929 Lecture, Seasons, Greenhouse

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Transcript 929 Lecture, Seasons, Greenhouse

Dr. Michael MacCracken
Is Climate Enginering an Option for Dealing with
Climate Change?
Why the Earth has 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
Basic Earth-Sun Geometry
• Fig 3.5
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
68.5o on June 21 to 21.5o on December 21
Duration of the day changes over the course of the
Above the Arctic circle, the duration of the day on June
21 is 24 hours; on December 21 it is 0 hours.
Earth from
Space, June
Fig. 3.6
suitably tilted
Other Factors
The fact that the Earth’s orbit is not a perfect circle makes little difference at
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.
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
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.
Basic Earth-Sun Geometry
• Fig 3.5
Annual Radiation Surplus and Deficit as a Function of Latitude
Greenhouse Effect
Climate Change
Global Warming
READ Somerville Chapter 3
Is the Greenhouse Effect Real?
Without greenhouse gases in the atmosphere (CO2 and
H2O) the Earth would be about 33 degrees cooler than at
The average global temperature is now 287 K
Without greenhouse gases it would be 254 K
We would have an ice-covered planet!
The Greenhouse Effect
Is the Climate Changing?
It has always been changing!
And it is expected to continue to change
Temperature Trends, 1850-1995
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
Atmospheric CO2 concentrations