Monday, February 3, 2014

Another hypothesis for the origin of the Little Ice Age

I just stumbled upon another hypothesis to explain the cooling - global or regional - experienced around 1700 A.D. I found it surprising, suggesting not only an anthropogenic cause for the LIA, but a Spanish cause.  As the linked  text in Abandoned footnotes  is very well explained, I leave it to the readers  to comment with any further ado.


Karl Kuhn said...

The hypothesis that a once much more numerous population in the New World kept substantial amounts of carbon dioxide in the atmosphere, thus warming the planet during the MWP, and that this was stoppped by smallpox etc, is a bold speculation. I do not believe that could add up. Such huge numbers of people, organized in civilizations, leave sufficient archeological traces ... which do not seem to be there.

hvw said...

The (main?) research putting forward this hypothesis is here:
Kaplan et al., 2010

This is considered and put into context in WG1-AR5, Chapter 6.

This idea is fascinating. Why are ideas fascinating that propose how low-tech humans, by going about their little human occupations, inadvertently but significantly impact global scale environmental systems?

eduardo said...

Let us say that the hypothesis, though indeed fascinating, is far from being proven.

I cannot say much on the dynamics of carbon cycle, but my feeling is that a drop of say 5 to 10 ppm in CO2 concentrations would not be sufficient to cause any detectable drop in Northern Hemisphere or global temperatures

From the IPCC Report WG1:

One of the possible explanations for the drop in atmospheric CO2 around
year 1600 is enhanced land and/or ocean carbon uptake in response
to the cooling caused by reduced solar irradiance during the Maunder
Minimum (Section However, simulations using Earth System
Models of Intermediate Complexity (EMICs)(Gerber et al., 2003; Brovkin
et al., 2004) and by complex Earth System Models (ESMs) (Jungclaus
et al., 2010) suggest that solar irradiance forcing alone is not sufficient to explain the magnitude of the CO2 decrease. The drop in atmospheric
CO2 around year 1600 could also be caused by a cooling from increased
volcanic eruptions (Jones and Cox, 2001; Brovkin et al., 2010; Frölicher
et al., 2011). A third hypothesis calls for a link between CO2 and epidem-
ics and wars associated with forest regrowth over abandoned lands and
increased carbon storage, especially in Central America. Here, results are
model and scenario dependent. Simulations by Pongratz et al. (2011a)
do not reproduce a decrease in CO2, while simulations by Kaplan et al.
(2011) suggest a considerable increase in land carbon storage around
year 1600. The temporal resolution of Central American charcoal and
pollen records is insufficient to support or falsify these model results
(e.g., Nevle and Bird, 2008; Marlon et al., 2008).

Karl Kuhn said...

I can't help but feeling that the conclusions of the paper drip of unjustified hyperbole:

"Carbon emissions as a result of anthropogenic land use over the preindustrial Holocene could have had a very substantial impact on the global carbon cycle."

What is meant by "very substantial"? The next sentence does not enlighten me:

"Even before 1000 bc (3 ka), up to 102 Pg of carbon could have been emitted into the atmosphere caused by human land use."

Why is that value not put into any perspective, into relative terms? In the next sentence they row back a bit:

"This amount of carbon could have contributed to, but cannot fully explain, the increase in atmospheric CO2 that occurred over the middle Holocene."

So to what extent could it explain the temperature rise. I mean, there are estimates of TCR out there ...

"By ad 1850, this amount increases to roughly 360 Pg, which equates to a ~25 ppm increase in atmospheric CO2."

Aha ... 25 ppm is merely a tenth of the much-feared doubling-of-CO2-content.

"[...] As a result, the world is much warmer than it would otherwise be."

The world is or has been much warmer? By how much? As a reviewer, I would not have approved such imprecisions and weaseling.

Karl Kuhn said...

I was commenting on the Kaplan (2011) paper, sorry ...