We typically think of the ocean's color as blue, but in some places, it looks blue-green. That's because those areas are teeming with single-cell plants called phytoplankton, which contain chlorophyll and reflect the green in sunlight. Though tiny, phytoplankton collectively absorb almost as much carbon dioxide as all the trees and land plants on Earth. They have an enormous impact on our climate, and scientists study that impact by measuring the color of the ocean with satellites and sea-based sensors.