ABSTRACT: Instrumental climate records suggest that summer precipitation

The City of Los Angeles was one of the first cities in the United States to address air quality concerns. Pressured by citizens to recognize the harmful health effects of smog, the city formed the Los Angeles Air Pollution Control District in 1947 and began a public campaign to control industrial emissions. In 1959, the State of California became the first state to establish air quality standards for public health (California Air Resources Board 2007). At the same time, California state legislators were refining pesticide regulations in response to an increased use of pesticides and a growing awareness of their toxicity. Just a few years later Rachel Carson completed her landmark text Silent Spring, exposing the environmental and human health risks of pesticides. For many, the publication of Silent Spring marked the beginning of America’s modern environmental movement, leading to the first Earth Day in 1970 and a series of federal regulations for environmental

wildfire activity and carbonaceous aerosol concentrations in the western United States D. V. Spracklen

concentrations in the western United States

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6 the proposed new Corporate Average Fuel Economy (CAFE) standards and reasonable alternative

Air quality in the Lake Tahoe basin is known to affect lake water quality, forest health, and human health. The Lake Tahoe Watershed Assessment provided an initial summary of the status of our scientific knowledge regarding the factors leading to the observed decline in water quality (Reuter and Miller 2000) and steps

The goal of simulating the Earth’s climate with mathematical models, using the most powerful computers available, is valid scientifically and fully consistent with the approaches being taken in many other fields of science dealing with very complex systems. These climate simulations provide the frame within which improved understanding of climate-relevant processes and improved observations are naturally merged into coherent projections of future climate change. The science of climate models has matured to the point that many aspects of current climate models and simulations are very convincing. These form a growing set that intersects significantly with, but does not completely cover, the set of processes that are centrally important for the attribution of past climate changes and the projection of future climate. The set of the most recent climate simulations, referred to as the CMIP3 models and utilized heavily in the Working Group 1 and 2 reports of the 4th IPCC Assessment, have received unprecedented scrutiny by hundreds of investigators with differing areas of expertise. While there are a number of systematic biases across the set of models, more generally the strengths and weaknesses of the simulations, when compared against the current climate, vary substantially from model to model. It is clear from many perspectives that an average over the set of models provides a superior climate