2. Global Warming
Signs of global warming
The term “global warming” has both a general and specific meaning. It is used in a general sense to indicate that the Earth is getting hotter. Global warming can also specifically imply that a warmer planet is the result of an increase of heat-trapping gases in the atmosphere, resulting from human activity. There is still serious scientific disagreement on both the magnitude and timing of this type of global warming.
It is likely, but not certain, that some of the recent rise of the Earth’s surface temperature is due to such an increase in greenhouse gases. However, much, if not all of the warming could be caused by natural temperature variations that are not related to humankind. Scientists are currently struggling to determine how much of the warming is caused by natural factors or human beings.
Over the past century, the planet as a whole has warmed up in fits and starts, fluctuating between warm and cool periods. For example, the world heated up by about 0.5 degrees Celsius, or 0.9 degrees Fahrenheit, between 1920 and 1940; but the average temperatures dropped by about half that amount between 1940 and 1970, leading some experts to predict a coming ice age. Then the heat turned on again, rising by another 0.5 degrees Celsius in the last three decades of the 20th century. By the 1990s, the world had become hotter than any time in recorded history.
There are now many other signs that indicate a warming atmosphere. The oceans are rising; mountain glaciers are retreating; polar sea ice is melting; the very timing of the seasons is changing, and erratic and severe weather is more common than just a few years ago. Taken alone, these events are no proof of global warming, but in combination they provide song evidence for a warmer climate.
Rising seas provide additional evidence for a hotter world. As the water in the sea gets warmer, it will expand as most substances do when heated. The sea will then ascend to higher levels, in much the same way that heating the fluid in a thermometer causes the fluid to expand and rise up. This is because warm water or other fluids occupy a greater volume than cold ones. Measurements indicate that the global sea level increased somewhere between 10 and 25 centimeters during the 20th century. However, you could not have noticed the change, for the sea level was only rising between 1.0 and 2.5 millimeters per year.
The melting of ice that now covers land, such as mountain glaciers, also contributes to the sea-level rise and most likely results from global warming. By the end of the 20th century, glaciers were retreating throughout the world, and those in Alaska were typically becoming about a meter thinner every 4 or 5 years.
As it melts and shrinks, the glacial ice releases water into streams and rivers that add to the sea. Such melt waters from mountain glaciers boosted the sea level between 2 and 5 centimeters in the 20th century.
Contrary to popular belief, the melting of floating icebergs will not raise the level of surrounding sea. When ice cubes in your drink at home melt, they similarly do not cause any change in its level, for the melted ice produces the same volume of water as it displaces.
Because most of Antarctica’s ice now covers land, it would add to the ocean’s volume if it melted or broke off in blocks, in a process known as calving. Since the average surface temperature of the Antarctic Peninsula rose by 2.8 degrees Celsius, or 5.0 degrees Fahrenheit, in the last two decades of the 20th century, people are worried about the future meltdown of the Antarctica ice pack.
Long hot summers and warmer nights provide another mark of the warming trend. In the Northern Hemisphere, winter shortened by about a week on both ends during the last two decades of the 20th century. So spring arrives earlier than it used to, autumn arrives a bit longer, and the land stays greener. The longer, warmer summers are beginning to thaw the northern tundra, or permafrost, releasing methane that might further exasperate the build up of heat-trapping gases.
Something strange, it’s claimed, is going on with the weather, providing other unwelcome signs that the heat is on, affecting our everyday lives. In fact, such events were predicted as an early symptom of a rise in the average global temperature, and now they are here. These warming signs include record rainfalls, and severe snowstorms; extraordinarily destructive hurricanes and tornadoes; heat waves unique in weather annals; widespread droughts and devastating forest fires; unseasonable warmth and cold; and some of the worst floods in recorded weather history.
A hotter ocean surface produces such weather extremes by accelerating the water cycle. The cycle begins when some of the ocean evaporates, releasing warm fresh-water moisture into the air. This water vapor rises high into the colder atmosphere where clouds are formed. Winds drive the clouds for great distances over sea and land. Rain or snow from the clouds can then fall to land, refreshing streams, lakes or underground reservoirs. The water then runs down to the sea, where the cycle begins once more.
The rising temperatures heat the oceans more than they used to be, causing more water to evaporate. In addition, a warmed atmosphere holds more moisture than a cool one. As a result, there is more water in the air. When this extra water condenses, heavy rains, severe thunderstorms, blizzard-like snowstorms, and damaging floods become more frequent, intense and severe.
While the oceans are being heated, so is the land. It can become highly parched in dry areas, resulting in droughts that are more severe and persist longer than they used to. The rising temperatures also enlarge heat and pressure differences across the land, causing winds to develop and leading to more tornadoes and hurricanes.
Thus there are many indications that the Earth is getting hotter. Climate change has definitely arrived and the future is bound to be different from today. But it is still unclear how much of this warming and extreme weather is attributable to our accelerated release of greenhouse gases, and how much too natural causes. Many scientists think that both are involved and that human activity must be at least party responsible. There is also scientific uncertainty about the future consequences of global warming.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University