CHEMISTRY 2000
Topics of Interest #6:
Greenhouse Gases
What They Are and How They Work
An End to Outdoor Hockey in Canada?
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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?
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We’ve all read/heard about greenhouse gases in the news and
their effect on the global climate. So, what is a greenhouse gas?
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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?
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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?
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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?
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So, why so much attention on carbon dioxide?
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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!