"Global change". "Greenhouse effect". "Global warming". The media are full of statements, concerns, guesses, and speculation about these phenomena, as scientists and policy-makers around the world struggle to address recent scientific observations that indicate that human activities have an impact on our environment. And yet, each of these is a "natural" phenomenon, as are many others. Hurricanes, droughts, monsoons, all occur without any control through human attempts to initiate, forestall or moderate such atmospheric events.

We can learn about the inner workings of our planet's interacting physical systems by observing the results of such natural phenomena, and use that knowledge in our explorations of human-induced changes. Consider, for example, the eruption of a volcano, such as occurred with Mt. Pinatubo in the Philippine Islands in 1991, that happens without human intervention.

When a volcano erupts it spews millions of tons of ash, debris, and gases into the atmosphere, not to mention the lava flows from some volcanoes that erupt with such outflows. Because of the presence of instruments--on the ground, at the ocean's surface, and in space--we were able to determine that a cloud of sulfur dioxide (SO₂) emitted by Pinatubo began to make its way westward, extending well past India within 12 days of the original eruption. By three months that cloud had completely encircled the Earth, as shown from space (below), and inside of a year SO₂ particles in the atmosphere were providing gloriously colored sunsets all over the globe and lowered global temperatures as well. Clearly, the eruption of such a volcano has impact on more people and places than just within its immediate vicinity.

Another fascinating example of a "natural" phenomenon is known as El Niño, because it has been found to occur with some regularity (although not complete predictability) around Christmas time. El Niño refers to the baby Jesus, whose birthday is celebrated at the end of December. When an El Niño occurs a pool of warm water that usually exists in the western Pacific Ocean moves eastward, taking up residence off the western coast of South America. In the process, weather patterns around the world change--often to the detriment of human populations--as do populations of fish off the coast of South America. In non-El Niño years, fish are abundant in this region, owing to the cold, nutrient-filled waters that are found there. When an El Niño occurs that cold water is forced deep into the Pacific, and fish populations decline dramatically, with concomitant effects on humans whose livelihood depends on those fish.

Other such citations abound. Some are readily amenable to observations from space sensors, particularly those dedicated to meteorological and oceanographic measurements (see Section 14). As examples, consider these near-global plots, the top from the Seasat Radar Scatterometer of prevailing wind patterns over the oceans at supercontinental scales and the lower, the mean day, night, and day - nite temperatures of the Earth's land and sea surfaces averaged for January of 1979 from Nimbus 6 HIRS 3.4 and 4.0 mm channels integrated with MSU microwave and infrared data. Suffice to say that even without human contributions--the Earth is a dynamic system, one that changes routinely and often drastically.

In addition to the observations made of the phenomena described above, other relevant recordings have accumulated. For example, it is now well documented that levels of several "trace" gases in our atmosphere have been rising and continue to rise. One of these, carbon dioxide (CO₂) has been on the increase since the middle of the last century after many, many years of essentially stable levels. Why is this? An easy explanation is that the Industrial Revolution began at about the time these increases were found to start. With that social upheaval came the use of biomass and coal for fuel to support these new industries. Burning such material results in the generation of CO₂. Other trace gases have been on the rise, as well. Methane (CH₄), from rice paddy production and enteric fermentation, is increasing, as is the presence of chlorofluorocarbons (CFCs) that have been used for many years in the production of foam products and as a refrigerant.

These gases all contribute to the greenhouse effect that is responsible for warming our atmosphere to the levels we now enjoy. The greenhouse effect results from the trapping of solar radiation reflected from the surface of the Earth by these (and other) gases. The atmosphere is essentially transparent to incoming solar radiation; after striking the Earth's surface the wavelength of this radiation increases as it loses energy. The gases we are discussing are opaque to this lower energy radiation, and therefore trap it as heat, thereby increasing the temperature of the atmosphere. As these gases increase--due to natural causes and as a result of human activity, they enhance the greenhouse effect, and make temperatures much warmer than we are used to or can even tolerate. As the temperature warms, the belts of our planet's surface where things grow will tend to move northward, changing ecological and biome patterns all over the globe. Other effects may be discerned in precipitation patterns, sea level changes, and more. Clearly, global mean annual temperatures are on the rise, and this conditioncan be monitored from space observations to help settle the question: how much is just a natural trend (e.g., inevitable interglacial warming) and how much is due to man's activities? A long-term data base needs to be established - we are now learning how to estimate past temperatures from CO₂ measurements of trapped air in glacial ice thousands of years old and other techniques (e.g., tree rings) - against which current and continuing measurements can be compared and judged.

Another role being played in global change by a class of one of these trace gases, the CFCs, is in the depletion of stratospheric ozone. Ozone in the stratosphere absorbs incoming solar ultraviolet radiation (UVR) of a type that is dangerous to living systems. This UVR is known to cause damage to the genetic material in living systems. Ozone prevents the UVR from reaching the surface of the Earth, and so protects us from its harmful effects. Ozone depletion has been observed during the Antarctic winter, an observation that helped determine the chemistry underlying this process. There, the right combination of cold stratospheric temperatures, ice crystals (or other solids that act as surfaces upon which the destruction chemistry occurs), and the nature of global wind patterns favor intensifying the process. Similar (although smaller) depletions have been noted in the Arctic, and some of the chemical agents involved have been shown to be increasing over mid-latitude regions, i.e., where most humans live.

Other human activities may be increasing the rate of global change. One now grabbing attention is the process of deforestation, whereby humans slash and burn or just clear-cut huge tracts of trees to either use the land for agriculture or the wood from the trees for building shelter. As these large regions become denuded biodiversity decreases, and land-use, water run-off patterns, and local weather phenomena are moderated. Already, satellite remote sensing has produced dramatic images of progressive deforestation, as witnessed in these two scenes taken years apart over the State of Rondonia in the Brazilian Amazon Basin by NOAA's AVHRR.

Thus, many observations and other data seem to point to humans as a major causative source having at least the potential for modifying global phenomena. However, we are not always sure of this: Are some of the observed changes--such as an apparent increase in atmospheric temperature--really due to human activities? Or are they part of a natural cycle that we are only now observing in detail because of the presence of instruments and sensors that were hitherto not available?

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