Question

Describe how ice cores are used to establish the CO₂ concentration in the earth’s atmosphere over the last 800,000 years

Answer 1

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.

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.