DECEMBER 4th 2005


Alarm over dramatic weakening of Gulf Stream,3605,1654803,00.html?gusrc=rss

. Slowing of current by a third in 12 years could bring more extreme weather
. Temperatures in Britain likely to drop by one degree in next decade

Ian Sample, science correspondent
Thursday December 1, 2005
The Guardian

Yet another scare!

This is what the real scientists say


Geophysical Research Letters 32, no.12 (28 JUN 2005) p. 1-5

A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO2 concentration.

J. M. Gregory 1,2, K.W. Dixon 3, R. J. Stouffer 3, A. J.Weaver 4, E. Driesschaert 5,
M. Eby 4, T. Fichefet 5, H. Hasumi 6, A. Hu 7, J. H. Jungclaus 8, I.V. Kamenkovich 9,
A. Levermann 10, M. Montoya 11, S. Murakami 12, S. Nawrath 10, A. Oka 6, A. P. Sokolov 13,
R. B. Thorpe 2

Abstract: As part of the Coupled Model Intercomparison Project, integrations with a common design have been undertaken with eleven different climate models to compare the response of the Atlantic thermohaline circulation (THC) to time-dependent climate change caused by increasing atmospheric CO2 concentration. Over 140 years, during which the CO2 concentration quadruples, the circulation strength declines gradually in all models, by between 10 and 50%. No model shows a rapid or complete collapse, despite the fairly rapid increase and high final concentration of CO2. The models having the strongest overturning in the control climate tend to show the largest THC reductions. In all models, the THC weakening is caused more by changes in surface heat flux than by changes in surface water flux. No model shows a cooling anywhere, because the greenhouse warming is dominant.


J.M. Gregory, Centre for Global Atmospheric Modelling, Department of Meteorology, University
of Reading, PO Box 243, Reading RG6 6BB United Kingdom


Geoscience Canada, June, 2004

by Andrew J. Weaver and Claude Hillaire-Marcel


The recent IPCC (2001) assessment stated that

"Most models show weakening of the Northern Hemisphere Thermohaline Circulation (THC),
which contributes to a reduction of surface warming in the northern North Atlantic.
Even in models where the THC weakens, there is still a warming over Europe due to
increased greenhouse gases."

However, there is still a widespread misunderstanding of the possible consequence of climate change on the Atlantic Ocean Meridional Overturning. In particular, it is often touted, especially in the media, that a possible consequence of anthropogenic greenhouse gas emissions is: "the onset of the next ice age". Here we document the history of this misconception and quantitatively show how it is impossible for an ice age to ensue as a consequence of global warming. Through analysis of the paleoclimate record as well as a number of climate model simulations, we also suggest that it is very unlikely that the Atlantic Meridional Overturning will cease to be active in the near future. We further suggest that a region where intermediate water formation may shut down is in the Labrador Sea, although this has more minor consequences for climate than if deep water formation in the Nordic Seas were to cease.



While much has been made in the media of global warming potentially leading to the onset of the next ice age, we believe we have shown that this is simply not possible. A more relevant question is what will happen to the AMO during the course of the next century. Some models assessed in IPCC (2001) find no reduction of the AMO during the 21st century while others find a slight reduction in its strength. Such a reduction leads to a negative feedback to anthropogenic warming in and around the North Atlantic. That is, through reducing the transport of heat from low to high latitudes, SSTs are cooler than they would otherwise be if the AMO was left unchanged. As such, warming is reduced over and downstream of the North Atlantic. It is important to note that in all models where the AMO weakens, warming still occurs downstream over Europe. While admittedly model-dependent, even in the case where we added an additional external freshwater perturbation to the North Atlantic for 500 years, whose magnitude was chosen to account for the upper estimate of the observed rate of global sea level rise this past century, we still do not get a cessation of the AMO and cooling down stream over Europe.

Worth mentioning here is the fact that the most sophisticated non flux-adjusted model of Wood et al. (1999) suggests that the freshening of North Atlantic surface waters presently observed (Curry et al., 2003) could be associated with an increasing AMO (Wu et al., 2004). This same model, praised by Rahmstorf (1999) as giving "for the first time a realistic simulation of the large scale ocean currents", also suggests that eventually it is only Labrador Sea Water formation that is susceptible to a collapse as it did during the most recent warmer episodes of the Earth Climate history (Hillaire-Marcel et al., 2001).

Climate change is offering decision makers and society as a whole many important challenges that need to be assessed and addressed, including the possibility of a reduction in the strength of the AMO or the very remote possibility of its cessation this century. One thing that they need not concern themselves with is Global Warming causing the onset of the next ice age. Unfortunately, such a conclusion is far less newsworthy than the one warning of the occurrence of an impending ice agc.


Nature 428, 601, April 8, 2004

Sir - Your News story "Gulf Stream probed for early warnings of system failure"
(Nature 427, 769 (2004)) discusses what the climate in the south of England would
be like "without the Gulf Stream." Sadly, this phrase has been seen far too often,
usually in newspapers concerned with the unlikely possibility of a new iceage in
Britain triggered by the loss of the Gulf Stream.

European readers should be reassured that the Gulf Stream's existence is a consequence
of the large-scale wind system over the North Atlantic Ocean, and of the nature of
fluid motion on a rotating planet. The only way to produce an ocean circulation
without a Gulf Stream is either to turn off the wind system, or tostop the Earth's
rotation, or both.

Real questions exist about conceivable changes in the ocean circulation and its climate
consequences. However, such discussions are not helped by hyperbole and alarmism. The
occurrence of a climate state without the Gulf Stream anytime soon - within tens of
millions of years - has a probability of little more than zero.

Carl Wunsch
Earth, Atmospheric and Planetary Sciences,
Massachusetts Institute of Technology

That is probably enough to be going on with,

Vincent Gray
75 Silverstream Road
Crofton Downs
Wellington 6004
New Zealand
Phone/Fax 064 4 9735939
"It's not the things you don't know that fool you.
It's the things you do know that aint so"
Josh Billings