Debates Over Global Warming

    While the behavior of the climate system and the processes that cause global warming are well understood and grounded in basic scientific principles, scientists are still working to understand certain details of the climate system and its response to increasing greenhouse gases. Scientific uncertainty is inevitable with a system as complex as Earth’s climate. However, advancements in measuring, analyzing, and modeling techniques have helped clarify many uncertainties in recent years.

   For example, there had been uncertainty regarding why the warming trend stopped for three decades in the middle of the 20th century. Records even showed some cooling before the climb resumed in the 1970s. The lack of warming at mid-century is now attributed largely to the sulfate aerosols in air pollution, which have a cooling effect because they reflect some incoming sunlight back to space. Continued warming has now overcome this effect, in part because pollution control efforts have made the air cleaner.

   Satellite measurements of atmospheric temperature, which became available around 1980, originally were thought to measure much less warming in the lower region of the atmosphere than surface thermometers. This led to some doubt about the accuracy of the warming detected at the surface. Eventually, other researchers reanalyzed the satellite data using more advanced techniques and concluded that the satellites were detecting warming quite similar to surface measurements. While there is still some uncertainty, scientists examining the satellite data now agree that the record is consistent with a warming world.

    For many years global warming was portrayed in the media as an issue with two sides, with some scientists arguing that global warming is occurring and others arguing that it is not. However, this portrayal was an oversimplification of the scientific debate. Skeptics of global warming, including some scientists, pointed to lingering scientific uncertainties to question whether global warming is actually occurring. However, there is now undeniable evidence that global temperatures are increasing, based on direct temperature measurements and observations of other impacts such as melting glaciers and polar ice, rising sea level, and changes in the lifecycles of plants and animals. As the scientific evidence on rising global temperature became indisputable, skeptics focused their argument on whether human activities are in fact the cause of global warming. They argued that the observed warming could be caused by natural processes such as changes in the energy emitted by the Sun. However, the Sun’s influence has been found to have contributed only slightly to observed warming, particularly since the mid-20th century. In fact, there is overwhelming evidence that greenhouse gas emissions from human activities are the main cause of the warming.

 In 1988 the United Nations Environment Program (UNEP) and the World Meteorological Organization (WMO) established the Intergovernmental Panel on Climate Change (IPCC). The panel comprises thousands of the top climate scientists from around the world and releases a report every six years describing the state of scientific knowledge on global warming. The IPCC’s Fourth Assessment Report, released in 2007, offered the strongest scientific consensus to date on global warming. The panel concluded that it is “very likely” (more than 90 percent probability) that human activities are responsible for most of the warming since the mid-20th century; that it is “extremely unlikely” (less than 5 percent probability) that the warming is due to natural variability; and that it is “very likely” the warming is not due to natural causes alone. This level of certainty is extremely high, given the complexity of the climate system and of the influence of human activities on the climate.