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Global Warming


What's the greenhouse effect?

Earth's greenhouse effect is a natural phenomenon that helps regulate the temperature of our planet. Simply put, the sun heats the Earth and some of this heat, rather than escaping back to space, is trapped in the atmosphere by clouds and greenhouse gases, such as water vapor and carbon dioxide. If all of these greenhouse gases were to suddenly disappear, our planet would be 60F colder and uninhabitable.

Is our planet warming?

The global temperature record shows an average warming of about 1.1F over the past century . This warming has been recorded in both the northern and southern hemispheres, and over the oceans, with some areas substantially warmer and others actually cooler.

The ten warmest years have occurred since 1983, with seven of them since 1990. Recent evidence shows the 20th century was the warmest in the last 1,000 years. The 1990s were the warmest decade and 1998 was the single warmest year of the past millennium.

Are human activities responsible for the warming?

Separating out the impact of human activity from natural climate variation is extremely difficult. Nonetheless, the IPCC concluded there is a "discernible human influence" on climate. This means the observed global warming is unlikely to be the result of natural variability alone and that human activities are at least partially responsible.

How do we take Earth's temperature?
Earth's temperature is taken through a network of thermometers on ships, buoys and land-based weather stations. The data are compiled by organizations like the World Meteorological Organization, NASA and the U.S. National Oceanic and Atmospheric Administration.

This global temperature record dates back to about 1860. During this period, measuring techniques have changed, some weather stations relocated and others became surrounded by cities. Scientists have taken special care to address these problems to ensure the global temperature record is reliable and consistent.

To know what temperatures were like before 1860, scientists must rely on limited records or reconstruct Earth's temperature history by examining tree rings, pollen records and air locked away in ancient ice.


What are the most important greenhouse gases?
Where are they coming from?

Many greenhouse gases occur naturally, but human activities are adding gases to the natural mix at an unprecedented rate. Water vapor is the most abundant greenhouse gas; it occurs naturally and makes up about two thirds of the natural greenhouse effect. Fuel burning and other human activities, however, are adding large amounts of greenhouse gases to the atmosphere – the most important ones being carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). For an overview of greenhouse gases, see the following brochure: In Brief--The U.S. Greenhouse Gas Inventory. (2.2M pdf)

Since pre-industrial times atmospheric concentrations of CO2, CH4 and N2O have climbed by more than 31%, 151% and 17%, respectively. Scientists have confirmed this is primarily due to human activity. Burning coal, oil and gas, and cutting down forests are largely responsible.

What will happen to Earth's climate if emissions of these greenhouse gases continue to rise?

Because human emissions of CO2 and other greenhouse gases continue to climb, and because they remain in the atmosphere for decades to centuries (depending on which gas), we're committing ourselves to a warmer climate in the future. The IPCC projects an average global temperature increase of 1-4.5F (0.6-2.5C) in the next fifty years, and 2.5-10.4F (1.4-5.8C) in the next century. Temperatures in some parts of the globe (e.g., the polar regions) are expected to rise even faster. Even the low end of the IPCC's projected range represents a rate of climate change unprecedented in the past 10,000 years.

What are the potential impacts of global warming and a changing climate?

Our health, agriculture, water resources, forests, wildlife and coastal areas are vulnerable to global warming and the climatic changes it will bring. The IPCC concluded that "climate change is likely to have wide-ranging and mostly adverse impacts on human health, with significant loss of life."

A few degrees of warming increases the chances of more frequent and severe heat waves, which can cause more heat-related death and illness. Greater heat can also mean worsened air pollution, as well as damaged crops and depleted water resources. Warming is likely to allow tropical diseases, such as malaria, to spread northward in some areas of the world.

It will also intensify the Earth's hydrological cycle. This means that both evaporation and precipitation will increase. Some areas will receive more rain, while other areas will be drier. At the same time, extreme events like floods and droughts are likely to become more frequent. Warming will cause glaciers to melt and oceans to expand. The IPCC projects that sea level will rise between four inches and three feet (9 to 88 cm) over the next century, in addition to the local sea level changes caused by other factors such as land subsidence and plate tectonics. This threatens low-lying coastal areas. Scientists are also concerned that warming could lead to more intense storms.

Source: US EPA, Office of Air and Radiation


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