Controlling Earth's Climate
The Earth's climate is all about energy at or near the surface of the planet. So anything that affects this energy will influence climate. Some of the major influences include:
- Solar insolation, or the amount of energy that reaches us from the Sun. This amount varies over very long timescales (e.g. the Sun has gotten brighter as it ages) and very short, annual time scales.
- Earth's orbital geometry. The Earth is tilted on it's orbital axis, and also wobbles as it revolves. The amount of tilt and wobble vary over tens of thousands of years, and these quasi-cycles are called Milankovitch cycles. These variations affect the amount of sunlight that reaches various areas of the Earth's surface.
- Continent/ocean configuration. The contiguity and orientation (north-south, east-west) of the oceans affects their circulation, and ocean circulation is probably the most important surface influence on climate. The amount of continental landscape exposed (i.e. not submerged) is another major influence. Land heats up and cools down much more quickly and easily than does water.
- Volcanism and other tectonic activity. Volcanic activity can cool the planet down by spewing masses of particles into the upper atmosphere and shading out the Sun. The recent eruptions of Mt. Pinatumbo in the Phillipines is a great example of this. Volcanism can also release greenhouse gases though, such as carbon dioxide, and hence warm up the planet.
- And last but not least, the biggie that we are interested in, greenhouse gases. Greenhouse gases have the interesting property of being permeable to incoming solar radiative heat, but reflective of outgoing radiative heat from the Earth. Solar heat is high energy ultraviolet radiation. It passes through the atmosphere and warms up the surface during the day. Later on, the Earth's surface radiates it out into the cooler atmosphere, but now as lower energy infrared radiation. Greenhouse gases trap a lot of this radiation, and hence, more heat enters the Earth's system than leaves it. This is very important! If we did not have sufficient levels of greenhouse gases, Earth would be a brutally cold desert like Mars. Unfortunately, too much of these gases, and you have a runaway greenhouse hothouse like Venus.
Now, think of all these mechanisms acting at the same time. Each one affects climate in some way. But it's not a simple problem of adding up all the influences. These mechanisms can interact with each other. Sometimes they reinforce each other, sometimes they cancel, and how they interact can be affected by numerous other physical, chemical and biological processes. These are so-called feedbacks. Beginning to understand why climate science is so complex? One of the big challenges that scientists face today is to understand how these mechanisms behave over time and under various circumstances, how they may or may not interact, and what influences them. Some of the answers are easy; e.g. nothing here on Earth has much of an influence on what the Sun is doing. But guess what? A LOT of what goes on here on Earth can affect some of the other mechanisms. Can you figure out which one is most easily influenced? If you said greenhouse gases, then you are correct! In my next posting, we'll talk about greenhouse gases, what they are and where they come from.