d ~ 192'960'000 km: day 75
We're coming up to the spring equinox, the official start of spring for many people, and it really feels to me like the seasons are changing.The seasons on earth are, of course, the result of the 23 degree tilt in our axis of rotation.
But what might our world be like without that tilt, and without its changing seasons? We wouldn't experience the usual swings between summer and winter, obviously, but would we have a permanent spring or autumn climate, or might the lack of axial tilt have different implications for our environment?
The earth hasn't always rotated with a 23 degree tilt. Pretty much nothing about the Earth's climate stays constant if you wait long enough, and that tilt is no exception. It wobbles up and down by a couple of degrees every 41,000 years or so (at the moment the tilt is slowly decreasing), and the strength of the seasons the earth experiences changes with it. When the tilt is greater, summers are warmer and winters are colder, and when the tilt is smaller there's less of a difference in the seasons. These repeating cycles in the strength of the seasons probably play an important role in forcing the huge climate shifts of the glacial cycles that the earth has experienced over the last million years - and that's all with changes of just 2 or 3 degrees in the tilt.
For fun, I set up a relatively simple model to simulate what the climate on an earth with a 0 degree tilt might be like. There are a few details that make this more of a toy than a serious scientific study, but we can still use it to illustrate some of the things that could happen in a 0 degree world. To start with, of course, the seasons disappear: although the weather is still different from day to day, February is much the same as June and October. However, if you guessed that the earth's climate in a 0 degree tilt world would permanently be stuck halfway between our usual summer and winter, you'd be wrong!
Images courtesy of Dr Robin Smith/University of Reading
A good way of imagining what it would be like to live on the 0 degree tilt world is to see how the ecosystems that we know from our 23 degree world would fare if we and they moved there*. The top panel shows a very simple way to characterise the climate of our 23 degree world in this kind of scheme. Greens show areas predominantly suitable for types of forest, browns are drier areas and grasslands, with grey for tundra, yellow for deserts and barren areas and ice caps in blue. There's a lot of fertile vegetation in this view of our world, with some desert in the hotter, drier areas and tundra and polar ice right up in the north.
The bottom panel shows what our toy simulation of a 0 degree world looks like. This climate is much less suited to our usual types of vegetation, with much larger barren desert areas, and a huge expansion of polar ice over Asia and North America. The
area suitable for vegetation at in the northern hemisphere shrinks dramatically, and northern Europe swaps its forests for tundra. The average temperature here in Britain sinks to a cool 7 degree C all year round, only varying by a couple of degrees warmer or cooler at most. Not everything would change for us, though - we'd still get about as much rain every year in a 0 degree tilt climate as we do now.
So, the earth's 23 degree tilt doesn't just give us the variations of the seasons and all the wonderful things we'll be seeing from this series - it's really important for setting the basic foundations of the environment we take for granted in our part of the world. As you can see, we'd have a very different planet without those 23 degrees.
Dr Robin Smith NCAS-Climate Dept. of Meteorology, University of Reading
* if the earth really had a non-seasonal climate, totally different types of vegetation would certainly evolve, so this is just a simple way of visualising what the different climates would be like.