No attempt was made to resolve those differences or develop consensus positions. The report that follows summarizes the detailed presentations given and discussions held at the meeting. Earth's climate has varied significantly in the past and will continue to change in the future for natural reasons that are not fully understood. The emerging understanding of this natural variability still poses fundamental challenges for efforts to identify and quantify the effects that humans might have on climate. A balance between incoming sunlight and escaping infrared energy controls the global heat balance.
Greenhouse gases affect climate by absorbing and emitting infrared radiation.
Earth's surface by about 30 C, making life on Earth possible. Higher concentrations of greenhouse gases absorb greater levels of infrared radia-tion, enhancing the natural greenhouse effect, and can be expected to lead to gradual warming of Earth's surface, assuming that other climate processes remain unchanged. Most greenhouse gases remain in the atmosphere for periods ranging from decades to thousands of years. Consequently, the effects of past and current increases in atmospheric concentrations of carbon dioxide and other long-lived greenhouse gases could persist for centuries. Other human emissions, particularly sulfate aerosols, tend to cool Earth's surface.