Global Warming

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The term global warming simply means that the global climate is warming. Humans are popularly assumed to be the cause of global warming. Further, global warming is usually assumed to be harmful to humans and to plant and animal life. Global warming is a commonly discussed and debated scientific topic both in the media and in the scientific community.

Nearly all of the scientific community agrees that based on surface temperature observations, the global climate warmed by about 0.5° C in the twentieth century. Satellite observations of global temperatures show warming trends between 1970 and 1990 similar to those found in surface observations. Decreases in sea ice cover and global glacier retreat provide corroborating evidence of global warming. A few scientists believe that the warming at weather stations is due to the development of cities around weather stations, but analysis in the late 1990s has shown that warming is similar at urban and rural areas. Different areas of the world and different seasons have warmed more than others. Due to global atmospheric circulation patterns that transport heat from the tropics to the poles, warming has been greatest in high latitudes. In some areas of Alaska and Asia, average temperatures have warmed by over 4°C. Warming has also been greatest in spring months, particularly March, and in nighttime minimum temperatures much more than in daytime maximum temperatures.

While the presence of global warming is not seriously debated, its causes are. Greenhouse gases, including carbon dioxide, trap heat radiated from Earth and reemit some of it back to the ground. Without greenhouse gases, our planet would be uninhabitably cold. As shown by research on air bubbles trapped in ice cores, high temperatures have been associated with high levels of greenhouse gases during the geologic past. Most human-made carbon dioxide is produced by cars and industrial activity. As a result of fossil fuel burning, carbon dioxide levels have increased from 280 parts per million in preindustrial times to more than 360 parts per million by the late 1990s. Scientists believe that the pace of greenhouse gas emissions in the late twentieth century is partially responsible for the recorded temperature increases. This is called the enhanced greenhouse effect. A minority of scientists believe that natural processes, such as increased sun spot activity, account for the observed warming. However, statistical analysis shows that compared with the enhanced greenhouse effect, these other processes are highly unlikely causes of the observed warming.

Year 1 of a simulated greenhouse effect. The temperature is coldest at the poles and graduates to the highest temperature around the equator.

Computer models called global circulation models (GCMs) predict that warming will continue to increase. Due to processes called negative feedbacks that reduce global warming, there is some uncertainty over how rapidly warming will occur. Clouds, for example, are a result of evaporation. In a warmer climate, more evaporation will occur, leading to more clouds. Most types of clouds reflect solar radiation; this would act to cool temperatures. Other processes can act as positive feedbacks that increase global warming. Sulfur particles emitted from factories block radiation and thus cool temperatures. In an ironic twist, making factories cleaner could actually increase global warming. Accurately considering the complex network of feedbacks is a critical field of global warming research. In spite of these uncertainties, GCMs consistently predict further global warming.

The pace at which record high temperatures are being broken is increasing. The years 1998, 1997, 1995, and 1990 were the warmest since at least 1400 C. E. The twentieth century was also the warmest century since at least 1000 C. E. The bulk of observational evidence shows that not only is warming occurring, but that it is occurring at a progressively more rapid pace. Scientists researching past climates have shown that during glacial and interglacial periods, global temperatures (and carbon dioxide levels) have changed by more than the 0.5 degree (and 25 percent) currently seen. If plants and animals have survived these past changes, why should we be concerned about present changes? The answer is that the rate of greenhouse gas and temperature increase appears to be unprecedented. There is no guarantee that ecosystems and the people that depend on them will be able to adjust to the predicted levels of global warming.

Global warming effects on plants depend on the existing climate and vegetation, but in general, there are three main categories of potential effects. First, for plants existing in climatic extremes, global warming may havedrastic impacts. For example, plants requiring cold temperatures may be forced off of mountain peak habitats. If this is their only habitat, extinction will occur. Second, global warming is likely to cause large shifts in biome distribution. Coniferous forests will shift farther north and grasslands and deserts will expand. Third, global warming can alter how plants function in their existing environment. For many areas, global warming is likely to lengthen the growing season, causing an increase in photosynthesis. Multiple observations show that the growing season has already significantly lengthened, especially in northern latitudes. For most plants, alteration in plant function without causing their extinction or displacement is the most likely consequence of global warming.

Year 12 of a simulated greenhouse effect. Note the irregularly shaped warm areas over Asia, North America, and Antarctica.

While people usually think about the effects of global warming on plants, plants can also moderate the effects of global warming. Only about half of the carbon dioxide put in the atmosphere remains there; the other half is taken up by Earth. Some of it is dissolved in the ocean, but plants take up some of it too. Plants are therefore acting to slow the pace of the enhanced greenhouse effect. Plants and especially trees are storing some of the carbon dioxide in wood. Unfortunately, this process is unlikely to continue forever. Eventually, when trees and shrubs die, the carbon stored in wood will enter the soil and will begin to decompose. Increased temperatures will cause high rates of decomposition, leading to an accelerated release of carbon dioxide from the soil. Consequently, it is likely that at a global level, ecosystems will begin to release carbon dioxide. Only by reducing fossil fuel emissions, a very difficult task both politically and economically, will we reduce greenhouse gasses.

Atmosphere and Plants; Carbon Cycle; Human Impacts.

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