I’m amazed that each winter I hear this, “It’s cold, so global warming must not be real”. Do I have to remind people that generally speaking it is colder during the winter months than it is during the summer? And that cold spells do happen, regardless of any current climate trend? But I can understand the confusion a big cold spell like this can cause.

A lot of the confusion comes from the fact that most people think that Global Warming means that it gets warmer every single year, for any given month in a year and at every single location on the globe. Unfortunately it’s not that simple.

For example if you take a look at the global temperature records in the past you can see it’s not a smooth graph with every year being warmer than the previous. And you see exactly the same pattern for local temperature records. So if a few subsequent years haven’t been warmer, or you get a couple of frigid winters in a row, it does not mean that global warming has stopped. Or even isn’t happening.

So what exactly is happening now. With all the snow storms and cold we are currently getting in Europe?

One of the weather systems that determines how warm or cold a winter will be in Europe is the NAO (North Atlantic Oscillation). It has two states, a positive and a negative. And each of these states cause very different weather.

The positive phase happens when air pressure is low over Iceland, but high down south over the Azores off West Africa. The pressure difference this has as a result will cause strong westerly winds, which whip up storms and sometimes cause floods. Essentially the engine behind the strong storms that can hit Europe from the west during winter.

But during a negative phase the pressure difference is low, weather systems slow down and get blocked by what meteorologists call ‘blocking highs’. These are big zones of high pressure systems that spread down from Greenland in winter and stick around for weeks at a time. And these pressure systems bring cold air from the arctic straight to Europe and bring warm air into the arctic region.

Just like during the previous winter this is exactly what is happening now. The NAO is again in a negative phase bringing very cold weather to Europe. And if you take a look at temperature maps of the northern hemisphere you can see the tell tale signs of a negative NAO. Temperatures of up to 15 degrees centigrade higher than normal in Greenland and parts of Canada to the west. In those regions people are actually wondering when winter will start.

Also the very same phenomenon that makes it so cold in Europe and warm in the arctic is also responsible for the cold in the U.S. and large parts in Asia.

And despite the current cold weather in certain areas of the northern hemisphere there is a good chance that 2010 as a whole will be the warmest year on record. In the very least it will be a tie with 2005 for the warmest year 2010 has actually tied with 2005 for the warmest year on record.

There is however a second part to this whole story about the cold weather we’ve been having lately. A lot of the skeptics mock the idea that this cold weather and snow might be caused by global warming. It’s not as silly as you might think it is at first glance.

The first thing you need to realise is that global warming causes the oceans and air to warm. Warm water evaporates more easily and warm air can hold more moisture. This subsequently means that when there is precipitation there’s more of it in a shorter time frame. And you can image what the effects are when this air hits a cold region. As cold air cannot hold as much moisture, it drops it as bucket loads of snow.

This is also indicated by a known statistic among researchers, which is that we get more snow in warm years. This is completely counter-intuitive to what you would expect, but the data does indicate this trend.

However in recent years there have been publications that are pointing in the direction that the melting of sea ice in the arctic might be the cause for the recent cold winters. The proposed mechanism for this goes as follows:

Sea ice in the Arctic has two main effects on the weather. It acts as a reflector for incoming heat from the sun preventing it from warming the ocean. It also creates a barrier between the warmer water and the atmosphere, reducing the amount of heat that escapes from the sea into the air.

During the autumns of 2009 and 2010, the coverage of Arctic sea ice was much lower than the long-term average. The open sea, being darker, subsequently absorbed more heat from the sun in the warmer months. With the ocean remaining ice free for longer than usual the air in the arctic could absorb more of the heat in the ocean. Causing higher air pressures which have as a result a reduced pressure gradient between the Iceland Low and the Azores High. In essence creating a negative NAO.

The last word on this hasn’t been spoken yet in the scientific literature. As years with similarly low arctic sea ice extend didn’t produce these cold winters. There’s still a lot we need to research before we get a better understanding of how the reduced sea ice extend is influencing the climate. And as such it’s long term consequences are not yet fully understood. But it does show that global warming isn’t as straightforward as many people believe it is. And the current coverage of this in the media is just because it makes a nice story.

But the global temperature trend is clear when you look at the data, for example the last decade was the hottest on record. And for Europe 7 of the last 10 winters, and 10 of the last 10 summers, were warmer than you would expect from the climatology.

Looking out of a window is not a good measure of what is actually happening to the worlds climate systems. Just like an early and warm spring does not indicate a warming trend, a single cold spell does not indicate a cooling trend.