One of many powerfull threats to life on Earth, but we have an "evolutionary" option to assure that at least some of our descendents survive. Perhaps advanced technology can even prevent all of these potential disasters if we give our civilization enough time and resources to develope it.

To reprise comments back at http://www.habitablezone.com/space/messages/21630.html which may have been made too late to contribute.

Supervolcanos can be as devastating as 1 km impacts and we have no potential defense or good warning system. The disaster frequency may even be similar as the last one was 74,000 years ago and yellowstone is due for another anytime in the next few million years. A second branch of civilization off planet would greatly speed recovery from such a disaster.

Mankind is of course both potential destroyer and savior of Earth's ecosystem. Learning to better shepard Earth's ecosystem by building one on another planet provides an opportunity to avoid pitfalls, repair past damage, and even improve Earth without risking damage to Earth during the learning curve.

Intentional human malice is another potential doom. An off-planet civilization protects against any human induced collapse of civilization. I firmly hope that a more technologically dependent civilization will help generate a more rational culture. This secondary effect may also reduce the potential for human malice, but more technology may mean fewer nuts are needed to generate the same level of mayhem.

Gamma ray bursts and supernovas require interstellar colonization to mitigate, but we have to start somewhere first. Colonizing near space then the deeper solar system mitigate the first three hazards above and mitigate the hazards of an aging sun. Such technology is sufficent for a slow spread across the galaxy similar to Polynesian island hopping.

The problem with people fixated on one risk like EP and Barry D is that they ignore the risks on other fronts that their single minded approaches would neglect. They also ignore the possibility that we only have a limited window to colonize space before a resource crunch closes the door. It is also quite ironic that BD ignores natural transfers from Mars to Earth. If martian life exists and is potentially hazardous we should study it under controlled conditions just as we do diseases on Earth. Similarly, EP ignores our limited impact diversion ability which would be greatly enhanced by advances in space technology which are unlikely to be funded based solely on the low probability of an impact in the near future.

I firmly believe that science and technology have made a better world even with some of the negative side effects. Continued advances in science and technology will mitigate the problems with current technology and negative consequences of newer technology. Spurring on these advances is another reason for space exploration and colonization. In addition, once we are firmly established in space the doors are blown off any potential resource crunch.

I also think those that believe mankind is a plague are nuts. All our "base" emotions are "natural" ones derived from our animal heritage. Those traits we are most proud of are either unique to humans or at least higher animals or greatly enhanced in us. Our culture is more moral than most of those in the past even by most PC standards and I believe it will continue to advance as long as we survive.

Regards,
Mark