Ice Cores: A Window into Climate History
Interview
ric Wolff, British Antarctic Survey: "We all take for granted that CO2 and methane levels have increased in the last 200 years. But only from ice cores can we be absolutely sure"
Three kilometers below the Antarctic’s surface Earth’s climate history is written in ice. Eric Wolff, leader of the British Antarctic Survey’s Chemistry and Past Climate team, explains how to decipher the ultimate time capsule.
How important are ice cores for climate science?
There are things ice cores can tell you that nothing else can. In particular, they allow us to see the composition of the atmosphere in the past. There is no other way of getting that information.
Ice cores are an unusually pure way of measuring the atmosphere but when it comes to climate they really only tell you about the polar climate. People have collected ice cores from mountains in the tropics but they tend to be partly melted and refrozen and are hard to interpret.
So ice cores are historical climate documents. How far back can they take us?
The oldest ice collected is from a core in Antarctica, which is three kilometers deep and contains 800,000 years of ice. We would like to get to ice 1.5 million years old. I’ll explain why.
In the last 800,000 years ice ages came and went roughly every 100,000 years but before that we know from marine sediments that ice ages came and went every 40,000 years.
Why this change? One theory is that CO2 levels decreased so the ice sheets got larger, and the Earth system had a longer time scale. If we could get an ice core from that era we could say whether that is correct.
How do you drill ice cores?
To drill for very old ice you go somewhere with a low snowfall rate, like ice domes on top of ice sheets where the ice has not flowed from anywhere else.
We only drill about two to three meters of ice at a time. The drill is actually quite short. It is on the end of a wire. It takes about one hour for the drill to go down three kilometers, so most of our time is spent waiting for the drill to go down and come back up again. The longest core took three summer seasons to drill.
The core comes to the surface and we then cut the ice into sections with saws. It then goes back to Europe frozen. We want to keep it frozen to the last possible moment before analysis.
How do you date an ice core?
If somewhere has quite a high snowfall rate you can count back the summer and winter snowfalls like tree rings. In Greenland researchers have counted back 60,000 years.
But in the oldest ice cores there is not enough snowfall. There we estimate age by measuring how much snow falls each year, calculating how it thins through ice physics, and checking the results by comparing with other data.
For example, the 800,000-year old ice core can be dated because we know that 780,000 years ago the Earth’s magnetic field reversed. That increased the amount of cosmic rays that entered the Earth’s atmosphere and formed the radioactive isotope beryllium-10, which can be measured in ice cores.
How exactly do ice cores tell us about the past climate? Aren’t they just water and air?
First, the water molecules tell us about temperature because the balance of different isotopes of water reveals the temperature when the snow fell. We can see the composition of water changing from summer to winter, and from warm period to ice age.
Second, every snowflake has at its center a particle of sea salt or dust, and particles attach to it as it falls. Those tell us about the composition of the atmosphere. For example, after a bigvolcanic eruption the amount of sulfuric acid in the ice increases for a couple of years.
Finally there is the air itself. In Antarctica snow never melts, it gets buried by more years of snow and the weight crushes the snowflakes into a solid matrix which traps air bubbles. Those air bubbles contain a record of every stable molecule in the air: nitrogen, oxygen, carbon dioxide, argon, methane.
So what key climate facts have ice cores revealed?
We all take for granted that CO2 and methane levels have increased in the last 200 years. But only from ice cores can we be absolutely sure. Regular measurements of CO2 in the atmosphere started just 50 years ago.
Ice cores show CO2 levels in the atmosphere started increasing in about 1800, from about 280 parts per million (ppm) to 390 ppm today. Methane concentrations have more than doubled. And ice cores have shown that the recent warming of the last 50 years in the Antarctic Peninsula region is unusual in the context of the last few hundred years.
In Greenland ice cores from the last ice age—between 80,000 and 30,000 years ago—have revealed climate jumps of 10 degrees Celsius within 30 to 40 years. Nobody would have believed that possible.
How could the climate transform so quickly?
People think that the ocean circulation system that transports heat in the oceans—what we in Europe experience as the Gulf Stream—was switching from a strong to a weak mode and back again.
The reason was probably because in the last ice age large amounts of ice melted off the North American ice sheet into the North Atlantic. That can’t happen now because we don’t have that ice sheet.
Can ice cores tell us much about the climate future?
Yes and no. What climate history can tell us is what has happened and what can happen; and we can then compare climate models with the ice core record.
What it can’t tell is what will happen under precisely the conditions we have now because we don’t have any ice core samples with levels of CO2 as high as they are today.
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