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II Clouds and Climate So the planetary system of clouds interacts strongly with the radiation passing both upward and downward through the atmosphere. Though the atmosphere itself lets sunlight pass through, clouds reflect significant portions of that light back to space. This reflective cooling, operating by itself, lowers the planet’s surface temperature by more than 20°C. A blanket of clouds can also introduce warming at the surface by blocking the passage of infrared radiation (heat) from the ground. High, thin clouds are the most effective heaters. Lack of reliable data on the heating and cooling by clouds has long hindered the study of climate and climate change. Recently, however, the heating and cooling by clouds was measured by V. Ramanathan (now at the Scripps Institution of Oceanography) and colleagues at the University of Chicago with instruments aboard the Earth Radiation Budget Satellite. They found that the reflective “white clothes” cooling of clouds was about 50% greater than their “blanket warming” effect. It appears, then, that the net effect of Earth’s clouds is to cool the planet. Earth’s clouds are so deeply interwoven into the climate system that they cannot be ignored in even the most rudimentary climate models. But where they fit in is the question. We know they have both cooling and heating effects. How do these effects balance? A glance at the cloud-covered planet from space illustrates how clouds brighten, and therefore cool, the planetary environment by reflecting sunlight. If we could somehow cleanse the atmosphere of all cloud condensation nuclei and prevent the formation of clouds, the reflectivity, or albedo, of the cloudless planet would drop from its present value of 30% to around 10%. That would mean an increase in the planet’s temperature. It is even possible that the oceans might evaporate if clouds went away. But clouds also warm the Earth by holding in outgoing long-wave (infrared) radiation. We have already noted how, as farmers know, a cloud layer is an excellent preventative for frost. In deserts, though solar heating during the day might be very extreme, loss of infrared radiation to the cloudless star-filled skies can lead to surprisingly cool nights. Now consider the opposite: a hypothetical Earth cloaked everywhere with a uniform, thin, high-altitude cloud deck. Let us assume that these clouds have low reflectivity, so that the planetary albedo is unchanged from its present value of 30%. (The reflectivity of clouds is frequently higher than 30%, but here we wish to concentrate on their heating effect.) On this cloud-covered planet the surface temperature would be 27° C (49° F) higher than in the real world. It would be a hot planet indeed! Changes in Earth’s Climate from Clouds and from CO2 Changes in climate from pollution or natural causes are sometimes expressed in terms of the heating of the Earth system that would attend a doubling of carbon dioxide in the atmosphere. Our burning of wood, coal, gas, and oil will almost certainly cause a doubling of atmospheric CO2 over its 1950 levels sometime in 9 CLOUDS AND CLIMATE CHANGE the middle of the next century, and that will cause a change in climate. We know that, on the average, clouds cool the Earth. Satellite measurements of cloud radiation indicate that the cloud cooling effect is about four times as large as the estimated heating introduced by the doubling of atmospheric CO2. So small alterations in the way clouds and radiation interact can play significant roles as feedback mechanisms. a certain point, the thermostat senses it and turns on the furnace, the temperature rises, and when the thermostat again senses a temperature threshold, the furnace shuts off. It has even been suggested by the British naturalist James Lovelock that the planet has evolved feedback loops to stabilize its climate system. This socalled Gaia Hypothesis is described in greater detail later. Figure 8 is a wiring diagram for a simple feedback control system. Each block represents a component or machine in the system. Part of the output of the second block is fed back through block B to provide a transformed signal that is added to or subtracted from the system’s input. The important point is that the system’s output affects the input. Feedback systems have some interesting and frequently surprising characteristics. The way the system behaves depends on whether the output from Block B is added to (positive feedback) or subtracted from (negative feedback) the input. Systems with negative feedback tend to be sluggish and respond more slowly to input than do systems with positive feedback. Positive feedback, on the other hand, is associated with amplification, rapid response, even overshooting and instability. Some Cloud/Climate Feedbacks Scientists believe that during past ice ages cool oceanic currents flowing toward the equator caused a migration toward the tropics of cooling clouds like the oceanic clouds mentioned above and that this might well have amplified the general cooling trend. This is an example of positive feedback. There are numerous positive and negative feedbacks in the planetary climate system. This section discusses some of those involving clouds. Feedback Loops in General A feedback is a process that responds to a system change by enhancing or diminishing the change. When a fraction of the output from a system feeds back to the system, it either speeds it up (positive feedback) or slows it down (negative feedback). Feedback loops are ubiquitous in nature. The concept lies at the heart of all biological systems. Any time a living organism causes a change in its environment and then reacts to that change there is feedback. A dog is following a trail. If he deviates from the trail, the odor decreases (negative feedback), causing him to adjust his direction until the odor becomes stronger. Feedback control systems also play important roles in such human enterprises as engineering, economics, sociology, and political science. A household heating system is a feedback system. The temperature drops to ELEMENTARY FEEDBACK CONTROL SYSTEM B ± Input A Output Figure 8. General block diagram of a feedback loop. Box A is the forward, or main, transfer function; block B is the feedback transfer function. The feedback system’s input is to the left, and its output is on the right. 10 CLOUDS AND CLIMATE Center for Atmospheric Research in Boulder, Colorado, using global climate models called general circulation models (GCMs). These simulations have suggested that the central regions of North America may experience severe drying when atmospheric carbon dioxide doubles. This would decrease cloudiness, which would lead to increased solar heating on the soil of the Great Plains, amplifying the tendency toward drying. systems exhibit both negative and positive feedback and can be more responsive to some kinds of inputs than others. Feedback Loops in Climate Some of the feedback loops in the control system of Earth’s climate machine are: 1. Ice/albedo feedback. As planetary temperature falls, ice sheets and oceanic pack ice build. The bright ice reflects more solar radiation back into space, and the cooling is amplified. 5. Cloud height and amount/temperature feedback. Computer simulations show that increases in greenhouse gases cause clouds to be higher and deeper, especially in the tropics. Surprisingly, there tends also to be a reduction in cloud cover, leading to greater heating by sunshine and to higher clouds, which also heat the Earth. Some scientists have estimated that these two feedbacks together provide a slight net positive feedback, making the Earth slightly warmer than it would otherwise be. 2. Water vapor/temperature feedback. This is another positive feedback loop. We learned earlier in this module that the saturation vapor pressure of water increases with temperature. Water vapor, like carbon dioxide, is a greenhouse gas, absorbing infrared radiation. Increasing global temperatures cause additional water to evaporate, and the water vapor content of the atmosphere rises. The greenhouse effect of the increased water vapor “feeds back,” and amplifies, the warming, by holding heat. 6. Cloud wetness/temperature feedback. Global warming will lead to increased atmospheric water vapor, so one might expect the volume of clouds to increase accordingly. Larger clouds are brighter, so they would reflect more solar radiation and counter the warming trend, a negative feedback. 3. Ocean temperature/cloud feedback. During the ice ages, low clouds and midlatitude cyclonic clouds over oceans may have been driven toward the equator by cool oceanic currents. During the height of the last ice age, the midlatitude Atlantic Ocean was 5° to 10°C cooler than at present. The new cloud banks over the cooler oceans would have amplified the cooling tendency (positive feedback) in a way similar to the ice/albedo feedback. If the positive feedback is strong enough, there is the possibility of runaway; fortunately, in this case, there were apparently negative feedbacks that prevented the ice age from amplifying endlessly. The cloud–climate control system for Earth, with some of the feedback loops mentioned above, is outlined in Figure 9. The strength of the feedbacks is uncertain. In a recent review paper on greenhouse warming, John Mitchell, of the United Kingdom Meteorological Office in Bracknell, England, says that the formation of clouds and their radiative properties depend on many small-scale processes that cannot be determined in large-scale models. Current models have only extremely crude representations of clouds, cloud water, and cloud radiative properties, and this is one of the largest sources of uncertainty in scientists’ attempts to simulate Earth’s climate. Resolving this 4. Drought/cloud feedback. Scientists carry out climate simulations on large computers like the CRAY Y-MP supercomputer at the National 11 CLOUDS AND CLIMATE CHANGE uncertainty has become the number one priority for research in the area of global change. processes. The sulfur gases spread throughout the lower part of the atmosphere in the form of dimethylsulphide (DMS), which provides cloud condensation nuclei. Charlson and colleagues suggest that as the planetary temperature alters, the production of DMS and, therefore, cloud droplet concentration, would also change. A rising temperature means more DMS, which means more reflective clouds, which, according to Ramanathan, tends to cool the planet. The result would be to dampen out swings in planetary temperature. A Gaia Feedback Loop Robert Charlson, a chemist and cloud physicist at the University of Washington, and colleagues have suggested a possible feedback loop that brings Earth’s biosphere into the climate picture. Tiny organisms in the sea, phytoplankton, produce sulfur-containing gases as a byproduct of their physiological THE CLIMATE SYSTEM + + B Ice Albedo 0.4 W m-2/°C C H2O VaporTemperature 1.5 W m-2/°C A Forward Transfer Fluctuation in Solar Radiation + – D Cloud Height-Temperature Cloud Fraction-Temperature E Cloud MicrophysicsTemperature 0.27 °C/W m-2 Fluctuation in Surface Temperature 0.9 W m-2/°C 0 to -1.9 W m-2/°C Figure 9. A diagram showing four climate feedback loops: ice/albedo, water vapor/temperature, cloud height/temperature, and cloud microphysics. “A” in the center represents transfer of solar radiation out of the system. 12