"Human Impacts on Climate
"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."
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