Thanks for the lift!

Winters in Ireland had been becoming progressively milder with no "sticking" snow some winters - until the last two - which were exceptionally cold and long lasting.  But we have never seen snow like this in late November early December before.  As I write this, (a week after landing) it is still snowing heavily and the only means of getting anywhere near a main road is on foot or by tractor.

I love snow, find it extraordinarily pretty and peaceful, and enjoy the sense of isolation it can convey.  I can always blog, listen to the radio, and have even started to switch on the TV again occasionally, having previously almost given up on it as a source of news or entertainment.  Most of the (voluntary) work I do can be done by internet/e-mail/phone although the list of missed meetings is starting to grow.  So I am not one to belly-ache about the disruption caused by snow although I appreciate the problems experienced by those who must travel for work/school or to acquire the basic necessities for life.  A three mile walk through snow/ice is not much of a hardship if you are fit and well but almost impossible if you are not.

However I am really appreciative of the home insulation project I undertook last spring which has meant that a previously inadequate (carbon based) home heating system is keeping the house comfortable and I can now look forward to some reduction in my carbon footprint and cost base as well. Unfortunately neither a thermal solar panel or domestic windmill would have been of much benefit during the current (calm and cloudy) cold spell with the roof covered by snow and under soil based heat pumps are still prohibitively expensive to retrofit.  So the search for an even moderately cost effective and more carbon neutral heat source goes on.

Having more time on your hands than usual also gets you thinking about whether there is any longer term significance in this change in weather patterns. Yes, I know all the usual caveats about climate change science being about long term trends and that you cannot claim statistically significant correlations between short term weather patterns and longer term climate trends.  There have been ice ages and mini-ice ages caused by reduced sun spot activity and all sorts of other terrestrial factors long before any anthropogenic climate changes become an issue.

But the particular phenomenon that has tickled my interest of late is the possibility that Global warming and the melting of polar ice caps may actually result in a cooling of Ireland and western Europe through a weakening of the North Atlantic Drift (NAD) which brings warm waters and winds to Ireland and western Europe and results in us having a much milder climate than other regions at a similar latitudes such a Newfoundland.

The NAD is driven by the natural circulation of ocean waters caused by prevailing westerly winds, the Coriolis force, temperature and salinity differences, and tides caused by the gravitational pull of the Moon and the Sun.  It is the salinity differences engine behind the NAD - a phenomenon known as Thermohaline circulation, which is said to be under threat from global warming.

Thermohaline circulation - Wikipedia, the free encyclopedia

The term thermohaline circulation (THC) refers to the part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents (such as the Gulf Stream) head polewards from the equatorial Atlantic Ocean, cooling all the while and eventually sinking at high latitudes (forming North Atlantic Deep Water). This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters (with a transit time of around 1600 years) upwell in the North Pacific (Primeau, 2005). Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's ocean a global system. On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.

So what is the possible impact of global warming on the thermohaline component of the engines which drive the NAD?

Shutdown of thermohaline circulation - Wikipedia, the free encyclopedia

There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localised cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. This would particularly affect the areas of the British Isles and the Nordic countries that are warmed by the North Atlantic drift.

Basically, the theory postulates that increased freshwater released by melting polar icecaps will reduce the density of polar waters, preventing them from sinking to the deep ocean basins, and thus interfering with one of the crucial components of the engines driving the global oceanic circulatory system. If polar waters don't sink to the ocean floor, they will effectively block or slow down surface waters flowing north as part of the NAD. This is believed to have happened before, during previous ice-ages - particularly in Europe:

Shutdown of thermohaline circulation - Wikipedia, the free encyclopedia

Some even fear that global warming may be able to trigger the type of abrupt massive temperature shifts which occurred during the last glacial period: a series of Dansgaard-Oeschger events - rapid climate fluctuations - may be attributed to freshwater forcing at high latitude interrupting the THC. The Younger Dryas event may have been of this sort, too. (See the discussion of chaos theory for related ideas.) However, these events are believed to have been triggered by massive freshwater discharges from the Laurentide ice sheet, rather than from the melting of polar sea-ice and precipitation changes associated with the increased open water in global warming.

So is there any evidence that this might be happening now?

Shutdown of thermohaline circulation - Wikipedia, the free encyclopedia

In April 2004, the hypothesis that the Gulf Stream is switching off received a boost when a retrospective analysis of U.S. satellite data seemed to show a slowing of the North Atlantic Gyre, the northern swirl of the Gulf Stream.^[7]

In May 2005, Peter Wadhams reported to The Times about the results of investigations in a submarine under the Arctic ice sheet measuring the giant chimneys of cold dense water, in which the cold dense water normally sinks down to the sea bed and is replaced by warm water, forming one of the engines of the North Atlantic Drift. He and his team found the chimneys to have virtually disappeared. Normally there are seven to twelve giant columns, but Wadhams found only two giant columns, both extremely weak.^[8][9]

----snip

 The NewScientist.com news service^[13] reported on 30 November 2005 that the National Oceanography Centre in the UK found a 30% reduction in the warm currents that carry water north from the Gulf Stream from the last such measurement in 1992.

----snip

Detlef Quadfasel reinforces the point that the uncertainty of the estimates of Bryden et al. is high, but says other factors and observations do support their results. Quadfasel continues by pointing out the significance of the possible implications, with palaeoclimate records showing drops of air temperature up to 10 °C within decades, linked to abrupt switches of ocean circulation when a certain threshold is reached.

There doesn't appear to be enough conclusive evidence at the moment to generate a scientific consensus, and in any case, the global impact of global warming may mask or overwhelm any local cooling caused by a weakening of the NAD. [Update: see last paragraph.]

However there are two points to note:

1. Paleoclimatic records indicate that quite dramatic changes in temperature have occurred wery rapidly in the past. A 10 °C change in average temperatures in north western Europe could trigger a mini-ice age, and these changes have occurred within decades in the past.

2. The scope of global climate change could be much greater than the changes which resulted in freshwater discharges in the past resulting in an increased frequency of extreme weather events and a much greater variability in the European climate.

Climate change is thus not a very gradual process of increasing average global temperatures, but could result in very dramatic and rapid fluctuations in particular regions of the globe.  It appears from the paleoclimatic records that the European climate is particularly unstable and prone to such rapid fluctuations. By the time sufficient evidence accumulates to demonstrate that a localised cooling event is happening in Western Europe, we could be well on the road to another ice age.

[Update] More recent evidence - highlighted by Nomad in the comments below - indicates that any previously observed variability in the NAD and deep water currents is within the natural and ongoing variability observed for those currents, and there is as yet no evidence of any long term trends. Indeed it could be as late as 2030-2050 before we have sufficient trend data to draw such conclusions in any case. The current scientific consensus is therefore that the NAD is not weakening, and also that, even if it did, it would not, of itself, be sufficient to cause another mini-ice age.

Phieu! That's a relief! And just to prove it, the snow has begun to melt...:-)