After the medieval warm period (WMP) the world experienced a distinctly chilly era between 1500 and 1750. This period is sometimes called the “Little Ice Age” though it was not a true ice age.

See: http://en.wikipedia.org/wiki/Little_Ice_Age

Well, could Christopher Columbus be to blame? According to Stanford researchers Richard Nevle and Dennis Bird the answer is a definite maybe. To be more precise, the viruses the Europeans brought with them may have caused an ice age.

Several viruses, notably smallpox and measles, depopulated the Americas. This resulted in agricultural land being abandoned and reforestation. Reforestation in turn pulled billions of tons of CO2 from the atmosphere resulting in global cooling.

See: Post-pandemic reforestation in New World helped trigger Little Ice Age, Stanford researchers say

http://news.stanford.edu/pr/2008/pr-manvleaf-010709.html

The cause of the LIA has been a bit of a mystery. One theory is sunspot activity. Another is increased volcanic activity. However the timing of these events does not fit.

On the other hand the timing of the observed drop in CO2 levels does match the timing of the observed drop in temperature.

>>Nevle and Bird don't attribute all of the cooling during the Little Ice Age to reforestation in the Americas.

"There are other causes at play," Nevle said. "But reforestation is certainly a first-order contributor.">>

CORRECTION:

I wrote:

>>Several viruses, notably smallpox and measles, depopulated the Americas. This resulted in agricultural land being abandoned and reforestation. Reforestation in turn pulled billions of tons of CO2 from the atmosphere resulting in global cooling.>>

Intended to write:

>>Several viruses, notably smallpox and measles, depopulated the Americas. This resulted in agricultural land being abandoned. THE TREES RECLAIMED THE ABANDONED LAND RESULTING IN REFORESTATION. Reforestation in turn pulled billions of tons of CO2 from the atmosphere resulting in global cooling.>>

Apologies.

It's called evolution !

I like that theory, Steven, however, was there really such deforestation in the pre-Columbian Americas? There really weren't that many people living there and unlike the Australian Aborigines, I don't think they practised firestick cultivation.

Moreover tropical rainforests are not easy to keep down. The deforestation of the Amazon has taken industrial methods to accomplish. I PNG slash and burn farming areas are typically recovered to be almost invisible within a few years. Would the rainforest have noticed if even quite large amounts of firewood were being taken to support communities?

Additionally, industrial man has deforsted far more significantly than pre-columban Americans could have ever dreamt of doing. Why has there been no sudden jump in temperatures since 1500?

It strikes me as a bit of wishful thinking, to be honest: an hypothesis looking for support.

Well it is certainly just as reasonable to blame him, as it is to blame an innocent little gas.

Antiseptic there is a report by a chaplain with the conquistadors, of cities of millions of people along much of the Amazon. I was recently reading of some archaeologists finding evidence of vast settlements, & agricultural works.

Apparently a band of Spanish became separated from their main group, & traveled down the Amazon by long boat, with a journal describing these cities.

Hi Antiseptic

The article I linked from the Stanford University website explains Nevle’s and Bird’s research.

Quote

>>Nevle and Bird synthesized published data from charcoal records from 15 sediment cores extracted from lakes, soil samples from 17 population centers and 18 sites from the surrounding areas in Central and South America. They examined samples dating back 5,000 years.

What they found was a record of slowly increasing charcoal deposits, indicating increasing burning of forestland to convert it to cropland, as agricultural practices spread among the human population—until around 500 years ago: At that point, there was a precipitous drop in the amount of charcoal in the samples, coinciding with the precipitous drop in the human population in the Americas.>>

So it seems the first Americans did clear-burn forest land for AGRICULTURAL (not firewood) purposes and there is some solid basis for Nevle’s and Bird’s hypothesis.

Here's a link to the piece on the Stanford website again:

http://news.stanford.edu/pr/2008/pr-manvleaf-010709.html

In answer to your second question, we have been on a warming trend since 1750 which has coincided with rising atmospheric carbon dioxide and methane levels.

Incidentally, you may be interested in one VERY controversial theory by Prof. William Ruddiman.

See: http://cires.colorado.edu/events/lectures/ruddiman/

See also Scientific American: How Did Humans First Alter Global Climate?; March 2005; Scientific American Magazine; by William F. Ruddiman;

http://www.sciamdigital.com/index.cfm?fa=Products.ViewIssuePreview&ARTICLEID_CHAR=F9374686-2B35-221B-635B1D2A02A8B6D5
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Ruddiman’s theory does not have much support. However it has survived all attempts by its critics to falsify it.

Of course the fact that a theory cannot be falsified does not make it true.

I am agnostic on Ruddiman’s theory. In one way it makes sense. But the supporting evidence is sketchy and for practical purposes it does not much matter whether anthropogenic warming stared 8,000 years ago or 250 years ago.