Transcript Topic #6
CHEMISTRY 2000 Topics of Interest #6: Greenhouse Gases What They Are and How They Work An End to Outdoor Hockey in Canada? Monday morning radio announced that “The future of Canadian outdoor ice hockey -- a sport synonymous with the country's culture -- is being threatened by anthropogenic climate change, new research suggests.” Scaremongering? Actually, no. Climate change does not affect all countries equally since it does not have the same effects at all latitudes. Researchers from McGill and Concordia universities used data from 142 weather stations across Canada to calculate the length of the outdoor skating season in various regions of Canada from the 1950s to present. Some parts of Canada have seen their outdoor skating season shrink by more than others. The scientists predicted that, if the trends in the data hold, British Columbia and southern Alberta will no longer have any significant outdoor skating season. http://esciencenews.com/articles/2012/03/05/ice.hockey.feels.heat.canada http://esciencenews.com/articles/2012/03/05/warming.2.degrees.inevitable.over.canada What is a Greenhouse Gas? We’ve all read/heard about greenhouse gases in the news and their effect on the global climate. So, what is a greenhouse gas? Carbon dioxide (most talked-about – though not the most potent) Methane (including leaks from fracking) Nitrous oxide (N2O) Ozone Water vapour What do these gases all have in common? And why are N2, O2 and Ar (some major atmospheric gases) not on the list? What is a Greenhouse Gas? Greenhouse gases all contain polar bonds. (C-H is only slightly polar but C and H have different electronegativities. The bonds in ozone are polar due to formal charges – recall the Lewis structure.) This means that these gases are all IR-active. The dipole of the molecule changes when the molecule is excited into a higher vibrational energy level. How Do Greenhouse Gases Work? When a molecule is excited into a higher vibrational energy level, it absorbs infrared radiation headed toward Earth from the sun. Shortly after absorbing it, the molecule relaxes, emitting the same amount of radiation. It does not release it back out toward the sun, though. Instead, it releases it in a random direction. So, the IR-active molecules are said to “scatter” the radiation. This increases the temperature of the air. Without any greenhouse gases, Earth would be cold and uninhabitable (about 33 C colder than now). Too many greenhouse gases, though, could also make the planet hot and uninhabitable (like Venus). Long before it got to that stage, though, there would be serious repercussions like melting of glaciers, arctic and antarctic ice and raising of sea levels. How Do Greenhouse Gases Work? So, why so much attention on carbon dioxide? We emit a lot. It absorbs different wavelengths from most other greenhouse gases. But if we keep emitting lots of methane, we’ll have to watch for that too!