Section 8.6: The Natural and Anthropogenic

(human-induced) Greenhouse Effect

A- Natural Greenhouse effect

How a greenhouse works?

What is the greenhouse effect?1) The ultimate source of energy is from the

sun2) This energy makes its way down to the

Earth in the form of radiation

3) Much of the high energy radiation is able to pass through the atmosphere, making its way down to the Earth’s surface. As it is absorbed, it becomes thermal energy which then warms up the Earth’s surfaces.

4) The warmer Earth then re-emits energy in the form of infrared radiation

What happens to the energy that is re-emitted by the Earth?

• It gets trapped by certain gases (called greenhouse gases) in the Earth’s atmosphere, which then re-radiate this energy in all directions.

• About half of this radiation returns to the Earth.

• The Earth then acquires more thermal energy which then causes warmer temperatures.

The natural greenhouse effect is very important. Without the natural greenhouse effect this average temperature would be -20oC which would result in very little life on Earth.

•N2 and O2 in the atmosphere do not absorb IR radiation and hence are not part of the greenhouse effect.

•The gases that are involved in the natural greenhouse effect exist in very low concentrations.

Greenhouse gases Molecular structure Notes

Water vapour Most abundant GHGs

CO2

Can trap more heat than water vapour

N2O (nitrous oxide)CH4 (methane)

O3

Carbon Dioxide as a GHG• Earth’s atmosphere contains 0.0385% CO2

• What % of the natural greenhouse effect is thought to be caused by carbon dioxide?

0.0385% CO2 in the atmosphere is enough to contribute to 25 % of natural greenhouse effect

•CO2 is an important part of the CARBON CYCLE. In this process, living things interact with the lithosphere, atmosphere and hydrosphere to move carbon dioxide and convert it into various forms.

Sources of CO2 in the atm.

• Atmospheric CO2 is naturally produced by

–Volcanic eruptions–Burning organic matter–Cellular respiration

of plants and animals

Carbon sinks- places that store carbon1. Soil: microorganisms in soil break down dead plants

and animals, thereby storing C in soil.2. Plants and permafrost also store great amount of CO2

3. Forest and vegetation (thanks to a process called?)4. Ocean:

- phytoplankton: microscopic organisms that also perform photosynthesis to incorporate C into the food chain

How do plants and oceans act as carbon sinks?

Water Vapour as a GHG• H2O vapour causes 66 % of the Greenhouse Effect

• How is the amount of water vapour and temperature related?WarmerTemp.

More H2O evaporates.

More H2O in the atm.

More energy trappedGreater GH effect

This is called a feedback loop in which a cause creates an effect that further alters the cause

Is this a positive or negative feedback loop? How do you know?

Feedback Loop• In a positive feedback loop, the effect increases

the original cause• In a negative feedback loop, the effect

decreases the original cause

More water vapour produced

Water vapour act as GHG and traps heat

Is water vapor- temperature a positive or negative feedback loop? How do you know?- positive feedback loop as the more water further increases the temperature

Methane as a GHG• Although low in quantity in the atmosphere,

methane is considered to be a much more powerful greenhouse gas. Explain?

•A molecule of methane can absorb 23 times more IR radiation than a molecule of CO2

•If it absorbs more IR radiation, then it traps more heat, making it a very effective GHG

Methane as a GHG• Atmospheric CH4 has risen almost three times

since the Industrial Revolution• CH4 increased from 0.700ppm to 1.785 ppm

(now)

Nitrous Oxide (N2O) as a GHG• Laughing gas• A molecule of N2O can absorb 300

times more IR radiation than a molecule of CO2

• Extremely good at trapping heat but lower concentration than methane

Nitrous Oxide (N2O) as a GHG• What changes have there been in the

concentration of nitrous oxide in the atmosphere?

• The concentration has risen by 20% since the Industrial Revolution; i.e. from .0270 ppm to 0.321ppm

• N2O is naturally produced by bacteria in the soil and water, but is mostly produced by humans in crop management

B- Anthropogenic greenhouse effect= greenhouse effect caused by increased human-generated GHGs

To sum up...Some energy is reflected

Energy absorbed by atmosphere and clouds

50 % of energy emitted to space.

GHGs absorb the IR radiation emitted by the Earth and re-radiate back in all directions.

Some energy is absorbed by the Earth’s surface.

Absorbed energy warms the Earth’s surface.

Re-emitted IR radiation warms the Earth’s surface even more.

Warm Earth emits IR radiation.