In honor of April 22, consider this picture:
It’s taken from Zhu et al’s 2016 paper, “Greening of the Earth and its Drivers.” “Leaf Area Index” is a measure of the density of vegetation cover. It’s positive over most of the planet, and especially so in the purple‐shaded regions, which are mainly the tropical rainforest, the most diverse and revered of our ecosystems.
Only 4% of the surface shows the opposite, or a significant “browning.” The causes of the planetary greening? In a word, “us.” According to the Zhu et al., 91% of the greening is attributable to human activity. Perhaps it’s best to simply quote from the paper:
Factorial simulations with multiple global ecosystem models suggest that CO2 fertilization effects explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%). CO2 fertilization effects explain most of the greening trends in the tropics, whereas climate change resulted in greening of the high latitudes and the Tibetan Plateau.
Again, Happy Earth Day!