Continued climate change could have widespread impacts on California's economy and ecosystems, and on the health of its citizens. Climate change can impact meteorology, emissions, and chemical processing and thereby influence air quality and its associated effects on public health and welfare. Some mobile source emission control technologies in the distant future are likely to be very different than current ones; it is not possible to make quantitative predictions of California's mobile source emissions inventory for 2050 and 2100. However, investigating the response of the current on-road mobile source inventory to possible future temperatures can provide insights for the near future and suggest actions which are needed to protect California's air quality. The results of this study indicate that for California surface temperatures predicted by the B1 and A2 IPCC-carbon dioxide (CO2) emissions scenarios, summer-time on-road total (exhaust plus evaporative) reactive organic gases (ROG) emissions from the 2005 motor vehicle fleet will increase by 4 to 5% using temperature projections for mid-century and by 13 to 16% for end-of-century temperature projections. Increases of as much as 30% are seen in evaporative emissions in 2100. California Air Resources Board's (CARB) mobile source emission factor model (EMFAC) predicts decreases of similar magnitudes for oxides of nitrogen (NOX) emissions. The medium-high emissions scenario results in a positive feedback loop for greenhouse gas (GHG) emissions from on-road motor vehicles, with 4 to 5% increase in methane and 8 to 9% increases in CO2 by 2100. These emissions estimates are strictly a test of the sensitivity of the current California mobile sources emission inventory model to temperature increases, as they do not take into account future growth in mobile source activity, new control regulations, or possible vehicle or fuel technological advances.