The Past 150 Years Show an Increase in Greenhouse Gases
Levels of several important greenhouse gases have increased by about 40% since large-scale industrialization began around 150 years ago. During the past 20 years, about three-quarters of human-caused (anthropogenic) emissions came from burning fossil fuels. Concentrations of carbon dioxide in the atmosphere are naturally regulated by numerous processes collectively known as the "carbon cycle."
The flux, or movement, of carbon between the atmosphere and Earth's land and oceans is dominated by natural processes, including plant photosynthesis. While these natural processes can absorb some of the net 6.2 billion metric tons (7.2 billion metric tons less 1 billion metric tons of sinks) of anthropogeniccarbon dioxide emissions produced each year (measured in carbon equivalent terms), an estimated 4.1 billion metric tons are added to the atmosphere each year.
This positive imbalance between greenhouse gas emissions and absorption results in the continuing increase in atmospheric concentrations of greenhouse gases.
Greenhouse Gases Warm the Planet
Scientists know with virtual certainty that increasing greenhouse gas concentrations tend to warm the planet.1 In computer-based models, rising concentrations of greenhouse gases produce an increase in the average surface temperature of the Earth over time. Rising temperatures may, in turn, produce changes in precipitation patterns, storm severity, and sea level commonly referred to as "climate change."
Assessments by the Intergovernmental Panel on Climate Change (IPCC) suggest that the Earth’s climate has warmed between 1.1° and 1.6°F over the past century and that human activity affecting the atmosphere is "very likely" an important driving factor. The IPCC’s Fourth Assessment Report (Summary for Policymakers) states, "Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."
Did You Know?
Fossil fuels supply 84% of the primary energy consumed in the United States and are responsible for 99% of carbon dioxide emissions.
It goes on to state, "The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past 50 years can be explained without external forcing, and very likely that it is not due to known natural causes alone."
More About Where Greenhouse Gases Come From »
1. U.S. Environmental Protection Agency, Climate Change State of Knowledge.