Paleoclimatology: Today’s Science Explaining Yesterday’s Climate

The truth will set you free, but first it will piss you off.” —Gloria Steinem

I recently made a blog post online as well as wrote a letter to the editor in response to the March 19th Maclean’s cover story about Canada having a warmer and drier winter than it has in 65 years. It ended up getting more responses than I usually elicit. Some people expressed doubt about the whole concept of global warming and how the changes we’re seeing now are unprecedented. Because our planet has gone through various ice ages with interglacial periods between them, many question whether what we’re seeing now is any different. Should we really blame our greenhouse gas emissions?

In my book, I explain how subtle changes in Earth’s orbit help to explain many of those past changes. Properties of our planet’s annual trip around the sun changes its axis with time via changes in its tilt or obliquity, eccentricity and precession. All  of these contribute to changes our planet goes through over many thousands of years in something referred to as Milankovitch cycles. (You can see a nice visual demonstration of these properties here.) These cycles don’t play a part in the recent concerns about global warming, however, because the changes we’re currently experiencing are taking mere decades, not the millennia necessary for subtle changes in orbit to occur.

Changes in solar energy output—known as irradiance—can contribute to changes in shorter time intervals than the Milankovitch cycles, but that won’t explain the recent findings of global warming either. That’s because the last sunspot cycle was much lower than average. Referred to as a deep solar minimum, it was the lowest energy output our star has provided in nearly 100 years. Our global temperatures plateaued over the last decade because of this minimum: rising greenhouse gas emissions were offset by a decrease in average solar output. Our current sunspot cycle is also a bit lower than usual, but if we had an average or even robust sunspot cycle, we’d be in even more trouble than we are now.

Some people doubt the data. When I posted comments about some of these facts, I had a few responders question where the data were to support what the carbon dioxide levels were in years past. Roger Revelle and Charles Keeling started measuring these levels in 1958 in Hawaii and Antarctica, but what about older records? Some skeptics doubted we’d have anything more than a thousand years old.

So I had to educate them about the Antarctic ice cores. It’s been known for decades that by drilling deep into the ice in Antarctica, we can retrieve little pockets of air within the ice containing samples of our atmosphere as it was when the ice was formed. SInce Antarctic ice doesn’t melt from season to season, it slowly accumulates with time, preserving little samples of air indefinitely.

So how far back can we look? Well, the Dome C ice core drilled down more than three kilometres, and it turns out the oldest samples of atmosphere are 740,000 years old. As a result, we now have measurements of greenhouse gas levels spanning eight glacial cycles. This allows us to know exactly how high carbon dioxide levels were in years past. It’s interesting to see that in nearly 800,000 years of Earth’s atmospheric history, it never experienced levels higher than 300 parts per million (ppm). That is, not until after our human activities started to add to the mix. Combustion of fossil fuels, deforestation and agriculture for 7 billion people add 30 billion tons of carbon dioxide into the atmosphere each year, with most of it lingering there for centuries. The climb in levels started after the Industrial Revolution and the steepness of that climb hasn’t faltered.

We’re currently at carbon dioxide levels in our atmosphere of 392 ppm, a level not seen in nearly a million years, and very possibly much longer, perhaps as much as 20 million years. That level is climbing about 2 ppm each year, and with no end in sight to this rate of rise. Thanks to the Dome C ice core, “the grand-daddy of ice cores” as it’s sometimes called, we have the facts.

Skeptics can think what they want, but we’re the cause of these unprecedented changes to our atmosphere and we’re already seeing the consequences with climate change. Ancient ice ages due to Milankovitch cycles aren’t our fault; little ice ages due to decreases in solar output aren’t our fault. But melting ice caps and glaciers, and increases in extreme weather such as floods and droughts due to climbing greenhouse gas emissions? We have no one to blame for that but ourselves.