The molecules of carbon dioxide in theEarth’s atmosphere affect the heat balance of the Earth by acting as a one-wayscreen. Although these molecules allow radiation at visible wavelengths, wheremost of the energy of sunlight is concentrated, to pass through, they absorbsome of the longer-wavelength, infrared emissions radiated from the Earth’ssuce, radiation that would otherwise be transmitted back into space. For theEarth to maintain a constant average temperature, such emissions from theplanet must balance incoming solar radiation. If there were no carbon dioxidein the atmosphere, heat would escape from the Earth much more easily. Thesuce temperature would be so much lower that the oceans might be a solidmass of ice. Today, however, the potential problem is too much carbon dioxide.The burning of fossil fuels and the clearing of forests have increasedatmospheric carbon dioxide by about 15 percent in the last hundred years, andwe continue to add carbon dioxide to the atmosphere. Could the increase incarbon dioxide cause a global rise in average temperature, and could such arise have serious consequences for human society? Mathematical models thatallow us to calculate the rise in temperature as a function of the increaseindicate that the answer is probably yes. Under presentconditions a temperature of -18℃ can be observed at an altitude of 5 to 6 kilometers above theEarth. Below this altitude (called the radiating level), the temperatureincreases by about 6℃ per kilometer approaching the Earth’s suce, where theaverage temperature is about 15℃. An increase in the amount of carbon dioxide means that there aremore molecules of carbon dioxide to absorb infrared radiation. As the capacityof the atmosphere to absorb infrared radiation increases, the radiating leveland the temperature of the suce must rise. One mathematical model predictsthat doubling the atmospheric carbon dioxide would raise the global meansuce temperature by 2.5℃: This model assumes that the atmosphere’s relativehumidity remains constant and the temperature decreases with altitude at a rateof 6.5℃ per kilometer. The assumption of constant relative humidity isimportant, because water vapor in the atmosphere is another efficient absorberof radiation at infrared wavelengths. Because warm air can hold more moisturethan cool air, the relative humidity will be constant only if the amount ofwater vapor in the atmosphere increases as the temperature rises. Therefore,more infrared radiation would be absorbed and reradiated back to the Earth’ssuce. The resultant warming at the suce could be expected to melt snowand ice, reducing the Earth’s reflectivity. More solar radiation would then beabsorbed, leading to a further increase in temperature. According to thepassage, the greatest part of the solar energy that reaches the Earth is _____.