May 25, 2010 @ 6:26 pm

As a climate scientist (I reconstruct climates of the past using the fossil record; search for the terms ‘climate proxies’), I am always puzzled by comments like ‘climate change isn’t proven’. The so-called lack of ‘proof’ for climate change is a complete fallacy. Climate is always changing, and has been throughout earth history. If there is any debate, it is over the cause of that climate change.

For most of earth’s more than 4 billion years of history it has been ice-free at one or both poles. Having ice-covered north and south poles is the ‘normal’ state for the past 2.5 million years, with Antarctic ice likely for the last 40 million years. The northern polar ice-cap has oscillated in size over the last 2 million years (the glacial-interglacial cycles of the Pleistocene Epoch). That oscillation was due primarily to changes in earth’s orbit around the sun (Milankovitch cycles; google for an explanation), a process that has been going on throughout earth’s history. What is different about the last 2-3 million years, and also the last 200 years, is the level of CO2 in the atmosphere.

There are many natural causes for fluctuations in CO2 levels: volcanoes produce both ash (which acts as a temporary coolant) and CO2; marine organisms fix CO2 in their shells which fall to the ocean floor and in the right setting get turned into limestone or other carbonate rocks, ditto coral reefs. As the tectonic plates grind their way under each other at subduction zones, any carbonate rocks are melted and the resulting CO2 is vented by volcanoes. If these same limestones are uplifted on land by mountain building, erosion of those rocks also releases CO2 into the atmosphere. When large areas of peat-accumulating swamps are buried, additional carbon is trapped and forms coal. Accumulating organic matter in oceanic sediments may become oil and gas deposits. This is the carbon cycle and it also incorporates land-based plant photosynthesis and animal respiration, decay etc., but these are minor components of the carbon cycle compared to the volcano-mountain erosion marine limestone story and the formation of hydrocarbon reservoirs (see any intro geology textbook). It is the understanding of the power of the carbon cycle and the primary role played by geological processes that promotes doubt in some geologists minds that present-day climate change is caused by human-sourced CO2 in the atmosphere; they simply don’t believe that we humans have that much influence. I disagree.

Prior to about 4 million years ago (Pliocene epoch and older), CO2 by most lines of evidence sat somewhere about 400-2000 ppm (parts per million) in the atmosphere, and may have been much higher at times. 50-55 million years ago when both poles were forested, CO2 was about 600-800ppm (see Zachos et al. 2008, in Nature vol. 451, January 17th, or google ‘Eocene climate’; several scientific papers on this topic will be coming out over the next couple of months). Throughout the Pleistocene ice-ages CO2 varied in the range 120ppm (glacials) and 280ppm (interglacials). It sits now at about 389ppm, according to NOAA values (see http://www.esrl.noaa.gov/gmd/ccgg/trends/). Present-day levels of CO2 are well established as being from burning fossil fuels and other human activities; a shift from 280 to 389ppm in 150 or so years.

The question really isn’t ‘where did the CO2 come from’, so much as where did it go in the past. You see for most of earth history CO2 has been high. The last 2-3 million years have been an anomaly, with low CO2 the norm. Coal-forming, limestone forming, and similar processes appear to have stored massive amounts of CO2 over the last 50 million years. This period coincides with the appearance of grasslands (a major shift in vegetative cover, with consequences for the albedo of the earth’s surface, hydrology, and for how much carbon is trapped in soils), as well as the period when the Himalayas, Andes and Rocky Mountains were pushed up. Eroding mountains can trap CO2 when it is dissolved in rainwater (I over-simplify).

When CO2 falls below critical thresholds, its influence as a greenhouse gas is overshadowed by other processes, such as orbital shape (Milankovitch cycles). Under lower than 400ppm, the shape of the earth’s orbit became the primary influence, and ice ages resulted. So, we’re busy burning coal, oil and gas that represents carbon dioxide that was fixed by plants and algae on land (coal) and the oceans (most oil and gas) millions of years ago; this was stated by Revell & Suess in 1957, who warned then that releasing all this old CO2 would push up global temperatures. That addition of old CO2 into the atmosphere is returning the atmosphere to a CO2 level it hasn’t seen for at least 4 million years (again, what Revell & Suess said in 1957; watch out for a paper in the scientific journal ‘Geology’ in July on this topic), and ultimately to levels last seen in the Eocene, 50-55 million years ago. 4 million years ago, under CO2 levels a little higher than today, boreal forests extended as far north as northern Ellesmere Island (see Tedford & Harington, 2003 in Nature vol. 425, pp. 388-390); 45 million years ago, these same forests were subtropical. That is our future under a ‘business as usual’ carbon dioxide policy. We can’t influcene astronomical and geological processes, but we can influence how much CO2 is in the atmosphere. That is why we should act now.

All of the above is not conjecture, nor is it my personal theory or someone else’s that I have repeated. It is the current understanding backed up by many scientific papers published over the last 20 years, and especially the last 5 years. I have cited a few here.

Thank you, David, for that great explanation of the current knowledge of the paleo field. It gives us lots to chew over and think about. I am excited to read the paper in Geology this July. In 2009 a paper in Science by Honisch et al found that CO2 is higher now than in the past 2 million years…..and now that estimate is potentially doubling. Scary stuff, why are the revisions to these estimates always in the direction of things getting worse?! -Kate