Overview: Climate Change Drivers

Greenhouse gases are the major human-influenced drivers of climate change. These gases warm the Earth’s surface by trapping heat in the atmosphere.

Jul 12, 2018

The Earth’s climate is warming, mostly due to human activities such as changes in land cover and emissions of certain pollutants. Greenhouse gases are the major human-influenced drivers of climate change. These gases warm the Earth’s surface by trapping heat in the atmosphere.

International climate agreements aim to stabilize atmospheric greenhouse gas concentrations at a level that would prevent “dangerous anthropogenic interference with the climate system.” The 2015 Paris Agreement calls for keeping the rise in the global average temperature to well below 2 degrees Celsius (°C) above pre-industrial levels. The Agreement also commits to pursue efforts to further limit the increase to 1.5°C. These efforts would significantly reduce the risks and impacts of climate change.

California’s greenhouse gas emissions show promising downward trends, with emissions per capita and per dollar of gross domestic product declining since 1990. These trends are the result of California’s pioneering efforts to curb greenhouse gas emissions, and are occurring despite an increase in the state’s population and economic output. Greenhouse gases are emitted from fossil fuel combustion for transportation and energy, landfills, wastewater treatment facilities, and livestock. The major greenhouse gases are carbon dioxide (CO₂), methane, nitrous oxide, and fluorinated gases. CO₂ accounts for 85 percent of greenhouse gas emissions in the state, and transportation is its largest source, accounting for over a third of the total emissions in 2015.

Concentrations of black carbon in California’s air have dropped by more than 90 percent over the past 50 years despite a seven-fold increase in statewide diesel fuel consumption — its largest anthropogenic source. This is largely due to tailpipe emission standards, diesel fuel regulations and biomass burning restrictions. Black carbon is a “short-lived climate pollutant”. Unlike CO₂, it does not persist for long in the atmosphere. It is also a powerful global warming agent. Black carbon is the second most important contributor to global warming after CO₂.

Atmospheric concentrations of CO₂ continue to increase. Measurements at California coastal sites are consistent with those at Mauna Loa, Hawaii, where the first and longest continuous measurements of global atmospheric CO₂ concentrations have been taken. In less than six decades, concentrations of CO₂ have increased from 315 parts per million (ppm) to over 400 ppm in 2015. Since CO₂ persists in the atmosphere for centuries, its levels are expected to remain above 400 ppm for many generations.

As atmospheric concentrations of CO₂ increase, so do levels in the ocean, leading to ocean acidification. The ocean absorbs approximately 30 percent of the CO₂ released into the atmosphere each year. Monitoring off Hawaii from 1988 to 2015 shows CO₂ levels in seawater are increasing at a steady rate. The longest-running publicly available data in California from Point Conception, near Santa Barbara, began in 2010. While not measured long enough to discern a trend for California waters, values are similar to those measured at Hawaii at similar times.