Richard Alley’s The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future explains why ice cores are such a mine of information about past climates. He was right there when the ice cores from central Greenland were being extracted between 1989 and 1993. There had been earlier extractions in places easier of access, but ice sheet flow had affected the lower layers and it was not until drilling was set up in a more central location that good records were obtained for the past 110,000 years – and less reliable records for longer than that. His story of how the camps were established, how the drilling of the 5.2 inch (for the sake of his American readers he doesn’t use metric measurements) cores was done, how the core sections were transported and stored, is interesting in itself. But the riveting chapters of the book are his explanations of the annual layers of snow being compressed to ice and stretching and thinning over time as the ice flows (a cardinal fact, the flow of ice) and of the information those annual layers contain and how it is coaxed from them.
In broad terms he explains that snow is compressed into ice under the weight of more snowfall in the top 200 feet or so of the ice sheet over a century or two as most of the air is squeezed out of it (though a very important little bit remains). By the time that foot-thick layer of ice has buried half-way through the ice sheet the layer has been stretched and thinned to half a foot in thickness; by seven-eighths of the way down it is only one-eighth foot thick and so on. As the layers stretch the ends melt very near the coast or break off as icebergs. Layers near the bed of the ice sheet are very thin, stretch and thin only a little, and don’t move down much.
How are the annual layers distinguished from each other? There is a difference in appearance of winter and summer snow because of the transformation to coarser grained hoarfrost driven by the sun which only shines in summer. Readily observable in cores from shallow levels, the difference remains distinct even in the thinner annual ice layers where the remaining air has been trapped as bubbles. Complications arise in ice a mile deep, when bubbles are replaced by clathrates, but late-winter dusty layers of soil particles blown on to the ice sheet can aid observation – or observers can wait for a few months after the ice has reached the surface when the bubbles begin to reapppear as the clathrates break down. Apart from visible appearance there are other aids to dating the layers including volcanic fallout, electrical conductivity, and ice-isotopic ratios.
Once dated, what can we learn from the ice-cores? Past temperatures for one thing. The isotopic composition of water that fell as rain or snow gives a reliable indication of temperature at the time, and has been checked against temperatures measured in the borehole (in a more complicated way than this bald statement may suggest, which he explains with a fascinating kitchen analogy). We can also learn from the dust which the wind has deposited on the ice sheet (once dry and wet deposition have been teased apart) such things as how much sea salt and continental dust were blowing around, how many fires were occurring upwind, how well we were shielded from cosmic rays, how many meteorites were being dumped on earth, and much more. Finally, the level of atmospheric gases such as carbon dioxide and methane can be determined from the air bubbles trapped in the ice. These gases are normally mixed globally by the winds, and checking the Greenland record against Antartica and high mountain glaciers has revealed a high reliability – so high that ice-core gases can now be used to correlate cores.
At this nearly half-way point in the book Alley turns to illuminating discussions of past climates and some ideas as to why the changes happened. He announces his punch lines for the rest of the book. Past climate has been wildly variable with faster changes than anything agricultural industrial humans have ever faced. Climate can be rather stable if nothing is causing it to change, but when ‘pushed’ it often jumps suddenly to something different rather than changing gradually. Such ‘pushes’ in the past have included drifting continents, wiggles in Earth’s orbit, surges of great ice sheets, sudden reversals in ocean circulation, and others. Small ‘pushes’ have cause large changes because many processes amplify the pushes – greenhouse gases are pobably the most important of these amplifiers. We humans can foul our own nest – and we can clean it up.
I won’t follow this summary statement into the detail of the remaining chapters of the book. Suffice to say that he is a master of illuminating analogy, writes with admirable clarity and establishes a happy rapport with the reader. He doesn’t take background knowledge in his readers for granted, but supplies relevant explanation and information as he goes so that the book is readily accessible to the non-scientist prepared to make a reasonable effort to follow the acount. His discussions are moderate in tone and always acknowledge uncertainties.
The book was published some time back, in 2000. The science of climate change is advancing rapidly and the tentative nature of some of his prognostications has possibly firmed up somewhat since then. He has recently commented:
For me, the 2007 IPCC provided neither a best estimate nor an upper bound on sea-level rise because of lack of understanding of ice-sheet changes.
He made that comment to Andy Revkin of the New York Times who had contacted him after he was recently co-awarded the 2009 Tyler Prize for environmental achievement. He also said the following:
We know so much about climate science, and environmental science in general, and the gap between the knowledge of the scientific community and the general community is so large, and so much misinformation is in circulation, that the leading task now is probably education and outreach. We need to provide people, including policymakers, with the knowledge background that will allow them to do their jobs better.