Why is This Important?

Carbon dioxide (CO2), a greenhouse gas, is the primary cause of climate change. CO2 enters the atmosphere through nature's carbon cycle and human activities such as the burning of fossil fuels and deforestation. Human activities are the main driver of increased CO2 in the Earth's atmosphere; global atmospheric concentrations of CO2 are now 30 percent higher than they were before the industrial revolution. Anthropogenic climate change is causing extreme weather events (including drought and fl oods), changes in sea level, increasing temperature, glacial and polar ice melt, and species extinction or migration.

What is a Sustainable State?

In a sustainable state, humans have reduced CO2 emissions to a level that is in balance with nature's ability to absorb those emissions.

How Are We Doing?

• Countywide carbon dioxide emissions totaled 5.8 million metric tons of carbon equivalent in 2008. As in past years, this estimate does not include off-road transportation sources such as ships, airplanes, and locomotives. Total carbon emissions were about 2 percent higher than in 2007 and 4 percent higher than 2006, however carbon dioxide emissions are down 10 percent since 2000.
• Per capita emissions increased slightly in 2008, up less than 1 percent in 2008. Since 2000, per capita carbon emissions have decreased about 14 percent.

Transportation

• On-road transportation is the largest contributor to carbon emissions in the county, accounting for 51 percent of all carbon equivalent emissions in 2008. Emissions from this source have been decreasing though, down 21 percent since 2000 largely due to decreased miles traveled on area roads.
• In the Source Inventory of Bay Area Greenhouse Gases 2007, the Bay Area Air Quality Management District (BAAQMD) estimated that total mobile source emissions of carbon dioxide in San Mateo County, when including offroad vehicles such as trains, ships, and commercial airplanes, accounted for about 70 percent of all carbon emissions.

Natural gas

After transportation, natural gas combustion in homes and businesses is the second largest source of carbon emissions in the county. Natural gas is used for heating and powering appliances and equipment.
• In 2008, natural gas combustion generated 1.43 million metric tons of carbon dioxide. This is a 3.6 percent increase in carbon emissions from 2007.

Electricity

Emissions from electricity are driven by both total electricity use and the carbon content of the generation source of that electricity. Pacifi c Gas and Electric Company (PG&E), delivers most of the electricity in San Mateo County and has a mix of generation sources that is less carbon intensive than the electricity for the state as a whole (see Energy Use indicator for more information).
• In 2008, electricity use in the county accounted for 1.26 million metric tons of CO2 emissions, a slight increase from 2007. Electricity use is a growing source of carbon emissions, increasing 20 percent since 2005, and 35 percent since 2001.

Solid waste
Carbon dioxide is released from landfi lls as plant matter decomposes. International guidelines for accounting for greenhouse gas emissions, however, recommend that carbon dioxide released in this way (so-called "biogenic carbon") not be included in carbon emissions inventories. As plants grow, they take carbon from the atmosphere and turn it into solid matter; when they decompose, that carbon is merely returned to the atmosphere, and there is no net effect on concentrations of carbon dioxide in the atmosphere. Methane, however, is an extremely powerful greenhouse gas (21 times stronger than carbon dioxide) and large quantities of methane are released from landfi lls. Methane emissions are traditionally converted into "metric tons carbon equivalent" for the purposes of measuring greenhouse gas warming potential.
• In 2008, landfi lls in the county emitted 110,000 metric tons of carbon equivalent into the atmosphere. Landfill methane has been decreasing consistently since 2000, in line with reduced waste disposal in landfills and increased diversion rates in the county.