April 22, 2018 8:24AM # Happy Earth Day! By [Patrick J. Michaels](https://www.cato.org/people/patrick-michaels) --- SHARE In honor of April 22, consider this picture: ![Trends in Leaf Area Index, 1978-2009. Positive tones are greening, negative are browning, and the dots delineate where the changes are statistically significant. There is approximately 9 times more area significantly greening up than browning down. ](/sites/cato.org/files/styles/pubs_2x/public/wp-content/uploads/leafareaindexx.png?itok=JOwvzM8Z) It’s taken from [Zhu et al’s](https://www.nature.com/articles/nclimate3004) 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! ##### Related Tags [Energy and Environment](https://www.cato.org/research/energy-environment) [![Creative Commons License](/build/cato_2020/images/creative-commons.svg)](http://creativecommons.org/licenses/by-nc-sa/4.0/) This work is licensed under a [Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License](https://creativecommons.org/licenses/by-nc-sa/4.0/).