14 Green House Effect and Global Climate Change

Prof. K.S. Gupta

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Contents

  1. What is climate?
  2. The factors determining the climate
  3. The climate change in the past
  4. Greenhouse and Greenhouse effect
  5. Solar and Terrestrial spectra
  6. Radiation balance of earth and its atmosphere
  7. The Mean Temperature of the Earth
  8. Global warming potential of gases
  9. Real variation in global temperature
  10. Relative contribution of different Green house gases
  11. Radiative forcing
  12. Greenhouse gases and their sources
  13. Consequences of global warming
  14. Global climate change: The IPCC report
  15. Kyoto Protocol
  16. References

 

What is climate?

According to National Aeronautics and Space Administration (NASA), United States: Weather consists of the short-term changes (minutes to months) in temperature, clouds, precipitation, humidity and wind in the atmosphere of a region or a city. Weather can vary greatly from one day to the next, or even within the same day. Most people think of weather in terms of temperature, humidity, precipitation, cloudiness, brightness, visibility, wind, and atmospheric pressure, as in high and low pressure.’

 

Climate is the long-term behavior of the weather in a particular area. Its description includes the following parameters

  1. Averages of temperature and rainfall
  2. The extremes of temperature and rainfall and the frequency of their occurrence
  3. Meteorological measurements such as wind velocity, air-pressure, solar radiation, humidity, precipitation, atmospheric particle count, and cloud cover etc over a long period of time in a given region

The Factors Determining the Climate

Two major determinants of climate are as follows.

  1. The solar radiation is the most important factor. The angle at which Sun strikes the Earth’s surface determines the areas which will be hot or which areas shall be cold.
  2. The other important factor is the general circulation. The determinants of global circulation are ocean currents and global wind system. It is responsible for redistribution of heat. Wind system is broken into different cells by Earth’s topography and rotation to produce characteristic patterns of prevailing winds.

The drivers of drastic changes in climate are the following natural and anthropogenic factors.

Natural Factors

  1. Small natural change in Earth’s orbit around the Sun alters the amount of solar energy, which the Earth receives, and its seasonal redistribution.
  2. The Sun does not emit radiation at a constant flux. A slight variation in the output of the sun has strong effect on temperature.
  3. Volcanic eruptions send dust clouds high in the atmosphere and these scatter the solar radiation back in space.
  4. Atmosphere – ocean interactions

Anthropogenic Factor

Now, a new dimension to the above natural climate changes has been added by the increasing concentrations of greenhouse gases such as CO2, CH4, N2O, CFCs etc.

The Climate Change in the Past

The Earth has passed through several ice ages many times. Next ice age is due after about 5000 years. By measuring the ratio of stable isotopes 18O : 16O in samples drawn from ice cores and ocean sediments, the temperature record of past billion years has been made. Warming and cooling cycles have been found in past too. For example, between 1000 – 1400 AD there was warming, and between 1400 – 1900AD there was global cooling called as little ice age.

 

Greenhouse and Greenhouse Effect

A greenhouse is made up of glass walls and ceiling. Trees and plants are grown in it under controlled climatic conditions. Glass walls and ceiling are transparent to solar radiation, which are largely visible, so these are allowed to go into the greenhouse. Greenhouse does not allow most infrared radiation to go out, resulting its warming. The atmospheric gases which behave like glass walls and do not allow the infrared radiation released by Earth to go out to space are called greenhouse gases.

 

Likewise, atmosphere is transparent to solar radiation, so these radiation reach Earth surface and heat it. The infrared radiation emitted by Earth are not allowed by greenhouse gases, viz., CO2, water vapors, methane, etc., to go out, resulting in the warming of the atmosphere. This is greenhouse effect.

 

Solar and Terrestrial Spectra

To understand how and why the greenhouse gases are transparent to solar radiation and opaque to infrared radiation, it is necessary to understand the solar and terrestrial radiation spectra and the absorption of solar radiation and their re-emission by Earth.

Solar Radiation Spectrum

Sun acts as a good black body with a surface temperature of about 5800 K. Important characteristics of Solar Spectrum are as follows:

  1. Solar radiation peaks in the wavelength range 400 – 700 nm.
  2. Maximum radiation are of >500 nm.
  3. Nearly half the radiation are of >700 nm.
  4. Only a small fraction is in the UV range <400 nm.
  5. The radiation flux at sea – level is lower than that at top of the atmosphere due to reflection by clouds etc
  6. Major absorbers of UV radiation are O2 and O3 in the upper atmosphere. Water vapors along with other greenhouse gases are major absorber of IR.
  7. Absorption is 100% efficient in UV region by electronic transitions of O2 and O3 in upper atmosphere.
  8. Atmosphere is largely transparent to visible radiation because energy is too low for electronic transitions and too high for vibrational transitions.

Terrestrial Radiation Spectrum

Earth also behaves as a blackbody and some important characteristics of terrestrial radiation spectrum are as follows:

  1. The radiation spectrum is a combination of black body spectra of different temperature ranging from 220-320oC.
  2. The wavelength range of maximum emission is 5000-20000 nm. The absorption of terrestrial spectrum (IR) is almost 100% efficient due to greenhouse gases, except at a window between 8000 to 13000 nm near the peak of terrestrial radiation. This atmospheric window allows the direct escape of radiation from the surface of the earth to space.

Radiation Balance of Earth and Its Atmosphere

The temperature of Earth’s surface and atmosphere is maintained by global energy balance between the incoming short wave radiation from the sun, known as solar radiation, and the outgoing long wave radiation, called as infrared, thermal and terrestrial radiation. The incoming solar radiation at the top of atmosphere (TOA) is 342 W m-2 (where W = J s-1 ). For the sake of simplicity, assuming 342 Wm-2 equal to 100 units, in percentage terms the energy balance can be explained as follows.

 

The total 100 units of energy, received at the top of atmosphere, is appropriated as follows:

  1. About 9 units are reflected back by Earth’s atmosphere.
  2. About 49 units are absorbed by Earth’s surface.
  3. About 22 units are reflected back into space by clouds, aerosols and gases.
  4. About 20 units are absorbed by the atmosphere.

The Energy Balance of the Earth

The energy received by earth is as follows:

  1. About 49 units are received from solar radiation.
  2. About 95 units are received in the form of long wave radiation from the atmosphere. Thus, the total energy reaching Earth’s surface is 144 units.

This energy is appropriated as follows:

  1. About 114 units are radiated back to the atmosphere as long wave radiation.
  2. About 23 units are used in evapo-transpiration. These are ultimately released as latent heat of the atmosphere.
  3. About 7 units are transferred to the atmosphere by heat fluxes associated with turbulence, convention, etc.

The Energy Balance of Atmosphere

The energy balance in atmosphere is achieved as follows:

  1. The atmosphere absorbs about 20 units from solar radiation.
  2. The atmosphere absorbs about 102 units from long wave radiation released by Earth’s surface.
  3. The atmosphere receives about 30 units from latent heat and thermals.

Thus, energy received by atmosphere is 152 units. The gases, clouds and aerosols present in atmosphere re-radiate about 95 units back to Earth in the form of IR radiation, and about 57 units to outer space.

 

Thermal Equilibrium of Earth

At TOA, 100 units of energy are incoming and 100 units (69 from outgoing IR radiation and 31 from reflected radiation) are outgoing, which ensure Earth-atmosphere thermal equilibrium.

The observed global mean surface temperature is 288 K.

 

The Mean Temperature of the Earth

The total solar radiation, Es emitted by sun (temperature, Ts = 5800K) per unit time is equal to the product of the radiation flux, σTs4, and area of the Sun surface, 4πRs2.

where Rs= 7×105 km is Sun’s radius and σ = Stefen constant =5.76×10-8 W m-2 K-4

The distance of the Earth from the Sun, d, is 1.5 x 108 km. the solar radiation flux, Fs, is distributed uniformly over the sphere cantered at the Sun having the radius d between sun and earth, is given by following equation.

Because of its low temperature, the Earth radiates almost exclusively in the IR region. The emission flux from Earth, which approximately behaves as black body, is equal to σTe4, where Te is the temperature of the Earth.

 

The atmosphere present above the Earth surface behaves as an isothermal layer and absorbs a fraction, f, of the terrestrial radiation, owing to the presence of greenhouse gases. Thus, the radiation flux absorbed by atmospheric layer is equal to-

The atmosphere layer has both downward and upward facing surfaces, each of which emits a radiation flux equal to f σ Ta4, where Ta is the temperature of the atmosphere.

On substituting A = 0.28, f = 0.77 in equation (11), the global mean surface temperature, Te , is found to be 288 K. By substituting Te = 288 in equation (9), the value of Ta is found to be = 241 k, which is roughly the observed temperature of atmospheric at the scale height on 7 km.

 

Global Warming Potential of Gases

The relative warming effect of different greenhouse gases is terms of global warming potential (GWP), which is defined as the total global warming expected from 1 kg of a gas over a 100 year period. The GWP values given in Table1 indicate the CFCs to have the highest warming potentials.

TABLE 1: The values of GWP of different gases (U.S. Department of Energy, October, 2000)

Greenhouse gases Relative GWP
CO2 1
CH4 21
N2O 310
CFC-11 1320
CFC-12 6650

 

Real Variation in Global Temperature

According to NASA’s Goddard Institute of Space studies, the average global temperature on Earth has increased by about 0.8° C since 1880. The temperature has been increasing at ~ 0.15-0.20°C per decade and since 1975. Several international institutes are continuously studying the variation in global temperature. Figure 2 accessed from the Earth Observatory NASA’s Feature article, presents the yearly temperature anomalies from 1880 to 2014 as recorded by NASA, NOAA, the Japan Meteorological Agency, and the Met Office Hadley Centre (United Kingdom). The data from different sources universally agree on the rapid warming in the last few decades. According to IPCC (2007)(Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis), the temperature is likely to increase by o.6 to 4.0 with attached uncertainty since 2000 to 2100 depending up on change CO2 concentration and other factors,

 

Relative Contribution of different Green House Gases

 

Radiative Forcing

The radiation balance of the Earth-atmosphere system is perturbed by the changes in concentrations of greenhouse gases and aerosols. This perturbation in radiation balance is called radiative forcing and is defined as the perturbation to the net irradiance (Wm-2) at the top of the atmosphere.

An increase in the concentration of greenhouse gases makes the perturbation positive, i.e., there is net increase in irradiance to the Earth-atmosphere system thereby increasing the temperature of Earth-atmosphere system. On the other hand, an increase in the amount of aerosols leads to a negative value of perturbation leading to a cooling effect due to increased reflection of solar radiation back to space.

 

Greenhouse Gases and Their Sources

The relative GWP values show the main greenhouse gases to be CO2, CH4, N2O and CFCs . It may be pointed out that water vapor is an important greenhouse gas, but is not considered because human activities have little control or contribution to its atmospheric concentration. The sources of other gases are described below.

1. Carbon Dioxide, CO2

CO2 has largest amount in atmosphere among all greenhouse gases. The major obvious manmade contribution is the combustion of fossil fuels. Deforestation is another. The trees and plants work as carbon bank by storing carbon. The biological processes release CO2 and fortunately, this amount is balanced by photosynthesis. It is in equilibrium with oceans, which absorb and release it. Human activities have led to net increase in CO2 level. Beginning with industrial revolution, there is a rising trend in its concentration since 19th century. Since 1957, it is being continuously measured at Mauna Loa, Hawaii(USA). The estimated CO2 concentration is ~ 280 ppm before the industrial revolution. On May 10, 2013, NOAA & Scripps first reported daily averages that temporarily reached 400 ppm.

2. Methane CH4

Methane is continuously rising since the later part of 20th century, primarily due to increase in number of cattle and paddy fields. Methane is released by ruminants as stomach gas. In water -filled paddy fields, anaerobic biodegradations release methane. Municipal waste disposal landfills and coal- mines are other important sources.

3. Nitrous Oxide, N2O

The microbiological processes in soil and ocean are the main sources. Increased use of nitrogen –based fertilizers is believed to be another cause. Other sources are change in agricultural practices and industrial manufacture of nylon.

4. Chlorofluorocarbons, CFCs

These are manmade compounds and there is no known natural source. These are inert and non-toxic and have a long lifetime of about 100 years. Since their synthesis in1930s, these are widely used as coolant, insulator in refrigeration, blowing agents in foams, in cleaning of electronic equipment, and propellants for aerosols. There is no removal mechanism for these in troposphere, so their concentration has continued to increase and some of it passed into stratosphere leading to ozone depletion.

5. Tropospheric ozone and the greenhouse effect

Tropospheric ozone contributes to the greenhouse effect. According to the 4th IPCC assessment report(2007), it is the third main greenhouse gas after CO2 and CH4, because of its radiative forcing of 0.35 W / m2 (IPCC, 2007). The global warming due to   climate change is helpful in tropospheric ozone formation.

 

Consequences of Global Warming

Effect on Vegetation

  1. The yield from some plant species may increase their due to increase in CO2 and its greater availability during the photosynthesis. This is called CO2 – fertilization. The expected increase in case of plants, trees and C3 – crops is as follows: biomass = 40%, yield = 26%. These values are subject to the effect of climate change.
  2. There is strong likelihood of alteration in distribution of the species. Some species may be able to adapt the changed climate. For others, the change may be too rapid to cope up with and they may be unable to adapt genetically or migrate. In the whole process, some species are likely to vanish.
  3. New species, including weeds and pests, may be introduced.
  4. In colder areas, the growing season for grasses and trees could increase. This could help increase the viability of grasslands, animal farming and forestry.
  5. Change in rainfall pattern and increase in frequency of draughts might seriously affect the agricultural production.
  6. The climate change is likely to affect sereously the soil and consequently its workability. This will affect the growth of trees, plants and crops.
  7. Climate change together with other atmospheric changes will put additional stress on plant life.

 

  1. Thermal expansion of the oceans, water is known to expand when temperature is raised.
  2. Glaciers and land ice sheets will melt.

The obvious consequence of the above two factors is the rise in sea level. According to IPCC, the modeling indicates a likely range of global mean sea level rise lie in the range 0.26 – 0.82 m for 2081–2100 compared with 1986–2005, depending on emissions.

Sea level rise has several implications, some of which are:

  1. Increased frequency of storm surges and consequent flooding
  2. Reduction in quality of groundwater due to salination
  3. Loss and or disruption of coastal ecosystems

Global Climate Change: The IPCC Report

The Intergovernmental Panel on Climate Change, the Nobel Laureate(2007), has unequivocally reaffirmed that the warming of our climatic system is real, and is linked to human activity. The conclusions, based on solid scientific studies, are as follows.

  1. Warming of the Earth climate is real as evidenced by observations of increase in global average air and ocean temperatures, wide- spread melting of snow and ice and rising global average sea level.
  2. Observational evidence exists from all continents and most oceans that the many natural systems are being adversely affected by regional climate changes, particularly temperature increases.
  3. The concentrations of greenhouse gases have increased markedly in global atmosphere owing to human activities since 1970 and now far exceed the pre-industrial values determined from ice cores spanning many thousands of years.
  4. This century the temperatures are likely to increase by 1.1 – 6.4oC and sea levels by 18 – 59 cm.
  5. Even if the concentrations of greenhouse gases are to be stabilized at the current level, the warming and sea level rise would continue for centuries due to time scales associated with climate processes and feedbacks.
  6. Climate change is likely to affect Africa, the Arctic, small islands and Asian mega-deltas.
  7. If the future temperature rise exceeds 1.5 – 2.5 oC, about 20 – 30 % species will face extinction.
  8. Some abrupt or irreversible impacts are likely to occur due to warming.

 

Kyoto Protocol

It is an international treaty, which extends the 1992 United Nations Framework Convention on Climate Change (UNFCCC) that commits State Parties to reduce greenhouse gases emissions, based on the premise that (a) global warming exists and (b) man-made CO2 emissions have caused it.

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References

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  3. D. J. Jacob (1999), Introduction to Atmospheric Chemistry, Princeton University Press, 41, Princeton, NJ 08540, USA.
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