Reductions in emissions of black carbon since the late 1980s, mostly from diesel engines as a result of air quality programs, have resulted in a measurable reduction of concentrations of global warming pollutants in the atmosphere, according to a new study from the California Air Resources Board.

The study, funded by CARB and led by Veerabhadran Ramanathan of the Scripps Institution of Oceanography at the University of California, San Diego, estimates that reductions in black carbon as a result of clean air regulations were equivalent to reducing carbon dioxide emissions in California by 21 million metric tons annually, or taking more than 4 million cars off California roads every year.

“We know that California’s programs to reduce emissions from diesel engines have helped clean up the air and protect public health,” said CARB chairman Mary D. Nichols. “This report makes it clear that our efforts to clean up the trucks and buses on our roads and highways also help us in the fight against climat change.”

Critics of CARB, including those in trucking, say the agency has taken too tough a stance in trying to regulate diesel emissions, which has led to higher prices for goods moved by truck, along with forcing some trucking operations out of business due to the high costs of compliance.

Black carbon, which are the tiny soot particles released into the atmosphere by burning fuels, has been linked by CARB to adverse health and environmental impacts and also notes it is one of the major short-lived contributors to climate change. The major sources of black carbon in California are diesel-burning mobile sources, residential wood burning in fireplaces and heaters, agricultural burning and wildfires.

The three-year-study, titled Black Carbon and Regional Climate of California, was conducted by UC San Diego and the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and Pacific Northwest National Laboratory. CARB says it is the first comprehensive regional assessment of the climate impact of black carbon on California. In conducting the study, scientists used computer models and air pollution data collected by aircraft, satellite and ground monitors.

CARB claims the study’s results support a growing body of scientific evidence that suggests it is possible to immediately slow the pace of climate change regionally by reducing emissions of short-lived climate pollutants, like black carbon.

Study co-author Dr. Tom Kirchstetter of LBNL, says black carbon levels have decreased by about 90% over a 45-year period, beginning with the establishment of CARB in 1967. Researchers say they found the state’s efforts to reduce diesel emissions to have lessened the impact of global warming on California, supporting earlier theoretical computer modeling that reducing black carbon from diesel combustion is a potent ‘climate cooler.’

The reductions occurred during a time when diesel fuel consumption increased by about a factor of five, CARB says, attesting to the effectiveness of its regulations requiring cleaner fuels and vehicle technology.

A copy of the report can be found on the CARB website.