Drilling into history

Over the past 40 years, ice cores have revealed more about the earth’s climate than any other scientific technique. It was by chance that Danish climatologist Willi Dansgaard realised the benefit of analysing the ice caps and went on to develop modern ice core science in the 1950s. 

According to Dansgaard "a minor - but to me fateful - miracle" whilst watching the rain fall, he wondered whether its isotopic composition changed from one shower to the next.  He tested his hypothesis, collecting samples during a rainstorm which turned out to be an unusually well-developed front system. When the rain began in western Jutland, it had not stopped raining in Wales, 1,000 km to the west. The miracle was his decision to start the sample collection under these unusually favourable conditions.

Dansgaard discovered that the ratio between two isotopes of oxygen depended on the temperature at which the rain was formed within the clouds and he then reasoned  that the relationship between temperature and delta value might also hold going back in time: "Old water might reflect the climate at the time of formation of the water," he recalls.

His theory was further developed in conjunction with Norwegian zoologist Per Scholander who noticed that gas bubbles tended to develop under ice, and also that a chunk of ice chopped from a glacier gave off bubbles of gas when put into a drink to cool it. Scholander determined that ice is virtually impermeable to gases; this made him wonder whether the air trapped deep in the glaciers contained a permanent record of the composition of the atmosphere in earlier periods. Scholander developed ingenious methods to mine the tiny bubbles of ancient air.

In 2004 a 3,270 metre-long ice core taken from Antarctica - the oldest taken - has proved one of the most useful, stretching back 800,000 years. Dating an ice core is a bit like counting tree rings; near the surface of an ice core, annual layers are usually visible.

Ice cores have confirmed that a great chill reached its climax in the 1600s when London festivals were held on the frozen River Thames, and that when the Viking adventurer Eric the Red named "Greenland'' in 985AD the weather there was warmer.  These ice cores have also been valuable in finding details of volcanic activity.  Ash blasted into the atmosphere during a volcanic eruption is often dispersed over a wide area within a few days or weeks. With very few records of volcanic eruptions from before 10,000 years ago on land these ice cores add considerably to our knowledge of volcanic history much further back in time.

With so much still to be learned from ice cores about our weather, scientists hope to go back up to 1.4 million years. Antarctica has been covered in ice for around the past 30 million years.

So if the climate is constantly changing why do scientists believe our present climate changes are anything different?

With records reaching 800,000 years back in time scientists can see that the gas bubbles show that the concentration of CO₂ was stable over the last millennium until the industrial revolution. It then started to rise exponentially, and its concentration is now a massive 40% higher. Other measurements have revealed that the increasing CO₂ is from fossil fuel usage and deforestation.

The fastest large natural increase, measured in older ice cores, is around 20ppmv (parts per million by volume) over 1000 years, as the earth came out of the last ice age 12,000 years ago. However CO₂ concentration has increased by the same amount - 20ppmv - in just the last 11 years.

The evidence is very compelling and even more worrying for the next generation.

“The world will not be destroyed by those who do evil, but by those who watch them without doing anything”

Albert Einstein