What would happen if Earth wasn't tilted?

Donald on The Naked Scientists forum asked what would happen if the Earth wasn't tilted on its axis? Chris Smith directed this question at climate scientist James Pope from the British Antarctic Survey...

James - It’s a brilliant question. The short answer is that basically every single day would be 12 hours long, everywhere. But your latitude would affect how high the sun was in the sky. So if you're at the equator, you'd always have sort of 12 hours with a very high mid day, directly in the middle of the sky, sun. If you're at the poles, you'd have a very low right on the horizon sun for 12 hours a day. So the energy we get from the sun comes in, you can sort of imagine if you took a football or something and just put straight lines towards this football, that’s the energy from the sun. Without that tilt, all of the energy would go into the equator, there would be less energy going into the polar regions. So you'd have more energy transfer from the equator in the low latitudes up to the high latitudes, so in the mid latitudes, where we are, we'd probably get stormier, because there would just be more movement of air. And overall the UK it would probably be much like the week before Easter permanently.

Chris - What with the traffic you mean? But being more serious for a minute, so basically the temperature we experience is because it's the amount of solar input, the amount of energy falling on the land for that length of time. And so we have summer because the Earth is tilted and we see more sun during the summer period, and we have winter because we see a bit less because that part of the earth's surface is tilted away and the sun's not as high in the sky. So basically it's thanks to the fact that the Earth is tilted that we have seasons and therefore we have a seasonal climate.

James - Yes exactly.

Chris - Yeah. Because someone said to me the other day that Mars is also tilted about, because the Earth is tilted 23 and a half degrees, Mars is tilted about the same, so therefore Mars sort of has seasons as well but it's a great deal colder than it is here on Earth.

James - Earth has got very lucky, it’s in this really nice little slot in the solar system where if we were a few hundred thousand kilometres nearer or a few hundred thousand kilometres further away from the sun it may be almost uninhabitable, and it's that sort of that variation, we're just a real sweet spot. And again, our tilt aids that, but also that tilt is what added to things like the ice ages. So the glacial interglacial cycles where huge ice sheets waxed and waned over especially northern Europe, part of that was due to the changes in the tilt and also the change in the shape of our orbit around the sun.

Chris - So why should the tilt change?

James - I don’t actually know. I think, the tilt was very early on in the earth's formation, the earth was slightly knocked off its spinning axis, so put it onto that slight tilt, and it's just got a natural wobble, like like a spinning top essentially.

Chris - And so owing to the fact that it is just gently changing that degree of inclination over very long timescales, is that why we go through phases of a bit warmer for a while, it's a bit colder for a while, and that's what people call the natural cycles, or Milankovich cycle?

James - Yeah

Chris - So when people say the earth has always throughout its 4.5 billion year history gone through cycles, when the climate has changed, how do we know that the warming that we're seeing today is owing to our influence and not just the earth entering another of these natural variations. Because the Earth has completely frozen up in the past, it's completely melted it's poles and been really really hot in the past hasn't it?

James - It's one of the toughest parts of climate science in many respects. So we have different ranges of data. Some of that is modern day satellite observations, weather stations, so it's all very new, recent, last 50 years. We have proxy data which comes from things like tree rings, the ice cores, or even sediment cores from the deep ocean, that we can go back about 65 million years for some these sediment cores. And we can look at the fossilized plankton and the chemistry of the shells we can approximate what the temperature would have been, but ultimately what we can see is that nothing has happened as fast. Nothing that we've really seen happens as fast as what we are currently observing and it's that rate of change that makes the anthropogenic effect, the manmade effect, of climate change different to natural climate change which has always happened and will continue to always happen with our signal on top of it.