As the most populous state in the United States, California's climate policies influence both global climate change and federal climate policy. In line with the views of climate scientists, the state of California has progressively passed emission-reduction legislation.

California has taken legislative steps in the hope of mitigating the risks of potential effects of climate change in California by incentives and plans for clean cars, renewable energy, and pollution controls on industry. In California, climate change policy has been developed through both the executive and legislative branches of the state government. Many of the policies have specifically targeted greenhouse gas emissions, which have been shown to raise global temperatures and skew natural rhythms.

One of the most notable pieces of climate legislation in California was Assembly Bill 32, the Global Warming Solutions Act of 2006. This landmark piece of legislation required many actors in California's economy to reduce their greenhouse gas emissions to 1990 levels by 2020. The bill also appointed the California Air Resources Board (CARB) to devise policies and mechanisms for reaching the goal. CARB ultimately implemented the state's cap-and-trade program, a type of emissions trading, the first such program in the United States. California was able to reach the emissions target four years ahead of schedule, in 2016.

California (the world's fifth largest economy) has long been seen as the state-level pioneer in environmental issues related to global warming and has shown some leadership in the last four years. On July 22, 2002, Governor Gray Davis approved AB 1493, a bill directing the California Air Resources Board to develop standards to achieve the maximum feasible and cost-effective reduction of greenhouse gases from motor vehicles. Now the California Vehicle Global Warming law, it requires automakers to reduce emissions by 30% by 2016. Although it has been challenged in the courts by the automakers, support for the law is growing as other states have adopted similar legislation. On September 7, 2002, Governor Davis approved a bill requiring the California Climate Action Registry to adopt procedures and protocols for project reporting and carbon sequestration in forests. (SB 812. Approved by Governor Davis on September 7, 2002) California has convened an interagency task force, housed at the California Energy Commission, to develop these procedures and protocols. Staff are currently seeking input on a host of technical questions.

In June 2005, Governor Arnold Schwarzenegger signed an executive order calling for the following reductions in state greenhouse gas emissions: to reduce GHG emissions to 2000 levels by 2010, to reduce GHG emissions to 1990 levels by 2020, to reduce GHG emissions to 80 percent below 1990 levels by 2050. Measures to meet these targets include tighter automotive emissions standards, and requirements for renewable energy as a proportion of electricity production. The Union of Concerned Scientists has calculated that by 2020, drivers would save $26 billion per year if California's automotive standards were implemented nationally.

On August 30, 2006, Schwarzenegger and the California Legislature reached an agreement on AB32, the Global Warming Solutions Act. He signed the bill into law on September 27, 2006, saying, "We simply must do everything we can in our power to slow down global warming before it is too late... The science is clear. The global warming debate is over." The Act caps California's greenhouse gas emissions at 1990 levels by 2020, and institutes a mandatory emissions reporting system to monitor compliance, representing the first enforceable statewide program in the U.S. to cap all GHG emissions from major industries that includes penalties for non-compliance. It required the State Air Resources Board to establish a program for statewide greenhouse gas emissions reporting and to monitor and enforce compliance with this program, authorizes the state board to adopt market-based compliance mechanisms including cap-and-trade, and allows a one-year extension of the targets under extraordinary circumstances.Gov. Schwarzenegger Signs Landmark Legislation to Reduce Greenhouse Gas Emissions. California Office of the Governor, 2006-08-27. Thus far, flexible mechanisms in the form of project based offsets have been suggested for five main project types. A carbon project would create offsets by showing that it has reduced carbon dioxide and equivalent gases. The project types include: manure management, forestry, building energy, SF6, and landfill gas capture.

Additionally, on September 26, Governor Schwarzenegger signed SB 107, which requires California's three major biggest utilities – Pacific Gas & Electric, Southern California Edison, and San Diego Gas & Electric – to produce at least 20% of their electricity using renewable sources by 2010. This shortens the time span originally enacted by Gov. Davis in September 2002 to increase utility renewable energy sales 1% annually to 20% by 2017.

Gov. Schwarzenegger also announced he would seek to work with Prime Minister Tony Blair of Great Britain, and various other international efforts to address global warming, independently of the federal government.

Legislation overview See California Climate Executive Orders for a detailed outline of executive orders signed by California governors that focus on climate change.

California lawmakers are among leaders in the U.S. in enacting climate change policy. Starting in the early 2000s, several executive orders focused on climate change were signed by California governors. The California State Legislature has passed numerous bills to enact the changes and regulations that were necessary to meet the goals outlined in executive orders. These policies address issues in emissions measurement, transportation, waste, and energy in California.

Climate change is driven by the accelerated amount of greenhouse gas emissions being put into the atmosphere by anthropogenic activity. To combat climate change, national and state governments around the world are struggling to control their emissions levels. Setting emissions reductions goals and using effective emissions-measurement technology is necessary to reduce emissions and keep track of progress across time. According to the 2022 IPCC report, the world needs to cut its emissions in half by the year 2030 to limit warming to 1.5o Celsius. The California legislative body has been paying attention to the importance of cutting emissions ever since the 2000s.

SB 1771 (2000) This bill created the California Climate Action Registry, however this organization officially closed in December 2010 and is encouraging its members to report their emissions to the Climate Registry instead.

SB 527 (2001) SB 527 was approved in October 2001, authorizing the California Air Resources Board to impose administrative penalties instead of civil penalties for violations of regulations to air pollution control, with a limit of 10,000 per day and 100,000 total. Before this bill was enacted, the California Air Resources Board had to rely on enforcement of their penalties of violations on air pollution regulations through an action by the Attorney General, now they would have the ability to assess and enforce administrative penalties. The California Climate Action Registry is required to record voluntary greenhouse gas emission reductions made by California entities after 1990, but many of its functions were changed with the passage of SB 527 including; ensuring the public can comment on board meetings, requiring protocols for monitoring and reporting emissions to be consistent with California Energy Commission protocols, and remove the requirement to report nationwide emissions in order to focus solely on in-state emissions.

AB 32 (2006) In September 2006, the California State Legislature passed AB 32, the Global Warming Solutions Act of 2006 with the goal of reducing man-made California greenhouse gas emissions (1.4% of global emissions in 2004) back to 1990 emission levels by 2020. The responsibility for implementing, enforcing, and monitoring progress to meet the emission goals was placed on the California Air Resources Board (CARB). Due to the extensive involvement of environmental justice groups, a cap and trade emission scheme was not specifically mandated.

Environmental justice proponents advocate for the reversal of the historical trend of dumping pollution on BIPOC, low-income, Hispanic and Latino communities. Communities of color are significantly more likely to live near major polluters, which emit both GHGs and particulate matter into the surrounding air. Environmental justice advocates assert that a cap-and-trade program does not call for sufficient protections for historically marginalized communities. Instead, it allows leaders the flexibility to act on the global issue of climate change without adequately addressing the more local issue of air pollution.  

Due to these concerns, decisions on what system would be most effective were left up to CARB, with mandated review and revision every five years. This granted CARB enough programmatic flexibility to successfully meet the emission reduction mandate. The emissions goal was reached in 2016, four years ahead of the 2020 deadline.

In order to reach emission reduction goals, the California Air Resources Board has adopted a variety of legislation including plans for greener transportation, waste reduction, a cap-and-trade program, the use of new energy efficient technology and the expansion of renewable energy resources.

The greenhouse gases that AB 32 targets include: Carbon dioxide Methane Nitrous oxide Hydrofluorocarbons Perfluorocarbons Sulfur hexafluoride Nitrogen trifluoride (was not included in the original AB scoping plan but was later listed as a targeted greenhouse gas via legislation)

The emissions target of AB 32 has been updated to a stricter target following Executive Order B-16-12 and B-30-15. Therefore, updates of the AB 32 Scoping Plan continue to introduce new measures in order to reduce greenhouse gas emissions even further.

Funding for the implementation of AB 32 is collected from greenhouse gas emitters. This includes approximately 250 fee payers from polluters such as electric power plants, oil refineries, cement plants, and other large industries. In addition, the revenue collected from auctioning permits to greenhouse gas emitters through the cap-and-trade system is also used to fund programs under AB 32.

Scoping plan Development of the scoping plan is a central requirement of AB 32, which is a bill that calls on California to reduce its greenhouse gas emissions to 1990 levels by 2020. The required Scoping Plan is intended to outline the approach California will take to reduce its greenhouse gas emissions. The comprehensive approach includes both new and existing measures in almost every sector of California's economy.