Evidence

According to NOAA , "human activity has been increasing the concentration of greenhouse gases in the atmosphere (mostly carbon dioxide from combustion of coal, oil, and gas; plus a few other trace gases). There is no scientific debate on this point. Pre-industrial levels of carbon dioxide (prior to the start of the Industrial Revolution) were about 280 parts per million by volume (ppmv), and current levels are about 370 ppmv. The concentration of CO2 in our atmosphere today, has not been exceeded in the last 420,000 years, and likely not in the last 20 million years. According to the IPCC Special Report on Emission Scenarios (SRES), by the end of the 21st century, we could expect to see carbon dioxide concentrations of anywhere from 490 to 1260 ppm (75-350% above the pre-industrial concentration)."

According to NASA , "the main human activities that contribute to global warming are the burning of fossil fuels (coal, oil, and natural gas) and the clearing of land. Most of the burning occurs in automobiles, in factories, and in electric power plants that provide energy for houses and office buildings. The burning of fossil fuels creates carbon dioxide, whose chemical formula is CO2. CO2 is a greenhouse gas that slows the escape of heat into space. Trees and other plants remove CO2 from the air during photosynthesis, the process they use to produce food. The clearing of land contributes to the buildup of CO2 by reducing the rate at which the gas is removed from the atmosphere or by the decomposition of dead vegetation.

Methane is twenty times as powerful a greenhouse gas as carbon dioxide, methane has more than doubled in the atmosphere in the last 150 years until today it totals about half the greenhouse effect caused by carbon.

"A small number of scientists argue that the increase in greenhouse gases has not made a measurable difference in the temperature. They say that natural processes could have caused global warming. Those processes include increases in the energy emitted (given off) by the sun. But the vast majority of climatologists believe that increases in the sun's energy have contributed only slightly to recent warming."

If anyone still had doubts, the Bush administration's Climate Change Science Program in May found "clear evidence of human influences on the climate system," echoing the world's leading science organizations on the causes of global warming.

The scientific community agrees global climate change is occurring and human activities are contributing to climate change. The following eight scientific reports and international agreements demonstrate this global consensus.

National Science Academies Issue Joint Statement on Climate Change   (June 7, 2005)

Eleven national science academies called on world leaders “to acknowledge that the threat of climate change is clear and increasing.”  In the statement Global Response to Climate Change, the science academies of Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, the United Kingdom, and the United States jointly declared that “there is now strong evidence that significant global warming is occurring” and “the scientific understanding of climate change is sufficiently clear to justify nations taking prompt action.” More information via National Academy of Sciences.


Kyoto Protocol Enters into Force, Ratified by over 150 Countries   (February 16, 2005)

One hundred fifty countries and regional economic organizations have ratified the Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC). Signatories to the treaty agree to legally binding targets to limit or reduce their greenhouse gas emissions--a major contributor to global warming. With Russia’s ascension to the treaty on November 18, 2004, countries accounting for 62 percent of the total carbon dioxide emissions in 1990 had ratified the protocol. It came into force on February 16, 2005. More information via UNFCCC.


The Scientific Consensus on Climate Change   (December 3, 2004)

An analysis published in Science of 928 peer-reviewed scientific papers on climate change issues found none disagreed with the “consensus of scientific opinion that Earth's climate is heating up and human activities are part of the reason.” The papers were drawn from a random sample of the more than 11,000 scientific papers on climate change written between 1993 and 2003. For more information see: Oreskes, Naomi. 2004. The scientific consensus on climate change. Science. 306, 1686.


Arctic Climate Impact Assessment   (November 9, 2004)

 

The Arctic Climate Impact Assessment (ACIA) is an intergovernmental report based on a four-year scientific study of the Arctic conducted by an international team of 300 scientists and sponsored by the eight arctic nations (Canada, Denmark, Finland, Iceland, Norway, Russia, Sweden and the United States) and six indigenous people’s organizations. The assessment determined that “the Arctic is now experiencing some of the most rapid and severe climate change on Earth. Over the next 100 years, climate change is expected to accelerate, contributing to major physical, ecological, social, and economic changes, many of which have already begun.” In response to the assessment, the eight nations of the Arctic Council agreed to pursue mitigation, adaptation, research and monitoring and outreach strategies to improve awareness and implement successful responses to climatic challenges in the Arctic. More information via Arctic Climate Impact Assessment.



Our Changing Planet, the US Climate Change Science Program’s Report   (July 2004)

The US Climate Change Science Program’s 2004 report to Congress, Our Changing Planet, US Climate Change Science Program for Fiscal Years 2004 and 2005, signed by the Secretary of Commerce, the Secretary of Energy, and the Director of the Office of Science and Technology Policy, found that the global temperature increases observed in the latter half of the 20th century can only be replicated if human influences are included in the models. Simulations by the Department of Energy, shown on page 47 of the report, “show that observed globally averaged surface air temperatures can be replicated only when both anthropogenic forcings—for example, greenhouse gases—as well as natural forcings such as solar variability and volcanic eruptions are included in the model.” More information via US Climate Change Science Program. 


National Academy of Sciences’ Review of IPCC Third Assessment Report   (July 1, 2001)

At the request of the White House, the National Academy of Sciences Committee on the Science of Climate Change reviewed the Intergovernmental Panel on Climate Change’ (IPCC) third assessment report on climate change and produced a report, Climate Change Science: An Analysis of Some Key Questions. The report determined that “the IPCC’s conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community on this issue.” Furthermore, “greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise.”  More information via National Academy of Sciences. 


Intergovernmental Panel on Climate Change Third Assessment Report   (2001)

According to the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report-Climate Change 2001, the consensus of scientific opinion agrees that human activities are affecting the Earth’s climate. “Human activities—primarily burning of fossil fuels and changes in land cover—are modifying the concentration of atmospheric constituents ... that absorb or scatter radiant energy.” In addition, “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.” The Third Assessment Report was written by 637 authors and reviewed by 420 experts. More information via Intergovernmental Panel on Climate Change.


United Nations Framework Convention on Climate Change   (1992)

One hundred eighty-eight countries and the European Union have ratified the United Nations Framework Convention on Climate Change (UNFCCC), agreeing that they are “concerned that human activities have been substantially increasing the atmospheric concentrations of greenhouse gases, that these increases enhance the natural greenhouse effect, and that this will result on average in an additional warming of the Earth's surface and atmosphere and may adversely affect natural ecosystems and humankind.” The United States ratified the UNFCCC on November 15, 1992 and it entered into force on March 21, 1994. More information via UNFCCC.