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What is global warming?
Global warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Climate has always changes from natural causes, but it is becoming clear that human activities have caused most of the past century‟s warming by releasing heat trapping gases – called greenhouse gases – into the atmosphere.
The greenhouse effect
The “greenhouse effect” is the warming that happens when certain gases in Earth‟s atmosphere trap heat. These gases let in light but keep heat from escaping, like the glass walls of a greenhouse .
First, sunlight shines onto the Earth‟s surface, where it is absorbed and then radiates back into the atmosphere as heat. In the atmosphere, “greenhouse” gases trap some of this heat, and the rest escapes into space. The more greenhouse gases are in the atmosphere, the more heat gets trapped. Scientists have known about the greenhouse effect since 1824, when Joseph Fourier calculated that the Earth would be much colder if it had no atmosphere. This greenhouse effect is what keeps the Earth‟s climate livable. Without it, the Earth‟s surface would be an average of about 60 degrees Fahrenheit cooler. In 1895, the Swedish chemist Svante Arrhenius discovered that humans could enhance the greenhouse effect by making carbon dioxide, a greenhouse gas. He began 100 years of climate research that has given us a complex understanding of global warming. Levels of greenhouse gases (GHGs) have gone up and down over the Earth‟s history, but they have been fairly constant for the past few thousand years. Global average temperatures have stayed fairly constant
over that time as well, until recently. Through the burning of fossil fuels, humans are enhancing the greenhouse effect and warming Earth.
Global warming and climate change
Scientists often use the term “climate change” instead of global warming. This is because as the Earth‟s average temperature climbs, winds and ocean currents move heat around the globe in ways that can cool some areas, warm others, and change the amount of rain and snow falling. As a result, the climate changes differently in different areas.
Aren’t temperature changes natural?
The average global temperature and concentrations of carbon dioxide (one of the major greenhouse gases) have changed on a cycle of hundreds of thousands of years as the Earth‟s position relative to the sun has varied. As a result, ice ages have come and gone. However, for thousands of years now, emissions of GHGs to the atmosphere have been balanced out by GHGs that are naturally absorbed. As a result, GHG concentrations and temperature have been fairly stable. This stability has allowed human civilization to develop within a consistent climate. Occasionally, other factors briefly influence global temperatures. Volcanic eruptions, for example emit particles that temporarily cool the Earth‟s surface. But these have no lasting effect beyond few years. Other cycles, such as El Niño, also work on fairly short and predictable cycles . Now, humans have increased the amount of carbon dioxide in the atmosphere by more than a third since the industrial revolution. Changes this large have historically taken thousands of years but are now happening over the course of decades.
Why is this a concern?
The rapid rise in greenhouse gases is a problem because it is changing the climate faster than some living things may be able to adapt. Also, a new and more unpredictable climate poses unique challenges to all life.
Historically, Earth‟s climate has regularly shifted back and forth between temperatures like those we see today and temperatures cold enough that large sheets of ice covered much of North America and Europe. The difference between average global temperatures today and during those ice ages is only about 5 degrees Celsius (9 degrees Fahrenheit), and these swings happen slowly, over hundreds of thousands of years. Now, with concentrations of greenhouse gases rising, Earth‟s remaining ice sheets (such as Greenland and Antarctica) are starting to melt too. The extra water could potentially raise sea levels significantly.
When sea levels rises, weather can become more extreme. This means more intense major storms, more rain followed by longer and drier droughts (a challenge for growing crops), changes in the ranges in which plants and animals can live, and loss of water supplies that have historically come from glaciers. Scientists are already seeing some of these changes occurring more quickly than they had expected. According to the studies, eleven of the twelve hottest years since thermometer readings became available occurred between 1995 and 2006.
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جمعه سی و یکم شهریور 1391 03:54 بعد از ظهر
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دوست عزیز ترجمه برخی مطالب در وب قرار داده شده است . اما بقیه مطالب ان شاءالله ترجمه خواهد شد.
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