Earth's Climate System

Earth’s Climate System 2 Climate is the state factor that most strongly governs the global distribution of terrestrial A Focal Issue biomes. This chapter provides a general back- Human activities are modifying Earth’s ground on the functioning of the climate sys- climate, thereby changing fundamental con- tem and its interactions with atmospheric trols over ecosystem processes throughout the chemistry, ocean, and land. planet, often to the detriment of society. Some climatic changes subtly alter the rates of ecosystem process, but other changes, such as the fre- Introduction quency of severe storms have direct devastating effects on society. Climate warming, for example, Climate exerts a key control over the function- increases sea-surface temperature, which increases ing of Earth’s ecosystems. Temperature and the energy transferred to tropical storms (Fig. 2.1). water availability govern the rates of many bio- Although no individual storm can be attributed to logical and chemical reactions that in turn control climate change, the intensity of tropical storms critical ecosystem processes. These processes may increase (IPCC 2007). Other expected effects include the production of organic matter by plants, of climate change include more frequent droughts its decomposition by microbes, the weathering of in drylands such as sub-Saharan Africa, more fre- rocks, and the development of soils. Understanding quent floods in wet climates and in low-lying the causes of temporal and spatial variation in cli- coastal zones, warmer weather in cold climates, mate is therefore critical to understanding the and more extensive wildfires in fire-prone forests. global pattern of ecosystem processes. What determines the distribution of Earth’s major The amount of incoming solar radiation, the climate zones? Why is climate changing, and why chemical composition and dynamics of the atmo- do regions differ in the climatic changes they sphere, and the surface properties of Earth deter- experience? An understanding of the causes of mine climate and climate variability. The temporal and spatial variation in the climate sys- circulation of the atmosphere and ocean influ- tem facilitates predictions of the changes that are ences the transfer of heat and moisture around the likely to occur in particular places. planet and thus strongly influences climate patterns and their variability in space and time. This chapter describes the global energy budget and Earth’s Energy Budget outlines the roles that the atmosphere, ocean, and land surface play in the redistribution of energy The sun is the source of the energy available to to produce observed patterns of climate and eco- drive Earth’s climate system. The wavelength system distribution. of energy produced by a body depends on its F.S. Chapin, III et al., Principles of Terrestrial Ecosystem Ecology, 23 DOI 10.1007/978-1-4419-9504-9_2, © Springer Science+Business Media, LLC 2011 24 2 Earth’s Climate System Fig. 2.1 Satellite view of Hurricane Katrina over coastal the hurricane. Climate warming is expected to increase Louisiana. This tropical storm flooded New Orleans in the frequency of severe tropical storms like Hurricane 2005, killing approximately 1,570 people and causing Katrina. Image courtesy of NOAA (http://www.katrina. $40–50 billion of damage. Human-caused ecological noaa.gov/satellite/satellite.html) changes in coastal Louisiana contributed to the impact of temperature. Because it is hot (6,000°C), the sun transmits about half of the incoming shortwave emits most energy as high-energy shortwave radiation to Earth’s surface, radiatively active radiation with wavelengths of 0.2–4.0 mm gases (water vapor, CO2, CH4, N2O and industrial (Fig. 2.2). These include ultraviolet (UV; 8% of products like chlorofluorocarbons [CFCs]) absorb the total), visible (39%), and near-infrared (53%) 90% of the outgoing longwave radiation radiation. On average, about 30% of the incom- (Fig. 2.3). Of the approximately 10% of long- ing shortwave radiation is reflected back to space, wave radiation that escapes to space, most is in due to backscatter (reflection) from clouds wavelengths where longwave absorption by the (16%); air molecules, dust, and haze (6%); and atmosphere is small (referred to as atmospheric Earth’s surface (7%; Fig. 2.3). Another 23% of windows; Fig. 2.2). The energy absorbed by radi- the incoming shortwave radiation is absorbed by atively active gases in the atmosphere is re-radi- the atmosphere, especially by ozone in the upper ated in all directions (Fig. 2.3). The portion that is atmosphere and by clouds and water vapor in the directed back toward the surface contributes to lower atmosphere. The remaining 47% reaches the warming of the planet, a phenomenon known Earth’s surface as direct or diffuse radiation and as the greenhouse effect. Without these long- is absorbed there (Trenberth et al. 2009). wave-absorbing gases in the atmosphere, the Earth also emits radiation, like all bodies, but, average temperature at Earth’s surface would be due to its lower surface temperature (about 15°C), about 33°C lower than it is today, and Earth Earth emits most energy as low-energy longwave would probably not support life, except perhaps radiation (Fig. 2.2). Although the atmosphere at hydrothermal vents in the deep ocean. Earth’s Energy Budget 25 longwave absorption by radiatively active gases and by the absorption of some incoming (short- ) 2 wave) solar radiation; it is also heated from the − m surface by non-radiative fluxes of heat that are car- ried upward by atmospheric turbulence (mixing). These include latent heat flux, where heat that evaporates water at the surface is subsequently Energy (W released to the atmosphere as air parcels rise and cool, and the water vapor condenses, forming Solar incoming radiation clouds and precipitation. There is also an upward transfer of heat that is conducted from the warm surface to the air immediately above it and then Terrestrial outgoing radiation moved upward by convection of the atmosphere as thermals (sensible heat flux). These heat sources 0510 15 20 25 Near infrared collectively sustain the longwave emission to Visible region space, as well as a large flux of longwave radiation UV from the lower atmosphere back to Earth’s surface. 1 This back radiation to the surface represents the CH4 natural greenhouse effect described earlier. 0 1 Long-term records of atmospheric gases, N2O 0 obtained from atmospheric measurements since 1 O2 and O3 the 1950s and from air bubbles trapped in glacial ptivity 0 1 ice, show large increases in the major radiatively CO 0 2 active gases (CO , CH , N O, and CFCs) since Absor 1 2 4 2 H2O the beginning of the industrial revolution 250 0 1 years ago (see Fig. 14.7). Human activities such Atmosphere 0 as fossil fuel burning, industrial activities, animal 0 510 15 20 25 husbandry, and fertilized and irrigated agriculture Wavelength (µm) contribute to these increases (see Chap. 14). As concentrations of these gases rise, the atmosphere Fig. 2.2 The spectral distribution of solar and terrestrial radiation and the absorption spectra of the major radia- traps more of the longwave radiation emitted by tively active gases and of the total atmosphere. These spec- Earth, enhancing the greenhouse effect and tra show that the atmosphere absorbs a larger proportion of increasing Earth’s surface temperature. A small terrestrial radiation than solar radiation, explaining why imbalance thus exists in the radiative flows shown the atmosphere is heated from below. Redrawn from Sturman and Tapper (1996) and Barry and Chorley (2003) in Fig. 2.3, estimated to be about 0.26% of the incoming radiation. Most of this excess energy is absorbed in the ocean, causing water to expand As a global long-term average, Earth is and sea level to rise. The warming caused by normally close to a state of radiative balance, radiative imbalance also contributes to wide- meaning that it emits as much energy back to spread melting of glaciers and ice sheets space (as longwave radiation) as it absorbs. (Greenland and Antarctica) and arctic sea ice. However, human activities are changing the com- The globally averaged annual energy budget position of the atmosphere enough to increase outlined above gives a sense of the critical factors the heat retained by the planet, as described controlling the global climate system. Regional later. Assuming balance, the longwave radiation climates, however, reflect spatial variation in emitted to space must equal the sum of the energy exchange and in lateral heat transport by solar radiation absorbed by both the surface and the atmosphere and the ocean. Earth is heated the atmosphere. The atmosphere is heated by more strongly at the equator than at the poles and 26 2 Earth’s Climate System Fig. 2.3 The average annual global energy balance dur- and absorbed longwave radiation (104 units) and ing 2000–2004 for the Earth-atmosphere system. The latent + sensible heat flux (29 units) are balanced by long- numbers are percentages of the energy received from wave emission to space (58 units) and longwave emission incoming solar radiation. At the top of the atmosphere, the to Earth’s surface (98 units). At Earth’s surface, the incom- incoming solar radiation (100 units or 341 W m−2 [global ing shortwave (47 units) and incoming longwave radiation average]) is balanced by reflected shortwave (30 units) (98 units) are balanced by outgoing longwave radiation and emitted longwave radiation (70 units). Within the (116 units) and latent + sensible heat flux (29 units). Data atmosphere, the absorbed shortwave radiation (23 units) are from Trenberth et al. (2009) rotates on an axis that is tilted relative to the plane of its orbit around the sun. Its continents are The Atmospheric System spread unevenly over the surface, and its atmospheric and oceanic chemistry and physics are Atmospheric Composition dynamic and spatially variable.

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