2. Components of the Climate System

The atmosphere is usually the first part of the climate system we naturally think of. It is literally the air we breathe: mostly inert nitrogen (78 %) with oxygen (16 %) and then other trace gases (argon, water vapor, carbon dioxide). The oxygen is a by-product of the respiration (“breathing”) of plants and other organisms: It is evidence of life on earth. The oxygen in the atmosphere did not exist before the emergence of living organisms.¹ Oxygen is emitted by plants as an outcome of photosynthesis that removes carbon from carbon dioxide. Oxygen reacts with materials (rock and ore) at the earth’s surface (oxidation) and disappears from the atmosphere. One of the most common reactants is iron (iron oxide = rust), which is responsible for the red color of many rocks. Unless organisms continue to produce oxygen, it will disappear from the atmosphere. It would take a long time however: hundreds of thousands to millions of years. But it is the trace species—water vapor, carbon dioxide and methane—known as the greenhouse gases, that are most important in understanding the climate system and how climate might change.

The term greenhouse gas refers to gases that are transparent to visible light emitted from the sun, but opaque (absorptive) to the “infrared” radiation emitted by the earth. This follows from fundamental physical principles described by the Boltzmann law, after Ludwig Boltzmann, a 19th-century Austrian physicist. All mass radiates energy, depending on temperature. The hotter a body (could be your body, the earth, or the sun), the more energy it radiates: and the peak energy occurs at different wavelengths. The peak radiation of the sun (surface temperature = 6,000 Kelvin² [K] or 10,000 °F: very hot either way) radiates a lot of the light we call visible. It is no accident our eyes evolved to be able to ‘see’ in the visible where the maximum solar emission is. Objects with cooler temperatures such as the temperature of the earth at about 300 Kelvin (80 °F) radiate in the infrared (longer wavelengths with much lower energy). These objects include our bodies, the earth itself, the ocean surface or clouds.

Greenhouse gases, like the glass in a greenhouse, allow in visible light from the sun, but absorb (and re-emit back down) the infrared energy from the earth.³ The higher the concentration of these gases in the atmosphere, the greater the warming effect. The most important greenhouse gas is water vapor. Water vapor also has some other important effects in the climate system. The second most important gas is carbon dioxide.

The basic picture of the flows of energy in the atmosphere is illustrated in Fig. 2.2. The arrows represent approximate sizes of the energy flows ⁴ in the energy budget. Solar energy comes in; the atmosphere, clouds and the earth absorb some and reflect some. The technical term for the ratio of reflected over total energy is albedo. White/light surfaces reflect a lot and have a high albedo: snow, ice, bright sand. Black/dark surfaces absorb a lot and have a low albedo: dark green trees, the ocean, asphalt. This is why a black car is hotter than a white car or asphalt is hotter than concrete in the sun. The energy absorbed warms the object (clouds, atmosphere, surface of the earth/ice/ocean) and it ‘re-emits’, but now at longer wavelengths and lower energy since the temperature of the surface is much lower than the sun. For common temperatures at the earth’s surface (0–100 °F, –20 to 40 °C), this emission occurs in the infrared. Infrared radiation is absorbed in the atmosphere by greenhouse gases and clouds, and some returns to the earth. The earth warms the atmosphere above it in two ways. The direct conduction of heat from a warm surface occurs by sensible heat. The energy hitting the surface, particularly the ocean surface, also can evaporate water vapor. Since this water vapor contains the energy used to evaporate it, it also is a way of transmitting ‘heat’ (energy), without changing the temperature. The energy in water vapor is called “latent heat”.

This is the cycle of radiative energy in the earth system, flowing through the atmosphere. The ultimate source of the energy is the sun. One of the biggest complications is water, in all its forms: from the oceans, to water vapor in the atmosphere, to clouds. Put a cloud over a dark surface (ocean or forest), and you change the energy in the system. The energy changes by reflecting solar energy from the top of the cloud back to space (cooling). Then energy also changes by absorbing infrared energy radiated from below in the cloud and sending some of it back down to the surface (warming).

The movement of water through the atmosphere, the surface and the ocean is called the hydrologic cycle (Fig. 2.3), which is also important for moving energy in the atmosphere. Solar energy hitting the ocean causes evaporation of vapor into the atmosphere, and plants also move water from liquid phase in the soil through their leaves, back into the atmosphere in a process called transpiration. This water carries the energy necessary for evaporation in the form of latent heat. The energy is used to make water molecules in liquid move fast enough to separate from each other into a gas. They conserve this heat of evaporation as the water vapor moves through the atmosphere. This heat of evaporation is released when water vapor undergoes condensation into cloud drops (or ice crystals). This can happen a long way from the liquid source (oceans), and clouds eventually form enough drops that they get heavy and fall (precipitate). The water thus completes a cycle, landing back on the surface.

Water then enters other components of the earth system, such as glaciers, snow, soil or rivers. Soil moisture can go through plants to get back into the atmosphere by transpiration. Or it can saturate the soil, become runoff and eventually get back into the ocean, where the cycle repeats. The key is that water changes the energy budget because it forms clouds, because it is a greenhouse gas and because it moves heat around. Water evaporated in the tropics moves as water vapor with the winds to higher latitudes (see Chap. 5), where it condenses as clouds and deposits heat there. Water is magical stuff in the earth system. The cycle of water is linked to the energy budget (through latent heat), and it links the atmosphere to other pieces of the system.

We have discussed the ocean briefly as a source of water. The world’s oceans (or combining them into a single “ocean”) are a critical part of the climate system.⁵ The ocean is a tremendous reservoir of heat and holds a lot more mass than the atmosphere. Water is denser than air, and the mass of the top 30 feet (10 m) of ocean is equal to the mass of the atmosphere above it (90 % of the atmosphere is in the first 55,000 feet or about 17 km). The ocean is critical as a very large store of heat that can move around and come back into contact with the surface after long periods of time. The ocean has currents that are both shallow (near the surface, the first few hundred feet of depth) and deep (the rest of the ocean). Unlike the atmosphere, the ocean has “edges”: Land sticks up in places, dividing the seven seas into, well, seven seas (actually five ocean basins). The ocean surface currents are forced by the rotation of the earth and winds. These currents are constrained by the topography in ocean basins and generate circulation patterns of surface currents.

In addition to the near surface ocean, there is a complex vertical structure to the ocean. The ocean is divided into a near surface region that is generally warmer and less salty (fresher) than the deeper ocean below it. Colder and saltier water is denser, so it tends to sink. The process often is self-reinforcing and is seen in lakes as well: The surface warms, gets less dense and mixes less with the deeper water, thus warming more over time (this is called stratification). Density is a critical part of the structure of the ocean because of changes in density due to heat and salinity, which do not change very quickly in the ocean. Density helps control the deep ocean circulation. The deep ocean has one large circulation globally, driven by the sinking of surface water that gets saltier and colder (denser) until it sinks from the surface to the deep ocean. This thermohaline circulation (thermo = heat, haline = salt) has sinking motion in small areas of the North Atlantic and the Southern Ocean near Antarctica, where water is cooling as it moves poleward, and where salt is expelled from formation of sea ice, increasing salinity. Both processes make the surface water denser than the water beneath it, causing it to sink.

The ocean also contains parts of two other spheres. The ocean has a biosphere, consisting of plants and animals living in the ocean and nutrients that flow through the ocean. Most of the biosphere in the ocean is composed of small organisms, mostly small floating plants (phytoplankton, algae) and animals that form the basis of the marine food chain. The ocean is an important part of the flows of carbon through the earth system as well. The ocean also holds (or supports) part of the cryosphere as sea ice.⁶ Sea ice forms in winter in high latitudes in the Arctic Ocean and in the Southern Ocean. The sea ice moves around and can grow and melt. The ice may last several years and is a year-round feature in most of the Arctic today.

Most of the land surface is not covered by ice, however. Most of it is covered by plants: the terrestrial biosphere. There are also lakes and rivers that channel precipitation and return it to the ocean. The atmosphere and ocean cause changes over time in the composition of the land by erosion and processing of rock material. Water, wind, and waves break up rock. The rock reacts with the atmosphere (oxidation) and with water: Sometimes minerals in rocks dissolve in water. The land evolves by moving slowly as it floats over the crust. For purposes of climate, the land is often considered static, but we know it changes over long timescales. Volcanoes and earthquakes are evidence of this motion.

The terrestrial surface is intimately connected with the plants and the biosphere that lives on it: Dead plant (and a bit of dead animal) material makes up a significant part of the soil. Plants regulate the water in the soil through uptake into plant structures and release water into the atmosphere. Plants also can be darker or lighter than the surface soil, thus absorbing (and reflecting) a different amount of energy than in their absence. Plants react to their environment (rainfall), but plants and the biosphere also create their environment. Plants cycle water vapor back into the atmosphere from the soil by moving water from the soil into the plant, where it is lost in the process of photosynthesis back to the atmosphere. Plants also change the reflectance of the surface, altering the energy absorbed in the system, and the temperature of the surface. The biosphere cycles nutrients used by plants from the soil into plant material and back again, sometimes also into the ocean. Central to the biosphere are nutrients, and the elements that make up the structure of organic molecules: oxygen (O) and hydrogen (H) and carbon (C).

But some of the terrestrial surface, particularly at high latitudes, is frozen. This, of course, brings us to a discussion of ice and snow at high latitudes and altitudes: the cryosphere. Snow may be present seasonally on land at high latitudes or high altitudes. If snow lasts over the summer, and continues to pile up, it compacts into ice and forms glaciers. If glaciers get big enough, they merge into ice sheets, ranging in size from small ice sheets filling a few valleys in New Zealand or Alaska, to ice sheets covering entire land masses (Greenland) or continents (Antarctica). The Greenland and Antarctic ice sheets are up to 10,000 feet thick; 99 % of the fresh water on earth is in ice sheets. Greenland holds enough frozen water to raise the sea level 20 feet (6 m). Antarctica holds enough frozen water to raise the sea level 200 feet (60 m).⁷

Finally, while physically humans are part of the biosphere, our impact on the planet is large enough that they are often treated as a separate sphere, the anthroposphere. Human emissions as a by-product of our societies now play a significant role in the climate system. Humans alter the structure of ecosystems on continent wide scales (deforestation, and subsequent afforestation). By-products of our societies add to the greenhouse gas loading of the atmosphere, and change atmospheric chemical processes near the surface (creating extra ozone, which is bad for humans, plants and animals). In the upper atmosphere some of these compounds destroy ozone in the stratosphere necessary to block ultraviolet radiation from reaching the surface. These human actions are often modeled using economic models, or models of entire societies and their emissions. These models are sometimes now treated as part of the climate system because the outputs from societies can alter the climate system.