The Big Picture
 
 

The Earth occupies a unique situation in the solar system. Due to its position 149 million kilometers from the sun, physical conditions on Earth are conducive to life. At this position, the temperature range at the Earth's surface allows water to exist in solid, liquid, and vapor states and atmospheric gases to exist in an appropriate mix and concentration. The solar radiation that reaches the Earth is either reflected by the atmosphere or temporarily absorbed by the oceans, waters, soils, or biota. Ultimately, all solar energy that reaches the Earth is re-radiated to space; in the meantime, however solar radiation drives weather, climate, and ecosystems. The amount and timing of solar radiation that is absorbed is unevenly distributed. Equatorial regions receive more total solar radiation annually than temperate or polar regions. The excess of energy is redistributed poleward by oceanic and atmospheric circulation. This redistribution of energy and the insulating properties of the atmosphere prevent the extreme variations in temperature and moisture that inhibit most life forms. However, climatic differences do exist (mainly due to temperature and moisture variations) and are the most important factors determining the distribution and abundance of species. Humans are the most dominant species on Earth. Due to technological advances and sheer population numbers, most scientists believe that humans are beginning to alter the planet's climate. Since the beginning of the industrial revolution, humans have become increasingly reliant upon fossil fuel combustion as an energy source. The greenhouse gases produced by fossil fuel combustion and from other anthropogenic sources coupled with deforestation and changing land use patterns are probably responsible for recent increases in average annual global temperature. The consequences of global warming are enormous. Changing temperature and moisture patterns will have a significant effect on natural and agricultural ecosystems, sea level, and human population distribution. There is considerable and unresolved debate as to the causes, outcomes, and solutions to the global warming situation.
 
 

Frequently Asked Questions
 
 

What is the Earth's atmosphere composed of?

• Seventy-eight percent of the atmosphere is free nitrogen (N₂), an inert gas.
• Twenty-one percent of the atmosphere is free oxygen (O₂).
• The remaining 1% is most inert argon (Ar) (0.9%), carbon dioxide (CO₂) (0.03%), and trace amounts of other gases and particulates.
• The relative percentage of these gases remains constant throughout the atmosphere, but the density of atmospheric gas molecules decreases with altitude.
• Water vapor constitutes from 1% to 4% of the troposphere, the layer nearest the Earth's surface.
• Atmospheric pressure is a measure of gas density. For example, atmospheric pressure at sea level is approximately 10⁵ N/m² or 14.7 lb/in.²
• Life on Earth is dependent upon three basic physical conditions of the atmosphere; gases at the appropriate ratios and atmospheric pressure, atmospheric temperature within an appropriate range (Figure 21.1), and relative humidity.

What is relative humidity?

• Relative humidity, expressed as a percentage, is a measure of the actual amount of water vapor in the atmosphere relative to the potential amount of water the atmosphere can contain.
• Relative humidity, expressed as a percentage, is dependent upon three basic factors: air temperature, air pressure, and water availability. An air mass containing the maximum amount of moisture possible given these three factors is referred to as saturated, and has a relative humidity of 100%.
• A rapid drop in air temperature, such as typically occurs at dusk, lowers the saturation point and water vapor condenses as dew.

What are atmospheric conditions like in the troposphere and the stratosphere?

• The troposphere is the lowest stratum of the atmosphere ranging from the Earth's surface to 10-12 km. The troposphere is the only stratum with an abundance of life, water, and weather. Air temperature and pressure decrease with increasing altitude (Figure 21.2).
• The stratosphere extends from the tropopause (~11 km) to 50 km in altitude; temperature is a relatively constant -55^o C throughout. The stratospheric ozone layer between 20 km and 25 km protects the Earth from harmful exposure to ultraviolet radiation.

What causes global air circulation and wind?

• Air moves from areas of relatively high air pressure to areas of relatively low air pressure. The stronger the gradient between high pressure air masses and low pressure air masses, the higher the wind speed.
• Differential heating and cooling of the Earth's surface and atmosphere creates differences in air pressure.
• At the equator, air near the surface is warmed, causing it to rise and creating a low pressure near the surface. To compensate for the low pressure area that is created, air masses move horizontally across the Earth's surface toward the equator from the north and south. In the upper troposphere above the equator, rising air cools and moves poleward then descend between 25^o and 30^o latitude. Similar circulating cells become established in the temperate zones and polar zones.
• Additional directionality is given to global wind patterns as a result of the Coriolis Effect. The Earth's rotational speed at the equator is faster than at higher latitudes. This tends to deflect surface winds to the right in the northern hemisphere and to the left in the southern hemisphere.
• The global wind belts are the:
• doldrums: equatorial, low pressure, ascending air, little horizontal air movement
• horse latitudes: ~30^o N and S, high pressure, descending air, little horizontal air movement
• trade winds: tropical zone, horizontal movement in an easterly direction
• westerlies: temperate zones, horizontal movement in an westerly direction
• polar easterlies: polar zones, horizontal movement in an easterly direction

What are weather and climate?

• Weather is short-term (daily to weekly) atmospheric conditions.
• Climate consists of long-term, characteristic weather patterns (Figure 21.4).
• Neither weather nor climate is static both change over time, albeit climatic changes occur at a much slower rate.
• Precipitation and temperature are the two factors that most strongly influence climate.
• Climate, in turn, exerts a strong influence on the distribution and abundance of the Earth's biota.
• While biogeography is determined largely by global climate, individual organisms are influence by microclimate (very localized atmospheric conditions).

What is urban microclimate?

• Cities alter local weather and climate.
• Tall buildings, concrete and asphalt surfaces, few trees, and fossil fuel emissions exert influences on the urban microclimate.
• Compared to adjacent rural areas, cities are generally warmer, dustier, and cloudier, with a lower relative humidity, higher precipitation, less solar radiation, and calmer winds (Table 21.1).
• One effect of stagnant, polluted air is the development of urban heat domes and dust domes over cities (Figure 21.5).

Has global climate changed over time?

• There have been numerous major changes in global climate throughout geologic history of the Earth (4-5 billion years). During the Pleistocene (2 million years BP to 10,000 years BP) four major glaciations occurred (the most recent of which formed what is now Long Island, NY). Figure 21.6 illustrates changes in global temperature during the past 1.0 million years.
• Glaciations (also called glacial events or ice ages) are separated by interglaciations. It is unknown if the ice ages have ended or we are simply in an interglaciation.
• On a shorter time frame, periods of warming and cooling, and wet and drought have occurred. These cycles have had a strong influence on human settlement and history.
• In the last 100 years, the global mean annual temperature has increased approximately 0.5^o to 0.7^oC.
• The decade from 1986 to 1995 is the warmest decade on record since global temperatures have been monitored (135 years). However, there is not sufficient evident to indicate whether this increase is a result of human influences on the global climate or part of a natural cycle of climatic change. None-the-less, there is ample evidence that humans have changes the composition of the atmosphere (increasing greenhouse gases) and disrupted global biogeochemical cycling.

What are global warming and the greenhouse effect?

• Global warming is a natural or human-induced increase in average global temperature.
• The temperature of the Earth near its surface is dependent upon the amount of solar energy that is: 1) received, 2) reflected by the atmosphere and surface, 3) retained by the atmosphere, and 4) is transferred in evaporation and condensation.
• The greenhouse effect is a natural phenomenon. Atmospheric gases, primarily water vapor, traps radiant heat in the lower atmosphere. The greenhouse effect moderates daily and seasonal temperature fluctuations.
• Water vapor is a natural greenhouse gas; of greater concern for global warming are anthropogenic inputs of other greenhouse gases.

What is the relative contribution of the major anthropogenic greenhouse gases to the greenhouse effect?
 

Greenhouse Gas	Relative Contribution
CO2	50% - 60%
CH4	12% - 20%
CFC	15% - 25%
N2O	5%

 

What is the status of atmospheric CO₂?

• Carbon dioxide (CO₂) concentrations can be measured glacial ice to determine long term trends.
• Based on this evidence, prior to the industrial revolution in the 1700's, background concentrations of atmospheric CO₂ were 200-300 ppm.
• In 1860 CO₂ concentration were 280 ppm.
• Today, CO₂ concentrations are ~400 ppm.
• Rate of CO₂ increase is 0.5% per year.
• Total carbon emissions (from CO₂, CH₄, other hydrocarbons, particluates, etc.) have increased 4.3%, annually.
• The discrepancy between total carbon emissions and CO₂ concentration suggests that an unknown carbon sink (probably the oceans and/or terrestrial plants) is accumulating some of the excess carbon.

What is the status of atmospheric CH₄?

• Methane (CH₄) is increasing 1% per year.
• Sources of CH₄ are termites, wetlands, cattle, rice cultivation, and biomass and fossil fuel combustion.

What is the status of atmospheric CFCs?

• Chloroflourocarbons (CFCs) are increasing at approximately 5% per year.
• CFCs are chemicals have been or are used as aerosol spray propellants and refrigerants.
• CFC molecules are especially significant contributors to the greenhouse effect because they absorb wavelengths that would otherwise exit the atmosphere via an "atmospheric window."
• CFCs have a long residence time in the atmosphere.

What is the status of atmospheric N₂O?

• Nitrous oxide (N₂O) is increasing as much as 5% per year.
• Sources include agricultural fertilizers and fossil fuel combustion.
• N₂O also has a long residence time in the atmosphere.

Is global warming really occurring?

• Because global atmospheric conditions apparently undergo cycles of warming and cooling, and wet and dry, it is difficult to attribute atmospheric conditions of any single year (or decade) to global climate change. Consequently, there is considerable debate among and between scientists, legislators, economists, and consumers about the causes and consequences of global warming.
• However, certain facts are apparent:
• The greenhouse effect is a well-understood atmospheric phenomenon.
• Anthropogenic processes increase the concentrations of greenhouse gases.
• A correlation between atmospheric CO2 concentrations and global temperature has been established.

What scenarios and models of global warming have been developed?

• Most scientists expect that average annual temperature will be 2^o - 4^o C higher by mid 21^st century.
• If global warming occurs, positive and negative feedback loops could increase or decrease global temperature.
• Positive feedback scenarios are as follows:
• increased global warming increases evaporation, increased water vapor (a major greenhouse gas) causes an increase in the greenhouse effect and increased global warming.
• increased global warming increases permafrost melting thus releasing methane (a greenhouse gas) causing additional warming.
• increased global warming initiates increased fossil fuel use for cooling, thus the emission of more greenhouse gases.
• Negative feedback scenarios are as follows:
• increased CO2 increases oceanic algae or terrestrial plant CO2 uptake, the total amount of CO2 would decrease and the greenhouse effect would diminish.
• increased CO2 increases evaporation rates causing increased cloud cover, less solar radiation reaches the surface thus cooling the lower atmosphere.
• Global Circulation Models (GCMs) use atmospheric temperature, relative humidity, and wind conditions to predict the regional effects of global warming.
• GCM models are crude but indicate that by the year 2030 an increase of 1^o - 2^o C rise will occur but this may be higher in polar regions.
• In central North America, a predicted increase in winter precipitation and decrease in summer rains will result in decreased soil moisture during the growing season and decreased crop yields.
• In Canada and Russia, the growing season may be increased but due to poor soil, crop yields may not result.
• Sea level will rise with global warming. (Actually, sea level is already rising at a rate of 2.5 - 3.0 mm per year.)
• Two factors contribute to sea level rise, thermal expansion and the melting of glacial and polar ice.
• Regional geology also determines the net effect of sea level rise, areas experiencing geologic subsidence will be more rapidly inundated, areas being uplifted may not be directly affected.
• Sea level rise will cause a landward migration of coastal and maritime ecosystems and saltwater intrusion into aquifers and surface waters.
• The economic costs of attempting to prevent inundation by building seawalls or abandoning coastal areas will be enormous.

What other variables in the global warming question need to be better understood?

• Sunspot cycles are correlated with global temperature trends but correlation does not necessarily mean cause-and-effect. Increased sunspot cycle length correlates with decreased global temperature and vice versa.
• Aerosols (particles <10um) provide condensation sites for water vapor. Increased cloud cover can result; this would increase reflectance of solar radiation and cause a cooling trend. Volcanic eruptions increase aerosol concentrations.
• El Nino reduces the amount of CO₂ outgased from the ocean to the atmosphere, thus altering the global carbon cycle.

What do we need to know about global warming in order to make the best possible decisions?

• There are many uncertainties surrounding our understanding of global climate and human interaction, chiefly:
• we do not know with certainty if the global temperature increase that has occurred during the past 100 to 150 years will continue,
• we do not know with certainty if the continued combustion of fossil fuels and increased atmospheric CO₂ concentrations will cause global warming to continue,
• we do not with certainty if policy decisions and economic sacrifices to curb emissions of greenhouse gases will have a beneficial effect.
• Given these uncertainties, we have two basic options.
• Attempt to mitigate the severity of global warming by reducing greenhouse gas emissions. ("erring on the side of caution")
• Accept that change will occur and adapt to the new conditions.

Ecology In Your Backyard
 
 

• Differences in microclimate can be extreme on hot summer days. Measure and compare air temperature at an elevation of 1 m and at 10 cm at a variety of different urban and rural sites. Readings should be taken at approximately the same time of day for valid comparisons. An interesting comparison would be an asphalt parking lot and a shaded park or a field compared to a forest on a sunny summer day.
• Search the Ecolinks below to find out what climate change predictions have been proposed for your region.
• If you live in a low-lying coastal area, what effects might be seen if sea level rises as much as 30 cm (1 foot) over the next century?
• Please respond to the questions or send your thoughtful examples and comments to:

• Barron, E.J. 1995. Climate Models: How reliable are their predictions? Consequences 1 (3): 16-27.
• Kareiva, P.M., J.G. Kingsolver, and R.B. Huey. 1993. Biotic Interactions and Global Climate Change. Sinauer Associates, Inc. Sunderland, Massachusetts. 559 pp.
• Karl, T. R., R. W. Knight, D. R. Easterling, and R. G. Quayle. 1995. Trends in U. S. climate during the twentieth century. Consequences 1 (1): 2- 12.
• Lean, J. and D. Rind. 1996. The sun and climate. Consequences 2 (1): 26-36.

Ecolinks On The Web
 
 

• http://liftoff.msfc.nasa.gov/academy/space/atmosphere.html - The Atmosphere. This NASA web site provides a basic description of the atmosphere.

• http://www.nhes.com/home.html - World Climate Report. This web site addresses current issues about the changing climate.

• http://bang.lanl.gov/solarsys/earth.htm - Clouds from Space. This NASA web site provides you with facts about Planet Earth. See the Clouds from Space selection.

• http://www.usia.gov/abtusia/posts/CS1/wwwhpx39.html - U.S. Department of State - Global Climate Change Fact Sheet.

• http://www.wri.org/wri/enved/trends/atm-home.html - World Resources Institute. This web site contains information on the greenhouse effect, CO₂ trends, and fossil fuel combustion.

• http://science.nas.nasa.gov/Services/Education/Resources/TeacherWork/Ozone/CFC.html - Chloroflourocarbons are described at this NASA web site.

• http://www.usgcrp.gov/ - USGS Global Climate Change. This web site contains links to global climate change information (See also: http://www.usgcrp.gov/usgcrp/GCWEBPGS.html ).

• http://gcrio.ciesin.org/ - Consortium for International Earth Science Information Networks (CIESIN). This CIESIN web page provides access to several global climate change links.

• http://gcrio.ciesin.org/agci-home.html - The Aspen Global Climate Change Institute is dedicated to advancing climate change science and education. Be sure to check out the EarthPulse NEWS subdirectory: http://gcrio.ciesin.org/ASPEN/epnews/EPNews.html

• Note: If any of these links are not working, please see if alternative links are available at the Ecolink Update Site.

Ecotest Online
 
 

1. In the troposphere, water vapor concentration is variable but ____ is the usual range.

a. 1% - 4%

b. 5% - 7%

c. 9% - 11%

d. 10% - 13%
 
 

2. Most of the coterminous United States lies within the ______ wind belt.

a. northeast trades

b. westerlies

c. easterlies

d. southeast trades
 
 

3. Which statement about the troposphere is false?

a. the troposphere contains almost all of the water vapor in the atmosphere

b. the troposphere contains almost all of the terrestrial life on the planet

c. the troposphere is characterized by changeable weather

d. the troposphere contains the "ozone layer"
 
 

4. Highly localized climatic factors that individual organisms are exposed to is called ______.

a. habitat

b. life zone

c. microclimate

d. range
 
 

5. According to the Council on Environmental Quality and the Department of State, all of the following statements about urban compared to rural climatic conditions are true except:

a. annual mean urban temperatures are 0.5o -1.0o C higher in urban areas

b. annual mean relative humidity is 6% lower in urban areas

c. annual mean wind speed is increased by 20-30% in urban areas

d. total radiation on horizontal surfaces is decreased by 15-20% in urban areas
 
 

6. Natural "greenhouse" conditions are largely a consequence of the heat trapping by ______, which is responsible for about 85% of the natural greenhouse effect.

a. CO2

b. CH4

c. H2O (vapor)

d. O2
 
 

7. This atmospheric gas resulting from human-related activities is responsible for 50-60% of the anthropogenic "greenhouse" conditions affecting global warming.

a. CO2

b. CH4

c. H2O (vapor)

d. O2
 
 

8. All of the following trends have been substantiated (using acceptable scientific methods) about global climate change except:

a. atmospheric CO2 concentrations increased from 280 ppm to 880 ppm in the past 50 years

b. global mean annual temperature has increased 0.5 to 0.7oC in the past 200 years

c. the decade from 1986-1995 was the warmest decade on record (dating back 130 years)

d. sea level is rising at a rate of 2.5 to 3.0 mm per year
 
 

9. Which of the following global climate change scenarios is an example of a negative feedback loop?

a. increased CO2 concentrations cause an increase in oceanic algae or terrestrial plant CO2 uptake, the total amount of atmospheric CO2 would decrease and the greenhouse effect would diminish

b. increased global warming increases evaporation, increased water vapor (a major greenhouse gas) causes an increase in the greenhouse effect and increased global warming

c. increased global warming increases permafrost melting thus releasing methane (a greenhouse gas) causing additional warming.

d. increased global warming initiates increased fossil fuel use for cooling, thus the emission of more greenhouse gases
 
 

10. According to your author, in addition to emissions of anthropogenic greenhouse gases, each of the following processes may contribute to the global climate change, except:

a. sunspot cycles

b. Earth's gradual reduction in rotational velocity

c. El Nino

d. aerosol inputs from volcanic eruptions and other sources
 
 

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