John Shirley has interviewed a group of SF writers, asking how their conceptions of the future have changed. The formatting's a little buggy, but it's a great read.
3) What do you think people in the future will regard as being the greatest overall mistakes made during our time?
Pat Murphy: Id say that our worst blunder has been the destruction of the environment particularly as it relates to our consumption of fossil fuels. Over the next few decades, I believe that we will increasingly experience the consequences of global warming in the form of extreme weather (heat waves, drought, severe storms), new patterns of disease (West Nile and the Hantavirus are just the beginning), rising sea levels, extinctions due to climate change, catastrophic weather in the last 100 years. For more on all this, check out www.Exploratorium.edu/climate.
Bruce Sterling's response is in the same ballpark: Ignoring the Greenhouse effect and neglecting public health measures.
Kim Stanley Robinson's response is related. Our greatest mistake, he says, is: The mass extinction event we are causing.
Some of that die-off is a result of sheer human sprawl. This connects with Ken Wharton's answer regarding our biggest mistakes: The worldwide population explosion. Being in the middle of it for so long, it's hard to remember that exponential growth can never sustain itself forever. 50-100 years from now population will have mostly stabilized at something, and that number will be the primary determinant on what sort of long-term future is in store for humanity. In hindsight, will there have been a way to stabilize at a lower number? Probably... and someday we might be viewed as criminal for not doing just that.
...Cory Doctorow: I think the Ashcroftian terrorist witchhunts, coupled with the fiscal irresponsibility of massive tax-cuts and out-of-control cronyist military adventurism will be regarded as the world mistake in this part of the American century by debtor generations to come who find themselves socially and economically isolated from the rest of the world. When the US dollar starts to drop against the laser-printed post-Saddam occupation Dinar, an unbacked currency, you know that your economy is in the deepest of shit.