영어명문/The World is Getting Warmer. Why? (Carl Sagan)
The World is Getting Warmer. Why?
by Carl Sagan
Today, we live in a balmy epoch, 10,000 years after the last major glaciation.
In the climatic spring, our species has flourished, we now cover the entire planet and are altering the very appearance of our world.
Lately - within the last century or so - humans have acquired, in more ways than one, the ability to make major changes in that climate upon which we are so dependent.
The Nuclear Winter findings are one dramatic indication that we can change the climate - in this case, in the spasm of nuclear war.
But I wish here to describe a different kind of climatic danger, this one slower, more subtle and arising from intentions that are wholly benign.
When sunlight strikes the Earth, part is reflected back into the sky;
much of the rest is absorbed by the ground and heats it - the darker the ground, the greater the heating.
The ground radiates back upward in the infrared.
Thus, for an airless Earth, the temperature would be set solely by a balance between the incoming sunlight absorbed by the surface and the infrared radiation that the surface emits back to space.
When you put air on a planet, the situation changes. The Earth's atmosphere is, generally, still transparent to visible light.
That's why we can see each other when we talk,
glimpse distant mountains and view the stars.
But in the infrared, all that is different.
While the oxygen and nitrogen in the air are transparent in both the infrared and the visible, minor constituents such as water vapor(H2O) and carbon dioxide(CO2) tend to be much more opaque in the infrared.
Accordingly, if you add air to a world, you heats it:
The surface now has difficulty when it tries to radiate back to space in the infrared.
The atmosphere tends to absorb the infrared radiation, keeping heat near the surface and providing an infrared blanket for the world.
There is very little CO2 in the Earth's atmosphere - only 0.03%. But that small amount is enough to make the Earth's atmosphere opaque in important regions of the infrared spectrum.
CO2 and H2O are the reason the global temperature is not well below freezing.
We owe our comfort - indeed, our very existence - to the fact that these gases are present
and are much more transparent in the visible than in the infrared.
Our lives depend on a delicate balance of invisible gases.
Too much blanket, or too little, and we're in trouble.
The greenhouse effect - what it is and isn't.
This property of many gases to absorb strongly in the infrared but not in the visible, and thereby to heat surroundings, is called the "greenhouse effect".
A florist's greenhouse keeps its planty inhabitants warm. The phrase "greenehouse effect" is widely used and has an instructive ring to it, remaining us that we live in a planetaryscale greenhouse and recalling the admonition about living in glass houses and throwing stones.
But, in fact, florist's greenhouses do not keep warm by the greenhouse effect; they work mainly by inhibiting the movement of air inside, another matter altogether.
When humans burn wood or "fossil fuels"(coal, oil, natural gas, etc), they put carbon dioxide into the air. One carbon atom(C) combines with a molecule of oxygen(O2) to produce CO2.
The development of agriculture, the conversion of dense forest to comparatively sparsely vegetated farm, has moved carbon atoms from plants on the ground to carbon dioxide in the air.
About half of this new CO2 is removed by plants or by the layering down of carbonates in the oceans. On human time-scales, these changes are irreversible; Once the CO2 is in the atmosphere,
human technology is helpless to remove it. So the overall amount of CO2 in the air has been growing - at least since the industrial revolution.
If no other factors operate, and if enough CO2 is put into the atmosphere, eventually the average surface temperature will increase perceptibly. As the climate warms, glacial ice melts.
Over the last 100 years, the level of the world's oceans has risen by 15 centimeters.
A global warming of 3°... or 4°... over the next century is likely to bring a further rise in the average sea level of about 70 centimeters.
An increase of this magnitude could produce major damage to ports all over the world and induce fundamental changes in the patterns of land development. A serious speculation is
that greenhouse temperature increase of 3°... or 4°... could , in addition, trigger the disintegration of the West Antarctic Ice Sheet, with huge quantities of polar ice falling into the ocean. This would raise sea level by some 6 meters over a period of centuries, with the eventual inundation of all cities on the planet.
The importance of thinking globally. The problem is difficult for at least three different reasons:
(1) We do not yet fully understand how severe the greenhouse consequences will be.
(2) Although the effects are not yet strikingly noticeable in everyday life, to deal with the problem, the present generation might have to make sacrifices for the next.
(3) The problem cannot be solved except on an international scale; The atmosphere is ignorant of national bundaries. South African carbon dioxide warms Taiwan, and Soviet coalburning practices affect agricultural productivity in America. The largest coal resources in the world are found in the Soviet Union, the United States and China,in that order. What incentives are there for a nation such as China, with vast coal reserves and al commitment to rapid economic development, to hold back on the burning of fossil fuels because the result might, decades later, be a parched American sunbelt of still more ghastly starvation in sub-Saharan Africa?
Would countries that might benefit from a warmer climate be as vigorous in restraining the burning of fossil fuels as nations likely to suffer greatly? Fortunately, we have a little time. A great deal can be done in decades.
Some argue that government subsidies lower the price of fossil fuels, inviting waste;
more efficient usage, besides its economic advantage, could greatly ameliorate the CO2 greenhouse problem. Parts of the solution might involve alternative energy sources, where appropriate: solar power, for example, or safer nuclear fission reactors, which, whatever their other dangers, produce no greenhouse gases of importance. Conceivably, the long-awaited advent of commercial nuclear fusion power might happen before the middle of the next century.
However, any technological solution to the looming greenhouse problem must be worldwide.
It would not be sufficient for the United States or the Soviet Union, say, to develop safe and commercially feasible fusion power plants: That technology would have to be diffused worldwide,
on terms of cost and reliability that would be more attractive to developing nations than a reliance on fossil fuel reserves or imports. During the last few million years, human technology, spurred I part by climatic change, has made our species a force to be reckoned with on a planetary scale. We now find, to our astonishment, that we pose a danger to ourselves.
The present world order is, unfortunately, not designed to deal with global scale dangers.
Nations tend to be concerned about themselves, not about the planet; they tend to have short-term rather than long-term objectives.
In problems such as the increasing greenhouse effect, one nation or region might benefit while another suffers.
In other global environment issues, such as nuclear war, all nations lose.
The problems are connected:
Constructive international efforts to understand and resolve one will benefit the others.
Further study and better public understanding are needed, of course.
But what is essential is a global consciousness - a view that transcends our exclusive identification with the generation and political groupings into which, by accident, we have been born.
The solution to these problems requires a perspective that embraces the planet and the future.
We are all in this greenhouse together.