Global Warming


  1. Introduction
  2. Climate Change or Global Warming?
  3. The Greenhouse Effect
  4. How Does Global Warming Occur?
  5. U.S. Climate Policy
  6. Greenhouse Gas Emissions
  7. The Impact of Climate Change Can Be Seen Now
  8. What You Can You Do Now?
Global Warming | Climate Change


The Earth's climate has changed many times during the planet's history, with events ranging from ice ages to long periods of warmth. Historically, natural factors such as volcanic eruptions, changes in the Earth's orbit, and the amount of energy released from the Sun have affected the Earth's climate. Beginning late in the 18th century, human activities associated with the Industrial Revolution have also changed the composition of the atmosphere and therefore very likely are influencing the Earth's climate.


Climate Change or Global Warming?

The term climate change is often used interchangeably with the term global warming, but according to the National Academy of Sciences, "the phrase 'climate change' is growing in preferred use to 'global warming' because it helps convey that there are [other] changes in addition to rising temperatures."

Climate change refers to any significant change in measures of climate (such as temperature, precipitation, or wind) lasting for an extended period (decades or longer). Climate change may result from:

  • natural factors, such as changes in the sun's intensity or slow changes in the Earth's orbit around the sun;
  • natural processes within the climate system (e.g. changes in ocean circulation);
  • human activities that change the atmosphere's composition (e.g. through burning fossil fuels) and the land surface (e.g. deforestation, reforestation, urbanization, desertification, etc.)

Global warming is an average increase in the temperature of the atmosphere near the Earth's surface and in the troposphere, which can contribute to changes in global climate patterns. Global warming can occur from a variety of causes, both natural and human induced. In common usage, "global warming" often refers to the warming that can occur as a result of increased emissions of greenhouse gases from human activities.


The Greenhouse Effect

Energy from the Sun drives the Earth's weather and climate. The Earth absorbs energy from the Sun, and also radiates energy back into space. However, much of this energy going back to space is absorbed by “greenhouse” gases in the atmosphere. Because the atmosphere then radiates most of this energy back to the Earth’s surface, our planet is warmer than it would be if the atmosphere did not contain these gases. Without this natural "greenhouse effect," temperatures would be about 60ºF lower than they are now, and life as we know it today would not be possible.


How Does Global Warming Occur?

During the past century humans have substantially added to the amount of greenhouse gases in the atmosphere by deforestation and burning fossil fuels such as coal, natural gas, oil and gasoline to power our cars, factories, utilities and appliances. The added gases — primarily carbon dioxide and methane — prevent heat from escaping to space and enhance the natural greenhouse effect, and contribute to an increase in global average temperature and related climate changes.

Greenhouse gases are necessary to life as we know it, because they keep the planet's surface warmer than it otherwise would be. But, as the concentrations of these gases continue to increase in the atmosphere, the Earth's temperature is climbing above past levels. According to NOAA and NASA data, the Earth's average surface temperature has increased by about 1.2 to 1.4ºF in the last 100 years. The eight warmest years on record (since 1850) have all occurred since 1998, with the warmest year being 2005. Most of the warming in recent decades is very likely the result of human activities. Other aspects of the climate are also changing such as rainfall patterns, snow and ice cover, and sea level.

If greenhouse gases continue to increase, climate models predict that the average temperature at the Earth's surface could increase from 3.2 to 7.2ºF above 1990 levels by the end of this century. Scientists are certain that human activities are changing the composition of the atmosphere, and that increasing the concentration of greenhouse gases will change the planet's climate. But they are not sure by how much it will change, at what rate it will change, or what the exact effects will be.


U.S. Climate Policy

The United States government has established a comprehensive policy to address climate change. This policy has three basic components:

  • Slowing the growth of emissions
  • Strengthening science, technology and institutions
  • Enhancing international cooperation

To implement its climate policy, the Federal government is using voluntary and incentive-based programs to reduce emissions and has established programs to promote climate technology and science. This strategy incorporates know-how from many federal agencies and harnesses the power of the private sector.

In February 2002, the United States announced a comprehensive strategy to reduce the greenhouse gas intensity of the American economy by 18 percent over the 10-year period from 2002 to 2012. Greenhouse gas intensity is a measurement of greenhouse gas emissions per unit of economic activity. Meeting this commitment will prevent the release of more than 100 million metric tons of carbon-equivalent emissions to the atmosphere (annually) by 2012 and more than 500 million metric tons (cumulatively) between 2002 and 2012.

EPA plays a significant role in helping the Federal government reach the United States' intensity goal. EPA has many current and near-term initiatives that encourage voluntary reductions from a variety of stakeholders. Initiatives, such as ENERGY STAR, Climate Leaders, and our Methane Voluntary Programs, encourage emission reductions from large corporations, consumers, industrial and commercial buildings, and many major industrial sectors.


Greenhouse Gas Emissions

In the U.S., our energy-related activities account for three-quarters of our human-generated greenhouse gas emissions, mostly in the form of carbon dioxide emissions from burning fossil fuels. More than half the energy-related emissions come from large stationary sources such as power plants, while about a third comes from transportation. Industrial processes (such as the production of cement, steel, and aluminum), agriculture, forestry, other land use, and waste management are also important sources of greenhouse gas emissions in the United States.

For a better understanding of where greenhouse gas emissions come from, governments at the federal, state and local levels prepare emissions inventories, which track emissions from various parts of the economy such as transportation, electricity production, industry, agriculture, forestry, and other sectors. EPA publishes the official national inventory of US greenhouse gas emissions, and the latest greenhouse gas inventory shows that in 2005 the U.S. emitted over 7.2 billon metric tons of greenhouse gases (a million metric tons of CO2 equivalents (MMTCO2e) is roughly equal to the annual GHG emissions of an average U.S. power plant.)


The Impact of Climate Change Can Be Seen Now

Climate change affects people, plants, and animals. Scientists are working to better understand future climate change and how the effects will vary by region and over time.

Scientists have observed that some changes are already occurring. Observed effects include sea level rise, shrinking glaciers, changes in the range and distribution of plants and animals, trees blooming earlier, lengthening of growing seasons, ice on rivers and lakes freezing later and breaking up earlier, and thawing of permafrost. Another key issue being studied is how societies and the Earth's environment will adapt to or cope with climate change.

In the United States, scientists believe that most areas will to continue to warm, although some will likely warm more than others. It remains very difficult to predict which parts of the country will become wetter or drier, but scientists generally expect increased precipitation and evaporation, and drier soil in the middle parts of the country. Northern regions such as Alaska are expected to experience the most warming. In fact, Alaska has been experiencing significant changes in climate in recent years that may be at least partly related to human caused global climate change.

Human health can be affected directly and indirectly by climate change in part through extreme periods of heat and cold, storms, and climate-sensitive diseases such as malaria, and smog episodes.

What You Can You Do Now?

Greenhouse gases are emitted as a result of the energy we use by driving and using electricity and through other activities that support our quality of life like growing food, raising livestock and manufacturing. Greenhouse gas emissions can be minimized through simple measures like replacing energy guzzling incandescent light bulbs in your home with CarbonNeutralBulb™ and SaniBulb™ Air Sanitizing and Energy Saving Compact Fluorescent (CFL) Bulbs.

State and local governments and businesses play an important role in meeting the national goal of reducing greenhouse gas intensity by 18 percent by 2012. For example, major corporations, states and local organizations are taking action through participation in a wide range of EPA and other federal voluntary programs.

You can start by assessing your own contribution to the problem, by using our Personal Greenhouse Gas Emissions Calculator to estimate your household's annual emissions. Once you know about how much you emit, you use the tool to see how simple steps you take at home, at the office, on the road, and at school can reduce your emissions.

Glossary of Climate Change Terms


* Adapted from the EPA website