Grímsvötn volcano erupts over Iceland in May. The world's volcanoes spew about 200 million tons of carbon dioxide into the atmosphere each year—an amount that human activities produce in less than three days. | Photo by Johann Ingi Jonsson/NordicPhotos
"It suddenly struck me that that tiny pea, pretty and blue,
was the Earth. I put up my thumb and shut
one eye, and my thumb blotted out the planet Earth.
I didn't feel like a giant. I felt very, very small."