Luisa Cristini, PhD, University of Hawaii at Manoa
[Note from the editor: This is the fifth in a series of blog entries that will focus on introductory topics in climate dynamics and modeling, and will be a great insight into the current understanding of the science.]
Nearly all the energy entering the climate system comes from the Sun in the form of radiation. At the top of the Earth’s atmosphere, a surface perpendicular to the rays receives roughly 1368 W/m2 (Watts per meter squared). This value is called the Total Solar Irradiance (TSI) or Solar Constant. Some of this incoming radiation is reflected straight back to space by the atmosphere, the clouds, and the Earth’s surface.
In order to achieve a heat balance, the heat flux coming from the Sun must be compensated for by an equivalent heat loss. If this were not so, the Earth’s temperature would endlessly rise or fall.
The radiation emitted by the Earth has a much longer wavelength than those received from the Sun and is termed ‘longwave radiation’, while the radiation from the Sun is called ‘shortwave radiation’.
The atmosphere only absorbs about 20% of the incoming solar (shortwave) radiation, with the rest of its absorption taking place at the Earth’s surface. The atmosphere is almost opaque to infrared (longwave) radiation. This is related to the radiative properties of some constituents of the atmosphere, especially water vapor, carbon dioxide, methane, and ozone. These gases constitute only a small fraction of the atmospheric composition, while the two dominant components (molecular nitrogen and oxygen) play nearly no part in this opacity. Nevertheless, a significant fraction of the energy emitted by the Earth’s surface is absorbed by the atmosphere and re-emitted both upward to space and downward back to the Earth, significantly increasing the temperature of the system. In a garden greenhouse, panes of glass are transparent to visible light but opaque to infrared radiation, “trapping” part of the energy emitted by the surface and resulting in warming of the air. In the same way, the alteration of the energy budget by some minor atmospheric constituents described above is called the greenhouse effect and those minor constituents are the greenhouse gases that allow the near-surface atmosphere to behave in the same manner as a greenhouse.
The instantaneous insolation, defined as the energy received per unit time and unit surface on a horizontal plane at the top of the atmosphere, depends on the geographical position on Earth as well as the position of the Earth relative to the Sun. Earth’s trajectory around the Sun is an ellipse with the Sun at one focus. The point of the Earth’s orbit that is the closest to the Sun is called the perihelion, while the aphelion is the point that is farthest from the Sun. The parameters of the Earth’s orbit, defining its eccentricity, vary with time, thus varying the amount of solar radiation reaching the Earth.
The geographical distribution of the net incoming solar radiation at the top of the atmosphere (i.e. the incoming minus the reflected solar radiation) that is absorbed by the Earth is a function of the insolation distribution as well as of the regional variations of Earth’s albedo (reflectivity).
The thermal radiation emitted by the Earth’s surface is a function of the surface temperature. A difference of about 50°C (122°F) between the equator and the poles roughly corresponds to a variation in the emitted thermal radiation of about 50 W/m2.
The presence of clouds and water vapor has a large influence on Earth’s energy balance. Indeed, water vapor is a strong greenhouse gas. It absorbs part of the radiation emitted by the surface before re-emitting radiation, generally at a lower temperature since clouds are located higher in the atmosphere where the ambient temperature is colder. This results in less outgoing thermal radiation. As a consequence, the maximum outgoing radiation on the Earth is found above warm, dry areas such as subtropical deserts. More generally, wet equatorial areas emit less radiation than dry tropical areas.
The net radiative heat flux at the top of the atmosphere must be balanced by the sum of the horizontal heat transport, the heat exchanges with the deep ground, and the contribution to the heat budget associated with changes in the heat storage in the atmosphere, the ocean, and the ground. Since the ground has a very low thermal conductivity, only the top few meters interact with the surface on seasonal to decadal timescales. On daily and seasonal timescales, the heat storage by the climate system plays a large role in mitigating the influence of the changes in the radiative flux at the top of the atmosphere. The ground has a specific heat capacity similar to that of the ocean but only a few meters are affected by the seasonal cycle. As a consequence, the effective heat capacity of the ground is much lower than that of the ocean on this time scale.
The effective thermal capacity of the sea is an order of magnitude larger than that of the atmosphere and the ground on a seasonal timescale. As a consequence, the sea stores much more energy during summer than any other surface, energy that is released during winter. This moderates the amplitude of the seasonal cycle over the sea, in comparison with the land. A strong difference in the amplitude of the seasonal cycle is also seen in land areas that are directly influenced by the sea compared to land masses far away from sea (for example, Seattle vs. Minneapolis).
For decadal to centennial variations, such as the warming observed since the mid-19th century, thermal heat storage in the first hundred meters of the ocean (and at greater depths in regions of deep water formation) also moderates the transient temperature changes. On much longer time scales, such as glacial to interglacial cycles, we have to take into account the full depth of the ocean (~4000m).
Locally, heat storage by the climate system cannot compensate for the net radiative flux imbalance at the top of the atmosphere and, annually, the balance is almost entirely achieved by heat transport from regions with a positive net radiative flux to regions with a negative net radiative flux. When the balance is averaged over latitudinal circles (zonal mean), this corresponds to a heat transport from equatorial to polar regions.
In the tropics, the majority of the atmospheric poleward heat transport is achieved by the convective Hadley circulation. By contrast, the mean circulation plays a much smaller role at mid- to high-latitudes where nearly all the transport is effected by eddies, whirlpool-like transient features. In the ocean, both the wind-driven and deep-oceanic circulations are responsible for a significant part of the oceanic poleward heat transport, the latter having a dominant influence in the tropics. The role of the oceanic eddies is less well known, but they can be significant in some regions (such as the Southern Ocean).
The thermohaline circulation is an additional source of longitudinal asymmetry since, in the Northern Hemisphere, deep-water formation occurs only in the North Atlantic. The associated circulation transports cold water southward at great depths with the mass balance ensured by a corresponding northward transport of warmer water in the surface layer. The thermohaline circulation is also responsible for the northward oceanic heat transport at all latitudes in the Atlantic, even in the Southern Hemisphere.
An analysis of the Earth’s global heat balance shows that more than 70% of the reflection takes place in the atmosphere, mainly because of the presence of clouds and aerosols, which are small solid or liquid particles floating in the atmosphere. The remaining 30% is reflected by the surface. By contrast, the majority of the absorption of solar radiation occurs at the surface, which absorbs 2.5 times more solar energy than the atmosphere. This demonstrates that the majority of atmospheric warming occurs from below, and not by direct absorption of solar radiation. This important property of the system explains the major characteristics of the Earth’s atmosphere, including the vertical temperature profile and the large-scale circulation of the atmosphere
As previously mentioned, the outgoing radiation required to balance the Earth’s Budget at the top of the atmosphere is mainly emitted by the atmosphere and clouds. The majority of surface radiation is absorbed by the atmospheric greenhouse gases and re-emitted toward the surface where the downward radiation flux becomes the largest term in the surface heat balance.
In addition to radiative fluxes, the surface and the atmosphere exchange heat through direct contact between the surface and the air (sensible heat flux or thermals) as well as through evaporation and transpiration (latent heat processes).
Goosse H., P.Y. Barriat, W. Lefebvre, M.F. Loutre and V. Zunz, (2012). Introduction to climate dynamics and climate modeling. Online textbook available at http://www.climate.be/textbook.