ES100 Fall 1998
Introduction to Environmental Studies (Bubier and Ellison)
Mid-term III: Technology and Science (exam key)
1) ( 4 points total): Compare and contrast each pair of terms (1 point each):
a. stratosphere vs. troposphere: The troposphere is the lower atmosphere near the earth's surface (up to 10 km). The stratosphere is the next layer of the atmosphere above the troposphere, where ozone intercepts short-wave radiation from the sun.
b. weather vs. climate: Climate is a region's long-term characteristic atmospheric conditions (seasons, years, decades), while weather occurs on shorter time scales (days or weeks).
c. flux vs. reservoir: A reservoir (or pool) is the total amount (storage) of an element in one phase of its biogeochemical cycle (e.g. the amount of water in a lake), while flux is the rate of transfer (mass per unit of time) into and out of the reservoir.
d. energy conservation vs. efficiency: Conservation involves lower total energy demand (e.g. using fewer lights or watching less tv); Efficiency is defined as the amount of work accomplished per unit of energy used (ratio of energy output to energy input)(e.g. getting more miles per gallon in your car).
2) (1 point total): Calculate the residence time for a molecule of water in a lake in the following example. A lake holds 3 million cubic meters (m^3) of water. Runoff from the surrounding upland goes into the lake at a rate of 3,000 cubic meters (m^3) per day. The lake loses about the same amount of water in evaporation each day.
Residence time = reservoir/flux; RT = 3,000,000 m^3/3,000 m^3/day = 1,000 days = 2.7 years
3) (2 points total):
a. What is the greenhouse effect? Gases, such as water vapor, carbon dioxide and methane in the troposphere absorb long-wave radiation that is emitted from the earth, warming the earth's atmosphere.
b. Name two anthropogenic greenhouse gases? Carbon dioxide (CO_2), and trace gases such as methane (CH_4), cholorfluorocarbons (CFCs), and nitrous oxide (N_2O) are some of the anthropogenic (human-induced) gases emitted by human activities such as fossil fuel and biomass burning.
4) (2 points total): Give examples of a) positive and b) negative feedback to global climate change. In your answer, be sure to explain the mechanisms for contributing to either climate warming or cooling. a) One positive feedback (destabilizing and self-enhancing) is the melting of permafrost with climate warming. As frozen soils thaw, they emit CH_4 and CO_2 as rates of microbial decomposition increase with increasing temperatures. b) Negative feedbacks (stabilizing and self-regulating) include increased CO_2 uptake by vegetation, and formation of clouds which reflect a portion of incoming solar radiation. (But clouds can also be a positive feedback by increasing H_2O vapor in the atmosphere, which is a greenhouse gas.)
5) (2 points total): a) Name two ways that humans could reduce the potential for climate warming: Several possible answers including reducing anthropogenic greenhouse gas emissions (by conservation and efficiency measures, and use of alternative energies that don't produce greenhouse gases such as solar energy), slowing the rate of deforestation, planting trees (being aware that planting trees may only work as a negative feedback temporarily).
b)What are two problems associated with our dependence on fossil fuels? Burning fossil fuels emits greenhouse gases, these fuels are not renewable, and we are dependent on the economy and political climate of the countries who export a large percentage of these fuels to the U.S.
6) (3 points total)
a. Names 2 types of alternative, renewable energy (1 point): Possible answers include solar energy (passive and active), wind, hydropower, tidal power, biomass burning.
b. What is one advantage and one disadvantage of each in terms of environmental impact (2 points): Solar (doesn't emit greenhouse gases, but is quite variable depending on the climate of the particular region); hydropower (is renewable, but alters the habitat of freshwater ecosystems; wind (like solar energy, wind doesn't emit greenhouse gases, but supply varies depending on the regional climate); biomass burning (renewable, but emits greenhouse gases and may disrupt forest ecosystems by overcutting).
7) (4 points total): a) (2 points)Would a lake ecosystem be more affected by an excess input of carbon or phosphorus? Why? Phosphorus would have a greater impact because it is a more limiting nutrient in lakes than carbon. Excess phosphate can result in high rates of algal production, lower oxygen supplies at depth, and fish death.
b) (2 points)Which element cycles faster in its biogeochemical cycle: nitrogen or phosphorus? Why? Nitrogen cycles faster because it has a gas phase in the atmosphere, whereas phosphorus does not have a gas phase. It becomes available for plant growth only by weathering of bedrock and soils.
8) (2 points total): Give two reasons why the global supply of usable freshwater is diminishing:
(from Sandra Postel's lecture): Overpopulation results in greater demand for freshwater supplies for e.g. drinking water and agriculture; development of hydropower dams alters the flow of freshwater, so that much of the water necessary for maintaining healthy aquatic habitats is diverted or kept in reservoirs. Other answers were possible.