A） According to the National Academy of Sciences, the Earth’s surface temperature has risen by about l degree Fahrenheit in the past century, with accelerated warming during the past two decades. There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities. Human activities have altered the chemical composition of the atmosphere through the buildup of greenhouse gases-primarily carbon dioxide, methane, and nitrous oxide. The heat-trapping property of these gases is undisputed although uncertainties exist about exactly how earth’s climate responds to them. 
What Are Greenhouse Gases?
B） Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Certain human activities, however, add to the levels of most of these naturally occurring gases. Carbon dioxide is released to the atmosphere when solid waste, fossil fuels(oil, natural gas, and coal), and wood and wood products are burned. Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from the decomposition of organic wastes in municipal solid waste landfills, and the raising of1ivestock. Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of solid waste and fossil fuels. 
C） Very powerful greenhouse gases that are not naturally occurring include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride(SF6), which are generated in a variety of industrial processes. Each greenhouse gas differs in its ability to absorb heat in the atmosphere. HFCs and PFCs are the most heat-absorbent. Methane traps over 21 times more heat per molecule than carbon dioxide, and nitrous oxide absorbs 270 times more heat per molecule than carbon dioxide. Often, estimates of greenhouse gas emissions are presented in units of millions of metric tons of carbon equivalents(MMTCE), which weights each gas by its GWP value, or Global Warming Potential. 
Changing Climate
D） Global average surface temperatures have increased 0. 5-1. 0。F since the late 19th century. The 20th century’s l0 warmest years all occurred in the last l5 years of the century. Of these, 1998 was the warmest year on record. The snow cover in the Northern Hemisphere and floating ice in the Arctic Ocean have decreased. Globally. sea level has risen 4-8 inches over the past century. Worldwide rainfall over land has increased by about one percent. The frequency of extreme rainfall events has increased throughout much of the United States. Calculations of climate change for specific areas are much less reliable than global ones, and it is unclear whether regional climate will become moil variable. 
What’s Known for Certain?
E） Scientists know for certain that human activities are changing the composition of Earth’s atmosphere. Increasing levels of greenhouse gases, like carbon dioxide(C02), in the atmosphere since pre-industrial times have been well documented. There is no doubt this atmospheric buildup of carbon dioxide and other greenhouse gases is largely the result of human activities. It’s well accepted by scientists that greenhouse gases trap heat in the Earth’s atmosphere and tend to warm the planet. By increasing the levels of greenhouse gases in the atmosphere, human activities are strengthening Earth’s natural greenhouse effect. The key greenhouse gases emitted by human activities remain in the atmosphere for periods ranging from decades to centuries. A warming trend of about l F has been recorded since the late 19th century. Warming has occurred in both the northern and southern hemispheres, and over the oceans. Confirmation of 20th century global warming is further verified by melting glaciers, decreased snow cover in the northern hemisphere and even warming below ground. 
What Are the Uncertainties?
F） Like many pioneer fields of research, the current state of global warming science can’t always provide definitive answers to our questions. There is certainty that human activities are rapidly adding greenhouse gases to the atmosphere, and that these gases tend to warm our planet. This is the basis for concern about global warming. The fundamental scientific uncertainties are these：How much more warming will occur? How fast will this warming occur? And what are the potential advantages and disadvantages? These uncertainties will be with us for some time, perhaps decades. Some of the largest uncertainties are associated with evens that pose the greatest risk to human societies. IPCC cautions, “Complex systems, such as the climate system, can respond in non-linear ways and produce surprises. ” There is the possibility that a warmer world could lead to more frequent and intense storms, including hurricanes. Preliminary evidence suggests that, once hurricanes do form, they will be stronger if the oceans are warmer due to global warming. However, the jury is still out whether or not hurricanes and other storms will become more frequent. 
G） More and more attention is being aimed at the possible link between El Nino events—the periodic warming of the equatorial Pacific Ocean—and global warming. Scientists are concerned that the accumulation of greenhouse gases could inject enough heat into Pacific waters such that El Nino events become more frequent and fierce. Here to0, research has not advanced far enough to provide conclusive statements about how global warming will affect El Nino. Global warming poses real risks. The exact nature of these risks remains uncertain. Ultimately, this is why we have to use our best judgment—guided by the current state of science—to determine what the most appropriate response to global warming should be. 
Impacts of Global Warming
H） Rising global temperatures are expected to raise sea level, and change rainfall and other local climate conditions. Most of the United States is expected to Warm, although sulfates may limit warming in some areas. Scientists currently are unable to determine which parts of the United States will become wetter or drier. but there is likely to be an overall trend toward increased rainfall and evaporation, more intense rainstorms, and drier soils. Unfortunately, many of the potentially most important impacts depend upon whether rainfall increases or decreases, which cannot be reliably projected for specific areas. 
What’s Being Done?
I） Today, action is occurring at every level to reduce, to avoid, and to better understand the risks associated with climate change. Many cities and states across the country have, prepared greenhouse gas inventories and many are actively pursuing programs and policies that will result in greenhouse gas emission reductions. At the national level, the U. S. Global Change Research Program coordinates the world’s most extensive research effort on climate change. In addition, EPA and other federal agencies are actively engaging the private sector, states, and localities in partnerships based on a win-win philosophy and aimed at addressing the challenge of global warming while, at the same time, strengthening the economy. At the global level, countries around the world have expressed a firm commitment to strengthening international responses to the risks of climate change. The U. S. is working to strengthen international action and broaden participation under the support of the United Nations Framework Convention on Climate Change(UNFCCC).