Why is global warming so cold?

.”..and what is the difference between weather and climate?”

Cold weather gear by j3net, on FlickrThe difference between climate and weather is basically one of timescale. Weather is what you experience hour by hour, day by day. Climate is usually what has happened over a month, a year or even longer – perhaps a decade or a century.

For example, I live in the UK. The start of 2012 was really, really dry – although it seems hard to remember it now. We had several of the driest months on record, and a number of drought warnings and hosepipe bans. The government even appointed a Minister for Drought. Then it started to rain – if you believe in Weather Gods you may think of them having a wicked sense of humour and timing (but I think it was a coincidence). Large periods of the rest of the year gave us the wettest months on record and massive floods. It was in fact wet enough that over the year 2012 was colder and wetter than average.

As for why ‘global warming’ is so cold, the simplest answer is that it is complicated and the precise details of what you’re experiencing will depend on where you live.

But there are some relatively easy parts to understand, and I will look at the local conditions to illustrate some of the issues that make it rather unpredictable.

In the centre of the continents, air temperature and clarity of the air are main factors affecting the climate. Very large lakes have a local impact (but since lakes don’t really move around much, they have a relatively predictable effect). If you look at such places, their climate is certainly getting warmer, and they contribute to the average global temperature going up. In this example ‘climate change’ does equate to ‘global warming’ – the planet surface temperature rising.

However, a lot of people, in many continents, live pretty close to the coast. There are exceptions of course – Chicago is a huge inland city – but there are a lot of big US cities on the South, East and West Coast compared in the middle. There’s some argument about the exact numbers, but as a rule of thumb, if you’re within about 1,000km of the coast you’re probably affected by the sea, and within 250km or so, you almost certainly are. The closer you get to the sea, the greater the impact the sea makes.

The prevailing winds in the Northern hemisphere blow West to East, and so Western coastal areas get a lot more water landing on them (because moist air from the sea forms clouds). This is why Ireland tends to be wetter than the UK, and the Western part of Great Britain wetter than the Eastern part. The same is somewhat true of the US, where Portland and Vancouver are very wet, and San Francisco is cool, wet and foggy; however the East coast is often affected by storms blowing up out of the Caribbean, so they’re not as dry as you might expect.

I live in the East of the UK and we’re still close to the water. Cold though the North Sea may be, it helps keep us improbably warm in the winter. You can look on a globe if you don’t believe me – I live in York (53º57’N), which is appreciably further North than Calgary (51º 3′ North). The average winter high in Calgary is -3ºC, whereas here in York it is 6ºC – appreciably warmer. New York City, much further South at 40º 40’North is much closer to our average winter highs, although their lows are much lower, and up to 24cm of snow is much, much more common than here (we don’t actually have a recorded average snowfall). Calgary, inland, high, and a long way from the sea gets a similar amount of snow to New York in their worst month, but more over the course of the year.

Looking back at York, some of those differences are directly thanks to the water, some of them are due to a massive current that pumps nice warm water from the Caribbean across the Atlantic to hit the shores of Western Europe and keep us relatively warm. Or, so it has been for as long as we’ve been keeping good records. However, as global warming takes effect there are changes in the ocean too. Cold water coming from the melting ice caps is one reason, and this is shifting currents. There are other reasons, but they’re harder to explain clearly and quickly. For cold water though, if you put ice cubes into a coloured drink – traditional lemonade or orange juice – you can see that as the ice cube melts currents form in the drink. That’s the same principle as the melting of ice caps: if we lose the warm current that hits North West Europe, then you lose the warm, wet air that tends to come with it. When this is lost, other weather patterns exert more influence. One of the weather patterns that effects the UK is a high pressure area over Greenland and Iceland. It tends to bring cold winds from the North Pole over Britain, and when it does the UK suddenly gets long, cold spells..

So, average global temperatures go up, but thanks to the complexities of the factors that affect the weather, you may have more frequent cold spells.

And, while we’re thinking of good news about global warming, the changes seen in the UK in 2012 might be repeated in future. Warmer air tends to carry more water vapour than colder air before it falls as rain. However, it is harder to for warmer air to reach the dew point (the point at which rain starts to fall). This can mean the UK gets rain less often. But when it does rain, it will pour because there’s just more water there to fall.

Have a good summer!

Answer by Lewis Pike

Question from Chris via Twitter

Article by Lewis Pike

May 29, 2013

Lewis studied BioMedical Sciences at Liverpool John Moores University and got a PhD in Microbiology and Immunology at University of York. He now spends more time helping people understand science writing than he should and wishing his colleagues would write more clearly for the public as well as each other.

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