Topics in Depth
The Theme of Earth's Climate in Biogeography
As far as we know, our planet is the only one in our solar system that supports life. Earth's distance from the sun makes the planet not too hot and not too cold. Goldilocks would choose us if she could. However, the equator, the belt around the widest part of the globe, is always very hot (some might say too hot) and the poles, which are the top and bottom of the globe, are always very cold. This lets us have icy tundra at the poles and tropical rainforests around the equator. But why?
The distance away from the equator is called latitude and is measured in degrees, like angles in geometry class. The equator is at 0°, the North Pole is at 90° North and the South Pole is at 90° South. Latitude is different than longitude, which runs north to south (vertically) around the globe. Climate varies with latitude because of Earth's shape, tilt, and rotation.
Shape: Earth is round. Duh. Well it wasn't so obvious to people back in the day, but we are ever so advanced now. Sunlight that reaches Earth is most intense around the equator because it hits most directly there; sunlight comes in at more of an angle at the poles, so it is not as intense.
Tilt: As you have probably heard before, Earth is tilted on its axis. This means for half of the yearlong rotation around the sun, the northern half of the globe, known as the northern hemisphere, is leaning toward the sun. This results in longer days and shorter nights, and summer vacation. During the other half of the year, the northern hemisphere tilts away from the sun, and days are shorter than nights. Of course, the southern hemisphere experiences the opposite pattern—northern hemisphere's winter happens during the southern hemisphere's summer.
Sunlight is also most constant in the tropics, thanks to the Earth's tilt. No matter what time of year it is, the tropics receive the same amount of sunlight. The poles have short days during the winter, and sometimes no sunlight at all.
Due to the Earth's shape and tilt, sunlight reaches the Earth at different angles throughout the year. Image from here.
Since sunlight is important for plants and algae, this has consequences for the types of living things that inhabit the poles and the tropics. Might there be some variation between them? You betcha.
The intense sunlight in the tropics makes the temperature hot hot hot at the equator. Heat rises, so hot air masses move up in the atmosphere, bringing water with them. As air moves up and farther away from the ground, it cools down, releasing water with it. (Cold air cannot hold as much water as warm air.) The form this water takes is of course, rain.
Climate patterns don't stop in the tropics though. The air that dumped all its water over the tropics now starts falling toward the ground again at about 30 degrees latitude, warming up as it nears the ground. But now the air is dry. Very very dry. The type of dry that scorpions, cacti and kangaroo rats do well in. At 30 degrees latitude, the dry air creates deserts.
Major deserts of the world, including the Sahara, Arabian, Gobi, Great Victoria, Kalahari, Atacama, and the Chihuahuan/Sonoran deserts occur around 30° N or 30° S. Deserts also occur at the poles for the same reason (Arctic desert in the north and the Antarctic desert in the south). Other factors influence desert formation, like having mountains nearby and prevailing wind patterns, which is why deserts also occur at other latitudes (for example the Great Basin desert in the US) and not every point at 30° is a desert. Hello, Florida.
Image from here.
The air circulation pattern that we just described (wet air rising at the equator, and dry air falling around 30°) is called a Hadley cell, named after the guy who discovered it, George Hadley. There are six cells like this from the north to the south pole, creating deserts at 30° and the poles and rainy areas at the equator and 60°.
Rotation: Earth rotates on its axis, which creates wind patterns. As the planet rotates, the equator moves faster than the poles thanks to the round shape of the Earth. Think about it—the Earth is much wider at the equator than at the poles. It takes 24 hours to do one full rotation (one spin around the axis). A point on the equator has much farther to go around in 24 hours than does a point at the pole, so it must be traveling faster. The increased speed at the equator causes winds to move away from the equator. Rather than just going west, the winds are deflected so that they move north above the equator and south below the equator. This is called the Coriolis effect, and makes for some pretty consistent wind patterns, like the Westerlies and the tradewinds that you see in this picture:
The tradewinds are consistently blowing in the same direction. During the colonial period, ships traveling between Europe and the Americas relied on them for trade between the continents, giving these winds their name.
The shape, tilt and rotation of the Earth are really important in determining temperature and precipitation, which in turn create biomes. A biome is an area with a particular climate that plants, animals and other living things are adapted to. Examples of biomes include tropical rainforest, desert, temperate grassland, chaparral, wetlands, coral reefs and tundra. As you can see, biomes are usually named as a combination of abiotic and biotic descriptions.
If mountains or a large body of water are nearby, these can also influence climate. In fact, oceans have a big impact on climate. Evaporation from the ocean is where most rain comes from, and since 75% of the Earth is covered in ocean, that's a lot of rain. Ocean water doesn't heat up or cool down very quickly, which means ocean temperatures are usually pretty constant. On land next to the ocean, this allows oceans to moderate the temperatures on land. Warm air on the land rises, then cools as it gets higher in the atmosphere. The cooler air sinks back down over the ocean and moves over land, cooling the land off, and usually making it foggy, too.
This air pattern happens over the Mediterranean Sea, making for dry summers and wet winters, but also happens in California, South Africa, and Australia, giving each of these places a similar Mediterranean climate. Plants that grow in one Mediterranean climate usually grow well in anther Mediterranean, which is why eucalyptus trees, native to Australia, seem so at home in their non-native California. Now they just need some koalas.
Eucalyptus trees are native to Australia but have become invasive in other Mediterranean climates.
In addition to influencing climate, oceans are also home to the largest biome on Earth. The oceanic pelagic zone covers 70% of the Earth's surface. The pelagic zone is the open ocean, with consistent currents and is home to tiny photosynthetic organisms called phytoplankton.
Now that we have explained climate, we know why biomes occur where they do. That pretty much explains biogeography, right? Well, almost. We haven't actually covered the organisms that live in these biomes. Keep reading for more.
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