It's easy to sit back and say you don't believe this. No one looked at the thermometer 100,000 years ago and wrote it in their journal. Or to ask "How can an old chunk of ice tell us the temperature back then?". That is the Science that eustis mentions. It is possible to tell how old a layer of ice is. It is possible to determine with good confidence the chemical composition of the atmosphere when that layer was formed, and it is possible to correlate that composition with environmental temperatures based on deposited plant life and other fossil records from the same time. Thousands of scientists, engineers, and researchers have come to this conclusion. These are people that have spent their life learning how to look at and understand this evidence. They present their findings and let other throw rocks at them through the peer review process. Sometimes, the rocks uncover an error. That is why the reviews happen and it improves the quality of the science and findings. Some ( but not all) of the research is funded by or carried out in government labs and that may make it difficult for some of you to believe it, fearing that it is a ploy to grab control of the people. But non government results, and the results from other governments, are in good agreement. Hopefully that reduces the concern of the US government conspiracy.
It is also possible that I am not convincing anyone with the argument. Probable even! But the fact that we are having this discussion without disparagement is a great thing.