Lecture 41: Carbon and Global Warming

Reading: Economy of Nature, pp. 631-636.

The transformation relationships in the carbon cycle are shown. The potential energy of carbon compounds is indicated by their oxidation state (Ricklefs, 1996, p 156, Fig. 7.6).

Atmospheric carbon dioxide has changed in historical times. Since the mid-1700’s atmospheric carbon dioxide concentrations (parts per million) have increased exponentially (data from Worldwatch Database, Worldwatch Institute, 1996).

This increase in atmospheric carbon dioxide concentration is the result of burning fossil fuels (coal, natural gas, and petroleum), and deforestation. Total world combustion of fossil fuels (coal, natural gas, and petroleum in units equivalent to tons of crude oil) is shown below for the period 1955-1995 (data from Worldwatch Database, Worldwatch Institute, 1996).

The significance of this atmospheric carbon dioxide increase concerns biosphere level considerations of energy flux and climate.

Solar energy that reaches the outer atmosphere of earth can be:

• absorbed
• scattered
• reflected

Energy is always conserved, so any energy absorbed must be converted to another form, as in photosynthesis (light energy to chemical energy), or reemitted at another wavelength.

Carbon dioxide (and water vapor, methane, ozone, nitrous oxides, and halocarbons) naturally absorb and reemit infrared radiation. [Halocarbons include: chlorofluorocarbons (CFC), hydroflurorocarbons (HCFC), perfluorocarbons (PFC), and sulfur hexafluorane (SF6).] These gases are called greenhouse gases because they permit visible light to pass through the atmosphere but limit the escape of heat (much like the glass of a greenhouse).

Greenhouse effect is a naturally occurring, abiotic process that predates human existence.

Small changes in the concentration of carbon dioxide has a large effect on the intensity of the greenhouse effect because carbon dioxide constitutes only a small fraction of the total atmospheric composition.

Atmospheric Gas Composition

Nitrogen	78.1%
Oxygen	20.9%
Argon	0.9%
Others	0.1%	including:
		Carbon Dioxide	0.03%
		Ozone	3 x 10-6%
		Water Vapor	variable

A balanced solar energy budget for the earth is shown with heat retention (the greenhouse effect) due to carbon dioxide, water vapor, and ozone (after Schneider, 1990).

Mean global temperature has increased during the past 150 years (0.3°C - 0.6°C per year) but the increase has not been steady as has the increase in carbon dioxide concentration during the same span of years.

Global temperatures are increasing as carbon dioxide concentrations are increasing. This suggests that the human induced changes in carbon dioxide concentration will yield an intensified greenhouse effect and global warming.

Evidence of Global Warming

• sea level increase 10 - 25 cm
• retreat of mountain glaciers
• decreased snow cover in the northern hemisphere
• increased sub-surface ground temperature
• retreat of polar ice caps

Global Circulation Models

GCM computer simulations currently predict that global warming will increase as the concentrations of greenhouse gases increase. The models can accurately describe past climate conditions.

Geological History of Carbon Dioxide Concentration and Global Temperature

Variation in carbon dioxide and global temperature are based on air bubbles sampled from the Vostok ice core from Antarctica. Temperatures are based on oxygen-18 to oxygen-16 isotope ratios in the ice (data from Worldwatch Database, Worldwatch Institute, 1996).

Global temperature increases of only ten degrees Celsius correspond to the climate extremes of geological history.

Small changes in global temperature will have significant local effects:

• changes in rainfall patterns
• sea level rise (10 - 25 cm since 1800)
• changes in wind patterns

Rate of Global Warming

Changes in global climate are occurring faster now than at any time in our past geological history.

Predicted Outcomes (1990-2100)

• 1°C - 3.5°C increase
• 15cm - 95cm sea level rise

Predicted Impacts of Global Warming

Agriculture

precipitation changes, temperature extremes, severe weather

negative effects: irrigation limits, pest eruptions, production disruptions and famine risks

positive effects: longer growing season, carbon dioxide fertilization

Water Resources

freshwater resource distributions will likely change as climate changes

water resources will become (more) limited in some regions

Coastal Cities

50% -70% of world human population lives in coastal areas

46 million at risk of storm surges (at current sea level)

50cm rise puts 92 million at risk

100cm rise puts 118 million at risk

addressing this risk will be costly because it either requires moving cities to higher ground or building sea wall protection systems

Energy Demands

negative effect: increased summer demand for air conditioning

positive effect: decreased winter demand for heat

Severe Weather

frequency of severe storms will likely increase

Natural Ecosystems

Risks to estuary and reef ecosystems

rapid sea level rise puts shallow water marine ecosystems at risk

Risks to coastal terrestrial ecosystems:

some major habitats will be inundated (Florida, Cape Cod)

Risks to non-coastal terrestrial ecosystems:

rapid changes in climate prevent ecosystem adjustments

animal migrations: problems for isolated habitats, human produced barriers

plant migration: plant species cannot migrate as fast as required

Plant migration in fossil record have occurred at rates of 0.04km - 2km/yr, but the rate of climate change occurring today would require plant species to migrate at rates of 1.5km - 5.5 km/yr. The maps below show the northern migration of four tree species following the retreat of the continental glaciers approximately 10,000 years ago. The jagged line shows the maximum glacial advance and the smooth contour lines show the northern limits of each species at thousands of years before the present. The shaded areas are the current species distributions (Ricklefs, 1996, p 580, Fig. 25.4).

Synergistic Effects

Polar methane hydrate

Methane presently locked in frozen methane hydrate will be released to the atmosphere as polar ice melts and add to the greenhouse gas concentrations in the atmosphere. This would further intensify the warming trend.

Boreal respiration vs. photosynthesis

Warming will increase both respiration (decomposition of accumulated boreal humus) and photosynthesis (removal of carbon dioxide). Recent research suggests that carbon dioxide releases will exceed removals in boreal ecosystems as temperature increases.

Increased evapotranspiration and increased water vapor and cloud cover

Increased temperature increases rates of water vapor transport into the atmosphere. Recall that water vapor is a greenhouse gas.

Deforestation decreases rates of carbon fixation

The removal of forests causes increases in carbon dioxide concentration since those trees are not removing carbon dioxide from the atmosphere in photosynthesis. Continued trends in deforestation will intensify the greenhouse effect.

Slowing Global Warming

Minimize carbon dioxide releases

• alternatives to fossil fuels
• energy efficiency improvements

Increase rates of carbon dioxide removal from atmosphere

• reforestation
• ocean photosynthesis

Kyoto Protocol (1997)

This international agreement specifies that countries reduce emissions of all the major greenhouse gases: carbon dioxide, methane, nitrous oxide, chlorofluorocarbons (CFC), hydroflurorocarbons (HCFC), perfluorocarbons (PFC), and sulfur hexafluorane (SF6).

The relative contributions of these different gases to the total warming effect is shown for different time intervals since 1765. Carbon dioxide increases have always been the most important factor contributing to global warming, but other gases are gaining in importance (Ricklefs, 1996, p 635, Fig. 27.12).

Global Cooling

Particulate matter (dust) in the atmosphere reflects light away from the earth and has a cooling effect on the atmosphere. Increases in particulate matter cause global cooling.

Sources of particulate matter

• volcanoes
• dust storms
• fires
• explosion
• fossil fuel combustion
• asteroid impact

Nuclear Winter Hypothesis

Observations on the global cooling effects of massive dust storms on Mars lead a group of researches (including the late Carl Sagan) to hypothesize on the risk of injecting large quantities of particulate matter into earth’s atmosphere as would result from a limited nuclear war. The nuclear winter hypothesis suggests that even a limited nuclear war would cause significant global cooling for a period of years, and put agriculture production at risk world-wide. These risks are separate from the risks associated with radiation exposure and blast-zone effects in a nuclear explosion.

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