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.
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% [by 1995] 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. see also here.
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.
In a comprehensive survey of the litertature on this subject, which I recommend as a short cut to many of the individual studies and the continent-by-continent details, Craig Idso reaches the following conclusions:
In the ensuing report we present a meta-analysis of the peer-reviewed scientific literature, examining how the productivities of Earth's plants have responded to the 20th and now 21st century rise in global temperature and atmospheric CO2, a rise that climate alarmists claim is unprecedented over thousands of years (temperature) to millions of years (CO2 concentration). Based on that analysis, we find the following:
- The productivity of the planet's terrestrial biosphere, on the whole, has been increasing with time, revealing a great greening of the Earth that extends throughout the entire globe.
- Satellite-based analyses of net terrestrial primary productivity (NPP) reveal an increase of around 6-13% since the 1980s.
- There is no empirical evidence to support the model-based claim that future carbon uptake by plants will diminish on a global scale due to rising temperatures. In fact, just the opposite situation has been observed in the real world.
- Earth's land surfaces were a net source of CO2-carbon to the atmosphere until about 1940. From 1940 onward, however, the terrestrial biosphere has become, in the mean, an increasingly greater sink for CO2-carbon.
- Over the past 50 years, for example, global carbon uptake has doubled from 2.4 ± 0.8 billion tons in 1960 to 5.0 ± 0.9 billion tons in 2010.
- The observed global greening has occurred in spite of all the many real and imagined assaults on Earth's vegetation that have occurred over the past several decades, including wildfires, disease, pest outbreaks, deforestation, and climatic changes in temperature and precipitation, more than compensating for any of the negative effects these phenomena may have had on the global biosphere.
- There is compelling evidence that the atmosphere's rising CO2 content - which alarmists consider to be the chief culprit behind all of their concerns about the future of the biosphere (via the indirect threats they claim it poses as a result of CO2-induced climate change) - is most likely the primary cause of the observed greening trends.
- In the future, Earth's plants should be able to successfully adjust their physiology to accommodate a warming of the magnitude and rate-of-rise that is typically predicted by climate models to accompany the projected future increase in the air's CO2 content. Factoring in plant productivity gains that will occur as a result of the aerial fertilization effect of the ongoing rise in atmospheric CO2, plus its accompanying transpiration- reducing effect that boosts plant water use efficiency, the world's vegetation possesses an ideal mix of abilities to reap a tremendous benefit in the years and decades to come.
Matt, an acclaimed author and former Science and Technology Editor for the Economist blogs at www.rationaloptimist.com.