Weather cycles and climate trends

It’s still February, and 2014 has continued the run of extraordinary weather events that have struck over the past few years. Britain has had the wettest winter on record, with extensive flooding. Why?

The MET Office tells us that Britain’s wet weather and the American cold snap freak are linked, both due to “a persistent pattern of perturbations to the upper tropospheric jet stream”. Something called Rossby Waves have pushed the jet stream around and are apparently “fundamental to understanding this winter’s weather”. I don’t understand them, so that scuppers my chances of explaining anything to you.

Furthermore, it’s a La Nina year, although being in a negative phase of the Pacific Decadal Oscillation means La Nina isn’t particularly pronounced. The North Atlantic Oscillation is positive, meanwhile, which has accelerated something called the ‘polar night jet’. The “polar vortex has been stronger than normal” too, as you’d expect. There’s also an 11-year solar cycle to consider.

You’d also want to factor in the activity of the Quasi-Biennial Oscillation, a two-year tropical weather cycle. It doesn’t normally affect us, unless it’s westerly. And guess what, it’s been in an “unusually strong westerly phase throughout this winter.”

“In conclusion,” say the MET Office, “the evidence suggests that the Pacific Ocean has been a major driver of this winter’s severe weather, whilst the strong polar vortex and its influence on the Arctic Oscillation, potentially influenced by the unusually strong westerly phase of the QBO, has also been an important contributor to the very strong North Atlantic jet stream.”

So there you have it. Tell your friends.

In case you hadn’t guessed already, I didn’t mention any of that to actually attempt to explain the weather. What I really wanted to highlight is the complex interactions of natural cycles involved.

When people say that climate change is natural, and that the climate has changed all through history, this is not news to earth scientists. They’re smart people. Neither is it news that there are warm years and cold years, or that some parts of the world are warming more than others. This complex interplay of natural cycles means that warming is never going to be a linear process.

Big ‘C’ global climate change, the kind caused by human activity, takes places within the context of all of these other cycles and oscillations. They impact each other, with the various natural rhythms either accelerating or mitigating the underlying warming effect of rising greenhouse gases. Working the other way, scientists are trying to piece together how climate change will affect all of these climate and weather patterns.

In this particular incidence, the sea level around the British coast has risen by 12cm in the last 100 years, making floods more likely. That’s one way that climate change has contributed to this year’s storms. Another is that while the number of storms hasn’t increased, their intensity has. The chances of a heavy deluge have increased. “What in the 1960s and 1970s might have been a 1 in 125 day event is now more likely to be a 1 in 85 day event.”

As always, it’s impossible to directly attribute weather events to climate change. It’s just that the long term warming of the planet makes certain things more likely. For those in coastal towns, on tidal rivers like the Thames or low lying places like the Somerset Levels, you could think of it as a game of Russian roulette. The difference between 1 degree of warming or 2 is the number of bullets in the gun.