The relative greenness of the planet is shown here using a time-smoothed Normalized Difference Vegetation Index (NDVI). There are many types of indices that measure vegetation and many are calculated by using satellite data to compare the relative difference between how much energy is absorbed by the land surface versus how much is reflected back into space. Plants absorb visible light to undergo photosynthesis, so when vegetation is lush, nearly all of the visible light is absorbed by the photosynthetic leaves, and much more near-infrared light is reflected back into space. However, for deserts and regions with sparse vegetation, the amounts of reflected visible and near-infrared light are both relatively high. In this imagery the seasonal effect of temperature and moisture can be seen as areas change from sparsely vegetated to lush.