DEPARTMENT COLLOQUIUM | Global Warming: Simple Physics in a Complex System

Speaker: Richard B. Rood (U-M Atmospheric, Oceanic and Space Sciences)

Despite the continuing politically motivated arguments centered on global warming and its impact, the quantitative basis that the surface of the Earth will warm due to increasing concentrations of carbon dioxide and other human-generated greenhouse is simple. The underlying principle is the conservation of energy and the balance of energy received by the Earth from the Sun and emission of thermal energy from the Earth back to space. Greenhouse gases, such as carbon dioxide, hold energy close to the surface much like a blanket holds energy close to the body. The radiative transfer of thermal infrared energy can be calculated with high accuracy. Thus it is simple to describe a zero-order effect that increasing carbon dioxide will heat the Earth's surface and cool the upper atmosphere. This zero-order warming occurs in a system that is enormously complex. One level of complexity in the physical climate system comes from water. Water exists in all three phases, and has profound impact on the distribution of energy. First, water is itself a greenhouse gas, and it is responsible for most of the warming the surface of the Earth above radiative equilibrium. When water changes phase it takes up or releases energy. As liquid or ice in the atmosphere, water is highly reflective and reduces the energy absorbed by the Earth. This complexity is increased because of the role of water in sustaining life, which causes transfer of mass and energy between the land and biosphere. Life, in turn, is strongly linked to carbon dioxide. Therefore, it is not possible, even, to isolate the study of climate to the physical climate system; it is necessary to consider biological and chemical aspects of the Earth's climate. Given this daunting complexity and the presence of both natural and human-caused variability and change, how do scientists reach the conclusion of the 2007 Intergovernmental Panel on Climate Change that climate change is "unequivocal?" This talk will focus on the correlated behavior of physical quantities and coherent responses of the biosphere to explore the concept of fingerprinting. This is a concept that poses on theory and prediction, for example, how would warming from increased solar irradiance compare with warming from greenhouse gas increases? Examining observations from a variety of sensors in the atmosphere, the ocean, and in the ground as well as continental-scale changes in ecosystems, it will be shown that the Earth is warming and that the probability that the warming is due to causes other than the activities of humans is very small.