The Geological Society has prepared a position statement on climate change, focusing specifically on the geological evidence (here's a pdf version of the statement). The geological record contains abundant evidence on the ways Earth’s climate has changed in the past and give us vital clues on how it may change in the future. Their statement is based on geological evidence, not on recent temperature or satellite data or climate model projections. The statement is a must-read, featuring a wealth of information and many useful peer-reviewed references (my to-do list has just gotten longer). I've summarised some of their key points below:
The Earth’s temperature changes naturally over time scales ranging from decades, to hundreds of thousands, to millions of years. In some cases these changes are gradual and in others abrupt. Evidence for climate change is preserved in a wide range of geological settings, including marine and lake sediments, ice sheets, fossil corals, stalagmites and fossil tree rings. Cores drilled through the ice sheets yield a record of polar temperatures and atmospheric composition ranging back to 120,000 years in Greenland and 800,000 years in Antarctica. Oceanic sediments preserve a record reaching back tens of millions of years, and older sedimentary rocks extend the record to hundreds of millions of years.
Evidence from the geological record is consistent with the physics that shows that adding large amounts of carbon dioxide to the atmosphere warms the world and may lead to higher sea levels, greatly changed patterns of rainfall, increased acidity of the oceans and decreased oxygen levels in seawater. Life on Earth has survived large climate changes in the past, but extinctions and major redistribution of species have been associated with many of them. When the human population was small and nomadic, a rise in sea level of a few metres would have had very little effect. With the current and growing global population, much of which is concentrated in coastal cities, such a rise in sea level would have a drastic effect on our complex society, especially if the climate were to change as suddenly as it has at times in the past.
Sudden climate change has occurred before. About 55 million years ago, at the end of the Paleocene, there was a sudden warming event in which temperatures rose by about 6ºC globally and by 10-20ºC at the poles. This warming event, called the Paleocene-Eocene Thermal Maximum or PETM, was accompanied by a major release of 1500 to 2000 billion tonnes or more of carbon into the ocean and atmosphere. This injection of carbon may have come mainly from the breakdown of methane hydrates beneath the deep sea floor, perhaps triggered by volcanic activity superimposed on an underlying gradual global warming trend that peaked some 50 million years ago in the early Eocene. CO2 levels were already high at the time, but the additional CO2 injected into the atmosphere and ocean made the ocean even warmer, less well oxygenated and more acidic, and was accompanied by the extinction of many species on the deep sea floor. It took the Earth’s climate around 100,000 years or more to recover, showing that a CO2 release of such magnitude may affect the Earth’s climate for that length of time.
When was CO2 last at today’s level, and what was the world like then? The most recent estimates suggest that between 5.2 and 2.6 million years ago, the carbon dioxide concentrations in the atmosphere reached between 330 and 400 ppm. During those periods, global temperatures were 2 to 3°C higher than now, and sea levels were higher than now by 10 to 25 metres, implying that global ice volume was much less than today. The Arctic Ocean may have been seasonally free of sea-ice.
Human activities have emitted over 500 billion tonnes of carbon to the atmosphere since around 1750. In the coming centuries, continued emissions of carbon could increase the total to 1500 to 2000 billion tonnes - close to the amounts added during the 55 million year warming event. The geological evidence from the 55 million year event and from earlier warming episodes suggests that such an addition is likely to raise average global temperatures by at least 5 to 6ºC, and possibly more. Recovery of the Earth’s climate in the absence of any mitigation measures could take 100,000 years or more. In the light of the geological evidence presented here it is reasonable to conclude that emitting further large amounts of CO2 into the atmosphere over time is likely to be unwise, uncomfortable though that fact may be.
I recommend everyone read the full position statement.
Posted by John Cook on Wednesday, 3 November, 2010
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