Chapter 11.  Air, Climate and Ozone
[Emphasizing Section 11-2, 11-3, 11-4 on Greenhouse Effect and Global Warming]

• Climate change has occurred many times in Earth's  history - there have been gradual and abrupt changes (See Figure 11.6)
• Greenhouse effect: temperature of the earth's atmosphere due to amount of heat trapped by "greenhouse effect" gases.  Here's how it works (Fig 2-12):
• Sun emits shortwave radiation, this passes through atmosphere, reaches surface of earth.
• Earth emits mostly longwave (infrared, heat) radiation.  Much of this is absorbed by greenhouse effect gases.
• This is normal  - without these gases, earth's temperature would be about 0F, all water would be frozen, life as we know it impossible.
• Global warming is consequence of amplification of greenhouse effect due to emissions of greenhouse effect gases due to human activity.
• Chemistry of the atmosphere
• Dry air = 99% nitrogen (N2, 78%) and oxygen (O2, 21%).  The remaining 1% includes trace gases (argon, CO2, CH4, etc.)
• Water vapor concentration varies in concentration seasonally and geographically from .01% (poles, winter) to 5% (tropics, rainy season).
• Greenhouse effect gases: effect of each gas depends on total concentration in the atmosphere and the per molecule heat trapping capacity of the gas.  See Figure 11-8.
• Water vapor (H2O) - this accounts for most of the greenhouse effect.  However, since there is so much water vapor in the atmosphere, human activities have little effect on its concentration, therefore human-caused changes in H2O concentration will contribute little to global warming.
• Carbon dioxide (CO2) - presently comprises 0.036% or 360 parts per million (ppm) but is increasing.  There is an annual oscillation due to the the seasonal balance between photosynthesis and respiration.  Responsible for 50-60% of global warming.  Main sources of emissions are fossil fuel combustion and deforestation.  Absent human activity, photosynthesis and respiration are in balance, with no net increase in CO2 concentration.
• Methane (CH4) - produced by bacterial decomposition of organic material in anaerobic environments (e.g. wetlands, cow and termite guts).  Also some leakage from natural gas production.  Each CH4 molecule traps 25X as much heat as a CO2 molecule.
• Nitrous oxide (N2O) - produced by bacterial activity in wetlands, also from combustion of organic matter.  Each N2O molecule traps 230X as much heat as a CO2 molecule.
• Chloroflourocarbons (CFC's) - synthetic molecules, component of refrigerants and aerosol propellants. Each CFC molecule traps 1500-7000X as much heat as a CO2 molecule.
• Note that these gases stay in the atmosphere for years (data in Figure 9-6).  Even if current rates of emission are reduced, effects of gas on atmospheric temperatures will persist (decades? centuries?)
• History, CO2 levels and atmospheric temperatures
• There have been repeated glacial cycles over past million years or so (Fig. 11-6).
• Data from ancient air trapped in ice bubbles show correlation between temperature and CO2 levels (Fig. 11-7).
• Pre-industrial (pre ca. 1800) CO2 levels were about 280 ppm.  Current levels (360 ppm) represent a 28% increase.  Projections predict that CO2 concentrations will double to about 700 ppm by 2100.
• It's getting hotter already:
• From 1860 to 2000 there was an increase of 0.5-1.1F.
• Over this period the 12 hottest years were in the period 1979-1998.
• The 4 hottest years were 1990, 95, 97, 98.
• Fig. 11-11 shows the range of projected temperature increase over the next century: best case = 1.8F, worst case = 6.3F, and these figures could be off by 2X.
• Signs that global warming is occurring:
• Melting of polar ice
• Glacial retreat in mountains (e.g. Alaska, Himalaya)
• Northward/upward spread of range of species.
• Coral death in warming ocean waters.
• Factors that could amplify or dampen global warming:
• Changes in solar energy output - this varies unpredictably.
• Changes in water vapor content and cloud cover - as temperatures rise, oceanic evaporation rates will increase, putting more water vapor in atmosphere, resulting in more cloud cover.  Problem: cloud cover can either amplify or dampen greenhouse effect.
• Rates of photosynthesis and respiration - CO2 could have a fertilization effect on photosynthesis and plant growth (if water and nutrients are not limiting), damping global warming. Increased soil temperatures could increase soil respiration, amplifying global warming.
• Changes in polar ice - melting will decrease reflectivity at pools, resulting in more heat absorption, resulting in more melting, and so forth.  This would amplify global warming.  Or - if increased water vapor in atmosphere results in more precipitation (snowfall)) over poles, reflectivity will increase, damping global warming.
• Release of methane from wetlands, tundra (melting of permafrost) and ocean sediments - release rate will increase with temperature, amplifying global warming.
• Release of CO2 from oceans - if oceans warm, their capacity to hold CO2 in solution diminishes.  Release of CO2 by oceans will amplify global warming.
• Modification of ocean currents (See Fig. 11-12) - melting of polar ice will reduce density of sea water at poles, less sinking will occur at poles, "conveyor belt" effect will weaken.  This could have dramatic effects on climate (warming and cooling)
• Changes in air pollution - pollutants can trap heat at night, shield earth from radiation during day, so effect could be complex.
• Will change be gradual or slow?  Geological record shows that some climate changes can be very rapid (decades, not centuries).
• Some possible effects of a warmer world:
• Rise in sea levels: this could be modest (ca. 19 inches) if thermal expansion of oceans is only process, or major (several feet) if polar ice melts.  In either case, coastal areas will suffer major impact.
• Shift in climate belts pole ward (latitude) and upward (altitude) - for example, Iowa will become more like Texas, North Dakota will become more like Iowa.  This may result in:
• Increases and/or decreases in crop production.  Most projections predict a net loss in production.
• Extinction of species whose migration rates cannot keep up with rate of climatic change.  (We can estimate migration rates from recent interglacial records.)
• Reduction of water supplies in some areas, increase in others.
• Increase in frequency/severity of wildfires.
• Increase in extreme weather, e.g. heat waves, hurricanes, violent storms, etc.
• Solutions - dealing with the threat of global warming.
• Three points of view:
• There is no problem, the threat of global warming is bogus.  Let's carry on business as usual. This is view of Exxon/Mobil Corp.
• Global warming could occur, but we do not have information about it to justify the expense of changing how we live.  We need more research before acting.  This appears to be the view of current administration.
• It's better to be safe than sorry, and since the evidence in support of global warming is strong, the responsible course of action is to take action.  This is the view of the vast majority of atmospheric scientists and environmental groups.
• Some tactics to slow global warming, listed in order of possible impact:
• Slow CO2 emissions by reducing dependence on fossil fuels.
• Reduce rates of deforestation, replant deforested and desertified areas.
• Technofixes:
• Fertilizing the oceans with iron to stimulate algal photosynthesis.
• Launch mirrors in space.
• Inject sulfate particles into atmosphere to reflect sunlight.
• International conventions/treaties: countries agree to reduce CO2 emissions:
• 1992 Convention on Climate Change, signed by 106 countries in Rio de Janeiro.
• 1997 Kyoto Accords, sets goals for reducing emissions, most burden on developed countries, less on developing countries.  Present US administration doesn't support the treaty.