Study guide
Populations
 Increases are recent, driven first by agriculture then by the industrial/technological
revolution and by the green revolution in agriculture
 When they are not limited by resources, populations grow exponentially according to
the equation dN/dT = rN where r is the rate of growth and N is the number of
individuals in the population. The equation can also be expressed as N = N0erT, a
formulation that makes the exponential growth explicit.
 The rate, r, in per cent increase per year, has not been constant. Especially in the last
200 years it has rapidly increased, giving us an unprecedented world population size.
In the last 45 years r has begun a gradual decrease.
 Population pyramids help to visualize changes in birth and death rates. They also aid
predictions of future changes in population.
 The carrying capacity, symbolized by K in the equation dN/dT = rN(K - N)/K, is a
useful concept in evaluating the maximum number of individuals that an environment
can support.
 It’s especially useful in thinking about local populations as they go through a
demographic transition.
 The populations of several developed nations went through growth spurts as a result
of industrialization but have now reached or almost reached a new plateau value. We
expect many other, developing nations to follow this path.
 I discussed several factors that have lead to the recent population growth –
agriculture, improved public health, the industrial revolution, the green revolution –
and factors that place limits on populations – prosperity, disease, food supply, water
supply, pollution.
 Climate change, caused by a previously underappreciated form of pollution, emission
of greenhouse gases, may become the dominant limit on populations.
Global warming
 It’s unequivocally happening as shown by air temperature increases, melting of snow
and ice, and rising sea level.
 Long term records show that our species has survived warmer and much colder
periods, but our agriculture and civilizations have developed during an extended
period of moderate temperatures. Rapid anthropogenic warming and its
consequences will likely disrupt many facets of our societies as well as the natural
world around us.
 Greenhouse gases trap a portion of the sun’s heat in our atmosphere.
 Recent global warming is largely caused by humans. The rapid accumulation of CO2
(and other GHGs) in the atmosphere results mainly from burning fossil fuels such as
coal, petroleum, and natural gas.
 CO2 will continue to warm our planet long after we have reduced emissions. About
80% of the CO2 will be removed from the atmosphere in 200 years, the remainder
will last >10,000 years.
Effects on the biosphere, including humans
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As the planet warms, plants and animals will be forced to move northward. We
discussed several examples: trees and shrubs in the land (Alaska, Canada,
Scandinavia, Russia) bordering the Arctic Ocean, checkerspot butterflies and crabs
along the Pacific coast.
Plants and animals in mountains will attempt to move to higher elevations, at least
until they are forced off the mountaintops.
Movements like these will scramble ecological communities, allowing “invasive’”
species to move in, causing others to depart (or die off). Examples: thistles, weasels,
pikas, food plants for the checkerspot, coral reefs.
Ecological disruptions like fires and insect infestations are spurred by primary results
of global warming – early snowmelt and drier, hotter summers in the case of
wildfires, milder winters allowing range expansion and increased population size in
the case of the pine mountain beetles.
Several results of global warming lead to positive feedbacks that amplify the initial
effects. For example both fires and beetle infestations lead to further release of CO2
that will amplify the warming.
More effects on humans
 Water shortages will become important because of the combination of increased
population, increasing development in many countries, and increasingly frequent
droughts caused by global warming. GW droughts are likely because of glacial
melting and changes in weather patterns.
 Paradoxically GW will also cause increased flooding from rapid snow/glacier
melting, from increasingly intense storms, and from sea level rise.
 The Tibetan Plateau provides a striking example of glacial –fed rivers that are in
jeopardy because of warming.
 As temperatures increase, tropical diseases are spreading northward and to higher
altitudes as their insect vectors are able to survive through the winter.
 Another important effect of GW is more frequent heat waves and associated deaths.
At least in the US, the number of heat-wave-associated deaths dwarfs the numbers
caused by other natural disasters.
 Crop failures from floods, droughts, increased temperatures will cause malnutrition
and famine, especially in the underdeveloped countries of the world.
Reducing CO2 emissions
 Since 85 % of our energy comes from fossil fuels and only 1% from renewables, our
top priority has to be reducing fossil fuel burning and massively expanding the
renewable energy industry.
 We must begin to cut back on CO2 emissions. Alarmingly, the last ten years have
seen an acceleration of emissions - from 1.5% increase per year to 3% per year –
rather than a decrease. Every year that we delay means that the decreases in ensuing
years will have to be larger.
 There are many different ways to reduce emissions; one study mentioned in class
identified 15 alternatives each of which could lead to the reduction of 1 GtC. At
present (actually 2004 when that study was published) we only need 7 of the
alternatives to be successfully used to stabilize the atmospheric CO2 level at about
500 ppm.
Efficiency
 Energy efficiency is the easy way to get this started; every efficiency-related
reduction in energy usage decreases costs to individuals or to corporations.
 Refrigerators provide a simple but extremely effective example of how increases in
efficiency can benefit both consumers and manufacturers.
 Automobile efficiency has lagged far behind even though there are large increases in
fuel efficiency to be gained by reducing weight and using modern lightweight but
extremely strong materials.
Carbon capture and storage
 Although coal causes the most CO2 release per energy generated, it will be
impossible to eliminate its use quickly because it is such a large part of electricity
generation, both in the US and increasingly in industrializing nations like China.
Carbon capture and storage (CCS) offers a possible solution because it can reduce
CO2 emissions by 80-90%.
 CO2 capture is an established technology, but storage has so far only been
demonstrated on a small scale.
 Because CCS will increase the price of coal electricity, there will need to be strong
incentives for its rapid adoption. These incentives will likely include tax credits for
its installation, caps on CO2 emissions or a carbon tax or all three (we didn’t have
time to discuss this in class, so not on the exam).
 Increases in the price of coal electricity will catalyze renewable energy development
by making wind, solar, geothermal electricity more cost competitive with coal.
Solar electricity
 Photovoltaic panels convert sunlight directly to electricity and are wonderful for local
power generation. At present their cost makes them unattractive for large industrial
plants that can contribute substantially to replace fossil fuel burning. In addition, PV
panels only supply power when the sun shines and therefore cannot be used as a
reliable overall electricity source.
 Concentrating solar power (CSP) is ideal for large industrial applications. The price
of CSP electricity reduces as plants get larger; at present it’s about 2-3 times as
expensive as traditional coal electricity.
 Since CSP heats up a liquid, it is possible to store the hot liquid for later use in power
generation, even when the sun is not shining.
 There is a vast resource of direct solar irradiation in arid parts of the world, that if
developed, could supply all the world’s electricity needs.
Biofuels
 Using recently grown crops to make liquid fuels that can be burned in cars and trucks
was originally pushed as carbon neutral because the CO2 released by burning ethanol
or biodiesel had been absorbed by plants from the atmosphere in the previous
growing season. It was thought that the absorbed and released CO2 balanced each
other.
 This analysis ignored all the energy required to produce the crops, refine the ethanol
or biodiesel, and distribute the fuels. Thus, none of the biofuels are actually carbon
neutral.
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Corn ethanol, the biofuel of choice for American agriculture (and government
subsidy) is a particularly bad choice. It produces as much or nearly as much CO2 as
gasoline and raises the price of corn that is used for food.
Ethanol from cellulosic sources is better but still will affect food prices and cause
deforestation.
Thus I think that biofuels will be an important but transient substitute for petroleum.
Some biofuel enthusiasts disagree with me and think that biofuels will outcompete
petroleum and drive its price down.