Page - 32 - in Water, Energy, and Environment - A Primer

Several of these impacts will be discussed in more detail in succeeding chapters on specific energy technologies. 4.2 MOREONCLIMATECHANGE Asmentionedabove,climatechangehasbeen identifiedbysome as themost important challenge facingmankind. Iwould pair it with the threat posed by potential use of nuclear weapons in warfare as our most challenging issues. Nevertheless, climate change isworthyof ourmost careful attention. Whatcausesglobalwarmingandtheresultingclimatechange? It is not hard to understand using only basic physics: it is the same physical process that occurs in a car on a hot day that we all experience. Every warm body radiates energy. The visible light rays from the sun, distributed in a frequency spectrum determined by the Sun’s surface temperature (about 5500° C/10,000°F), easily pass through the car’s glass windows and are absorbed by the car’s interior, which gets warm and often hot to the touch. Thesewarmor hot surfaces then reradiate in a spectrum different from the sun’s radiation because of their vastly different surface temperatures. The basic physics is the same – Planck’s Law, first proposed in 1900, specifies the spectral distribution and intensities of the radiation emitted by a black (perfectly emissive) body at temperature T. In a car the energy reradiated from the interior surfaces is mostly in the infrared region, which doesn’t pass easily through the glass. This trapping of the reradiated heat causes the car’s interior temperature to rise until, owing to the interior’s now higher temperature, enough reradiated infrared radiation gets through the glass to provide a balance between the energy of the incoming andoutgoing radiation streams. This is exactlywhat happens in the Earth’s atmosphere,with gases andwater vapor in the atmosphere playing the role of the glasswindshield anddetermining the atmosphere’s transmission characteristics. Important global warming (greenhouse) gases are CO2 (much arising from combustion of fossil fuels), Water,Energy, andEnvironment–APrimer32