To be a tourist is one of the characteristics of the “modem” experience. Not to “go away” is like not possessing a car or a nice house. It is a marker of status in modem societies and is also thought to be necessary to health. 47_____ In Imperial Rome, for example, a fairly extensive pattern of travel for pleasure and culture existed for the elite. In the thirteenth and fourteenth centuries, pilgrimages had become a widespread phenomenon practicable and systematized, served by a growing industry of networks of charitable hospices and mass-produced indulgence handbooks. Such pilgrimages often included a mixture of religious devotion and culture and pleasure. The Grand Tour had become firmly established by the end of the seventeenth century for the sons of the aristocracy and the gentry, and by the late eighteenth century for the sons of the professional middle class. 48_____ There was a visualization of the travel experience, or the development of the gaze, aided and assisted by the growth of guidebooks which promoted new ways of seeing. The eighteenth century had also seen the development of a considerable tourist infrastructure in the form of spa towns throughout much of Europe. The whole apparatus of spa life was designed to provide a concentrated urban experience of frenetic socializing for a dispersed rural elite. 49_____ In the countryside work and play were particularly intertwined in the case of fairs. People would often travel considerable distances and the fairs always involved a mixture of business and pleasure normally centered around the tavern. But before the nineteenth century few people outside the upper classes traveled anywhere to see objects for reasons unconnected with work or business. 50_____ Travel is now thought to occupy 40 percent of available “free time.” It is a crucial element of modern life to feel that travel and holidays are necessary.

     Throughout its long history, Earth has warmed and cooled time and again. Climate has changed when the planet received more or less sunlight due to subtle shifts in its orbit, as the atmosphere or surface changed, or when the Sun’s energy varied. 

     Global warming is the unusually rapid increase in Earth’s average surface temperature over the past century primarily due to the greenhouse gases released as people burn fossil fuels. The global average surface temperature rose 0.6 to 0.9 degrees Celsius (1.1 to 1.6°F) between 1906 and 2005, and the rate of temperature increase has nearly doubled in the last 50 years. Temperatures are certain to go up further. 

     Earth’s temperature begins with the Sun. Roughly 30 percent of incoming sunlight is reflected back into space by bright surfaces like clouds and ice. Of the remaining 70 percent, most is absorbed by the land and ocean, and the rest is absorbed by the atmosphere. The absorbed solar energy heats our planet. As the rocks, the air, and the seas warm, they radiate “heat” energy (thermal infrared radiation). From the surface, this energy travels into the atmosphere where much of it is absorbed by water vapor and long-lived greenhouse gases such as carbon dioxide and methane. When they absorb the energy radiating from Earth’s surface, microscopic water or greenhouse gas molecules turn into tiny heaters— like the bricks in a fireplace, they radiate heat even after the fire goes out. They radiate in all directions. The energy that radiates back toward Earth heats both the lower atmosphere and the surface, enhancing the heating they get from direct sunlight. This absorption and radiation of heat by the atmosphere—the natural greenhouse effect—is beneficial for life on Earth. If there were no greenhouse effect, the Earth’s average surface temperature would be a very chilly -18°C (0°F) instead of the comfortable 15°C (59°F) that it is today. 

     What has scientists concerned now is that over the past 250 years, humans have been artificially raising the concentration of greenhouse gases in the atmosphere at an ever-increasing rate, mostly by burning fossil fuels, but also from cutting down carbon-absorbing forests. Since the Industrial Revolution began in about 1750, carbon dioxide levels have increased nearly 38 percent as of 2009 and methane levels have increased 148 percent. The atmosphere today contains more greenhouse gas molecules, so more of the infrared energy emitted by the surface ends up being absorbed by the atmosphere. Since some of the extra energy from a warmer atmosphere radiates back down to the surface, Earth’s surface temperature rises.