Weather and climate<\/a> are related but they differ in the time scales of changes and their predictability. They can be defined as follows.<\/p>\n Weather<\/a> is the instantaneous state of the atmosphere around us. It consists of short-term variations over minutes to days of variables such as temperature, precipitation, humidity, air pressure, cloudiness, radiation, wind, and visibility. Due to the non-linear, chaotic nature of its governing equations, weather predictability is limited to days.<\/p>\n Climate is the statistics of weather over a longer period. It can be thought of as the average weather that varies slowly over periods of months, or longer. It does, however, also include other statistics such as probabilities or frequencies of extreme events. Climate is potentially predictable if the forcing is known because Earth\u2019s average temperature is controlled by energy conservation. For climate, not only the state of the atmosphere is important but also that of the ocean, ice, land surface, and biosphere.<\/p>\n In short: \u2018Climate is what you expect. Weather is what you get’<\/p>\n Earth\u2019s climate system<\/a> consists of interacting components (Fig. 1). The atmosphere<\/a>, which is the air and clouds above the surface, is about 10 km thick (more than two thirds of its mass is contained below that height). The ocean covers more than two thirds of Earth\u2019s surface and has an average depth of roughly 4 km. Contrast those numbers with Earth\u2019s radius which is approximately 6,400 km and you\u2019ll find that Earth\u2019s atmosphere and ocean are very thin layers compared to the size of the planet itself. In fact, they are about 1,000 times thinner. They are comparable perhaps to the outer layer of an onion or the water on a wet soccer ball. Yet all life is constrained to these thin layers. The major ocean basins are the Pacific, the Atlantic, the Indian, and the Southern Ocean. Ice and snow comprises the cryosphere, which includes sea ice, mountain glaciers and ice sheets on land. Sea ice is frozen sea water, up to several meters thick, floating on the ocean. Ice sheets on land, made out of compressed snow, can be several kilometers thick. The biosphere includes all living things on land and in the sea from the smallest microbes to trees and whales. The lithosphere, which is the solid Earth (upper crust and mantle), could also be considered an active part of Earth\u2019s climate system because it responds to ice load\u00a0and impacts atmospheric carbon dioxide (CO2) concentrations and climate on long timescales through the movements of the continents.<\/p>\n The weight of the ice depresses the lithosphere, which provides a feedback to the ice by lowering its surface elevation<\/a>. Traditionally it has been thought that this feedback acts on long (millennial and longer) timescales, but recent research indicates that in certain regions such as West Antarctica, where the lithosphere is thin and the mantle viscosity is low the lithosphere can respond on much shorter (decadal to centennial) timescales.<\/p>\n The components interact with each other by exchanging energy, water, momentum, and carbon thus creating a deliciously complex coupled system. Imagine water evaporating<\/a> from the tropical ocean heated by the sun (Fig. 1). The air containing that water rises and cools. The water condenses into a cloud. The cloud is carried by winds over land where it rains. The rain sustains a forest. Trees are dark, having a low albedo. This influences the amount of sunlight absorbed by the Earth. Dark surfaces absorb more sunlight and get warmer compared to bright surfaces such as desert sand or snow. Air warmed by the surface rises and affects the wind.<\/p>\n","protected":false},"excerpt":{"rendered":"The Climate System<\/h3>\n