Natural and anthropogenic climate change: Why paleontology matters
When I was an undergraduate, I became intimately familiar with a tiny fossil called Cycladophora davisiana. This organism is a radiolarian, one of a group of single-celled organisms that make their gorgeous shells out of silica. What makes Cycladophora davisiana special is that it likes cold water; in the far Northern Pacific it can make up almost half of the radiolarians. My job was to take a sample of sediment from a core taken in the Indian Ocean, prepare a slide covered with radiolarians, and count how many were Cycladophora davisiana. What stays with me to this day is that as I went farther down in the core and thus further back in time, the number of specimens of the species increased and rapidly reached a peak. I was seeing first hand, from a tiny fossil, direct evidence for the cold oceans of the last ice age.
One of the recurrent themes of the climate change denialists is that we can’t blame humans, since “climate is always changing!” My general response to this is a shrug and “aren’t we the ones who told you that?” The idea that climates of the past have been different than that of today is obvious to any paleontologist or other student of Earth’s past. Data from the fossil record is what first gave scientists an appreciation of changing global climates and remains essential for unraveling the causes, patterns, and consequences of these shifts. It is our appreciation of these changes that makes us so worried; we know the historical context of our modern climate and that it is increasingly anomalous.
Some of these shifts are major. Today’s reefs are found in warm, shallow, tropical waters. And yet this past weekend I showed a class a huge coral reef in a quarry just south of Chicago. The message from these fossils is that climate in Chicago 425 million years ago was remarkably different from the unseasonably cold spring of this year.
Not too distant from the ancient reef are the remains of trees and other plants that lived in the area during the Pennsylvanian, some 310 million years ago. Their buried remains, here and elsewhere in the world, form the huge coal deposits that characterize this time period. The signal from these fossils is that the Chicago area was once much warmer and wetter. Ironically, so much carbon dioxide was tied up in buried ancient forests that global temperatures dropped, triggering an ice age in the Southern Hemisphere.
The Chicago area also has deposits left behind after the great ice sheets last retreated from this region, beginning some 14,000 years ago. In these deposits are the pollen of plants that today are found only far to the north. So even after the glaciers left, it was still cold here, even colder than Chicago’s current notorious winters. Using the fossils, we can see how modern conditions developed. Again, we see direct evidence of climate change, with the details given to us by fossils.
Chicago’s fossil record of climate change is of course limited in area and very incomplete in time. When we combine it with information from around the globe, paleontologists and their colleagues can produce an incredibly detailed and thorough historical record of global environmental change over time. Some of this information comes directly from knowledge of the biology of the organisms and their relationship to climate. For example, the margins of leaves in tropical areas tend to be smooth, whereas those in colder areas are jagged. The proportion of the two types of leaves is an amazingly accurate thermometer for ancient temperatures. We can also look at the stomata (pores) of leaves to estimate the amount of carbon dioxide in the atmosphere.
Similarly, as was the case with Cycladophora davisiana, the relative proportions of the shells of different species of marine single-celled organisms can yield a temperature record for ocean waters over millions of years. One of the most important sources of climate information comes from analyzing the chemistry of those shells. The relative amounts of different isotopes of oxygen yield detailed insights into the temperature history of the Earth. We now know that there were not four advances and retreats of the continental ice sheets, as was long thought, but twenty in the last two million years. The timing of these glaciations gave the critical clue that the major control on the ice ages are variations in the Earth’s orbit. We also know that glacial periods begin slowly but end abruptly. Based on this history, we should be starting the long slow slide into another glacial period over the next tens of thousands of years.
The lesson of the fossil record of climate is that it can indeed change over many time scales. Climate from time periods such as the Silurian and the Pennsylvanian demonstrate the close tie between atmospheric carbon dioxide and global temperature. But they are otherwise poor analogs for today; the Silurian lacked forests and had extensive shallow seas, the Pennsylvanian had far larger continental areas and no flowering plants.
The really relevant time is the last two million years, the period during which humans evolved and developed civilization. It is in this context that that truly anomalous features of today’s climate stand out. Multiple lines of evidence, including many based on fossils, tell us that carbon dioxide levels in the atmosphere have been between 200 and 300 parts per million (ppm) for this entire interval, with carbon dioxide levels and temperature rising and falling in step. Last year, the value rose to above 400 ppm. We have to go back at least 4 million years, well before our species evolved, to find similar levels. It is predicted that these values might rise as high as 600–800 ppm by the end of the century, levels not seen for 25 million years, when apes first appeared, and the world was much warmer than today.
What will the world be like with carbon dioxide levels twice of today? We simply don’t know; it will strongly depend on how fast we get there. We do understand that it will be hotter. But if you want at least a clue to its impact, ask a paleontologist. We’ve been there.