I have an announcement to make: Canada's climate is about to change dramatically. It will get warm, the snow and ice will disappear and vegetation will emerge and flourish. And it has nothing to do with greenhouse gases.
You see, spring arrives next month. Sorry if that sounds anticlimactic, but in this country, changing seasons always mean changes in climate. Understanding why that happens can help us understand and dismiss one of the most commonly held fallacies about today's changing climate. Perhaps you've heard it: "the climate has always changed; this is just part of a natural cycle." That's incorrect, and here's why.
A nod to the sun
As Earth makes its journey around the sun each year, its northern hemisphere is tilted toward the sun for six months -- from March 21 to September 21, or spring and summer, the seasons of our longest, brightest and warmest days.
Spring doesn't arrive because of a rise in greenhouse gases in our atmosphere, it arrives because our part of the world gets longer days with more intense sunlight. Fall arrives because days get shorter and the sunlight is less intense. It's an annual cycle that has occurred as long as Earth has been going around the sun, and it's quite independent of greenhouse gases in the atmosphere.
(Our seasons are also influenced by the fact that Earth's orbit brings us closer to the sun in some months than in others.)
Seemingly in defiance of this annual cycle, however, our planet experiences periodic cold intervals we call ice ages. The last one ended about 20,000 years ago. At its maximum, it covered most of Canada with ice up to three kilometres thick.
How could this be? If our planet moves in a steady orbit around the sun, shouldn't our climate be steady?
Well, it turns out that our planet's movement around the sun is not perfectly 'steady'; it has three tiny irregularities.
First, its path around the sun varies: it's elliptical (not circular), but the shape of that ellipse changes over time, which affects how close we get to the sun. Second, Earth's axis tilts slowly back and forth within a range of about three degrees, varying our exposure toward the sun. Third, Earth has a slight wobble akin to the wobble of a spinning top, again varying our exposure toward the sun (as demonstrated in this graphic from NASA).
Image courtesy NASA
These three irregularities happen independently and have cycle times of thousands of years. Individually, each has only a small effect upon Earth's climate. However, the three cycles periodically overlap, and their maximum impacts happen at the same time. The result, formed gradually over many millennia, is an ice age. Then, equally slowly, the three cycles naturally desynchronize, eventually overlapping in the opposite direction, and things warm up again. (I'm simplifying somewhat; Google Milankovitch Cycles to learn more.)
Like our seasons, ice ages and the warm periods that follow them are not caused by greenhouse gases; they are caused by changes in our relationship to the sun. Whereas seasons happen annually, these natural cycles happen over tens of thousands of years.
Natural changes, unnatural changes
The bottom line? It's true that Earth's climate was changing long before humankind began consuming fossil fuels and generating greenhouse gases. But these earlier climate changes occurred because of tiny irregularities in Earth's orbit that are now well understood, and those irregularities cannot explain the global warming we are seeing now. (Globally, January 2015 was the second warmest on record, behind only January 2007.) As well, earlier climate changes occurred over many millennia, not years or decades as we are seeing now.
It is naive, simplistic and incorrect to dismiss today's climate changes as "just part of natural cycles." So the next time someone suggests to you that they are, feel free to set them straight.
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