Whenever we are struck by extreme weather these days, be it floods or droughts, heat waves or bitter cold, it is often the jet stream that is blamed. So why don’t we just stop it – would that be so bad? What would happen if we could ‘turn off’ the jet stream?
First, we should lay a common confusion to rest. In our wisdom, we use two quite similar names for two very different things. The Gulf Stream lies in the ocean; it is a powerful current of warm water that flows northward out of the Caribbean and along the coast of America.
Though often thanked for keeping Europe warm in winter, it actually breaks up into swirling eddies in the mid-Atlantic, long before reaching Europe.
Here, however, we are not talking about the Gulf Stream, but the jet stream: a great river of fast-moving air which encircles the globe, high above our heads. (The easy way to remember which is which is to think of jet planes – it is the jet stream which provides a handy tailwind for planes following its path.)
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So, we are not trying to stop the waters here, but the winds instead. What would happen then? To start with, we would be spared the familiar onslaught of storms; the swirling low-pressure systems which lash the shores of Europe. Today, these are guided eastward across the Atlantic by the driving force of the jet stream.
When storm after storm hits the same weather-beaten region, this is because the jet stream has stuck there. The odd storm might still pass by, in a world without a jet, but nothing like the cyclone-highway we have today.
Less appealingly though, we would also lose our mild, south-westerly prevailing winds, for these are simply the very underbelly of the great jet stream above us. We mentioned that the warm waters of the Gulf Stream don’t really get close enough to warm European winters much.
Instead, we should largely thank the prevailing south-westerlies for bringing us warm air from further south. And in a twist of fate, these same winds in summer bring relatively cool, maritime air onto the much warmer land.
So, the jet stream helps to keep those of us in western Europe warm in winter and cool in summer. Shut it down, and we’d lose all that.
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However, what goes around, comes around, and the jet’s effects are not so benign elsewhere. The meanders of the jet over North America are such that much of Canada and the north-eastern US endure frigid winters under the grip of polar north-easterly winds. There, they might be all too happy to wave the jet goodbye.
With different nations affected so differently by the jet, making the decision to turn it off would be a tour de force of international diplomacy.
What else would be disrupted by the great jet stream switch-off? It turns out that the jet is intimately tied to the air temperatures on either side of it. The jet, running from west to east, is pinned to the north-south temperature contrast. Hence, it runs along the boundary between warm, sub-tropical air to the south, and cold, polar air to the north.
This is why shifts and meanders of the jet can have such impact; if the jet moves north of you, then you are on the warm side, if it shifts to your south, you are suddenly on the cold side.
Without a jet, then, the whole pattern of global temperatures would be different, with the air cooling much more gradually across the latitudes. One of the clearest features of Earth’s climate, the striking temperature difference between equator and poles, would be gone. This may at first sound like a very pleasant world, and would certainly make life easier for all those migrating bird species, but all is not quite as nice as it seems.
Even in this state, the sun would still be beating down on the tropics from high in the sky overhead, while just landing glancing blows on the polar regions. Somehow, Earth would have to share out the heat, by taking some from the tropics and donating it to the poles, while still obeying the laws of physics. In fact, it is this very need which sets up the conditions for the jet stream in today’s climate.
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The balances can be satisfied, without the need for a jet stream, at least not like the one we have today. But it would be a radically different climate state, such as Earth hasn’t seen since the equable climates of the Cretaceous and Eocene periods at least 50 million years ago. It would be a world with no ice caps, and greatly increased sea levels.
Hopefully, it is becoming clear that turning off the jet stream would involve such incredible changes to Earth’s climate that not even the damage we are doing to it today will take us there. For the foreseeable future, we will just have to live with the jet stream.
To see why, let’s introduce the ground-breaking lab experiments performed by Dave Fultz and colleagues in the late 1940s. Fultz made a fluid model of the atmosphere, by fitting a small glass bowl inside a larger one, and filling the gap in between with water. Then he set the whole thing spinning about its axis and gently heated it from below.
Miraculously, a miniature jet stream was set up in his watery atmosphere, meandering its way around the bowls just like our own jet. The heating had caused the water to convect, and so mix the heat around. But then, as it moved in the void between the spinning spheres, the water had to obey the physical laws of momentum, and Fultz’s jet stream was born.
Fultz’s experiments reveal the two crucial ingredients responsible for today’s jet stream: heating from the sun and the spinning of the Earth. We can’t turn off the jet stream, thankfully. As long as the sun keep shining, and the planet keeps spinning, the jet will be with us. And if either of those things stops, we’ll have other problems to think about.
Jet Stream: A Journey Through our Changing Climate by Tim Woollings is out now (£25, Oxford University Press).