American Geophysical Union Council 

Human Impacts on Climate

The following is a policy statement adopted by the American Geophysical Union Council in December 2003.

Human activities are increasingly altering the Earth's climate. These effects add to natural influences that have been present over Earth's history. Scientific evidence strongly indicates that natural influences cannot explain the rapid increase in global near-surface temperatures observed during the second half of the 20th century.

Human impacts on the climate system include increasing concentrations of atmospheric greenhouse gases (e.g., carbon dioxide, chlorofluorocarbons and their substitutes, methane, nitrous oxide, etc.), air pollution, increasing concentrations of airborne particles, and land alteration. A particular concern is that atmospheric levels of carbon dioxide may be rising faster than at any time in Earth's history, except possibly following rare events like impacts from large extraterrestrial objects.

Atmospheric carbon dioxide concentrations have increased since the mid-1700s through fossil fuel burning and changes in land use, with more than 80% of this increase occurring since 1900. Moreover, research indicates that increased levels of carbon dioxide will remain in the atmosphere for hundreds to thousands of years. It is virtually certain that increasing atmospheric concentrations of carbon dioxide and other greenhouse gases will cause global surface climate to be warmer.

The complexity of the climate system makes it difficult to predict some aspects of human-induced climate change: exactly how fast it will occur, exactly how much it will change, and exactly where those changes will take place. In contrast, scientists are confident in other predictions. Mid-continent warming will be greater than over the oceans, and there will be greater warming at higher latitudes. Some polar and glacial ice will melt, and the oceans will warm; both effects will contribute to higher sea levels. The hydrologic cycle will change and intensify, leading to changes in water supply as well as flood and drought patterns. There will be considerable regional variations in the resulting impacts.

Scientists' understanding of the fundamental processes responsible for global climate change has greatly improved during the last decade, including better representation of carbon, water, and other biogeochemical cycles in climate models. Yet, model projections of future global warming vary, because of differing estimates of population growth, economic activity, greenhouse gas emission rates, changes in atmospheric particulate concentrations and their effects, and also because of uncertainties in climate models. Actions that decrease emissions of some air pollutants will reduce their climate effects in the short term. Even so, the impacts of increasing greenhouse gas concentrations would remain.

The 1992 United Nations Framework Convention on Climate Change states as an objective the "...stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system." AGU believes that no single threshold level of greenhouse gas concentrations in the atmosphere exists at which the beginning of dangerous anthropogenic interference with the climate system can be defined. Some impacts have already occurred, and for increasing concentrations there will be increasing impacts. The unprecedented increases in greenhouse gas concentrations, together with other human influences on climate over the past century and those anticipated for the future, constitute a real basis for concern.

Enhanced national and international research and other efforts are needed to support climate related policy decisions. These include fundamental climate research, improved observations and modeling, increased computational capability, and very importantly, education of the next generation of climate scientists. AGU encourages scientists worldwide to participate in climate research, education, scientific assessments, and policy discussions. AGU also urges that the scientific basis for policy discussions and decision-making be based upon objective assessment of peer-reviewed research results.

Science provides society with information useful in dealing with natural hazards such as earthquakes, hurricanes, and drought, which improves our ability to predict and prepare for their adverse effects. While human-induced climate change is unique in its global scale and long lifetime, AGU believes that science should play the same role in dealing with climate change. AGU is committed to improving the communication of scientific information to governments and private organizations so that their decisions on climate issues will be based on the best science.

The global climate is changing and human activities are contributing to that change. Scientific research is required to improve our ability to predict climate change and its impacts on countries and regions around the globe. Scientific research provides a basis for mitigating the harmful effects of global climate change through decreased human influences (e.g., slowing greenhouse gas emissions, improving land management practices), technological advancement (e.g., removing carbon from the atmosphere), and finding ways for communities to adapt and become resilient to extreme events.

For more information about AGU, please visit the AGU website at www.AGU.org.