Introduction

Global climate change is the subject of policy debate within most nations, and of negotiations within the Conference of Parties (COP) to the Framework Convention on Climate Change (FCCC). But, what do we mean by the words climate and climate change? Climate is usefully defined as the average of the weather we experience over a ten-or twenty-year time period. Long-term temperature and rainfall changes are typical measures of climate change, and these changes can be expressed at the local, regional, country, or global scale. When the global average temperature changes we call that global warming or cooling.

What produces global warming or cooling? Fundamentally, it can be driven by any imbalance between the energy the Earth receives, largely as visible light, from the sun, and the energy it radiates back to space as invisible infrared light. The greenhouse effect is a warming influence caused by the presence in the air of gases and clouds which are very efficient absorbers and radiators of this infrared light. The greenhouse effect is opposed by substances at the surface (such as snow and desert sand) and in the atmosphere (such as clouds and aerosols) which efficiently reflect sunlight back into space and are thus a cooling influence.

Easily the most important greenhouse gas is water vapor, which typically remains for a week or so in the atmosphere. Water vapor and clouds are handled internally in climate models, although with considerable uncertainty. Concerns about global warming, however, revolve around less important but much longer-lived greenhouse gases, especially carbon dioxide. The concentrations of carbon dioxide and some other long-lived gases (methane, nitrous oxide, chlorofluorocarbons) have increased substantially over the past two centuries due at least in part to human activity. When the concentration of a greenhouse gas increases (with no other changes occurring), it temporarily lowers the flow of infrared energy to space and increases the flow of infrared energy down toward the surface. The Earth is then temporarily receiving more energy, for example 1 percent more, than it radiates to space. This small imbalance, which is often called "radiative forcing", tends to raise temperatures at the surface and in the lower atmosphere. The rate of surface temperature rise is slowed significantly by uptake of heat by the world's oceans. The greenhouse effect as quantified by this radiative forcing is real and the physics relatively well understood. What is much more uncertain, and the cause of much of the scientific debate, is the response of the complex system that determines our climate to this radiative forcing. Feedbacks in this system can either amplify or dampen the response in ways which are only partially understood at present.

To inform

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