Global Energy Balance and Climate

Variations in solar radiation and the energy balance components strongly shape how the climatic conditions near

Solar Terrestrial Solar Terrestrial radiation radiation radiation radiation

Solar Terrestrial Solar Terrestrial radiation radiation radiation radiation

Heat transport = k • (Tp -Tt)

(i) No heat transport

Temperature Solar

.Terrestrial

(i) No heat transport

Latitude

Latitude

(ii) Some heat transport

Temperature Solar trial

(ii) Some heat transport

Temperature Solar trial

Latitude

Latitude

(iii) Maximum heat transport Solar

(iii) Maximum heat transport Solar

G G SO BO 9G North

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South

G G SO BO 9G North

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Heat transport coefficient k

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, that is, a state where the atmosphere works and dissipates kinetic energy as much as possible.

the surface vary in space and time. This linkage is important to understand for the present-day climate, but also how it reacts and interacts with change.

Geographic and Temporal Variations

The geographic variation of the main climatic variables, surface temperature and precipitation, and their seasonal variation are strongly connected to the energy balance (Figure 6).

The annual mean temperature distribution largely follows the variation of solar radiation with latitude. Some deviations in this pattern can be found at west coasts in the subtropics where cold ocean currents affect temperature, and in high altitudes, such as the Andes and the Tibetan plateau. The seasonality in temperature is much stronger over land than over oceans. This reflects the differences in heat storage (Table 2) and the lack of oceanic heat transport on land.

Mean precipitation is at its peak in the tropics and is associated with general upward motion of the atmospheric circulation. The subtropics are dominated by a lack of rainfall, which is associated with large-scale sinking motion, which prevents air from cooling and saturating. This shapes the large-scale distribution of deserts. Seasonality is strongest in the tropics, which is associated with the seasonal change of solar radiation.

The availability and seasonality of precipitation on land has large impacts on the surface energy and water balance as it limits the amount of water than can be evaporated or transpired by the vegetative cover, thereby affecting the latent heat flux.

Feedbacks

Global changes (e.g., orbital parameters, atmospheric concentration of carbon dioxide) affect energy balance components, which in turn alter climate. These changes can be amplified or reduced due to feedbacks. Feedbacks characterize the response of the global energy balance to a perturbation or external forcing. They formalize the non-linearities and interactions in the climate system. Feedbacks are classified into positive and negative feedbacks. Positive feedbacks enhance the response of a chosen variable (mostly temperature) to the external forcing, while negative feedbacks stabilize the system, making it less responsive. In other words, a positive feedback makes a positive (negative) external change more

180 -90

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r—«—I—'—^—I—1—I—1—I—1—I—1—I

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Temperature -90 0 90 180

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-40 -30 -20 -10 0 10 20 30 40 Delta temperature

Delta precipitation

Figure 6 Annual mean climate during the period 1980-90. (a) Annual mean near-surface air temperature and its seasonal variation (June-August average minus December-February average). (b) The 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. Data sets have been obtained from the ECMWF data server.

-40 -30 -20 -10 0 10 20 30 40 Delta temperature

Delta precipitation

Figure 6 Annual mean climate during the period 1980-90. (a) Annual mean near-surface air temperature and its seasonal variation (June-August average minus December-February average). (b) The 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. Data sets have been obtained from the ECMWF data server.

positive (negative). Examples for important climate sys- respiration are generally neglected in considerations of tem feedbacks are given in Figure 7. the surface energy balance.

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