Fossilized pollen can likewise pinpoint changes in climate. The evergreen red beech is adapted to warm locales. The abundance of its pollen 8,000 years ago implies that this time correlates with the maximum temperature since the end of the Cenozoic Ice Age. On the other hand, the abundance of pollen from Phyllocladus, a shrub that is adapted to the cold, at 26,000 years ago and again at 20,000 years ago, indicates these years as the coldest during the ice age.
Photosynthetic algae evolved in the ocean as early as four billion years ago. From this beginning, the first plants colonized land 410 million years ago, before animals. By 360 million years ago, the beginning of the Carboniferous period, plants had spread throughout the planet, forming lush forests. The temperature and amount of carbon dioxide in the atmosphere were both higher than today. So lush was the growth of plants that when they died, they formed one layer upon another. Under heat and pressure, these many layers of plants formed the vast deposits of coal, natural gas, and petroleum that humans are now exploiting. The immensity of these deposits underscores the massive growth of plants during the Carboniferous period.
The fact that plants absorb carbon dioxide during photosynthesis has an important consequence for the climate. Carbon dioxide correlates with temperature. A high concentration of carbon dioxide correlates with high temperatures, whereas a low concentration of the gas correlates with low temperatures. This relationship holds true, because carbon dioxide is a greenhouse gas: it traps sunlight that reflects from Earth, preventing light, in the form of infrared radiation, from returning to space. In trapping sunlight, carbon dioxide traps heat, thereby increasing the temperature of the atmosphere. Plants lower the concentration of carbon dioxide by absorbing it during photosynthesis. When plants absorb carbon dioxide faster than Earth produces it through vol-canism, the concentration of carbon dioxide dimin ishes, and temperatures decline. In this context, plants may have contributed to the onset of the Ice Ages by absorbing carbon dioxide.
The absorption of carbon dioxide is cyclical. In spring and summer, when plants grow vigorously, they absorb large amounts of carbon dioxide. In autumn, however, plant growth slows and in winter it stops. The decay of dead plants in autumn returns the carbon dioxide that they had absorbed while alive to the atmosphere. The Northern and Southern hemispheres contribute to the carbon dioxide cycle in opposite fashion, for when plants are growing vigorously in the Northern Hemisphere, they are dead and decaying in the Southern Hemisphere, and vice versa.
In another sense, the carbon dioxide cycle spans eons of time. In the Carboniferous period, lush forests absorbed prodigious amounts of carbon dioxide, storing it in their tissues as sugars. Upon the death of plants, their conversion to coal, natural gas, and petroleum locked up these vast amounts of carbon dioxide. Since the Industrial Revolution, humans have burned these fossil fuels for energy, and in the process liberated carbon dioxide, returning to the atmosphere the carbon dioxide that plants had absorbed during the Carboniferous period. The liberation of this carbon dioxide has increased global temperatures. Humans are burning fossil fuels constantly, leading climatolo-gists to predict that humans will, by 2080, double the amount of carbon dioxide in the atmosphere, further increasing temperatures and perhaps leading to the flooding of coastal cities.
Plants absorb carbon dioxide through their sto-mata, pores on the leaves. At the same time, plants shed water through their stomata. Plants transpire as water vapor more than 90 percent of the water that they absorb through their roots. Through transpiration, plants change the climate, though not in a straightforward way. On the one hand, water vapor is a greenhouse gas, absorbing as heat the sunlight reflected from Earth.
The warmth of the Carboniferous period, a time of abundant plant growth, was due to the greenhouse effect. Along with carbon dioxide, the water vapor transpired by plants contributed to a warm climate. On the other hand, the water vapor that plants transpire forms clouds, which reflect 30 percent of sunlight back into space before it can heat the Earth. In their role in forming clouds, plants cool the climate.
One scientist predicted that a doubling of carbon dioxide concentration might cool, rather than heat Earth, because plants will, in transpiring water vapor, hasten the formation of clouds.
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