Global Environmental Problems

Spring 2006

Problem Set #6

Due: 15 March

1.

Human interference in the hydrological cycle.

A.

NASA researchers recently estimated that the world's dams, reservoirs, and artificial lakes store about 5,000 km³ of water. Ninety percent of this storage capacity has been added since 1950. How does this compare to the amount of water stored in natural freshwater lakes? What effect does human storage of freshwater have on sea level?

B.

Because of their large surface area, and often because they are located in hot, arid climates, these reservoirs increase evaporation of water and reduce runoff. NASA researchers estimate that this has decreased total runoff by about 600 km³/y (apart from any net change in the amount of water stored). What effect does this have on sea level?

C.

What is the average residence time of water in reservoirs with respect to evaporation?

D.

In addition to storing and diverting runoff, humans also pump groundwater for irrigation and other uses. Some of this water eventually percolates back into the ground, but most evaporates or runs off into streams. The rate of groundwater pumping is highly uncertain. Suppose that 25% of all water used by humans is from groundwater, and that 25% of this groundwater becomes runoff. What effect does this have on sea level? (Find average per-capita water consumption in last week's class notes or at http://earthtrends.wri.org.)

Reference: http://www.giss.nasa.gov/research/intro/gornitz_06

2.

Human interference in the carbon cycle. 

A.

Current worldwide emissions of carbon dioxide from fossil-fuel burning currently total about 6.6 billion metric tons of carbon per year (GtC/y). If all this carbon dioxide remained in the atmosphere, at what rate would carbon dioxide concentrations increase, in ppmv/y?

B.

The preindustrial carbon dioxide concentration was about 275 ppmv. If all the carbon dioxide released by all fossil-fuel burning—estimated at about 300 GtC—remained in the atmosphere, what would the atmospheric concentration of carbon dioxide be today?

3.

Human interference in the nitrogen cycle. 

A.

As indicated in the class notes, current production of grain amounts to about 2 billion tons per year. Adding in all other crops, the total amount of dry agricultural biomass consumed amounts to about 3 billion tons. Using the empirical formula for biomass given in Harte I.8.1 (chapter I, problem 8, exercise 1), approximately how much nitrogen is removed in crop harvests each year? What proportion of this nitrogen requirement could manufactured fertilizers be supplying? World production of nitrogen fertilizer is about 90 millions tons of nitrogen per year (MtN/y).

B.

Suppose that half of the nitrogen in manufactured fertilizer blows or washes off farmland and enriches the growth of natural vegetation. If terrestrial ecosystems sequester this nitrogen in the form of additional biomass, how many billion tons of carbon (GtC) would be sequestered each year? What fraction of annual carbon emissions (6.6 GtC/y) does this correspond to?