Update: To read other articles in this series, click here.
Climate scientists who study the history of the Earth’s climate (also known as paleoclimatologists) know that modern carbon dioxide levels are at their highest level in the last 800,000 years. They tell us this because they’ve been able to measure the carbon dioxide in air that is actually 800,000 years old. So how do they do that?
Scientists know how much carbon dioxide was in the air hundreds of thousands of years ago because they actually have small samples of ancient air stored in glacial ice. To get a feel for how this works, consider the following examples.
Go to your freezer and pull out a few ice cubes. Look closely at the cubes and you’ll see bubbles frozen in the ice cube. My ice maker produced easily visible large bubbles (~ 1/8th inch across) in about every third cube. Those bubbles are filled with air, so it’s pretty obvious that ice can capture and store air. If you put that ice cube in a refrigerated time capsule for 800,000 years, some future scientist could pull it out and they’d have a sample of the air from your freezer that was 800,000 years old. The same thing happens naturally in lakes, as these frozen bubbles of methane show. But what about snow?
First, we know that fallen snow contains a lot of air – that’s why it’s so light when you shovel it. Even wet and heavy snow has a lot of air, as you can feel by how much you can compact it as you’re making snowballs – compacting the snow into a ball squeezes the air out of it. Even at the bottom of a deep snowdrift there’s still lots of air mixed in with the snowflakes. As snow keeps falling, eventually there is enough to keep the air several feet down from mixing with the atmosphere. When that happens, the air is trapped and essentially becomes a time capsule, storing a sample of air in snow much like that air bubble in the ice cube did. The trapped air is released back into the atmosphere when the snow melts.
Second, anyone who lives where it snows several inches in winter already knows that snow eventually turns to ice just by compressing it – it’s how snowy roads turn icy just by cars driving on them, even when the weather is so cold that the snow doesn’t melt. The same thing happens in a glacier or ice cap, but instead of cars compressing the snow into ice, the weight of years or decades of accumulating snow is what turns old snow into ice. And when the snow turns into ice, the trapped air forms small bubbles in the ice.
Scientists can extract the air from those bubbles and then use instruments (namely mass spectrometers) to directly measure how much of what gases are present in the air. The depth of the ice from the surface tells scientists how old the ice is. And from this information, scientists can plot a graph of the concentration of a gas (such as CO2, or methane) over time.
When scientists do this, they create images like the one above from the 2007 IPCC “state of the science” Working Group 1 report. Note those stars in the upper right corner – they’re the “present day” measured concentrations of nitrous oxide (green), methane (blue), and carbon dioxide (red). Note also that they’re on the same scale as their respective lines – so the blue methane star (1750 parts per billion, or ppb) is on the same scale as the blue methane line, which never gets above about 800 ppb concentration in the ice core record. Similarly, the red CO2 star (370 parts per million, or ppm) is on the same scale as the red CO2 line, which never gets above about 300ppm in the ice core record.
So why does this matter? Well, because it means that scientists can use air trapped in snow from hundreds of thousands of years ago to tell us how much of each greenhouse gas there was in the air back then. And that gives us a baseline to compare modern concentrations of greenhouse gases to old concentrations. And that tells us that CO2 and methane haven’t been this high in hundreds of thousands of years, even as ice ages have come and gone.
In the next installment, we’ll learn about how scientists can use these same ice cores to estimate how the global average temperature has varied over the last several hundred thousand years.