Weather is the state of the atmosphere at any given time. Climate, on the other hand, can be described as the average conditions over a length of time (referred to as a Base Period). For example, one measure of climate could be the average annual precipitation in a place for a period of 30 years, while one measure of weather could be the amount of precipitation that occurred on a given day. In addition to the average over time, measures of climate also include variability – so in the case of precipitation we would be interested in not only the annual (or seasonal or monthly) precipitation, but also how much those values change from year to year.
But there’s even more to climate than average and variability….
Although we normally think about climate as the aggregate of many weather events over time, constraints of Earth’s climate system exist, making it possible for only certain weather conditions. The major constraint on Earth’s climate is its energy balance. The Earth receives energy (in the form of short wave, visible radiation) from the sun. As a cooling mechanism, the Earth emits energy back to space (in the form of long wave infrared radiation). Ultimately, it is the balance of incoming and outgoing energy that determines the climate, and subsequently, what weather is possible on the Earth. For example, if the Earth is absorbing the same amount of energy that it is emitting, then the overall temperature of the Earth would stay constant. However, if the Earth were to absorb the same amount of energy and emit less energy, then the Earth would warm until it is able to emit more to balance the incoming sunlight. If the Earth were to absorb less solar energy and emit energy at the same rate it would cool until it achieved balance with incoming sunlight.
A useful analogy to consider…
…is a faucet and a sink. If the faucet (visible sunlight) adds more water than can drain out (infrared long wave) of the sink, then the water level (or “temperature”) will rise. If the sink can drain faster than the faucet adds water, then the water level will sink. If the speed at which the faucet adds water is equal to the rate at which the sink drains, the water level will stay the same. Climate on a global scale is constrained by this energy balance, and in this analogy day-to-day weather fluctuations would be like waves on the surface of the water – the waves are important at any given time, but they do not determine the depth of the water, just as weather does not constrain climate, but in a sense is constrained by it.
The sink analogy illustrates that it’s not just the incoming solar radiation (water inflow) that’s important, but also the amount of cooling to space (outflow from drain) that determines the climate (water level). The climate can change if either inflow or outflow varies, and the Greenhouse Effect works on the outflow part of the balance.
The Greenhouse Effect is a natural phenomenon that is very important in determining the climate of Earth. Greenhouse gases in the atmosphere, like Water Vapor, Carbon Dioxide and Methane, trap some of the infrared radiation energy that is emitted by Earth. This energy is absorbed in the atmosphere, and although a small percentage is emitted to space by the atmosphere, much of it is emitted back to further warm the surface. Because of this effect the average temperature of the Earth is warmer than it would be if Earth had no atmosphere. In our sink analogy (mentioned above) it would be similar to a piece of food clogging the drain of the sink. The outflow of water would slow while the input would stay the same. This would cause the water level to rise to achieve a new balance between inflow and outflow, just as Greenhouse Gases result in a warmer temperature of earth needed to balance the incoming sunlight.
The Human Factor
At present, human activities are increasing the amount of greenhouse gases in the atmosphere, which further reduces Earth’s ability to cool by sending infrared energy to space. This creates an imbalance between incoming sunlight and outgoing infrared radiation, which then will require Earth’s surface temperature to rise until balance is achieved. These human-induced changes in the climate system are referred to as Anthropogenic Global Warming.
Take the Yale Project on Climate Change Communication Survey here: