Geog412
3/6/2016
Lab 7: Global Dimming
We have learned about the radiation budget and how properties of the atmosphere
alter it through absorption and reflectance. We can take this one step further by looking
at an important area of Global Climate Change research to see how humans might change
this radiation budget.
Global dimming is the gradual reduction in the amount of global direct irradiance
at the Earth's surface. It has been observed since systematic measurement started in the
1950s. The effect varies by location, but worldwide it has been estimated to be of the
order of a 4% reduction over the three decades from 1960–1990. It is thought to have
been caused by an increase in particulates such as sulphate aerosols in the atmosphere
which are anthropogenic in nature. However, since 1990, the trend has reversed due to
the Clean Air Act.
As you have discussed in class, aerosols and other particulates absorb solar energy and
reflect sunlight back into space. The pollutants can also become condensation nuclei for
cloud droplets. Increased pollution causes more particulates and thereby creates clouds
consisting of a greater number of smaller droplets. The smaller droplets make clouds
more reflective, so that more incoming sunlight is reflected back into space and less
reaches the earth's surface.
Clouds intercept both shortwave radiation from the sun and longwave radiation from the
Earth. Their effects are complex and vary in time, location, and altitude. Usually during
the daytime the interception of sunlight predominates, giving a cooling effect; however,
at night the re-radiation of heat to the Earth slows the Earth's heat loss.
Pan evaporation data
Over the last 50 or so years, pan evaporation has been
carefully monitored. For decades, nobody took much notice
of the pan evaporation measurements. But in the 1990s in
Europe, Israel, and North America, scientists spotted
something that at the time was considered very strange, the
rate of evaporation was falling although they had expected it
to increase due to global warming. The same trend has been
observed in China over a similar period. A decrease in solar
irradiance is cited to be the driving force.
Pan evaporation experiments are easy to reproduce with
low-cost equipment, there are many pans used for agriculture all over the world and in
many instances, and the data has been collected for nearly a half century. However, pan
evaporation depends on some additional factors besides net radiation from the sun. The
other two major factors are vapor pressure deficit and wind speed. The ambient
1
temperature turns out to be a negligible factor. The pan evaporation data corroborates the
data gathered by radiometer and fills in the gaps in the data obtained using pyranometers.
Assignment:
1. Read the Introduction, section 2.6, and all of section 3 from:
Stanhill, G. and S. Cohen (2001). "Global dimming: a review of the evidence for a
widespread and significant reduction in global radiation with discussion of its probable
causes and possible agricultural consequences". Agricultural and Forest Meteorology
107: pp. 255-278.
2. Answer the following questions based on the reading:
Explain in your own words why Extra-terrestrial radiation, rayleigh scattering, gas
absorption and water vapor are not likely candidates for causing global dimming?
What are the differences between the radiative effects of aerosols estimated by the IPCC,
Harvey (2000), and Arking (1996)? How do these relate to the estimated magnitude of
global dimming?
What are 2 physical (and kind of common sense) reasons to think aerosols cause global
dimming?
After 9-11 air traffic was grounded for several days. Based on the discussion of cloud
transmissivity, how would this have affected global dimming?
3. Given pan evaporation and aerosol data, write a fortran program to graph the
two curves together and calculate a correlation.
Go to http://www.sjd.water.ca.gov/landwateruse/agroclimatic/index.cfm and open the
file for Fresno station, year 2000. Data isn’t always available in a nice digitized form.
For this particular data you will need to create a text file in your unix account and hand
type the 12 monthly “evaporation class A pan (inches)” number into it.
Now collect aerosol data from http://toms.gsfc.nasa.gov/aerosols/aot.html
Download the 12 monthly files of aerosols for the year 2000 and ftp them to your unix
account.
There is the beginning of a fortran program prepared for you that opens and reads in all
the data files. Go to the class folder on Haruka and ftp the program lab7_template.f95 to
your unix account. Change the name of the program using mv.
Ex: mv lab7_template.f95 daria_lab7.f95
2
Graph the two data sets together using GPLV2, and calculate a correlation coefficient as
well as a significance using any method you learned in GEOG250.
In your write-up discuss the graph and correlation value. Do you see what you expected
based on the first 2 parts of this exercise? Why or why not? What are potential
limitations to this study?
Upload a copy of your program as well as your discussion.
3