By Paul Homewood
Competition will be fierce, but we have an early contender for the Junk Science of the Year Award:
Colonisation of the Americas at the end of the 15th Century killed so many people, it disturbed Earth’s climate.
That’s the conclusion of scientists from University College London, UK.
The team says the disruption that followed European settlement led to a huge swathe of abandoned agricultural land being reclaimed by fast-growing trees and other vegetation.
This pulled down enough carbon dioxide (CO₂) from the atmosphere to eventually chill the planet.
It’s a cooling period often referred to in the history books as the "Little Ice Age" – a time when winters in Europe would see the Thames in London regularly freeze over.
"The Great Dying of the Indigenous Peoples of the Americas led to the abandonment of enough cleared land that the resulting terrestrial carbon uptake had a detectable impact on both atmospheric CO₂ and global surface air temperatures," Alexander Koch and colleagues write in their paper published in Quaternary Science Reviews.
What does the study show?
The team reviewed all the population data it could find on how many people were living in the Americas prior to first contact with Europeans in 1492.
It then assessed how the numbers changed in following decades as the continents were ravaged by introduced disease (smallpox, measles, etc), warfare, slavery and societal collapse.
It’s the UCL group’s estimate that 60 million people were living across the Americas at the end of the 15th Century (about 10% of the world’s total population), and that this was reduced to just five or six million within a hundred years.
The scientists calculated how much land previously cultivated by indigenous civilisations would have fallen into disuse, and what the impact would be if this ground was then repossessed by forest and savannah.
The area is in the order of 56 million hectares, close in size to a country like modern France.
This scale of regrowth is figured to have drawn down sufficient CO₂ that the concentration of the gas in the atmosphere eventually fell by 7-10ppm (that is 7-10 molecules of CO₂ in every one million molecules in the air).
"To put that in the modern context – we basically burn (fossil fuels) and produce about 3ppm per year. So, we’re talking a large amount of carbon that’s being sucked out of the atmosphere," explained co-author Prof Mark Maslin.
"There is a marked cooling around that time (1500s/1600s) which is called the Little Ice Age, and what’s interesting is that we can see natural processes giving a little bit of cooling, but actually to get the full cooling – double the natural processes – you have to have this genocide-generated drop in CO₂."
Where’s the support for the connection?
The drop in CO₂ at the time of the Great Dying is evident in the ice core records from Antarctica.
Air bubbles trapped in these frozen samples show a fall in their concentration of carbon dioxide.
The atomic composition of the gas also suggests strongly that the decline is being driven by land processes somewhere on Earth.
In addition, the UCL team says the story fits with the records of charcoal and pollen deposits in the Americas.
These show the sort of perturbation expected from a decline in the use of fire to manage land, and a big grow-back of natural vegetation.
Ed Hawkins, professor of climate science at Reading University, was not involved in the study. He commented: "Scientists understand that the so-called Little Ice Age was caused by several factors – a drop in atmospheric carbon dioxide levels, a series of large volcanic eruptions, changes in land use and a temporary decline in solar activity.
"This new study demonstrates that the drop in CO₂ is itself partly due the settlement of the Americas and resulting collapse of the indigenous population, allowing regrowth of natural vegetation. It demonstrates that human activities affected the climate well before the industrial revolution began."
For a start, the Little Ice Age did not start in the late 16thC, as the paper implies.
HH Lamb, along with many other climate historians, is quite clear that temperatures began to decline from the High Middle Ages as early as the 13thC. This cooling trend began in the Arctic, but soon spread elsewhere in the 14thC.
Where you demarcate the end of the MWP and the beginning of the LIA is of course academic. But the decline in global temperatures was an ongoing process from the 13thC to the late 17thC, when temperatures appear to have bottomed out. (Following a small amount of warming, temperatures again dropped to low levels in the mid 19thC).
Whatever caused this long term trend of declining temperatures, it certainly was not colonisation that started 300 years later!
This global cooling would be enough to explain the drop in atmospheric CO2 concentrations, without the need for any man-made involvement.
In any event, the authors reckon that this great dying reduced CO2 by a tiny 7 to 10 ppm. This, even by IPCC standards, would be fat too small to have a measurable effect on global temperatures.
According to the paper, their theory only accounts for a cooling of 0.03 to 0.08C during the 1500s and early 1600s, which would not be enough to account for the climatic changes observed during that time.
Lamb reckoned that English temperatures were 1.5C less than the early 20thC during the coldest phase in the 1690s. And the abundant evidence of massive glacial expansion throughout the world indicates that this was not just a local phenomenon.
One particular problem for their theory, which the authors don’t seem to address, is what brought about the gradual warming after 1700. Lamb identified that there was a sharp change to warmer conditions between 1700 and the 1730s, in places as varied as England, Greenland, central and northern Europe, China, California and New Zealand.
The authors fail to show how their theory explains this, and there is certainly no evidence that the forest regrowth, which they surmise took place in the 16thC, was somehow magically and suddenly reversed a century later.
The Little Ice Age, of course, poses huge problems for AGW obsessed climate scientists. Until they can explain its causes, and indeed earlier cycles of warming and cooling, they cannot explain 20thC warming.
This latest exercise looks to be just another attempt to marginalise it as a minor, man-made anomaly. This, as we know, is sheer nonsense.
We know, for instance, that ice cores in Greenland indicate that the 19thC was the coldest time since the Ice Age itself. And we also know that glaciers expanded hugely around the world during this era.
This was no mere blip, as the authors imply.