The greenhouse effect is an important natural process that makes life on earth possible. Because the atmosphere always contains heat-trapping gases like carbon dioxide, water vapor, and methane, much of the heat from the sun's radiation is "trapped" in the atmosphere rather than radiating back into space—much like how the panes of glass in a greenhouse hold heat inside the greenhouse. As a result, the atmosphere remains warm enough to support human, plant, and animal life as we know it. Without greenhouse gases, Earth would be about 60 degrees (F) colder and uninhabitable. Unfortunately, as humans burn fossil fuels like coal, oil and natural gas, we are increasing the amount of carbon dioxide and other greenhouse gases in the atmosphere. This is causing more of the sun's heat to be trapped, thus causing the intensification of the greenhouse effect known as global warming.

Scientists contend that anthropogenic additions of greenhouse gases, mainly CO₂, greatly enhance the natural warming of the earth. Use of fossil fuels (e.g. driving a car, drawing electricity from a coal-fired power plant, heating a home with oil or natural gas) is the main human source of CO₂ and other greenhouse gases released into the atmosphere. The second most important source of heat-trapping gases is land-use changes, such as deforestation. The concentration of CO₂ since pre-industrial times has increased by 31%. Also, agricultural activities such as growing rice and raising cattle has had a large influence in the 151% rise of atmospheric methane. With these considerable increases in greenhouse gases, more heat from the sun and earth's surface is trapped in the atmosphere, causing the phenomenon known as global warming.

Global Warming is likely to make summers hotter and drier, and winters hotter and wetter. Climate models project varied changes across the State. These changes include increased winter precipitation over the coast and the Sierras, a drier southeast corner, and a strong warming in the northern Sierras and the Central Valley. Although temperatures on a whole are expected to rise, California is likely to see a more dramatic increase in temperature during the winter. As a result, less precipitation will fall as snow, and more as rain. As snow packs decrease, less water will be available during the summer and much water during the winter will be lost as runoff. Sea level is expected to rise anywhere from eight to twelve inches, leading to coastal erosion, flooding, and permanent inundation.

In hot, dry summers, demand for water for agriculture, industry, and urban areas is high. A decreased stream flow would intensify competition for fresh water. Aquatic ecosystems, like that of the San Francisco Bay, would become more saline, hindering fish that use the Bay for spawning (an impact already evident). California depends on many water-intensive crops, principally grapes, cotton, and alfalfa. A shortage of water would greatly decrease their profitability. The agricultural industry will feel a heavy impact because important perennial crops such as fruit and nuts require many years to adapt to new conditions. Furthermore, dry summers, along with predicted enhanced Santa Ana winds, increases the risk of fires, threatening people and their homes, livestock, and farmland. However, during the winter, intense rains could produce severe flooding and landslides. Sea level rise will have a severe impact on coastal wetlands, housing, agriculture, roads, levees, and other public works. Heavier and possibly more frequent El Niño events threaten ocean productivity as well as human health because heavy rains promote insect and rodent populations that can carry harmful diseases.

Today the average number of days in a year in which the temperature in Los Angeles rises above 90° F is four. By the 2080s, this average is expected to rise to 14-37 days a year. This increase will exacerbate heat-related illnesses and mortality. According to studies linking heat-related illnesses and global warming, mortalities from the heat are projected to grow 62%-88%. Complete acclimation to hotter temperatures is unlikely because many urban buildings, especially residences of the poor, will not be modified quickly enough to offset the often extremely hot conditions. People likely to be most affected by the heat are senior citizens of 60 years old and up, people with previously existing cardio-respiratory problems, and people living in poor housing conditions.

Furthermore, ozone or smog levels will increase as the temperature rises. The inversion layer (acting like a blanket of hot air) traps pollution in the South Coast Air Basin. Heat and sunlight contribute to the formation of ozone (smog), which occurs as a heat-dependent chemical reaction between nitrogen oxides and volatile organic compounds (VOCs) in the atmosphere. Symptoms of ozone exposure are coughing, shortness of breath, wheezing, fatigue, throat dryness, chest pain, headache and nausea. Ozone has been shown to cause inflammation of lung tissue and reduced lung capacity. Development of asthma, increased lung cancer mortality rates, and accelerated lung aging have all been linked to ozone exposure. Lung damage from long-term exposure to ozone can be permanent, while short-term exposure appears to be reversible. Ozone reduces the respiratory system's ability to fight infection and remove foreign particles such as particulate matter.

Environmental Justice is a grassroots movement working toward improving living conditions of people of low socioeconomic status who have been found to face higher environmental health risks than the general public. Many low-income communities have suffered from various health problems due to the siting of toxic waste incinerators or disposal sites, power plants, and other hazardous facilities in their neighborhoods. As a result, Environmental Justice requires that public policy be based on respect for all people, free from discrimination or bias, and demands that low-income and minority communities be involved in all levels of environmental decision-making, including needs assessment, planning, implementation, enforcement, and evaluation.

Minorities and low-income groups are expected to suffer a disproportionate amount from climate change. Individuals from these communities are less likely to leave urban areas during extreme heat events. Today, hospitalizations for asthma attacks are much higher among minorities and low-income groups than for the general public. These communities are more likely to already suffer from respiratory illnesses, while less likely to receive adequate health care. Warmer temperatures will aggravate the problem, placing more stress upon low-income communities and communities of color. The poor will also suffer as food, water, and other goods and services become more expensive.

The people expected to be most impacted by global warming will be urban dwellers, farmers, and residents of low-lying areas such as Bangladesh, the Maldives, and the South Pacific Islands. These poorer countries have already started to experience the devastating impacts of sea level rise and intense storms. Impacts include forced evacuation and loss of homes, destruction of cropland, and contamination of drinking water. In the future, millions more could be forced to flee their homes, triggering extensive financial and cultural losses as well as physical hardship. According to the International Federation of Red Cross and Red Crescent Societies, developing countries lack the necessary health infrastructure and social resources to respond adequately to costly natural disasters and will therefore be less able to adapt to the climate changes produced by global warming. In 1998, there were more refugees created by environmental disasters than by armed conflicts. Today, 96% of all deaths caused by natural disasters occur in developing countries.

The Intergovernmental Panel on Climate Change (IPCC) predicts a warming in the range of 2.5 to 10.4 degrees Fahrenheit by the year 2100.

During the 20th century the global average surface temperature has increased about 1 degree Fahrenheit. This is likely to have been the largest increase of any century during the past 1,000 years. As a result we are already observing various effects of this warming. Snow and ice cover are decreasing, including a widespread retreat of mountain glaciers in non-polar regions during the 20th century. Sea level is rising and the heat content of the oceans has increased. Precipitation and extreme weather events also appear to be increasing.

The Intergovernmental Panel on Climate Change is a scientific organization established by the United Nations Environment Program (UNEP) and the World Meteorological Organization in 1988. The IPCC brings together the world's top scientists in all relevant fields, to survey and synthesize peer-reviewed scientific studies of climate change, and to provide an authoritative assessment of the state of knowledge regarding global warming. There have been three such assessments conducted. The first one was released in 1990, the second in 1995, and the third in 2001. The IPCC is made up of 2500 of the world's top climate scientists. There are three different "working groups" within the IPCC. Working Group I assesses available scientific information on climate change. Working Group II assesses the environmental and socioeconomic impacts of climate change, and Working Group III assesses possible solutions or response strategies.

Since the late 1950's we have been directly measuring the amounts of carbon dioxide in the atmosphere. We can also measure amounts from previous eras using ice core samples and other techniques. There is no scientific doubt that carbon dioxide levels have been increasing. According the IPCC's third assessment, the atmospheric concentration of carbon dioxide (CO₂) has increased by 31% since 1750. The current CO₂ concentration has not been exceeded during the past 420,000 years, and most likely was not exceeded during the last 20 million years. The atmospheric concentration of methane (CH₄), a potent greenhouse gas, has increased by 151% since 1750, and continues to increase.

According to the IPCC about three-quarters of the human-caused emissions of CO₂ is due to the combustion (or burning) of fossil fuels. The rest is primarily due to deforestation and other land use changes.

The nuclei of carbon atoms in carbon dioxide emitted by combustion of fossil fuels differ from the nuclei of carbon atoms in carbon dioxide emitted under natural conditions, in that the carbon dioxide emitted from natural sources on the Earth's surface retains a measurable radioactive portion, while the carbon dioxide emitted by fossil fuel combustion does not. As carbon dioxide has been emitted through fossil fuel combustion, the atmospheric levels of CO₂ have increased, but the radioactive fraction of carbon in the atmosphere has decreased.

The search for exactly that kind of proof has gone on during the 1990s, and is sometimes referred to as the search for the "Greenhouse Fingerprint." Typically, what we are looking for is some sort of "Greenhouse Signal" that emerges against a backdrop of noise. The noise is the natural variability in climate data or trends. To find such a signal requires identifying some distinctive results that would only be likely to occur as a result of warming caused by the increase of greenhouse gases in the atmosphere. Climate models could predict certain results that should be expected, and then actual data would be examined to see if they conform closely enough to the expected results to be statistically significant.

The science bearing on this issue has been reviewed in each of the 3 assessments conducted by the Intergovernmental Panel on Climate Change (IPCC). In its first assessment, in 1990, the IPCC concluded that we could not yet tell with any certainty that humans were responsible for the observed warming.

Between the first assessment and the second assessment, however, some new studies had begun to detect a clear "greenhouse fingerprint." In the second assessment the IPCC stated that its review of the science showed that "the balance of evidence suggests a discernible human influence on global climate," but also noted that "the anthropogenic signal was still emerging from the background of natural climate variability."

Presumably, over time, as the human causes of global warming increase (as CO₂ concentrations continue to rise), the signal should become more easily detectable. In fact, the third assessment released earlier this year, found that "there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

The United States is the world leader, producing almost 25% of the total CO2 emissions worldwide. China shows the most rapid increase in CO2 emissions, and Canada is the world leader in per capita CO2 emissions (truly a dubious distinction). Table #1 below, compares the CO2 emissions of the top 10 nations.

Although Global Warming and ozone depletion are two separate threats, they do bear some relation. The enhanced greenhouse effect causing global warming takes place in the lower part of the atmosphere known as the troposphere. However, ozone depletion is a crisis occurring in the upper atmosphere known as the stratosphere. Stratospheric ozone is responsible for keeping out the sun's ultraviolet radiation that causes harm to life on earth. Because the ozone layer traps heat, its destruction not only lets in dangerous radiation but also cools the upper atmosphere, thereby disturbing weather patterns of the upper atmosphere.

The two problems are related in that some human-made gases, called chlorofluorocarbons, both trap heat and rise into the stratosphere to destroy the ozone layer. Chlorofluorocarbons, however, are a larger contributor to ozone depletion than to global warming. Although any effort to reduce ozone-depleting gases is essential to preventing further devastation to the ozone layer, it will not solve the crisis of global warming at the same time. Nonetheless, working to eliminate many different types of emissions in order to regulate global warming may also have a positive effect on saving the ozone layer.

To be blunt, yes. Scientists of the IPCC have concluded that earth's temperature has already begun to rise unnaturally. The ten warmest years on record have occurred in the last 15 years. The 20th century experienced a global average surface temperature rise of about 1 degree Fahrenheit, possibly the largest increase experienced by any century in the last 1,000 years. The latter half of the 20th century saw a decrease in snow cover and the retreat of mountain glaciers. However, it is not too late to slow down the process of global warming, and to reduce the amount of warming that ultimately occurs. Scientists and economists have identified many cost-effective alternatives that would reduce greenhouse gas emissions. These alternatives include investments in energy-efficiency technologies, clean energy sources, and zero-emission vehicles, thus easing away from a dependence on oil and coal as main energy resources.

Americans have played a large role in the anthropogenic addition of greenhouse gases. California, the U.S., and the world need to achieve a large reduction of emissions from fossil fuels. Governments should adopt more stringent energy efficiency standards, eliminate subsidies that promote coal and oil use, and support a trend toward renewable sources of energy (e.g., solar and wind power). While cutting CO₂ emission levels is by far the most important factor in reducing global warming, the protection and restoration of forest ecosystems, which serve as important storehouses for carbon, can also help. Probably the single most important step that the federal government could take would be to increase federal fuel economy standards for new cars, trucks and SUVs. California should lead the nation in tackling global warming by formally authorizing the state's Air Resources Board (ARB) to reduce CO₂ emissions from California's passenger cars and SUVs. California should also adopt a "Renewable Portfolio Standard" which would promote renewable energy sources by requiring that energy suppliers generate 20% of their power from renewable sources (like solar and wind power) by 2010.

Yes, absolutely. Climate Change can be successfully reduced only if all levels of government participate in finding ways to reduce greenhouse gas emissions. Cities and counties can adopt many policies that promote energy efficiency and renewable energy resources. They can also adopt a municipal Climate Action Plan that inventories a city's own contribution to greenhouse gas emissions, and sets a goal for reducing municipal emissions of greenhouse gases. Such plans often include specific measures for improving the energy efficiency of city buildings and streetlights, vehicle fleets, and even commercial and residential buildings throughout the city. They can include provisions for bicycle patrols, solar-heating at municipal swimming pools, and capturing methane to create electricity at landfills. And the good news is that most of these ideas will pay for themselves in reduced energy costs in just a few years.