Climate Change began with Civilization

Bill Ruddiman and the early
anthropogenic hypothesis posits that human influence on global climate began not with the
industrial revolution a couple of hundred years ago but rather with the advent of
agriculture thousands of years ago Even though we know global populations
were very small at those times, we also know, increasingly so, that humans had
a significant impact on clearing the land. Probably fewer people per capita had
a larger impact on land clearance than they do today with more
efficient agricultural practices. We believe that over time,
hundreds, thousands of years, that was a big enough effect
to artificially increase the amount of methane and carbon
dioxide in the atmosphere and to actually reverse the natural trend
for the last several thousands of years which we would have expected to be downward actually
changed several thousand years ago and became upward. Even though that was long before
the advent of industrialization we think that those early farming practices had a significant
impact on global climate, artificially warming it. The strongest support for the hypothesis
I think in terms of the data is that if you look at the past many warm
interglacial periods like we’re in now what always happens is that the concentrations of
carbon dioxide and methane from natural processes is highest early in the interglacial
and then it drops over time. Right now we’re in one of those dropping periods,
when solar radiation during summer is waning. What should be happening, naturally, is that we would have
lower and lower levels of carbon dioxide and methane, which would produce more and more cooling
potentially beginning ice sheets. Instead, what we’ve found, not just in the last couple
hundred years with industrialization, but even before then, is that that natural trend changed
and it actually went in the opposite direction. The big question is why, and the only obvious
difference between the current interglacial and all these six or seven other recent
interglacials is humans and agriculture. People who argue that this
is just a natural process, that carbon dioxide and methane levels dropped
initially and then five or six thousand years ago they reversed direction
and went upward. The problem is, why is this interglacial
the only one naturally when that happened. So our hypothesis is that, therefore, it isn’t natural, it
actually is because of humans affecting the atmosphere. Basically, the deforestation produces
a lot of carbon in the atmosphere. The other big effect with methane
was rice paddy cultivation which began in SE Asia about six
or seven thousand years ago. There’s good, new, archeological evidence that shows a
rapid increase in the expansion of rice agriculture. That certainly should have increased the
amount of methane released to the atmosphere. Colleagues who have been working on this show that
right at the time, five or six thousand years ago, there was a big uptick in this
type of agricultural practice. That certainly should have increased the amount of
methane released artificially into the atmosphere. That coincides very nicely with the ice core records that
show that’s when global concentrations of methane did go up even though they had been going down
for a few thousand years before that.

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