Could humans have been influencing climate as long as 15 thousand years ago?

Could humans have been influencing climate as long as 15 thousand years ago?

When someone mentions "climate change" and people, most of us think about the start of the industrial revolution.  Those with a more anthropological viewpoint will think about the advent of slash-and-burn agriculture some 8 thousand years ago.  Now, some serious researchers have had some fun and examined whether the mammoth may have had an impact on climate; concomitantly, they look at the implications for the extinction of the mammoth.  [I should note that although it was trendy several years ago to talk about humans causing the extinction of the North American megafauna, there has been some recent evidence that such may not have been the case (but that is for another blog entry).]

Here is how the researchers structured the scenario.  Excavated intestines from frozen mammoths show that mammoths grazed on, among other things, small trees like birch.  So, if the mammoths disappear (for whatever reason), it is likely that deciduous trees proliferated.  The resulting darker landscape [recall that heat absorption was mentioned in a prior post on why the early earth did not freeze even though the Sun was much weaker 4 billion years ago] absorbed more sunlight, heating up the air above the Earth's surface. 

Now a little hard science comes into play.  The researchers looked at ancient records of birch pollen (pollen being one of the favorite tools of paleo researchers) in lake sediments from known mammoth breeding grounds (e.g., Siberia).  They found a spike upward coinciding with a drop in mammoth numbers roughly 15 thousand years ago.  To guestimate what influence the loss of mammoths would have had they used modern elephants as a stand-in.  The rate at which present day pachyderms eat plants and uproot trees was adjusted for presumed differences in the two species' behavior (yes, it is getting a little mushy at this point) and variation in vegetation between the tundra and African savannah.  Okay, let's face it, they made an educated guestimate. 

Conclusion?  The area of birch growth would have been 23% smaller (an average) if the mammoths were still around.  Plug all this into one of the climate models [prior posts have reviewed their variations, which is why a lot of modern predictions are built on averages derived from running a multitude of different models because they do not all come to the same exact conclusion], and the conclusion is, ta da, 0.13 C rise in Siberian climate over the course of several centuries following the mammoth's extinction (in some places the temperature increased, according to the model, by 1 C). 

Now, you are thinking, 1 C sounds so modest.  Add perspective.  The average global temperature is believed to have risen all of 0.74 C in the 20th century. 

So, what does this mean?  Well, if the model holds up (assumption), and if humans had a hand in eradicating the hirsute (okay, okay, covered with hair) herbivores [as I said, that is certainly less certain now than several years ago], then mankind's activities impacting the climate began 15 thousand years ago. 

A report on this exercise can be found at http://www.agu.org/news/press/pr_archives/2010/2010-15.shtml.