Few of us will ever venture past the 60-mile boundary that separates Earth and outer space. If you do, though, you're likely to experience something known as "the overview effect" -- a cognitive shift in how you perceive our planet. Political boundaries disappear, and our atmosphere, which seemed like a boundless expanse of blue from the ground, is suddenly revealed to be a paper-thin shield between life and the dark void of space.
Last week, the fragility of that thin blue shield was underscored by the news that we've reached a daily average of 400 parts per million (ppm) of carbon dioxide in our atmosphere. That's the highest level in at least 3 million years. In less than two centuries, we've increased atmospheric CO2 by 42 percent -- by burning fossil fuels, degrading our forests, and disturbing our soils. And it's still going up.