What with all the talk about the greenhouse effect recently, I decided it’s time for a quick review…
The term “greenhouse effect” is usually used today in reference to a predicted gradual warming of the Earth caused by an increase in various gases in the atmosphere, primarily due to human activity.
Really, however, the greenhouse effect has been at work for eons, which is a good thing, because it’s what keeps Earth’s mean surface temperature high enough (17 degrees Celsius) for life to thrive.
About 40 percent of the energy we receive from the sun arrives at such short wavelengths that it zips through the atmosphere unimpeded. It warms the ground, however, which then radiates heat back at a much longer wavelength–a wavelength which certain gases, especially carbon dioxide and methane, absorb. This heats them, and thus warms the entire atmosphere.
The amount of greenhouse gases and the mean surface temperature have varied over Earth’s lifetime. The temperature has been so cold much of the planet was covered with ice, and so warm sub-tropical flora and fauna flourished at the poles.
Currently, the amount of carbon dioxide in the atmosphere is on the rise, due primarily to the burning of fossil fuels and the cutting down of forests (trees remove carbon dioxide from the air, but once they’re dead, they release carbon dioxide back into the atmosphere as they burn or decompose). Before the Industrial Revolution, the atmosphere contained about 280 parts per million of carbon dioxide. Today, it contains 360 parts per million, and the Intergovernmental Panel on Climate Change, an assembly of Earth’s top climatologists, estimates that by the end of the 21st century, that level could be anywhere from 480 to 800 ppm.
Methane, produced when bacteria decompose organic matter, has also increased due to human activities, including raising livestock, wetland rice farming and the disposal and treatment of garbage and human and animal wastes.
The IPCC estimates that carbon dioxide levels of 560 ppm could cause an increase of 1.1 to 3.3 degrees Celsius in the planet’s average temperature next century. Considering Canadian winters, that doesn’t sound too bad or too extreme. But consider: during the last ice age, when three kilometres of ice covered most of North America, the average temperature was only 2.5 to 5 degrees cooler than it is now. Even slight changes can have huge effects.
The possible consequences are frightening. Some forests could become grasslands; some grasslands (southern Saskatchewan, perhaps?) deserts. Longer, hotter summers and shorter, milder winters could be punctuated by more extreme storms, leading to floods and other weather disasters. Species unable to adapt could become extinct; familiar songbirds and animals like polar bears and manatees could vanish forever. Coastal cities could suffer from rising sea levels and more powerful hurricanes. Mosquitoes could become more plentiful, and carry tropical diseases like malaria and dengue fever further north. Crop failures could result in a huge worldwide refugee problem.
Environmentalists can point to plenty of other horrendous possibilities. What they can’t do–yet–is prove that human-induced warming has already begun.
It’s true that the IPCC stated in 1995 that “the balance of evidence suggests that there is a discernible human influence on global climate.” It reached that conclusion because the average temperature of the planet has increased by about half a degree this century, and because that warming has occurred in the fashion predicted by the most sophisticated computer models.
However, since then, new models and new discoveries have added new uncertainty. Many scientists believe it will be another 10 years before we can say unambiguously that we have seen the “fingerprint” of human activity in global warming.
Those who fear the economic consequences of possible legislated efforts to reduce greenhouse gas emissions point to this uncertainty as good reason to hold off on any action. But even most of the scientists who say human-induced warming has not yet begun don’t doubt that it will. This century’s warming apparently falls within natural variations in climate–but that doesn’t mean humans didn’t cause it. And out basic understanding of the greenhouse effect tells us that increased levels of carbon dioxide in the atmosphere must eventually cause warming.
As a result, governments worldwide are faced with an unpleasant choice: do nothing, or too little, and face possible catastrophe, or reduce emissions of greenhouse gases, and face the wrath of an inconvenienced public.
I don’t envy them–but for all our sakes, I hope they make the right choice.