Evapotranspiration and Greenhouse Warming
In addition to the carbon cycle, vegetation plays a direct role in other aspects of the Earth's climate. Green leaves are relatively dark, allowing for vegetated land to absorb more of the Sun's energy than light-colored deserts or snow-covered surfaces, which reflect most incoming solar radiation. Vegetation takes up water from the soil and releases it back to the atmosphere as water vapor, a process called "evapotranspiration." New studies suggest that higher levels of carbon dioxide in the atmosphere, coupled with higher temperatures, could alter evapotranspiration rates, which would impact both the hydrological cycle as well as land plant biomes.

During photosynthesis, thousands of tiny valve-like pores (called "stomates") on a plant's green leaves open up to allow carbon dioxide to flow into the leaf interior. Consequently, water lining the stomatal cavity can escape from inside the leaf out to the open air. This flow of water into the atmosphere acts to cool the land surface. As the water in the leaf is depleted, it is replaced by a flow of liquid water taken up from the soil by the plant's root system. Plants appear to continuously modulate the width of their stomates so as to get a maximum rate of photosynthesis for a minimum loss of water.

Figure 2: Computer model calculation of the effect of carbon dioxide on plant physiology and global climate. As carbon dioxide increases, vegetation may evaporate less water which would cause the land to heat up (dotted areas). This map shows additional heating (over and above the conventional carbon dioxide greenhouse effect) over the continents due to this phenomenon, for doubled current carbon dioxide concentrations (700 ppm).

As concentrations of atmospheric carbon dioxide increase, plants may be able to reduce their evapotranspiration rates (water loss) to cause no reduction, or maybe even a slight increase, in photosynthesis. Some studies have modeled this effect and calculate that as carbon dioxide increases, evapotranspiration erasure over the continents will decrease, effectively reducing the amount of water vapor in the atmosphere. If this happens and greenhouse warming will be further amplified over the tropical land areas by as much as 50 percent above the predicted greenhouse warming effect alone (Figure 2).

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Changing Global Land Surface

Introduction

The Carbon Cycle

Greenhouse Warming

Plants, Snow & Ice

Terra & Landsat Observations