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Carbon concentrations in the atmosphere influence Earth's climate

The buildup of atmospheric CO2 that has resulted from the burning of fossil fuels is warming Earth. The concentration of CO2 in air trapped in the Antarctic and Greenland ice caps during the last Ice Age—between 15,000 and 30,000 years ago—was as low as 200 parts per million. During a warm interval 5,000 years ago, atmospheric CO2 may have been slightly higher than it is today. This long-term record shows that Earth was warmer when CO2 levels were higher and cooler when they were lower.

Complex computer models of Earth's system indicate that a doubling of the atmospheric CO2 concentration would shift climates toward the poles, would probably cause droughts in the central regions of continents, and would increase precipitation in coastal areas. Global warming could result in the melting of the Greenland and Antarctic ice caps, and would warm the oceans. If so, the oceans would expand, raising sea levels and flooding coastal cities and agricultural lands.

¡oiTi Both physical and biological processes I V control the carbon cycle

' * Over decades to centuries, the oceans, which have 50 times the amount of dissolved inorganic carbon as the atmosphere, determine atmospheric CO2 concentrations. The rate at which CO2 moves from the atmosphere to the oceans depends, in part, on photosynthesis by plankton in the surface waters. These organisms remove carbon from the water, thereby increasing its absorption of carbon from the atmosphere. In addition, many marine organisms form calcium carbonate (CaCO3) shells, which eventually sink to the ocean floor. This sedimentation increases absorption of carbon from the atmosphere by removing carbon from surface waters.

The rate of CO2 movement to deep ocean waters also depends on a circulation pattern called the ocean conveyor belt, which is driven by the sinking of dense, saline water in the North Atlantic Ocean. The ocean conveyor belt may weaken if melting of the Greenland ice cap discharges great quantities of fresh water into the North Atlantic Ocean. If this happens, the climate of Europe could become colder while climates elsewhere are warming.

Each year, photosynthesis by terrestrial vegetation, principally in forests and savannas, absorbs about 60 billion metric tons of carbon. About the same amount of carbon is released by respiration, about half by the plants themselves and half by microbes decomposing organic matter produced by the plants. Currently photosynthetic consumption of CO2 appears to exceed respiratory production of CO2 by 2 billion metric tons of carbon per year. Thus, Earth's forests are storing carbon that would otherwise be increasing atmospheric CO2 concentrations.

Ecologists are conducting experiments to determine the effects of higher atmospheric concentrations of CO2 on rates

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