Which gas is keeping the Earth warm?

The major natural greenhouse gases are

water vapor, which causes about 36-70% of the greenhouse effect on Earth ( not including clouds );
carbon dioxide, which causes 9-26%;
methane, which causes 4-9%, and
ozone, which causes 3-7%.

Note that it is not really possible to assert that a certain gas causes a certain percentage of the greenhouse effect , because the influences of the various gases are not additive. (The higher ends of the ranges quoted are for the gas alone; the lower ends, for the gas counting overlaps.) [3] [4]

rom http://en.wikipedia.org/wiki/Greenhouse_gas

So, what's the big deal if human CO₂ causes 1°C temperature increase?

An increase in atmospheric temperature (human or natural origin) will lead to the increase in the water vapor content of the troposphere.
Because water vapor is a strong greenhouse gas, the increase in H₂O vapor in turn causes enhanced greenhouse effect, raising the temperature more.
Higher atmospheric temperature will cause more evaporation of water
Which leads to even higher temperature…

⇒ Runaway Green House Effect!

How about Clouds and Ice?

Water vapor (water in gaseous phase) is one of the most potent and abundant greenhouse gas…but

Clouds (water in liquid form) reflect sunlight, decreasing the solar energy input into Earth's atmosphere during the day, but they trap IR radiation from the Earth during the night. It's net effect is not well know so far…

– Albedo of clouds range from close to 0 to 70%.

– Testing climate impact of clouds after Sept. 11, 2001…

Ice has a very high albedo , ~ 80 to 90%.

– Thus, reduction of the polar ice cap can cause more heating…