The Ice Age Now
By the standards of the geological past, we live in an Ice Age. The world has rarely been as cold as it is today. We don’t call it an Ice Age, because not too long ago the world was even colder than it is today – that is what we think of as “the” Ice Age. Before too long, unless human activities prevent it, the world will cool again, back into the Ice Age proper. Our perspective (the entire history of human civilization) embraces only a short-lived, temporary retreat of the ice, an Interglacial. The succession of relatively long-lived Ice Ages and relatively short-lived Interglacials is now known as an Ice Epoch, and lasts for several million years. It is the story of the discovery of the rhythms of the Ice Epoch, and the implications for life on Earth, that we tell here; but even an Ice Epoch is just a passing phase in the lifetime of a planet that has already been around for more than four billion years.
We think that it is normal to have ice at both poles of our planet. After all, there has been ice there for longer than there has been human civilization. But in the long history of the Earth, polar ice caps are rare, and having two polar ice caps at the same time may be unique. Indeed, it may be the presence of those polar ice caps which has made us human. And although we associate weather with the movement of masses of air around the globe, with high pressure systems bringing settled, dry conditions and low pressure systems bringing wind and rain or snow, as far as climate is concerned great ocean currents are far more important. Those currents carry warm water from the equatorial region to the poles, and the polar regions can only freeze at all if that flow of warm water is obstructed. Today, the South Pole is frozen because a great land mass, Antarctica, lies right over the pole, preventing any ocean currents from reaching it. The conditions around the North Pole are almost a mirror image of this, with a nearly landlocked Arctic Ocean almost surrounded by land masses which make it difficult for water to flow northward to the pole. But the warming power of that flow of water away from the equator is nowhere better seen than in northwestern Europe, where the current known as the Gulf Stream, which “ought” to be warming the pole, has been deflected eastward by the bulge of Canada and by Greenland to make Britain and its neighbourhood some six degrees, Celsius, warmer than it would otherwise be. The simplest way to envisage what the climate if northwestern Europe “ought” to be like is to look at any globe of the world, and cast your eye due west from Ireland to Canada. Anybody who has experienced a Canadian winter – at the same latitudes as Ireland – would be in little doubt that we are living in an Ice Age.
Six degrees is a lot of warming. If Greenland were not in the way, and the Gulf Stream could warm the Arctic by six degrees, the thin layer of ice floating on the surface of the Arctic Ocean would disappear, changing the climate of the temperate region of the entire Northern Hemisphere. In fact, just a fraction of the heat carried by the Gulf Stream would be enough to do the job. The changes resulting from Arctic warming would be bigger than you might expect at first sight, and would in many ways be unpredictable, because of the effect of positive feedback. Today, the shiny white surface of the ice covering the Arctic Ocean reflects away incoming solar energy, and helps to keep the polar region cool. Once the ice starts to melt, however, it exposes dark water, which absorbs the incoming solar energy and warms the region still further. If the world cooled for any reason, the feedback would operate in reverse, with dark ocean being covered by shiny ice that reflects away incoming solar energy and helps to keep things cold. If the Arctic icecap were removed, by magic, tomorrow, it would not reform. The world would be quite happy to maintain an ice-free Arctic Ocean. Either state is stable — with or without ice. But you can’t have half the north polar icecap; the feedbacks make it an all or nothing choice.
Curiously, this kind of process raises the possibility that one of the first effects of the global warming that is going on at present (which most climatologists explain as being at least partly caused by human activities) could be to cool northwest Europe. The argument runs like this. The warm water flowing on the surface of the ocean up the western side of the North Atlantic (carrying thirty million cubic meters of water every second) is part of a global system of ocean currents which flows all the way from the tropical Pacific, around South Africa’s Cape of Good Hope, picking up warmth from the Sun for most of its long journey. This warm water is less dense than the cold water of the deeper ocean, which is why it forms a surface current; but it is increasingly salty, because evaporation carries water away into the air. In the far North Atlantic, where the current is giving up its heat to the winds which blow from west to east at those latitudes, carrying the warmth towards Europe, the current becomes colder and more dense. With the added burden of its high salt content, this makes it sink into the depths, where it returns all the way back to its starting point before welling up again in the North Pacific. The whole system forms a kind of conveyor belt, driven by upside-down convection, pushed by the descending dense, salty water of the North Atlantic. The flow of this “river” in the ocean is twenty times greater than the flow of all the rivers on all the continents of the Earth put together.
If the Arctic icecap began to melt, though, fresh water would mingle with the surface flow of the Gulf Stream, diluting its saltiness and making it less dense. If this stopped the water from sinking, the push that drives the conveyor belt would be turned off, and the whole flow would stop, with warm water no longer being carried northward from Panama past Florida and up the North Atlantic. The whole flow could even reverse, with Europe cooling to the temperatures typical of Vancouver, and Vancouver enjoying the kind of climate now found in Ireland. If this claim seems over the top, consider that the rate at which heat is being transported northward by the Gulf Stream today is more than a million billion watts, as if the current were a real conveyor belt covered with a million million one-bar electric fires, all pouring their heat out into the atmosphere. And that is the only reason why it isn’t obvious to everyone in Britain, and the rest of northwest Europe, that we are living in an Ice Age now.
But there’s more to this story than the change of perspective it provides. Because of the positive feedback involving the shiny ice of the Arctic Ocean, small changes in the way the world is heated can produce effects on climate which seem, to someone not used to such feedbacks, out of all proportion to their size. Computer simulations of the so-called greenhouse effect show that under some circumstances an increase in the amount of carbon dioxide in the air which produces a warming equivalent to an increase of merely 0.0002 per cent in the Sun’s energy arriving at the Earth could flip the Arctic Ocean into an ice-free state – and, conversely, a comparably small cooling could flip it back again, if the conditions were just right. Because the real world is much more complicated than the computer models, nobody knows how close the real world is to being changed by such a climatic flip, but the message to absorb is that, thanks to positive feedback, small changes in the heat balance of the Earth can have big effects on climate.