How do scientists determine that global warming is occurring?

ice samples help determine the change of greenhouse gasses and temperature, among other things from time to time.
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On the Discovery channel you can find scientists and photographers who have taken pictures of the polar icecaps every day for years just to prove global warming is actually happening. Hop on the net and go to Discovery Channel and search for these videos, you won't be sorry. How much information do some people need, it is going on and doesn't show any signs of stopping. Please use electric or natural gas.