Dr. Michael MacCracken

Is Climate Enginering an Option for Dealing with Climate Change?

9/25/09

http://www.colby.edu/sts/20090925MaccrackenGeoengineeringColby.pdf

Why the Earth has Seasons

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.

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 year.

Above the Arctic circle, the duration of the day on June 21 is 24 hours; on December 21 it is 0 hours.

ANGLE:

Earth from Space, June

21Fig. 3.6

suitably tilted

Duration

Other FactorsThe 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?

Basic Earth-Sun Geometry

• Fig 3.5

Annual Radiation Surplus and Deficit as a Function of Latitude

Greenhouse Effect Greenhouse Effect Climate Change Climate Change Global WarmingGlobal Warming

READ Somerville Chapter 3READ Somerville Chapter 3

Is the Greenhouse Effect Real?

YES! Without greenhouse gases in the atmosphere (CO2 and H2O) the Earth would be about 33 degrees cooler than at present

The average global temperature is now 287 KWithout greenhouse gases it would be 254 K

We would have an ice-covered planet!

The Greenhouse Effect

Is the Climate Changing?YES!YES!

It has always been changing!It has always been changing!

And it is expected to continue to changeAnd it is expected to continue to change

Temperature Trends, 1850-1995

What is Humanity Doing to Enhance the What is Humanity Doing to Enhance the Natural Greenhouse Effect?Natural Greenhouse Effect?

The composition of the earth's atmosphere The composition of the earth's atmosphere is undergoing a major global change is undergoing a major global change

Human activities, such as air pollution, Human activities, such as air pollution, deforestation, the burning of fossil fuels, and deforestation, the burning of fossil fuels, and even agricultural practices are now starting even agricultural practices are now starting to alter the balance of radiatively active to alter the balance of radiatively active gases in our atmospheregases in our atmosphere

Atmospheric CO2 concentrations