You'll often hear environmentalists claiming that humans are a cancer that is slowly killing the planet. We upset ecological equilibriums, contribute to the greenhouse effect, and put other species in danger. You may also have heard of the recent discovery of a 300 mile wide crater (that's really big!) buried deep under the ice flows of Antarctica. I read up a bit on the effects of cataclysms like these (check out Space.com and Wikipedia), and it kind of put what we, as humans, can do to the planet in perspective. The ozone layer: We are extremely dependant on the layer of protective gases that surround the earth. Upsetting its fragile balance has a direct effect on the earth's surface temperature, which in turn can have terrible effects on the planet's biodiversity. There are huge masses of plankton living around oceanic fissures that metabolise the methane produced by the movement of tectonic plates. Without this plankton, the methane would slowly bubble out from beneath the ocean and make the atmosphere almost uninhabitable within a few years. However, that quantity of harmful gases would be nothing compared to the amount that would be released after the impact of an asteroid. Scientists have theorised about a possible global firestorm that might have helped wipe out the dinosaurs. The enormous shockwave created by an asteroid would have made huge cracks in the earth's crust from which huge quantities of toxic gases would have come gushing out, filling the atmosphere. Lighting bolts would then have literaly set the atmosphere on fire. Just try and picture that! The scorched earth would have stayed about 10 degrees hotter for hundreds of thousands of years afterwards. Talk about a greenhouse effect. Radiation: Think Chernobyl was bad? Every day scientists detect gamma ray bursts emiting from deep space. These bursts bombard our atmosphere with radiation from hundreds of thousands of light years away. Luckily they are usually at a sufficient distance for our atmosphere to block them out, but a single burst from a nearby star (nearby being several hundred light years away) 450 million years ago wiped out almost all life on earth and vaporized 70% of the earth's atmosphere. Just like that. My point is, I think it's amazing how fragile we are. Just as easily as your life could be ended by a car crash on your way to work tomorrow, on a larger scale, a flash of light from a star or a small pebble (by planetary standards) could come along at any time and annihilate everything that mankind has built in an instant. I'm not saying we shouldn't take care of the environment, but that we should keep in mind that we're not the captains of spaceship Earth. The very equilibrium that we depend on and want to protect will always be at the mercy of forces much larger than itself. Luckily, it seems life has survived far worse conditions than anything mankind could cause, although ultimately we'll still all be swallowed into the sun!