Climate Science

Scientists agree.

Scientists from the United Nations Intergovernmental Panel on Climate Change (IPCC) conduct research on climate-related topics and publish this research in peer-reviewed scholarly journals. In the largest consensus that scientists have ever assembled from around the world, the IPCC scientists agree that “most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic [human-generated] greenhouse gas concentrations.” The IPCC defines “very likely” as greater than 90 percent probability of occurrence.1

Due largely to the combustion of fossil fuels, atmospheric concentrations of carbon dioxide (CO₂), the principal human-generated greenhouse gas (GHG), have been rising and are at a level unequaled for at least the last 800,000 years.2 GHG from human activities—such as the burning of fossil fuels for use in buildings and transportation, and methane production from agricultural practices—are trapping more of the sun’s heat in the planet’s atmosphere and warming the Earth. Over the last century, average global temperatures rose by more than 1°F, and the arctic warmed about twice as much,3 with predictions for continued temperature increases in the coming years.4

Diagram of GHG Inventory
The Greenhouse Effect and Greenhouse Gases

Trend projections indicate that atmospheric concentrations of GHG emissions will continue to increase throughout this century. If these projections become reality, climate change will threaten our economic well-being, public health, and the environment.

In its fourth assessment of climate change, the IPCC provides a comprehensive overview of the impacts of climate change. This report describes potential global emission scenarios for the coming century. These scenarios vary from a best-case scenario, characterized by low population growth, clean technologies, and low GHG emissions, to a worst-case scenario, where high population and fossil-fuel dependence result in extreme levels of GHG emissions. While scientists indicate that some degree of climate change is inevitable, they also agree that atmospheric GHG concentrations need to be stabilized in order to avoid the most serious impacts.

  1. Climate Change 2007: The Physical Science Basis, IPCC, 2007. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.
  2. Climate Change 101: Understanding and Responding to Global Climate Change, Pew Center, Updated January 2011.
  3. Smith, T.M., R.W. Reynolds, T.C. Peterson and J. Lawrence, 2008: Improvements to NOAA’s historical merged land-ocean surface temperature. Journal of Climate 2009.
  4. Climate Change 101: Understanding and Responding to Global Climate Change, Pew Center, Updated January 2011.

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