Four degrees of separation: lessons from the last Ice Age

Posted on Posted in Climate Science

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The pace and magnitude of human-caused climate change is nothing short of remarkable.

dsc_8634The dramatic climate change we are now experiencing is a rapid and unexpected side effect of the astonishing ingenuity of humanity. It’s really quite amazing that industrialization – harnessing the power of machines to do our work – could have such far-reaching consequences. Not since the appearance of the first photosynthetic organisms, which introduced oxygen into the air for the first time some 2.5 billion years ago, has life had such a profound impact on the chemistry of the atmosphere.

There have, of course, been many times over Earth’s 4.5 billion-year history where natural events caused the atmosphere to change, in many cases significantly altering the climate. Understanding how these past changes were different from present-day, human-caused climate change is an important step towards realizing the profound consequences of our ongoing release of carbon dioxide and other greenhouse gases.

Let’s take a look at a relatively recent climate change, one that took place 100,000 to 10,000 years ago: a period that we now call the last Ice Age.


More than one way to change a climate

All life on Earth owes its existence to the sun. Similarly, how warm or cool the Earth becomes depends very strongly on two things: how much sunlight reaches the planet, and how much of the warmth generated by sunlight is trapped here.

This means that there are several things that can make climates change. Releasing greenhouse gases into the atmosphere is of course one of them: GHGs trap warmth in the atmosphere, heating up the planet. Other options include:

– changes to the surface of the Earth that make it more or less reflective (and therefore more or less warmed by sunlight)

– the relative brightening or dimming of the sun

– adding or removing aerosols – tiny bits of dust – in the atmosphere, which can block the sun’s rays

All of these processes do essentially the same thing: they affect the amount that sunlight warms our planet. They do so either by altering the amount of solar radiation that reaches Earth’s surface or by affecting the amount of heat energy trapped by Earth’s atmosphere.


Any guesses what caused the last Ice Age?

Did you guess changes in solar output? No, the sun wasn’t any dimmer back then. But the Earth’s orbit was a different shape and the tilt of Earth on its axis was at a slightly different angle compared to today. Taken together, that means that the amount of sunlight that reached the Earth was different than it is today.

This difference in the orbit and tilt of the Earth produced winters that were slightly warmer and summers that were slightly cooler than now. Warmer winters lead to more snowfall – because warmer air can hold more water vapour – while cooler summers prevented that snow from melting.

And thus, over many, many years, the two effects of more snow falling and less snow melting let snowfall accumulate into enormous glaciers that came to cover much of North America.

This is also an example of how climate change factors can work together: once the snow started to accumulate, more and more sunlight was reflected off the snow back into space. This amplified the cooling and so, over thousands of years, Earth became colder and snowier and settled into the long grip of the Ice Age.


How different was it?

Imagine the Canadian Prairies, covered with glaciers more than 1 km thick, as far as the eye could see. Imagine the world’s oceans up to 100 metres lower than today (because the water is frozen in the glaciers). How much cooler must Earth have been to cause these truly profound changes?



Earth’s atmosphere was, on average, just 4 degrees cooler than post-glacial (and pre-industrial) times.


Keeping it to 1.5

It took the Earth tens of thousands of years to cool by that much, and it’s taken tens of thousands of years for it to warm again. Humans, on the other hand, have managed to warm the planet by a little over 1°C in just the past couple of hundred years alone, and we’re on track to warm the planet by as much as 3 or 4 degrees by the end of this century.

It’s difficult to imagine a world 4°C warmer than pre-industrial times, but comparing the amount of change relative to the last Ice Age draws attention to the magnitude of the change. A 4°C change in climate is the difference between the world we live in and the Ice Age. This is one reason why climate experts make such a big deal out of what can sound like small changes in temperature.


The Paris Agreement of 2015 holds the nations of the world to limit global warming to just 2 degrees above pre-industrial values, and ideally just 1.5 degrees. 1.5 °C of warming is still a lot of change in a short period of time, and will have significant consequences for all life on Earth. And if we miss that target? Well, we just have to look back to the last Ice Age for lessons about how different the world might become.


Ryan Smith & Steve McCullough

Prairie Climate Centre