The atmospheric concentration of carbon dioxide (CO2) and the temperature of the earth's surface have been rising in parallel for decades, with the former recently reaching 400 parts per million, consistent with a 1.5°C increase in global warming. Climate change models predict that a “business as usual” approach, that is, no effort to control CO2 emissions from combustion of fossil fuels, will result in a more than 2°C increase in annual average surface temperature by approximately 2034. With atmospheric warming comes increased air pollution. The concept of a “climate gap” in air quality control captures the decreased effectiveness of regulatory policies to reduce pollution with a hotter climate. Sources of greenhouse gases and climate-forcing aerosols (“black carbon”) are the same sources of air pollutants that harm health. California has adopted robust climate change mitigation policies that are also designed to achieve public health cobenefits by improving air quality. These policies include advanced clean car standards, renewable energy, a sustainable communities strategy to limit suburban sprawl, a low carbon fuel standard, and energy efficiency. A market-based mechanism to put a price on CO2 emissions is the cap-and-trade program that allows capped facilities to trade state-issued greenhouse gas emissions allowances. The “cap” limits total greenhouse gas emissions from all covered sources, and declines over time to progressively reduce emissions. An alternative approach is a carbon tax. California's leadership on air quality and climate change mitigation is increasingly important, given the efforts to slow or even reverse implementation of such policies at the U.S. national level.