OK, so I've heard a couple people wondering about global warming and whether it occurs naturally or because of man. The simple answer is both, although currently it's being caused almost entirely by man.
I would like to begin by going over what global warming is. Put simply, it is the gradual increase of Earth’s average temperature. Scientists have shown that global warming is caused by the greenhouse gas effect. Let me explain: Light and heat pours into Earth’s atmosphere from the sun. Much of the heat leaves the atmosphere as infrared radtiation, although some of the heat is trapped, which is beneficial because it keeps the Earth a reasonable temperature. However, when greenhouse gasses such as carbon dioxide and methane build up, more heat is trapped in the atmosphere. This is because they reradiate heat that is leaving the atmosphere back onto the face of the earth, thus heating the earth more than usual. So the higher the levels of greenhouse gasses are, the higher the global temperature is. While I may have somewhat over-simplified it, that is basically what is currently happening to earth, causing temperatures to heat up.
Currently, CO2 levels are at an unprecedentedly high level (381 ppm as of November 2007, although doubtlessly higher now). This is the highest it's been for at least 650000 years! So what has caused carbon dioxide levels to rise above the historic average? Recent evidence shows that “the rise in atmospheric CO2 is entirely caused by fossil fuel burning and deforestation,” according to researcher Dr. Le Quere of the University of East Anglia. As shown in the graphic, there is a strong correlation btwn co2 concn and temp. WHile correlation certainly doesn't = Causation, because we know the greenhouse properties of CO2, methane, and other such gases currently being emitted by man, we know there is at least some degree of causation. One blip in correlation often brought up by skeptics was in the mid-20th centuries. Although co2 continued to rise, temperatures slightly declined. However, what they don't mention is that an increase in aerosols actually offset the affects of GHGs (greenhouse gases) by reflecting sunlight back into space essentially (some have even proposed using aerosols as a solution to global warming).
On the natural side of things (what climatologists call Natural Variation), the sun & degree of the earth's tilt have the most significant impact on global temperatures (things like El Nino have usually just a regional impact for limited period of time). Right now the degree of the earth's orbit isn't 'strange' enough to impact the temp to the degree that we are seeing it rise (it's "wobble" affects climactic temp).
THe sun is what is most often brought up by GW skeptics as the "real" cause of global warming. While this explanation appeals to common sense (I mean, the sun is th source of all our energy, so why wouldn't it be the sun?), as the graphic shows, it isn't really having too much of an impact currently. The sun has its energy output by less than .1% from 1955 to 2000. During this period, the oceans increased in temperature .7 degrees C, which means that much more energy than usual had to be captured in Earth's climate because water has a high specific heat capacity and thus it takes a lot of energy to increase it's temperature. The increase in solar output is "way, way too small, an order of magnitude too small" too small to explain recent temp change, according to Ben Santer of Lawrence Liverpool National Laboratory.
There is also a "human fingerprint" present on current warming that further shows that man is primarily responsible for recent warming. This fingerprint is the difference in the warming of the atmosphere -- the troposphere has warmed while the stratosphere has cooled. If GW were due to an increase in the sun's intensity, the layers of the atmosphere would have warmed uniformly. For more info about the sun and global warming, see this very convincing, scientific article: http://www.newsweek.com/id/33510/page/2
Hopefully that leads to a bit further understanding of global warming for some.
The graphic is a compilation of data from a few different sources which really illustrates the true nature of GW. Click the image to enlarge it.
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You do prevent the evidence simply and in a well-out manner but your science lesson leaves out many important factors global warming "skeptics", such as myself, believe to negate the reality of a anthropogenic global warming. First, you talk about the "greenhouse effect." You point out the usual infrared escapes and heat gets trapped because carbon dioxide is reflective of the heat rising from the earth. If the carbon dioxide is re-radiating the heat rays why aren't rays originally from the sun being blocked by the build up in "greenhouse gases"? Next, you continue to use the misleading phrase "the greenhouse effect" which is inaccurate in terms of the earth. The earth itself is very different from a greenhouse because the rotation of the earth causes winds to circulate throughout the earth. These winds can dissipate heat while in a greenhouse a main reason the greenhouse is warm is because there is no effect from wind. Finally, you talk about the oceans rising .7 degrees C when satellite temperature readings(which a consensus of scientists agree is more accurate than surface readings) show a global cooling of .1 degrees C.
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