Global warming - the idea that humans may be warming the air by adding gases such as carbon dioxide - is always in the news. Average global air temperatures have gone up by about a degree over the last century, but there's still a question of how much of this warming, if any, is related to increases in carbon dioxide.

Often overlooked in the global warming debate is the fact that on smaller scales, we've already changed the climate dramatically because of other things that we do - the best example is building cities. Concrete and asphalt are better absorbers of sunlight than the fields and forests they replace, and so urban areas tend to be warmer by at least several degrees. This so-called "urban heat island" isn't the global warming that researchers talk about, but it is a warming, it is real, and it's not natural.

So the next time that predictions of future global warming make you ponder what it would be like to live a world several degrees warmer than today, just remember that such warmer climates already exist. Just go downtown.