Putting the life back in science fiction

Apocalypse 3: More with Milankovitch

I’ve been having some fun reading up on Milankovitch cycles since the previous post in this series, and I’ve learned that I didn’t know what I was talking about in the previous post. However, there’s still an apocalypse involved.

Here are the basics about global warming. The global average temperature goes up when there’s more CO2 in the air, down when CO2 goes out. The temperature change is proportional (roughly) to the doubling of CO2. If we double the old concentration of about 280 ppm, temperature goes up 1.5-5 degrees Celsius. If we quadruple it, the temperature goes up about 3-10 degrees, and so forth. Currently, we’re following what the IPCC calls the BAU (Business As Usual) model, or the 5000 Gigatonne carbon release. This will crank CO2 levels up to about 1200 ppm or more, so we’re easily into the quadruple jeopardy mode.

Anyway, the Milankovitch cycles are composed of three components: Earth’s orbital eccentricity, it’s axial tilt, and the precession of the orbit, all of which change at different rates. Of these three, only eccentricity (how elliptical the orbit is) actually changes the annual amount of sunlight earth as a whole receives, and that by only a percent or two. Obliquity and precession don’t affect the average amount of annual sunlight across the globe, and in this I was wrong.

Here’s the picture from the last post, about insolation at 65 degrees north at midsummer) for reference:

What’s happening here is real, but it’s only true for the northern Arctic area. Variations at the equator are similar in direction but smaller in magnitude, while those at the Antarctic Circle are (very crudely) reversed.

Now, remember how I said that Earth wouldn’t be warming up at the peaks and valleys in this graph? That is true. However, there will be LOCAL increases and decreases in temperature. Variations in axial tilt and precession of the equinoxes cause substantial changes in the seasons. When there is a lot of sun in the north, the summers are warmer (and probably wetter), while the winters are cooler (and probably drier). At the local lows, the summers are cooler and drier, while the winters are warmer and wetter. This is all on a comparative level, of course: it’s the difference between, say, California and South Carolina. The California coast gets most of its rain and snow in the winter and has cool, foggy summers, while the Carolinas get most of their rain in the summer, and have relatively fewer rain or snow storms. The southern hemisphere, of course, follows the opposite pattern.

When we’re dealing with Ice Ages, cool summers and warm winters can be a problem. Warm winters mean more snow falls, while cool summers means the snow lasts longer. If the summers are cool enough, the snow never melts entirely, and glaciers start to form. If the summers are warm enough, the snow melts, and the glaciers go away. This is how (very crudely) Milankovitch cycles help control the onset and end of ice ages, at least during times when the climate is cold enough (due to low levels of CO2) that ice ages are possible. The northern hemisphere at 65 degrees north is a bit of a driver, because there’s more land at that latitude than there is in the southern hemisphere, and large ice fields help force global ice ages (more or less).

Now, getting back to the idea of 37 apocalypses. We’re dumping a lot of CO2 into the air, and it’s going to take a long time to come out. Therefore, the Earth will be warmer for a long time, until that carbon comes out of the air. However, the seasons can vary. Due to the Milankovitch cycles, the weather can vary between summer rain and winter rain. If the temperatures are tropical, this doesn’t particularly matter. Most tropical areas have a dry season and a wet season, but since the annual temperature doesn’t vary a huge amount, when the rain occurs doesn’t particularly matter. Milankovitch cycles don’t particularly matter.

However, closer to the poles, these matter, even if the world is very warm. Above the Arctic circle, there’s an entire season of darkness as the sun slips below the horizon (due to axial tilt). If most of the precipitation comes during the darkness, it will land as snow. If it comes during the daylit summer, it will come as rain. Different plants prefer these conditions, so people living there will have to grow different crops. To use the example of California and the Carolinas, California does great with winter vegetables and summer fruits, while the summer rain areas can grow things like corn and other late summer vegetables. Winter rainfall climates also tend to favor massive irrigation projects, because farmers have to capture the moisture that comes during the winter, and dole it out when the crops are growing.

The Milankovitch cycles do matter in that they dictate what the vegetation will be, due to when precipitation occurs and what form it comes in. Think the differences between Portland, Oregon and Madison, Wisconsin, for example . Plant communities will shift to follow the Milankovitch cycles, as will farming practices and things like irrigation. Classically, these are the kinds of shifts that cause civilizations to rise and fall, and I have no doubt this will continue into the future. As noted in the previous post, there will be times of future stability, and times of future change, and the times of change will likely bring down civilizations that adapted to the old conditions.

Considering how much I’m learning, I seriously doubt that this will be the last word on the subject, so if I don’t quite understand things now, feel free to straighten me out. My goal here is to think about what the deep future might look like, and I still think it looks like it’s going to keep changing for the foreseeable future, in ways that aren’t that favorable for stable, global civilizations.