2. Dr. Michael MacCracken Is Climate Enginering an Option for Dealing with Climate Change? 9/25/09 http://www.colby.edu/sts/20090925MaccrackenGeoeng ineeringColby.pdf

3. Why the Earth has Seasons

4. Seasons A season is a period of time during the calendar year characterized by or associated with a set of coherent weather or climate phenomena. Seasons are terrestrial responses to solar heating inequalities that are governed by the tilt of the Earth’s axis and the shape of its orbit around the Sun.

5. Basic Earth-Sun Geometry Fig 3.5

6. Angle and Duration of Sunlight Angle of the sun’s rays changes over the course of the year. In our latitude, the noontime sun angle varies from 68.5 o on June 21 to 21.5 o on December 21 Duration of the day changes over the course of the year. Above the Arctic circle, the duration of the day on June 21 is 24 hours; on December 21 it is 0 hours.

7. ANGLE: Earth from Space, June 21 Fig. 3.6 suitably tilted

8. Duration

9. Other Factors The fact that the Earth’s orbit is not a perfect circle makes little difference at all. The Earth is farther from the Sun in summer in the Northern Hemisphere (aphelion on July 5) and closer to the Sun in the Winter (perihelion on Jan. 5). The difference of 3.5% in distance results in 7% greater solar insolation in Winter than Summer. Overall, the angle and duration factors far outweigh this difference in distance. Moreover, Summer is actually longer than Winter! There are 185 days between March 21 and Sept. 22 but only 180 days between Sept. 22 and March 21. Why?

10. Basic Earth-Sun Geometry Fig 3.5

11. Annual Radiation Surplus and Deficit as a Function of Latitude

12. Greenhouse Effect Climate Change Global Warming READ Somerville Chapter 3

13. Is the Greenhouse Effect Real? YES! Without greenhouse gases in the atmosphere (CO 2 and H 2 O) the Earth would be about 33 degrees cooler than at present The average global temperature is now 287 K Without greenhouse gases it would be 254 K We would have an ice-covered planet!

14. The Greenhouse Effect

17. Is the Climate Changing? YES! It has always been changing! And it is expected to continue to change

19. Temperature Trends, 1850-1995

20. What is Humanity Doing to Enhance the Natural Greenhouse Effect? The composition of the earth's atmosphere is undergoing a major global change Human activities, such as air pollution, deforestation, the burning of fossil fuels, and even agricultural practices are now starting to alter the balance of radiatively active gases in our atmosphere

21. Atmospheric CO 2 concentrations