientists at the Lawrence Berkeley National Laboratory. They determined four years ago that the greenhouse gases produced from burning fossil fuels could be measured regionally, which would help sort out how much of the pollution can be blamed on humans.

Until now, government monitoring sites were designed to focus solely on global patterns.

"This is the first time that the complete suite of greenhouse gases from a mixture of urban, suburban and rural areas will be monitored in a systematic fashion," said physicist Marc Fischer, leader of the lab's Environmental Energy Technologies Division.

"California is a big player in total greenhouse gas emissions," said Fischer, who submitted a proposal to the California Energy Commission five years ago. The agency granted $700,000 toward the research.

California is the state that emits the most greenhouse gases a year -- one-fourth of the U.S. total.

Under a new state law, by 2020 California must cut its greenhouse gas emissions to the levels emitted in 1990, a reduction of 25 percent. State law says that by 2050, carbon emissions must be reduced to 80 percent below 1990 levels.

At Sutro Tower and the second site in Walnut Grove, scientists are sampling all of the greenhouse gases twice a day. In Walnut Grove, they also are measuring some of the gases continuously. The San Francisco spot was chosen to measure oceanic air, and the inland site will allow experts to measure air that is heavily influenced by the urban and rural areas.