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Global warming is caused when greenhouse gases build up in the atmosphere and trap heat inside. Some amount of greenhouse gases occurs naturally, but in the past century humans have dramatically increased the quantity of these gases in the atmosphere. The primary greenhouse gas is carbon dioxide, which is created when fossil fuels such as coal and oil are burned -- no surprise then that electric power plants and motor vehicles account for two-thirds of U.S. carbon dioxide emissions.

The accumulation of greenhouse gases has already reached the stage where changes in the planet's climate are both perceptible and environmentally significant. At current rates of greenhouse-gas production, climate change will become even more pronounced.

The United States' Disproportionate Share of Greenhouse Gases
Although the United States accounts for just 4 percent of the world's population, it produces 25 percent of the world's greenhouse gases. And the contributions of individuals are significant. In 1995, the United States emitted nearly twice the amount of carbon dioxide per person as the United Kingdom. The average American produces 15,000 pounds of carbon dioxide emissions per year, not counting the additional 25,000 pounds-per-person average share of our industrial emissions. And these numbers continue to rise: between 1990 and 1997, our per-person emissions increased 3.4 percent. Nearly 82 percent of individual (as opposed to industrial) contributions to emissions come from activities primarily related to generating electricity and powering cars.

Effects of Climate Change
Global climate change threatens to destabilize entire countries. For example, climate change will cause sea levels to rise, flooding coastal cities, shorelines, and wetlands. Temperature changes and increased precipitation will affect agriculture as well, because different crops require different temperature and water conditions to thrive, and many may not be able to adapt to new conditions or fend off new species that move in. Climate change may cause public health problems, imperil freshwater supplies, increase the occurrence of droughts and dangerous storms, hasten extinction of certain species, and more, as its impacts ripple through natural systems.

The Bay Area -- indeed, the entire state -- will feel many of these effects. California will likely lose coastal and bay wetlands as sea levels rise. Developed coastal areas could be flooded. Saltwater intrusion could threaten drinking water quality, wildlife habitats, and agricultural land in the delta, the area where the Sacramento and San Joaquin Rivers come together and flow into San Francisco Bay. More rain could increase flooding in some areas, decrease the effectiveness of existing flood-control measures, and increase the need to protect and expand floodways. If, as expected, global warming also reduces snowfall, there will be less spring runoff, more droughtlike conditions, and reduced water supplies. And insufficient fog and more forest fires could render coastal redwoods unable to reproduce, while other forest ecosystems could shift northward.


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