Earth has experienced many episodes of dramatic climate change, with different periods in Earth history seeing the planet much hotter or much colder than at present. There have been periods during which the entire planet has been covered in ice in a frozen, seemingly perpetual, winter; at other times Earth's surface has been scorchingly hot and dry; and at others, much of the planet has felt like a hot, wet sauna. Many scientists warn that the planet is currently becoming warmer at a rapid pace, and there will be significant consequences for the people and ecosystems on the planet.

There are many different variables that control climate and can change the planet rapidly from one condition to another. Most of these are related to variations in the amount of incoming solar radiation caused by astronomical variations in Earth's orbit. Other variables that can strongly influence long-term climate change are the amount of heat that is retained by the atmosphere and ocean and, on timescales of tens to hundreds of millions of years, the distribution of landmasses as they move about the planet from plate tectonics. Each of these changes operates with different time cycles, alternately causing the climate to become warmer and colder.

Very long-term climate changes include the gradual change of Earth's atmosphere from a global hothouse dominated by CO2 and other greenhouse gases when Earth was young to an atmosphere rich in nitrogen and oxygen over the next couple of billion years. Fortunately, during

Harvesting beans at a bend in the Yellow River, where it emerges from the Tibetan Plateau near Shapotou, China. Across the river are sterile dunes that form the limit of the Gobi Desert, and on this side the fertile sediment where the agricultural valley begins. (Corbis)

the early history of Earth, the Sun was less luminous, and the planet was not exceedingly hot. Over time, the motion of the continents has alternately placed continents over the poles, which causes the continent to be covered in snow, reflecting more heat back to space and causing global cooling. Plate tectonics also has a complex interaction with concentrations of CO2 in the atmosphere, for instance, by uplifting carbonate rocks to be exposed to the atmosphere during continental collisions. The CaCO3 then combines with atmospheric CO2, depositing it in the oceans. Thus, continental collisions and times of supercontinent formation are associated with drawdown and reduction of CO2 from the atmosphere, global cooling, and sea level changes.

Orbital variations seem to be the main cause of climate variations on more observable geological time scales. The main time periods of these variations have alternations of hotter and colder times, varying with frequencies of 100,000, 41,000, 23,000, and 19,000 years. To understand the complexity of natural climate variations, the contributions from each of these main contributing factors must be added together to obtain a very complex curve of climate warming and cooling trends. Built on top of these long-term variations in climate are some shorter term variations that can change rapidly, caused by changes in ocean circulation, sunspot cycles, and finally, the contribution in the last couple of hundred years from the industry of people, called anthropogenic changes. Deciphering which of these variables is the cause of a particular percentage of the present global warming is no simple matter, and many political debates focus on whom to blame. Perhaps it is just as appropriate to be focusing on how the human race needs to respond to global warming. Coastal cities may need to be moved, crop belts are migrating, climate zones are changing, river conditions are going to be different, and many aspects of life that people are used to will be different. Considerable effort is being spent to understand the climate history of the past million years and to help predict the future.

In this book, different aspects of climate change are examined. First, the causes of climate change on these different time scales are examined in detail in the first few chapters, with the first chapter examining natural long-term climate changes and the evolution of the atmosphere and the second examining natural medium- to short-term causes of climate change. The third chapter assesses human-induced climate changes over the past couple of hundred years, and incorporates findings from the Intergovernmental Panel on Climate Change report "Climate Change 2007." Climate change is causing many of the deserts of the world to expand. Chapters 4 and 5 examine deserts and climate changes that lead to drought and desertification. The glacial ice caps, mountain glaciers, and permafrost regions of the world are shrinking under the influence

Fort Proctor, Mississippi Delta, Louisiana, surrounded by water due to regional subsidence of the delta and sea level rise (Visuals Unlimited)

of climate change. In chapter 6, glacial environments are described, and the past several million years of climate history of advancing and retreating glacial periods is analyzed. Chapter 7 examines how Earth has switched over its entire 4.5-billion-year history from one climate extreme to another and balances this history with the current climate changes that are likely to be induced by human injection of greenhouse gases into the atmosphere. Details of what the different changing climate zones and intervals may be like in the future are examined by looking in detail at past- and present-day desert and glacial climate zones on Earth. This chapter paints a picture of what the future Earth climate may be and what the human race needs to do to adapt to the rapidly changing climate. Much of chapter 8 follows the recommendations of the Intergovernmental Panel on Climate Change, which in late 2007 and 2008 issued a series of predictions and guidelines on how nations should be preparing for climate change in the coming century.

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Growing Soilless

Growing Soilless

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