Greenhouse Effect

Greenhouse Effect

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The Earth’s climate is changing rapidly due to global warming, pollution and the depletion of the ozone layer but these are not the only working factors, Greenhouse Effect is also one of them and a major one apparently.

Before we discuss the Greenhouse Effect, let’s understand what a Greenhouse is.

A greenhouse is a building made with walls and a roof made of transparent materials like glass, in which flora is grown given regulated climatic conditions.

These buildings vary in size; they can be as small as sheds or like big industrial buildings. The miniature greenhouses are known as cold frame. The greenhouses are exposed to sunlight so that they become warmer on the inside, protecting the plants from cold weather.

There are commercial greenhouses which are production facilities for vegetables, flowers, and fruits. Some are controlled with equipment to control the heating, cooling, and lighting to provide optimal conditions for the plants to grow.

There are different techniques that are used to examine these optimal conditions to reduce the production risk before cultivating a specific crop.

Now let’s discuss the Greenhouse Effect in detail.

The term “Greenhouse Effect” is a slight misnomer, in relation to the physical greenhouses that warm using a different mechanism.

The greenhouse effect as an atmospheric mechanism works through radiative heat loss while a normal greenhouse as a building blocks the convective heat loss.

This results in an increase in temperature in both the cases.

Human activities lead to global warming by increasing the greenhouse effect. Greenhouse Effect is the process due to which radiation from the Earth’s atmosphere heats up the surface to a higher temperature than normal.

This happens when certain gases namely, Carbon dioxide, Methane, Nitrogen Oxide, and fluorinated gases collect in the Earth’s atmosphere.

These gases occur naturally. Greenhouse gases allow the sun’s light to shine on the Earth’s surface, but they trap the heat that reflects back into the atmosphere. They act like insulating glass walls of greenhouses.

The greenhouse effect maintains the Earth’s climate. Without this effect, the temperature on the surface would drop down about 33 degrees Celsius (60 degrees Fahrenheit), and numerous life forms would freeze to death.

After the Industrial Revolution in the late 1700s and early 1800s, people have been releasing huge amount of greenhouse gases into the atmosphere.

That amount has soared high in the previous century. Greenhouse gas emission has drastically increased by 70 percent between the years 1970 to 2004.

Emissions of carbon dioxide, the most imperative among the greenhouse gases, rose by about 80 percent during that period. The amount of carbon dioxide in the present-day atmosphere far exceeds the natural range observed over the last 6,50,000 years.

The majority of the carbon dioxide that people let out into the atmosphere is produced from burning fossil fuels such as oil, coal, and natural gas.

Vehicles like cars, trucks, trains, and planes use and burn all the fossil fuels. Many electric power plants also burn fossil fuels. Another way people release carbon dioxide into the atmosphere is by eliminating forests.

This happens for two reasons. The plant materials which are decaying, including trees, release tons of carbon dioxide into the atmosphere. Living trees absorb carbon dioxide.

The cutting down of trees allows the gas to remain in the atmosphere. The activities of human beings add greenhouse gases to the atmosphere which result traps more heat than required which leads to global warming.

The Greenhouse effect usually takes place in 6 steps. They are:

  1. Solar radiation approaches the Earth’s atmosphere; some of this radiation is reflected back into space.
  2. The remaining of the sun’s energy is absorbed by the land and the water bodies which ultimately heats the Earth.
  3. The heat starts to radiate from Earth towards space.
  4. Some of the heat is captured by greenhouse gases present in the atmosphere which keeps the Earth heated enough to sustain life.
  5. The excessive consumption of fossil fuels and the clearing of land is increasing the number of greenhouse gases that are in the atmosphere.
  6. This traps extra heat as a result and causes the Earth’s temperature to rise.
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The Earth absorbs energy from the Sun in the form of ultraviolet, visible, and near-infrared radiation.

Almost 26% of the solar energy which is absorbed by the Earth is reflected back to space by the atmosphere and clouds, and about 19% is absorbed by the atmosphere and clouds.

Most of the energy which remains is absorbed by the surface of Earth. As the Earth’s surface is much colder than the Sun, it radiates at wavelengths that are much longer than the wavelengths which were absorbed.

Most of this thermal radiation is absorbed by the atmosphere and it warms it. The atmosphere also absorbs heat by sensible and latent heat fluxes that come from the surface.

The atmosphere radiates energy both upwards and downwards; the part which is radiated downwards is absorbed by the surface of Earth. This results in a higher equilibrium temperature than the atmosphere radiated.

A normal thermally conductive blackbody at the same distance from the Sun as the Earth is from the sun would have a temperature of about 5.3 degrees Celsius (41.5 degrees Fahrenheit).

But the Earth reflects about 30 percent of the absorbed sunlight, this idealizes the Earth’s effective temperature (the temperature of a blackbody that would emit the same amount of radiation) which would be about −18 degrees Celsius (0 degrees Fahrenheit).

The surface temperature of this hypothetical blackbody is 33 degrees Celsius (59 degrees Fahrenheit) which is below Earth’s real surface temperature of approximately 14 degrees Celsius (57 degrees Fahrenheit).

The greenhouse effect is the contribution of greenhouse gases that makes this difference.

The visualized greenhouse model is just a simplification. In reality the atmosphere near the Earth’s surface is largely opaque to thermal radiation and most of the heat loss which happens from the surface is by convection.

But radiative energy losses become increasingly important higher in the atmosphere, mostly because of the decreasing concentration of water vapor, which is an important greenhouse gas.

Rather than the surface itself, it is more important to think of the greenhouse effect as an application to the layer in the mid-troposphere, which is effectively coupled to the surface by a lapse rate.

A simple picture also assumes a steady-state, but in the real world, the diurnal cycle, as well as the seasonal cycle and weather disturbances, complicate matters.

Solar heating applies only during the daytime. During the night, the atmosphere cools down a bit, but not great, because its emissivity is low. Diurnal temperature changes with height in the atmosphere.

Within the region where radiative effects are important, the description given by the visualized greenhouse model becomes realistic.

Earth’s surface, has warmed up to an “effective temperature” around −18 degrees Celsius (0 degrees Fahrenheit), which radiates long-wavelength, infrared heat in the range of 4–100 μm.

At these wavelengths, greenhouse gases that were largely transparent to incoming solar radiation are more absorbent now.

Each layer of the atmosphere with greenhouse gases absorbs some of the heat which is radiated upwards from lower layers. It reradiates in all directions, both upwards and downwards; in equilibrium (by definition) the same amount that it has absorbed.

This results in more warmth below. This increases the concentration of the gases which in turn increases the amount of absorption and re-radiation, and therefore warms the layers and ultimately the surface below.

Greenhouse gases which include most diatomic gases with two different atoms such as carbon monoxide, CO and all the other gases with three or more atoms are able to absorb and emit infrared radiation.

Though more than 99% of the dry atmosphere is IR transparent because the main constituents like N2, O2, and Ar are not able to absorb or emit infrared radiation directly, intermolecular collisions cause the energy to get absorbed and emitted by the greenhouse gases which is shared with the other, non-IR-active, gases.

Greenhouse Gases

Greenhouse gases are those gases which absorb and emit infrared radiation in the wavelength range emitted by Earth.

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Carbon dioxide (0.04 percent), nitrous oxide, methane and ozone are trace gases that account for almost 0.1 percent of Earth’s atmosphere and have an appreciable greenhouse effect.

Clouds also absorb and emit infrared radiation and in turn affect the radiative properties of the atmosphere. In ascending order the most major greenhouse gases are:

  • Water Vapour (H2O)
  • Carbon Dioxide (CO2)
  • Methane (CH4)
  • Nitrous Oxide (N2O)
  • Ozone (O3)
  • Chlorofluorocarbons (CFCs)
  • Hydro fluorocarbons (includes HCFCs and HFCs)

Role of the Greenhouse effect in Climate Change

The greenhouse effect has strengthened through human activities which is known as the enhanced or anthropogenic greenhouse effect.

This increase in radiative forces from human activity is attributable mainly to increased atmospheric carbon dioxide levels.

The 2014 Assessment Report from the Intergovernmental Panel on Climate Change reported that “atmospheric concentrations of carbon dioxide, methane and nitrous oxide are unprecedented in at least the last 800,000 years.

Their effects, together with those of other anthropogenic drivers, have been detected throughout the climate system and are extremely likely to have been the dominant cause of the observed warming since the mid-20th century”.

Carbon Dioxide is produced by the burning of fossil fuels and other activities such as cement production and forest deforestation.

Measurements of Carbon Dioxide from the Mauna Loa observatory show that concentrations have increased from about 313 parts per million (ppm) in 1960, passing the 400 ppm milestone on May 9, 2013.

The current observed amount of Carbon Dioxide exceeds the geological record maxima (~300 ppm) from ice core data.

Carbon Dioxide which is produced from combustion has an effect on the global climate, a special case of the greenhouse effect first described in the year 1896 by Svante Arrhenius, has also been called the Calendar effect.

Over the past 800,000 years, the ice core data shows that carbon dioxide has varied from values as low as 180 ppm to the pre-industrial level of 270 ppm.

Paleo Climatologists see variations in carbon dioxide concentration as a fundamental factor influencing climate variations over the whole time scale.

Anti-Greenhouse Effect

The anti-greenhouse effect is a mechanism which is similar to the greenhouse effect, but with the opposite consequence of cooling down the surface temperature of a planet.

If the gases in the atmosphere of a planet have a lesser transmittance for inbound radiation like the solar rays in the Solar System than for outbound radiation, that is the thermal radiation on the planet’s surface in the infrared domain, the surface temperature at which inbound and outbound heat fluxes are at equilibrium is relatively lower.

Runaway Greenhouse Effect

A runaway greenhouse effect occurs when a planet’s atmosphere contains greenhouse gas in such an amount that it is sufficient to block thermal radiation from the planet, preventing the planet from cooling down and from having liquid water on its surface.

A runaway version of the greenhouse effect can be defined by a limit on a planet’s outgoing longwave radiation which is reached due to higher surface temperatures boiling a condensable water body which often goes out as water vapor into the atmosphere, increasing its optical depth.

This positive feedback means the planet cannot cool down through longwave radiation which is stated in the Stefan–Boltzmann law and continues to heat up until it can radiate outside of the spectral lines of the condensable species.

The runaway greenhouse effect is often formulated with water vapour as the condensable species. In such case the water vapour reaches the stratosphere and escapes into space via hydrodynamic escape route, which results in a desiccated planet. This may have happened in the early history of Venus.

Conclusion

Therefore, the greenhouse gas emissions need to be reduced which is a critical step in slowing the global warming.

Protecting forests and planting new trees will help to balance the greenhouse gases in the atmosphere as they absorb carbon dioxide. The global temperature has changed and will continue to change in the coming years.

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