In a meaty lecture that serves as a concise and comprehensive primer on the twin challenge of energy and environment, John Holdren lays out the difficult options for contending with a world rapidly overheating.  “There is no question the world is growing hotter,” says Holdren, “and we do have a pretty good handle on influences on climate that are changing the average temperature of the Earth,” he says. Since the mid-19th century, there has been a 20-fold increase in the world’s use of energy, the preponderance of which comes from burning fossil fuels. The U.S. is 82% dependent on these fuels, and the rest of the world is racing to catch up. If all nations continue business as usual, says Holdren, by 2030 energy use will increase by about 60% over 2005 levels, with fossil fuels comprising about 70% of world energy use. While there is legitimate concern about the economic, political and security risks of fossil fuel dependence, he says, CO2 and other greenhouse gas emissions that result from fossil fuel combustion pose an immense, immediate threat to the planet. From urban and regional air pollution to massive wildfires and fierce storms that bring coastal inundation, dramatic climate disruption is upon us and demands action now. In order to avoid the biggest risks, such as a temperature increase of several degrees centigrade, we must “sharply change the ratio of energy used essentially immediately,” Holdren says. But it would cost around $15 trillion to convert the world’s fossil fuel dependent energy system into something less destructive, and this conversion would take too long, even if nations could agree on an alternative system. So we are confronted with striking a balance between mitigation and adaptation. Scientists think stabilizing CO2 emissions at 450 parts per million by 2030 might give humanity a shot at avoiding a planet with temperatures as high as those 30 million years ago (when crocodiles swam off Greenland and palm trees swayed in Wyom...