what is the reason behind global warming?

Answer 1

SHORT ANSWER
Greenhouse gases such as carbon dioxide prevent some heat from leaving the Earth's atmosphere and escaping into space. The build up of atmospheric greenhouse gases in recent years has led to an increase in the amount of heat that is trapped. The term 'Greenhouse Effect' has been coined to describe this phenomenum.


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LONG ANSWER
Our atmosphere acts like a blanket trapping heat and keeping Earth at a habitable temperature, it’s this retaining of heat that is referred to as the Greenhouse Effect. The greenhouse effect is caused by greenhouse gases that trap heat from the sun, the more greenhouse gases there are the more heat is retained

GREENHOUSE GASES
Water vapour (H20) is the most prevalent greenhouse gas and occurs naturally. Some greenhouse gases are both natural and manmade including carbon dioxide (C02), methane (CH4), nitrous oxide (NO) and tetraflouromethane (CF4). There are several synthetic gases consisting of carbon and halogens, many of the manmade greenhouse gases are also responsible for ozone depletion.

As with temperature, there is a natural cycle in the amount of CO2 in the atmosphere. Ice core samples extending back some 650,000 years show the minimum amount of atmospheric CO2 to have been around 190 parts per million by volume (ppmv) and the maximum about 300 ppmv. The worry is that the current levels of CO2 are considerably higher at around 385 ppmv .

LEVELS OF GREENHOUSE GAS
Excluding water vapour, carbon dioxide is by far the most prevalent of the greenhouse gases accounting for over 99% of all of them. The only other gas present in appreciable quantity is methane and this accounts for nearly 0.5% of the greenhouse gases.

By comparing levels of greenhouse gases in 2000 with levels in 1750 we can see that there has been a 31% increase in the amount of carbon dioxide, a 16% rise in levels of nitrous oxide and a 149% increase in the levels of methane.

Different greenhouse gases are more effective than others at contributing to the greenhouse effect, an effect called the Global Warming Potential (GWP). Although carbon dioxide accounts for 99.4% of the greenhouse gases by volume, the contribution it makes as a total of all the greenhouse gases is considerably lower at just 72.3%; this is because, as greenhouse gases go, it's not very good. Nitrous oxide on the other hand is nearly 300 times as effective and although it occurs in very small amounts when compared to carbon dioxide, it manages to contribute 18.4% towards the total greenhouse effect. The other gases which make sizeable contributions are methane and dichlorodifluoromethane, respectively these are responsible for nearly 8% and nearly 1% of the contribution to the greenhouse effect.

SOURCES OF GREENHOUSE GASES
One of the primary causes of greenhouse gas emissions for which humans are responsible result from the burning of fossil fuels (coal, oil and gas) and much of the remainder results from farming and agricultural activities. Fossil fuels are used in power generation, to heat homes and offices, to power factories, to fuel transport and many more uses.

Other manmade causes of greenhouse gases include deforestation, fertilisers, air conditioning units, open fires, fridges and freezers, numerous industrial and chemical processes, fire suppressants, coal mining, effluent, landfill sites, livestock and rice cultivation.

There are several natural causes of greenhouse gases including volcanic activity, the seas and oceans, natural decay of plants and animals and the natural melting of ice caps.

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To Gene (below): Please dispute global warming but I wouldn't advise using the article you have linked to as a means of doing so. The article isn't related to global warming and actually states "The new study shows that the TSI has increased by about 0.1 percent over 24 years. That is not enough to cause notable climate change". The article (dated 2003) also states that TSI (Total Solar Irradiance) will reach a minimum in 2006, that being the case then average global temperatures should have fallen in recent years. In understanding global warming account is taken of all factors including solar variation and, as the article in question states, total variation is very small.

Answer 2

Mostly us, mostly by burning fossil fuels.

"I wasn’t convinced by a person or any interest group—it was the data that got me. I was utterly convinced of this connection between the burning of fossil fuels and climate change. And I was convinced that if we didn’t do something about this, we would be in deep trouble.”

Vice Admiral Richard H. Truly, USN (Ret.)
Former NASA Administrator, Shuttle Astronaut and the first Commander of the Naval Space Command

Here are two summaries of the mountain of data that convinced Admiral Truly, short and long.

http://www.globalwarmingart.com/wiki/Image:Climate_Change_Attribution.png

http://www.ipcc.ch/SPM2feb07.pdf

The sun is only about 10% of it. See the first graph above. And here's a detailed analysis:

http://solar-center.stanford.edu/sun-on-earth/FAQ2.html

The sun is carefully measured. Climatologists include solar radiation in their analysis. To think they would ignore the sun is utterly ridiculous.

Answer 3

Mostly an increase in the sun's output in a normal climate cycle. The climate is always changing and the media just latched onto this one and is basically using the scare tactic to sell copy and advertising..

http://www.space.com/scienceastronomy/sun_output_030320.html