Understandings in physics provide solid evidence that an increase in atmospheric carbon dioxide (CO2) is influencing the earth's temperature, and that these higher CO2 levels are caused by human activities, said Paul Alivisatos, the fall 2011 Bethe lecturer.

"Physics addresses one of the biggest issues [global warming] facing humankind today," said Alivisatos, director of Lawrence Berkeley National Laboratory and a professor of nanotechnology at the University of California-Berkeley, Sept. 28 in Schwartz Auditorium, Rockefeller Hall. He presented some of the science behind the changes in the carbon cycle; an imbalance in the carbon cycle, he said, is what has led to global warming.

The natural carbon cycle, which includes such processes as photosynthesis and respiration, volcanoes spewing CO2 into the atmosphere and oceans absorbing CO2 from the atmosphere to form sediments, is what Alivisatos refers to as "Carbon Cycle 1.0."

He added: "Today, with human activity, the net flux of carbon into the atmosphere is 100 times the natural geological flux."

Carbon Cycle 2.0, an initiative of the Berkeley Lab that seeks creative solutions toward a carbon-neutral energy future, is an effort "to have a science basis that will allow people in the developing world to have a good economy and health, and at the same time restore the balance of the carbon cycle," Alivisatos said.

Alivisatos went into some detail discussing the work of three pioneering 19th-century scientists -- Joseph Fourier, John Tyndall and Svante Arrhenius, which laid the foundation for understanding how CO2 in the atmosphere traps heat. For example, in 1896, Arrhenius calculated that a doubling of atmospheric CO2 would raise the global temperature. "Today the best calculations [estimate] about 3.5 degrees Celsius," Alivisatos said.

As some evidence that human activity is behind the increasing CO2 concentration in the atmosphere, Alivisatos noted that scientist Charles David Keeling started measuring the atmospheric CO2 level at Mauna Loa, Hawaii, in 1958 and showed that it has increased dramatically in the past 50 years, reaching unprecedented levels never seen in the past 800,000 years. More recently, Ralph Keeling, son of Charles, has reported that the atmospheric oxygen level has decreased steadily in the past 20 years, showing that the combustion of fossil fuels (hydrocarbons) is causing the increased CO2 concentration.

Further evidence, Alivisatos said, is provided by way of isotopic concentrations of carbon in the atmosphere. "When plants do photosynthesis, they selectively assimilate C-12 in them. Since oil in the ground is derived from plant matter, it is richer in C-12 as compared to C-13," said Alivisatos. He presented data that clearly showed an increase in the ratio of C-12 to C-13 in the atmosphere over the past few decades, indicating burning of fossil fuels, he said.

So can't we simply adapt to any changes arising due to the increasing level of CO2? Alivisatos said the answer is not so clear. "There is a good probability that the changes [in the atmosphere] will be big enough, that the consequences would be large enough, that we have to do something."

The Bethe Lectures honor Hans A. Bethe, Cornell professor of physics from 1936 until his death in 2005. Bethe won the Nobel Prize in physics in 1967 for his description of nuclear processes that power the sun.

Graduate student Vivek Venkataraman is a writer intern at the Cornell Chronicle.