Energy Balance

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Abstract:

The global energy balance characterizes the planetary functioning of Earth and thereby sets the large-scale conditions for ecosystems. This article reviews the basics of the energy balance and the atmospheric greenhouse effect and provides current estimates of the global energy balance components for the presentday. The irreversible nature of the energy balance is highlighted by the global entropy budget with current estimates for the rates of entropy production associated with the various dissipative processes within the Earth system. Details are provided regarding the radiative exchange, including the processes that result in the scattering and absorption of radiation. The role of atmospheric and oceanic circulation is demonstrated in the energy balance and the applicability of the hypothesis of Maximum Entropy production is being discussed. The description of the energy balance terms are then related to the mean climatological variations of temperature and precipitation. The role of ecosystems in the energy balance is discussed as well as associated feedback loops. The article closes with a discussion of the controversial Gaia hypothesis and the associated Daisyworld model that represents the extreme form of biotic regulation of the global energy balance. All ecosystems are affected by and interact with their environment. At the global scale, the Earth"s environment is characterized by the global energy balance, the balance of all heating and cooling terms that shape the climatic variations in space and time, especially with respect to surface temperature, precipitation, and light. From an energy balance viewpoint, the interrelationship between ecosystems and their environment are threefold: (i) ecosystems utilize energy sources from their environment, and thereby are a part – though small – of the energy balance, (ii) ecosystem processes are affected by environmental conditions that are directly or indirectly connected to the energy balance (e.g. precipitation affects the levels of water limitation of terrestrial productivity) and (iii) the form and functioning of ecosystems affect energy balance terms. This article reviews the basics of the global energy balance, how it is reflected in the seasonal and geographic distribution of mean climatic properties, and how it interacts with life through ecosystem functioning.

Reference:

Encyclopedia of Ecology (5 vols.), S. E. Jørgensen and B. D. Fath (eds.), Global Ecology, Vol. 2, pp. 1276-1289, Elsevier, Oxford.
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Postprint of this manuscript (accepted version of the paper formatted by author).
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Figure 1: Earth"s global energy balance. The dominant energy fluxes and their brief description within the Earth"s climate system, expressed as percentage of the average amount of incoming solar radiation of 342 W m^-2.

Figure 2: The orbit of the Earth around the Sun and its relation to seasons. The orbit of the tilted Earth around the Sun results in the seasons, as indicated for the Northern hemisphere (NH). In the NH winter, the Earth"s axis of rotation is pointed away from the Sun, resulting in less incident solar radiation and the polar night at latitudes above the arctic circle. This situation is reversed in the summer.

Figure 3: Effects of the orientation of the Earth"s surfaces towards the Sun on the amount of incident solar radiation at different locations of the Earth"s surface. left: The amount of solar radiation that reaches the surface at a given latitude depends on the declination angle δ;. The declination angle measures the angle between the Earth"s axis of rotation to the vertical of the orbit, or, alternatively, the angle between the direction of solar radiation and the Earth"s equator. The declination angle defines Earth"s major regions: the tropics (latitudes -δ to +δ) and the polar regions (90° - δ to pole). Earth"s declination angle is currently at 23.45°. right: At a given location on Earth, the zenith angle measures the angle between the vertical and the Sun. It depends on hour, latitude, and time of year. In the situation shown on the left (northern hemisphere winter solstice), the zenith angle at location A at noon is 90°, i.e. the Sun does not rise above the horizon and no solar radiation is incident at the surface. At location B, the zenith angle is in between 0° and 90°. At location C at the tropic of capricorn, the zenith angle is 0° at noon, and the incoming solar radiation is vertical to the surface. In sloped terrain, a correction needs to be applied for the calculation of incident radiation to correct for the slope.

Figure 4: Zonally and annually averaged components of the energy - and water balance at the top of the atmosphere and the surface from the time period 1980-1990. top: The graph shows the fluxes of net solar radiation (incoming minus reflected, red line, “solar”), terrestrial radiation (outgoing longwave radiation, blue line, “terrestrial”), and the difference between both (black line labeled “net”) at the top of the atmosphere. The positive values of net radiation in the tropics (i.e. more solar radiation is absorbed than terrestrial radiation emitted to space) indicates that heat is transported by the atmosphere and ocean systems towards the polar regions, where net radiation is negative. middle: Surface energy balance components of absorbed solar radiation (red line, “solar”), net emission of terrestrial radiation (blue solid line, “terrestrial”), latent heat flux associated with evaporation (blue dashed line, “latent heat”), sensible heat flux (blue dotted line, “sensible heat”), and the residual (black line, “net”). The residual consists of the effects of ocean heat transport and heat fluxes due to freeze/thaw of sea-ice. bottom: The atmospheric water budget, reflected by annual mean precipitation (red line), evaporation (blue line), and the difference (“net”, black line). Regions where evaporation exceeds precipitation (“net” is negative) are regions where the atmosphere gains moisture, which is transported by the atmospheric circulation to regions where precipitation exceeds evaporation (“net” is positive). The plots were created using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data sets.

Figure 5: Heat transport, its effects on the radiative balance at the top of the atmosphere, and its effects on entropy production. A: Conceptual diagram to illustrate the effect of poleward heat transport on the radiation balance at the top of the atmosphere. With heat transport, less terrestrial radiation is emitted to space in the tropics, but more is emitted in the polar regions. This effect is indicated by the yellow arrows. B: Sketches of how solar and terrestrial radiation and surface temperature would vary for no, some, and maximum amount of heat transport. C: Conceptual model results that demonstrate the existence of a maximum in entropy production associated with poleward heat transport, i.e. a state where the atmosphere works and dissipates kinetic energy as much as possible.

Figure 6: Annual mean climate during the period 1980 - 1990. left: Annual mean near surface air temperature and its seasonal variation (June-August average minus December - February average). right: same, but for annual mean precipitation and its seasonal variation. The plots were created using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data sets.

Figure 7: Dominant feedback processes that shape the response of the global energy balance and surface temperature to external change. The diagrams show the variables involved in four important feedback processes. The (+/-) signs at the arrows indicate positive/negative influences. Figure 7a: The thermal radiation feedback. An external change in forcing that would increase surface temperature would also increase the emission of longwave radiation (a “+” influence). An increased emission would result in a lower surface temperature (a “-” influence). The enhanced emission of longwave radiation therefore counteracts the initial change, resulting in a negative feedback loop. The same line of reasoning also applies for an external change that would reduce surface temperature.

Figure 7b: The snow/ice albedo feedback. An external change that warms the surface reduces the presence of snow, lowers the surface albedo, thereby amplifying the warming (a positive feedback).

Figure 7c: The water vapor feedback: An external change that warms the surface heats the lower atmosphere. Since warmer air can hold more moisture, this enhances surface evaporation and the amount of water vapor in the atmosphere. More water vapor results in a stronger atmospheric greenhouse effect, thereby amplifying the initial change (a positive feedback).

Figure 7d: Two types of cloud feedbacks. Continuing from the water vapor feedback, more water vapor in the atmosphere can result in more clouds. Depending on the balance of increased cloud cover on shortwave reflection (path A) or increased greenhouse forcing (path B), cloud feedbacks can form both, positive and negative feedback loops on surface temperature.

Figure 8: Climatic differences of a “Desert World”. Annual mean differences in (top) near surface air temperature, (middle) precipitation, and (bottom) cloudiness between the simulated climate of a “Desert World” void of terrestrial vegetation and the simulated present-day climate. These climatic differences result from the effect of vegetation on surface albedo, aerodynamic surface roughness, and the depth of the rooting zone.

Figure 9a: Vegetation feedbacks on the surface energy balance. The diagrams show the two major feedback loops by which vegetation directly affects the physical functioning of the surface energy balance. A: The snow-masking feedback. An external change in forcing that would increase surface temperature in regions where temperature limits terrestrial productivity (such as the arctic) increases the length of the growing season. A longer growing season would result in higher productivity, which extends the boreal forest cover in temperature-limited regions. Enhanced boreal forest cover masks the presence of snow at the surface, thereby lowering the surface albedo. This results in enhanced absorption of solar radiation, which amplifies the initial change, resulting in a positive feedback loop.

Figure 9b: Vegetation feedbacks on the surface energy balance. The diagrams show the two major feedback loops by which vegetation directly affects the physical functioning of the surface energy balance. B: The water cycling feedback. An external change that results in enhanced precipitation in regions where water limits productivity (such as the semiarid tropics) increases the length of the growing season, resulting in higher productivity. This extends vegetative cover, and thereby evapotranspiration, atmospheric moisture content, resulting in more precipitation. The initial change is hence amplified, resulting in a positive feedback.

Figure 10: Emergence of temperature regulation in the conceptual “Daisyworld” model. The “Daisyworld” model is a conceptual model of a virtual world in which the planetary albedo is regulated by the population dynamics of black and white daisies. The top figure shows the fractional cover of black and white daisies for different values of solar luminosity, expressed as the fraction of its present-day value. The different proportions of daisies result in an overall planetary albedo that results in constant temperature conditions (bottom) over a wide range of solar luminosity values.