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EPA no longer updates EPA's Global Warming Site, but is maintaining this archive for historical purposes. Please see EPA's Climate Change site for current information on climate change and global warming.
Additional Questions
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How do scientists predict future climate?Basic physics tells us that greenhouse gases trap Earth's heat and cause warming. But Earth's climate is very complex, involving interactions among the air, land and oceans. That's why scientists use computer models to project the effects of global warming. Though the models are far from exact, they do a reasonable job of simulating our current climate and reproducing known changes from past climates.
Scientists are confident about the models' ability to simulate large-scale effects of global warming, such as global temperature increase and average sea level rise. The models are less reliable when it comes to simulating changes in other weather variables such as changes in rainfall. Also, current models are still ill-equipped to predict with any confidence what will happen in local areas.
Can you say the recent extreme weather is a manifestation of climate change?Given our knowledge of global warming and our changing climate, we can expect more extreme weather, including more frequent hot days and droughts, less frequent cold days, and more precipitation (including more snowfall in cold areas). But attributing any particular extreme weather event to global warming remains beyond the current limits of scientific capability.
Does El Niņo have anything to do with global warming and climate change?El Niņo is a natural phenomenon that has been occurring throughout the centuries, though not always with the same regularity; it now occurs about every two to seven years. El Niņo is the strong warming of the equatorial Pacific ocean. Its effects are felt worldwide, which demonstrates the interconnected nature of the Earth's climate.
Recent El Niņo events have been very strong and have contributed to the record-setting temperatures of the 1990s – evidence that El Niņo events can warm parts of the Earth. But now scientists are examining how human-induced global warming could affect El Niņo. Scientists are concerned that the accumulation of greenhouse gases in our atmosphere may inject enough heat into the Pacific Ocean such that El Niņo events become more frequent and fierce.
Does ozone layer depletion have anything to do with global warming and climate change?The human health and environmental concerns about ozone layer depletion are different from the risks we face from global warming. Nevertheless, the two phenomena are related in certain ways. Some pollutants contribute to both problems and both alter the global atmosphere.
Ozone layer depletion allows more harmful ultraviolet (UV) radiation to reach our planet's surface. While increased UV radiation is not the cause of global warming, it can lead to skin cancers, cataracts and a suppressed immune system in humans, as well as reduced yields for crops. Ozone layer depletion is mainly caused by chlorofluorocarbons (CFCs). CFCs are therefore no longer produced in industrialized countries, and will eventually be eliminated worldwide.
But like CO2, CFCs are also a strong greenhouse gas. CFCs can remain in the atmosphere for as long as a century, meaning that their contribution to both ozone layer depletion and climate change will persist for a long time.
How do we measure greenhouse gas emissions?The Greenhouse Gas Inventory Program collaborates with hundreds of experts representing more than a dozen federal agencies, many academic institutions, industry associations, consultants, and environmental organizations. The Program also works directly with industries and other government agencies to develop high quality emissions data and is supported by EPA's Clean Air Markets Division (CAMD) experience with the U.S. emissions trading programs and its network of continuous emission monitors for CO2 on most electric power plants in the United States.
Greenhouse gas emission inventories are developed for a variety of reasons. Scientists use inventories of natural and anthropogenic emissions as tools when developing atmospheric models. Policy makers use inventories to develop strategies and policies for emissions reductions and to track the progress of those policies. And, regulatory agencies and corporations rely on inventories to establish compliance records with allowable emission rates. Businesses, the public, and other interest groups use inventories to better understand the sources and trends in emissions. For an overview of the U.S. Greenhouse Gas Inventory, see the following brochure: In Brief--The U.S. Greenhouse Gas Inventory. (PDF, 8 pp., 1.2 MB)
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