Worldbuilding with real worlds

Lieutenant Elena Jones shivered with excitement as her Ecological Survey craft entered the atmosphere. She checked the readings in front of her carefully. This was her first solo mission, and she was determined to do everything just right. She came in over the major planetary ocean, following the prevailing winds. She stared out the window at the coast, her first sight of land not filtered by cameras and instruments. Her planet, hers! Not just her first sight of this Earthlike world, but the first sight ever. Elena guided the little survey shuttle over the high coastal mountain range, and leveled the craft out over the lush green tropical forest beyond.

Hold on, something’s wrong. Did you spot it? (Besides the cliches; we’re here for the science.) The worldbuilding here jams the components of a planet together randomly instead of fitting each piece into its proper ecological spot. A tropical forest doesn’t go downwind of the coastal mountains on an Earthlike planet. Major categories of vegetation and their associated animal life, called biomes, can only be found in particular places. At a global scale, the locations of desert, grassland, forest and tundra biomes are determined by temperature and precipitation. At a particular site other factors like soils and disturbance come into play, but let’s stick to whole planets for now.

If you draw a graph with temperature as one axis and precipitation as the other, each kind of biome can be added as a region.

Tundra is very cold and dry. Deserts are dry and can be warm or cold. Grasslands are dryish and warmish. Tropical rainforests are warm and wet. That’s pretty straightforward, but how are the different types distributed on a planet? Deserts and grasslands and rainforests all belong somewhere, but why?

We only have one sample to extrapolate from, but at the planetary scale it’s all physics, and the physical principles that determine biome placement are general to Earthlike worlds. Like so many other things, it’s all about energy. Our planet’s major source of energy is the sun, so insolation (the amount of solar radiation received) is the driving force in global climate.

If the sun is directly overhead, a sunbeam will be concentrated in the smallest area and providing the greatest amount of energy per square meter. If the sun is hitting at an angle, the same sunbeam hits a larger area of ground and the energy is more diffuse.

The angle at which the sun’s rays hit the planet varies with latitude. That’s why it is warmer at the equator than near the poles: more energy received. Seasonal patterns come from both latitudinal differences and the angle of the Earth’s axis: during the summer, the polar regions are receiving more direct sunlight, and so greater energy density.

The timing is reversed for the southern hemisphere, but the shape of the curves is the same: more energy overall near the equator, more energy in the summer and less in the winter at higher latitudes.

The geographic pattern of annual insolation looks just like that at the top of the atmosphere: highest in the tropics, moderate in the temperate zone, and lowest at the poles.

The surface of the planet is part land and part water so it doesn’t heat evenly, but the tropics are warmer than the poles.

So that takes care of temperature, but solar radiation is also responsible for the global atmospheric circulation – the prevailing patterns of winds.

Warm air rises and cold air sinks. At the equator, the land warms the most and heats the air above it. The warm air rises, and air from farther away flows in to fill that space. The warm air rises, moves away from the equator and cools, sinking again. There are three of these circulation cells in the northern hemisphere, and the same in the southern hemisphere. The direction of planetary rotation figures into it too (ours turns west to east).

The same patterns of heating and air movements control the dominant precipitation patterns. Warm air can hold more moisture than cold air, so when warm air rises and cools it has to drop its moisture.

That creates a rain shadow, and explains why deserts are downwind of mountains. The combination of temperature and precipitation explains why all the biomes are where they are.

And that’s all there is to it: the sun powers everything, driving temperature and global circulation patterns, which interact with topography to determine precipitation patterns. The physical principles are broadly applicable, and can be used to predict biomes of the late Jurassic, or of your latest fictional world.

Start with the location of the continents. Where are the mountains? Which way do the prevailing winds go? What parts are hot? Dry? Cold? The World Climate Charts site has excellent maps of all sorts of climate parameters that will help you get a feel for how everything goes together in a science-based world. Build a realistic geography, then use it to fuel your story.

But there’s no reason your planet has to be physically like ours: farther from the sun, or nearer; with a different axial tilt, or none, or just an entirely different continental layout. Any of those factors can shift the planetary proportions around on that first temperature-precipitation graph. Most of the planet could be hot and wet, or cold and dry, or whatever suits your story. Just make sure there’s science behind it, as well as imagination. (And I didn’t talk about it at all, but don’t forget that it gets colder as you go up in elevation too.)

Once the deserts and forests and grasslands are in the proper places, then comes the fun of figuring out what lives there: plants and animals or something entirely different, things adapted for hot or cold, wet or dry. Do these organisms need to conserve heat or get rid of it? Find water to drink or avoid it?

How much would an alien desert look like ours? Or a forest?

Climate affects so much of human or alien culture as well. What kind of shelter is necessary? Is water available (if needed)? The biome may determine what kind of food, building materials and other natural resources are available, and also the predators, pests, annoyances. If you follow these basic principles you’ll know that your planet is plausible.

Elena guided the little survey shuttle over the high coastal mountain range, and leveled the craft out, flying low and slow over the broad open grassland beyond. Look, elephants! If elephants had trifurcated trunks and irridescent spines, anyway.

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