In the atmosphere, various gases, including water vapor, methane, carbon dioxide and other trace chemicals, act like the glass of a greenhouse and trap heat near the Earth's surface. This natural "greenhouse effect" is essential for life on the planet. However, human activities are changing and enhancing this natural effect - thickening the walls of the "greenhouse" - with significant consequences for the global climate.
The burning of fossil fuels, deforestation, and certain agricultural activities and industrial practices unleash billions of tons of carbon dioxide (CO2) into the environment. Since the industrial revolution, atmospheric levels of carbon dioxide have increased by more than 30 percent to levels unsurpassed in the past 800,000 years. The United States shares much of the blame for these changes: although only 4% of the world's population, we are responsible for a quarter of worldwide greenhouse gas emissions.
The increase in the Earth's average temperature is often referred to as "global warming." But since higher temperatures may not be the only effect of increased pollutants in the atmosphere, many scientists and environmentalists now prefer to use the broader term "climate change." Scientists have discovered that in some places, climate change may cause temperatures to decrease, even if the earth's average temperature rises, and the term global warming does not include other atmospheric changes, such as severe weather patterns, that are predicted. The following are some statistics about this phenomenon.
- During the last century the earth was warmed by 1.4 degrees Fahrenheit. Based on current trends, greenhouse gas emissions may raise temperatures an additional 2.5-12 degrees in the 21st century.\
- 20 of the 21 hottest years on record have all occurred in the past 25 years. Additionally the years 2005 and 2010 are statistically tied for the highest average global temperature, and 2012 was the hottest year on record for the contiguous 48 states.
- From 1970 to 2005, the number of category 4 and 5 hurricanes increased by over 50%, and weather models predict increasingly extreme weather patterns.
- The World Health Organization estimates that 140,000 people a year were dying worldwide from the effects of global warming (i.e. heat stroke, increased diseases from insects that thrive in hotter climates, malnutrition from crop destruction) by 2004.
- As a direct result of climate change, ¼ of all plant and animal species could face extinction by 2050.
- Scientists believe that flooding could increase nine fold in the next century.