Did you know that the Earth is getting greener, quite literally? Satellites are now confirming that the amount of green vegetation on the planet has been increasing for three decades. This will be news to those accustomed to alarming tales about deforestation, overdevelopment and ecosystem destruction.
This possibility was first suspected in 1985 by Charles Keeling, the scientist whose meticulous record of the content of the air atop Mauna Loa in Hawaii first alerted the world to the increasing concentration of carbon dioxide in the atmosphere. Mr. Keeling’s famous curve showed not only a year-by-year increase in carbon dioxide levels but a season-by-season oscillation in the concentration.
John S. Dykes
Satellites show that the amount of green vegetation has been increasing or three decades straight.
During summers in the Northern Hemisphere, the Earth breathes in carbon dioxide as green plants (most of which are north of the equator) absorb the gas and turn it into carbohydrate. In the northern winter, the Earth breathes the gas out again, as the summer’s leaves rot.
Mr. Keeling and colleagues noticed that the depth of the breathing had increased in Hawaii by 20% since the 1960s: The Earth was taking in more carbon dioxide each northern summer and giving out more each winter. Since the inhalation is done by green leaves, they reasoned, the amount of greenery on the planet must be growing larger. In the 1980s forest biologists started to report striking increases in the growth rates of trees and the density of forests: in Douglas firs in British Columbia, Scots pines in Finland, bristlecone pines in Colorado and even tropical rain forests.
Around the same time, a NASA scientist named Compton Tucker found that he could map global vegetation changes by calculating a “Normalized Difference Vegetation Index” (NDVI) from the data produced by a satellite sensor. The data confirmed Mr. Keeling’s suspicion: Greenery was on the increase. At first, this was thought to be a northern phenomenon, caused by faster growth in the great spruce and birch forests of Siberia and Canada, but the satellites showed it was happening all over the world and especially strongly in the Amazon and African rain forests.
Using the NDVI, one team this year reported that “over the last few decades of the 20th century, terrestrial ecosystems acted as net carbon sinks,” i.e., they absorbed more carbon than they were emitting, and “net greening was reported in all biomes,” though the effect had slowed down in recent years.
The latest and most detailed satellite data, which is yet to be published but was summarized in an online lecture last July by Ranga Myneni of Boston University, confirms that the greening of the Earth has now been going on for 30 years. Between 1982 and 2011, 20.5% of the world’s vegetated area got greener, while just 3% grew browner; the rest showed no change.
What explains this trend? Man-made nitrogen fertilizer causes crops to grow faster, but it is having little effect on forests. There are essentially two possibilities: climate and carbon dioxide itself. Warmer, wetter weather should cause more vegetation to grow. But even without warming, an increase in carbon dioxide should itself accelerate growth rates of plants. CO2 is a scarce resource that plants have trouble scavenging from the air, and plants grow faster with higher levels of CO2 to inhale.
Dr. Myneni reckons that it is now possible to distinguish between these two effects in the satellite data, and he concludes that 50% is due to “relaxation of climate constraints,” i.e., warming or rainfall, and roughly 50% is due to carbon dioxide fertilization itself. In practice, the two interact. A series of experiments has found that plants tolerate heat better when CO2 levels are higher.
The inescapable if unfashionable conclusion is that the human use of fossil fuels has been causing the greening of the planet in three separate ways: first, by displacing firewood as a fuel; second, by warming the climate; and third, by raising carbon dioxide levels, which raise plant growth rates.