Without the greenhouse effect, Earth would be a cold, lifeless planet. Carbon dioxide and water vapor in the atmosphere absorb solar heat as it reradiates off Earth’s surface and keep the planet warm. The greenhouse effect, of course, also has a downside. Some researchers predict that in 60 years, global temperatures will rise by nearly 4 degrees Fahrenheit as the carbon dioxide concentration in the atmosphere doubles. Even worse, a new study says that the interaction of higher carbon dioxide levels with Earth’s vegetation could amplify global warming by as much as 50 percent.
How could plants worsen global warming? The culprits, says biologist Jim Collatz of nasa’s Goddard Space Flight Center, are the tiny pores called stomata on leaf surfaces. Stomata allow carbon dioxide to seep into leaves, where it is used in photosynthesis, and they also let water out. Normally, when the sun heats up a plant, water inside the plant gets warm and evaporates out the stomata. But to cut down on water loss, stomata close when a leaf has absorbed enough carbon dioxide, and as carbon dioxide concentration goes up, they stay closed for longer periods of time. With stomata closed, hot water inside the plant can’t escape. The water heats the plant, and the plant in turn warms its surroundings. So the sun’s energy, instead of being used to evaporate water from vegetation, heats plants--and the rest of the planet.
To find out just how significant this effect would be, Collatz and a team of researchers headed by Goddard atmospheric scientist Piers Sellers created a computer model that estimated the exchanges of heat, water, and radiation between the land, ocean, and atmosphere. The model also predicted the effect of increased levels of carbon dioxide on Earth’s vegetation.
If carbon dioxide levels double over the next 60 years, the model found, closed stomata will add up to 2 degrees to the 4 degrees of warming expected from the greenhouse effect alone. It is a significant amount of heating, Collatz says, and from a source that had not previously been addressed in any other model. Even though plants with closed stomata would release less water vapor--itself an important greenhouse gas--that effect would be outweighed globally by increased evaporation from the oceans in a CO2-warmed world. Meanwhile the drop in atmospheric water vapor over parts of the land could have a decidedly negative effect, the model found: it could cut rainfall over vegetated areas during the growing season.