tailieunhanh - The Earth’s Atmosphere Contents Part 9

Tầng điện li: từ 80–85 km đến khoảng 640 km, nhiệt độ tăng theo độ cao có thể lên đến °C hoặc hơn. Ôxy và nitơ ở tầng này ở trạng thái ion, vì thế gọi là tầng điện li. | 376 Chapter 14 Climate Change - L. 1850 1875 1920 1925 1950 1975 2000 Year FIGURE Changes in the average global land and sea surface air temperature from 1850 to 1998. The zero line represents the average surface air temperature from 1961 to 1990. We can see in Fig. that over the last hundred years or so the earth s surface has warmed by about C about F . The warming however is not uniform as the greatest warming has occurred over the mid-latitude continents in winter and spring while a few areas such as the North Atlantic Ocean have actually cooled in recent decades. The United States has experienced little warming as compared to the rest of die world. Moreover most of the warming has occurred at night. The changes in air temperature shown in Fig. are derived from three main sources air temperatures over land air temperatures over ocean and sea surface temperatures. There are however uncertainties in the temperature record. For example during this time period recording stations have moved and techniques for measuring temperature have varied. Also marine observing stations are scarce. Moreover urbanization especially in developed nations tends to artificially raise average temperatures as cities grow the urban heat island effect . When urban warming is taken into account and improved sea surface temperature information is incorporated into the data the warming over the past hundred years measures between C and C. A global increase in temperature between and C may seem very small but in Fig. we can see that global temperatures have varied no more than C during the past 10 000 years. Consequently an increase of C becomes significant when compared with temperature changes over thousands of years. Up to this point we have examined the temperature record of the earth s surface and observed that during the past century the earth has been in a warming trend. Most climate scientists believe that at least part of