If we are so convinced the planet is warming...?

With that short of a baseline measurement no one knows for sure.

Using core samples from Antartic Ice, scientists are able to extract samples of the atmosphere that were trapped in the ice tens of thousands of years. From these samples, scientists can determine the carbon dioxide composition of the atmosphere as it was during those tens of thousands of years. An analysis of the composition allows one to ascertain prevailing global temperatures.

Edit ~ I was addressing your comment, "we have recorded temperatures for only 150 years or so." That is not entirely accurate, as I explained in my answer. However, your overall assertion that the planet's warming is part of a natural trend is also wrong. Although the planets temperatures have risen and fallen many times, we know from the collected ice samples that the prior temperature changes have never occurred as rapidly as they are occurring now. The rate of temperature change is so dramatically larger than any prior climate change, that it suggests human activity must be the cause.

The temperature of the earth should be at the point where the polar ice caps don't keep melting. Otherwise, goodbye Miami, Tampa, New Orleans, Biloxi, and how about New York City? A 2 foot rise in the level of the oceans would be catastrophic for coastal cities all over the world.

Im pretty sure the fact that the polar ice caps are melting, pretty quickly i might add, is enough proof for me to believe in global warming. I live in New York City, and it would be nice if my home isnt underwater 20 years from now.

All global warming questions should be moved to the mythology category where they belong.