A few things:

Human society is based on agriculture, which is only possible on earth in moderate climates. We are concerned about climate change not because the earth has never been warmer, but because human society has never been warmer. That is why climate comparisons are based on baselines. Scientists usually say "hottest on record," but we also know it's now been millions of years since we've been this hot. We are worried because it will affect the basis of our society, security, and economy.

Climate is always changing, but at rates and for reasons. We know now that it is changing faster than any time in geologic history we've ever measured, and we know the reasons are almost entirely because of human caused CO2 increases. It's actually a quite straightforward set of evidence and arguments if you'd like, I can recommend readings.

And FYI, Denver hasn't been at sea level since about 65 million years ago, when the Fox Hills sandstone was deposited on the shores of an inland sea.