From a Usenet thread. …
A weather fan from the usenet group: uk.sci.weather wrote:
Originally posted by Les Hemmings:
Europe basked in unusually warm weather in medieval times – so much so that
wine was produced in England – but why has been open to debate. Now the
natural climate mechanism that caused the mild spell seems to have been
The finding is significant today because, according to Valerie Trouet at the
Swiss Federal Institute for Forest, Snow, and Landscape Research in
Birmensdorf, the mechanism is not now of sufficient strength to explain
The finding scuppers one of the favourite arguments of climate-change
deniers. If Europe had temperature increases before we started emitting
large amounts of greenhouse gases, the theory goes, then maybe the current
global warming isn't caused by humans, either.
To work out what the global climate was doing 1000 years ago during the
so-called "Medieval Warm Period", Trouet and colleagues started by looking
at the annual growth rings of Moroccan Atlas cedar trees and of a stalagmite
that grew in a Scottish cave beneath a peat bog. This revealed how dry or
wet it has been in those regions over the last 1000 years.
The weather in Scotland is highly influenced by a semi-permanent pressure
system called the Icelandic Low, and that in Morocco by another called the
Azores High. "So by combining our data, which showed a very wet medieval
Scotland and very dry Morocco, we could work out how big the pressure
difference between those areas was during that time," says Trouet.
Originally posted by Alastair McDonald:
"According to Trouet, a Pacific La Niña mode and a positive NAO mode could have reinforced each other in a positive feedback loop and this could explain the stability of the medieval climate anomaly."
Positive feedback loops are inherently unstable. It is negative feedback loops which are stable.
But even allowing for that being a slip by Trouet or the reporter, when they meant a negative feedback loop, there is still another fundamental error in their reasoning. If the MWP was caused by a change in the NAO or ENSO, what caused that? If it was not an increase in CO2 then the fallacious argument of the deniers still stands. The last max in global temperatures occurred during the 1998 El Nino.
The normal mode for the North Atlantic is positive where Lows are low and Highs are high and the weather is given to alternating periods of wet and dry whos duration depends on the times of the phases, how closely they match each other and how they interact.
A series of closely matched phases is broken by a spate of weather that is out of sequence with the time of the phase because of a severe tropical storm or because of an impending earthquake.
Something I'd consider impossible to decipher with statistics. But then a few decades back I'd have thought what I now consider fact (above) to be impossible.
Two years ago I'd have said quantifying the power in a convergence of low pressure systems and comparing them to the power of severe earthquake trains would also be impossible.
It turns out this latter isn't even difficult.
The severity of a Mag 7 earthquake for example, is not the severity of two Mag 3.5 earthquakes because of the amplifaction effect of a train of waves meeting to produce such large magnitudes.
A large quake is no more than the convergence of several acoustic waves of moderate and even of no severity.
The earth is a sump of sounds and they scatter in all directions. Sometimes they overlap, that's all there is to earthquakes.