Seasons, Ice ages and why greenhouse gases are good for the planet.

“So that’s Earth close up?”

That’s it. Pretty isn’t it. Nothing quite like it as far as the eye can see.

“Is it my imagination or is the earth titled over?”

No you are quite right it is leaning over. With maps showing north being straight up people usually think the earth spins upright, but without that tilt life on earth would be very different.

“In what way?”

Well, no seasons. There would just be hot places and cold places. The hot ones would stay hot all year around and the cold ones stay cold.

“Just because the earth is tilted a little?”

All because the Earth is tilted and by more than just a little. The most famous leaning thing on Earth, the leaning tower of Pisa, only leans over by 5.5^o. The earth leans over by more than four times as much, 23^o.

“So could the Earth fall over?”

Well it wobbles at bit, but it’ll never fall over. It’s like a spinning top; as long as it keeps spinning it will stay upright.

“But spinning tops fall over eventually?”

What makes a spinning top fall over is the fact that the bottom is rubbing on the table slowing it a fraction at every turn. The Earth is floating space, not touching anything so there is nothing to slow it down. It’s been spinning for 4 billion years which sort of proves the point. If it wasn’t tilted tree’s wouldn’t need to loose their leaves in winter in colder countries, there’d be no such things as summer holidays and swallows would only bother flying south if the north bored them.

“Hold on, you’ll have to explain that one. How can the earth’s spin cause all that?”

Well, because the Earth is tilted over as it circuits the sun every year then for half the year the top half of the earth is leaning towards the sun and for the other six months the bottom half. When the top half of the Earth is sloping towards the sun, it’s hotter because it’s a bit nearer the sun but more importantly the sun's rays hit the Earth head on. That’s why in summer the sun is higher in the sky. In winter the suns rays hit the earth at an angle and are spread over a bigger area, so it’s much colder. Of course whatever happens in the top half of the world, the northern hemisphere, is opposite to the what happens in the bottom half or southern hemisphere. When it’s winter in Europe, its summer down south in places like Australia. That's why in Australia they celebrate Christmas in shorts on the beach, at least for the time being.

“The time being? Is Australia going to move somewhere else?”

No but the seasons can change because the Earth wobbles as it spins. The bad news for Australia is that over the next 12,882 years the direction of the Earth’s tilt will change. That will make it winter at Christmas in Australia in the year 14,889, but everything should be back to normal by the year 27,772.


“Could that explain global warming?”

The earth’s temperature doesn’t seem to change in synchrony with that particular wobble, so, if the earth is getting hotter as it seems to be, then we have to look at other causes. The problem is that the earth’s temperature is a very fine balance of gaining heat from the sun and loosing to space. All the weather on Earth, even life itself depends on the fact that the Earth is in almost in perfect temperature balance. For all the energy arriving on Earth from the Sun, exactly the same amount has to be lost back into space. If a fraction more arrives than leaves then the Earth heats up. If more is lost than arrives then the Earth starts to freeze over.

“How can a planet loose heat?”

Some of the heat from the sun is reflected by clouds and the ice at the north and south poles. Some of it is sent back into space as infra-red rays. These are the type of invisible photon that carries heat - the same sort of photons that night vision goggles use. By turning infra-red photons into normal visible light, night vision goggles let you see the heat from anybody or anything even in complete darkness. How much infra-red escapes from the Earth depends on the greenhouse effect.

"You told me a while back that it’s the greenhouse effect that keeps Venus so hot. So it must be a bad thing?"

Well it depends. On Venus the greenhouse effect traps so much heat that it is horribly hot but Mars is so cold because there isn’t enough of a greenhouse effect. On Earth the greenhouse effect is there, but not to the same devastating degree as on Venus. You might read about how the carbon dioxide from cars, factories and airplanes is putting the world in danger, but without any greenhouse gases the Earth would be frozen solid and probably lifeless. The most important greenhouse gases for Earth are water in clouds and carbon dioxide gas (or CO₂). Clouds and CO₂ both absorb infra-red rays leaving the earth and heading off into space. Because the energy is absorbed the earth doesn’t cool down as much. If there was no greenhouse effect on Earth from the water vapour in clouds and CO₂ in the atmosphere, the world would be 40^oC cooler and most of it would be frozen solid. In all the recent global warming the Earth’s temperature has risen by less than a degree in the last hundred years.

"So what's the problem?"

The problem is that if the greenhouse effect is changed even a little bit that could put the Earth out of energy balance. What happens if you are leaning to ride a bicycle and you loose your balance?

“You fall over.”

Exactly, humanity is like a small child learning to ride but rather than getting the hang of bicycle it has to learn to keep a whole planet in balance. When things on earth get out of balance animals and plants start dying off so it is an important lesson for humanity to learn. The asteroid that hit the Earth 60 million years ago and caused the dinosaurs to die out didn’t kill them by blowing them up, but by changing the temperature balance from all the dust that was blasted into the atmosphere. Dinosaurs couldn’t keep up with the climate change that this dust caused. 245 million years there was an even bigger life wipe-out with 90% of all sea species dying out and that was probably climate related too.

“But that was millions of years ago, things have been pretty stable recently haven’t they?”

Well, not really. There have been major freezes or ice ages throughout the last few million years, following a pattern of an ice age every 40,000 years or so. But it’s not got any better recently. It’s only 10,000 years ago since the last proper ice age, with ice covering most of Europe for thousands of years. There was a mini chill just a few hundred years ago. Between the years 1645 and 1715 winters were so cold that it was called the little ice age, the River Thames in London froze so solid Londoners had winter fairs on the ice.

“What causes ice ages?”

There is a pattern in when ice ages happen and a mathematician Milutin Milankovitch from Serbia, in the 1930’s suggested that it was all to do with the wobbles in the Earth’s spin. But ice ages could be caused by lots of other reasons like changes in how brightly the sun shines, sudden shifts in ocean currents or volcano’s. The bottom line is that no-one is quite sure what causes ice ages. The worrying thing about the ice ages is that as scientists are not absolutely sure why they happened in the past, they can’t predict when they might happen in the future.

“Well, if the earth is warming up we could do with another mini-ice age right now.”

I’m not sure the earth can wait 30,000 years until the next one, but let’s talk more about global warming next time.