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439 The Scope of the Greenhouse Problem Global Warming: Facts and Guesses On the Ocean’s Response to Warming Some Lessons from the Past The Biological Pump The Carbonate Pump Debating the Future Our planet has life because of greenhouse gases in the air. They keep Earth warm, so that water can flow and clouds can form to bring rain. Without these gases, most of the planet would be covered with thick masses of ice, and the air would be dry. The two most important greenhouse gases are water vapor and carbon dioxide. Molecules of these gases intercept heat radiation that attempts to leave the planet. As a result, the planet’s radiation balance is achieved several kilometers up in the atmosphere, and the ground and lower atmosphere, where we live, are pleasantly warm and suitable for growing things. Within the last several decades, where we live has been getting warmer. Greenhouse gases have considerably increased since the industrial revolution. Each year, the burning of fossil fuels produces additional carbon dioxide—roughly 1 percent of what is in the air already. The ocean takes up a large portion of it, but nevertheless, the atmospheric content of this gas has been increasing by just under 0.5 percent annually. Methane, another greenhouse gas, also has been increasing substantially. In response to these changes, Earth is warming. Countervailing effects—heat uptake by the ocean, shading by effects from air pollution—are slowing the process somewhat, but warming continues.1 The physical interactions between the various elements of the climate system are very complex, and there are natural variations producing warming and cooling in the climate on the scale of decades and centuries. Thus, it is not possible to state precisely just how much of the observed warming owes to the man-made increase in greenhouse gases. Also, as the climate warms and the atmosphere changes composition (including an increase of particulate pollution), the rules are changing, especially concerning the formation of clouds and their role in the heat budget as heat traps and reflective umbrellas. Thus, projections of future FIFTEEN Global Warming and the Ocean HUMAN IMPACT ON A GREENHOUSE PLANET warming effects are subject to considerable uncertainty—things could change more, or they could change less than the best guesses offered by experts indicate. The uncertainties (but not the fact that man-made greenhouse gases produce warming) are the subject of much research and discussion among active geophysicists . Others, with little or no research background in the relevant sciences, also participate in the discussion, motivated by various political or economic concerns. (Since global warming has political and economic impacts this is a perfectly reasonable development.) What is of interest to ocean scientists is how the ocean will respond to warming. Since the ongoing warming is greater in high northern latitudes than in low latitudes, temperature gradients in the Northern Hemisphere will decrease, zonal winds will decrease correspondingly , and mixing and upwelling will decrease as a result. Productivity will drop. What will happen in the south is less clear: enormous ice masses on Antarctica are resistant to removal, which stabilizes the existing situation. Anticipated changes will affect the uptake of heat and carbon dioxide by the global ocean, and its patterns of productivity. In any case, sea level will continue to rise globally, and the rise might well accelerate to rates considerably higher than those of the twentieth century. On the whole, the weather will become less predictable. In addition, with continued warming from burning coal and petroleum, the risk from unanticipated and troublesome events will keep increasing .2 THE SCOPE OF THE GREENHOUSE PROBLEM Our planet is habitable because certain trace gases in the atmosphere of Earth keep it from freezing over. This benign and welcome consequence of having the atmosphere we have is known as the greenhouse effect. The trace gases keeping the planet warm—the greenhouse gases—are water vapor and carbon dioxide, mainly. At the ground, the difference in temperature with and without greenhouse gases is around 32 ⬚C.3 To satisfy our energy needs in a modern industrial society we burn fossil fuels: coal, oil, and natural gas. This produces prodigious amounts of carbon dioxide, a colorless and odorless gas that enters the atmosphere. As a greenhouse gas, carbon dioxide absorbs a portion of the heat radiation from the ground and lower atmosphere, radiation that would normally escape into space (fig. 15.1). Thus, the large-scale release of carbon dioxide into the air impacts the...

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