Climate and Climate Change: Certainties and Uncertainties Schwartz S. E., ISSA Lecture, Imperatore School of Sciences & Arts, Stevens Institute of Technology, Hoboken NJ, March 4, 2002.

"Climate and Climate Change: Certainties and Uncertainties" The Earth's climate is driven by radiant energy from the sun. This radiant energy, transmitted to great extent through the atmosphere, is absorbed by the surface resulting in heating of the planet. The planet in turn radiates, but at a lower temperature than the sun, in the infrared region of the spectrum. Some of this re-radiated energy is absorbed by infrared absorbing gases in the atmosphere and re-radiated downward, increasing the total flux of energy to the surface and heating the earth even more. This so-called greenhouse effect is a key feature of the Earth's climate and is responsible in large part for the temperate climate of the Earth. Carbon dioxide and other greenhouse gases have increased substantially in the last 200 years largely because of industrial activities. An increase in the concentration of greenhouse gases would be expected to lead to an increase in the Earth's surface temperature, but there is no a priori means of predicting the magnitude of this increase. The situation is complicated by the inhomogeneity of the earth and its atmosphere as a function of location and season, for example. Clouds are a further complication, as they are both highly reflective in the visible and highly absorptive (and radiative) in the infrared, so any change in cloudiness or cloud properties might exert a large influence on climate change. Aerosols (haze or smog) also scatter and absorb solar radiation, and changes in aerosol loading and properties over the industrial period are thought to have influenced radiation and cloud properties, but these influences are highly uncertain. These uncertainties are substantial in the context of the radiative influence of increased concentrations of greenhouse gases over the industrial period, challenging present capability to infer the sensitivity of the climate system to the greenhouse effect based on observations of climate change over the industrial period.

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