GHG (11) Flashcards
What is the greenhouse effect?
What is a GHG?
Retention of solar energy in the form of heat.
Atmospheric gas which can absorb infrared radiation.
What are the 2 types of light energy involved in the greenhouse effect?
- Shortwave radiation
- visible light
- short wavelengths
- comes from the sun: some of this energy is absorbed and then reflected by the earth. - Infrared radiation
- longer wavelength
- reflection of energy changes wavelengths from short to long, infrared wavelengths.
How do GHGs absorb and radiate heat?
- All molecules with 2 or more atoms vibrate (move in space).
- Two-atome gas molecule can only move by stretching.
- Molecules with >2 atoms can move both by stretching and bending.
Bending and stretching vibrations are not in the same wavelength as visible light, so solar radiation passes through atmospheric gasses without interacting.
Bending vibrations are slower than stretching vibrations.
- for some molecules, rate of movement for bending can fall within the wavelength range of infrared radiation.
- these molecules will interact with infrared radiation.
Infrared radiation interaction causes bending vibrations to move more vigorously.
- some energy from infrared radiation is captured and then radiated outward.
- some to space, lost.
- some back to earth = heat.
Explain the importance of oxygen to produce energy.
Oxygen as an electron donor - produces a large amount of energy.
All large multicellular life requires aerobic cellular respiration in order to produce energy to support metabolism and growth.
What are chemoautotrophs?
Life that uses inorganic substances as electron donors or acceptors.
Reactions where molecules other than carbon are the electron donor (O2 is the electron acceptor).
Give an example of a reaction where a molecule other than oxygen are the electron acceptor.
What are their impacts on the global cycle?
Denitrification or methanogenesis: the products are nitrous oxide and methane.
- Bacteria are the main driver of the global nitrogen cycle.
- Certain species produce GHGs as a waste product (methane)
Our ability to impact the concentration of these gases is dependent on how we impact the microbial community.
What are the two main metrics used to compare the relative impact of a gas on the greenhouse effect?
- Residence time (or atmospheric time): the amount of time a GHG remains in the atmosphere reservoir.
- Global warming potential (or relative radiative forcing): measure of the contribution of a gas on global warming, using CO2 as a standard.
- for a given time period, how much infrared energy 1 ton of gas will absorb, compared to 1 ton of CO2.
- often measured out of 100 years.
Why is the time period (100 years) used to measure GWP can be deceiving?
It depends on the residence time.
For example:
- methane: 25-28 GWP over 100 years
- but residence is only 10-12 years
- methane does most of its damage quickly, then leaves
If measured on a 20 year time scale
- GWP of methane = 80
So, using a long period of time makes the GWP seem smaller.
What are the 3 most impactful gases?
CO2:
- present in the atmosphere at the highest concentrations
Methane:
- Traps 25x more heat than CO2
Nitrous oxide:
- high GWP and resident time
- released from agriculture
What are the sources of methane?
- Natural sources (mainly biotic)
- methanogens: bacterial species (areas that lack oxygen) and animals that ferment their food during digestion - Anthropogenic sources
- agriculture: animals and crops (rice)
- energy: natural gas and processing of petroleum
Rates of methane release have increased twice as fast as CO2 in the 20th century
Why does methane fluctuate seasonably like CO2?
Like plants, bacteria freeze - when the ground freezes in the winter, there is a drastic reduction of methane production.
What are 2 large sources of methane?
- Cattle industry
- methanogenesis takes place during the digestion of foods in ruminant mammals
- high biomass of cows globally results in high impact - Permafrost: soil water which has remained frozen for a minimum of 2 continuous years
How does permafrost contain methane?
Every season, some of the permafrost melts. During the warm period, organic matter can build up, but does not have the time to fully decompose.
Decomposing matter builds up over time.
Complete melting of the deep permafrost = rapid decomposition of stored organic matter.
Decomposition in the absence of O2 produces methanogensis. In the absence of O2 in the soil, production of methane and CO2 occurs.
How is the melting of the permafrost a positive feedback loop?
Positive feedback between melting permafrost and increasing atmospheric temperature.
Melting permafrost —> Atmospheric GHG —> Increasing temperatures —> Melting permafrost
What are the sources of nitrous oxide?
- Natural part of the nitrogen cycle:
- produced by bacteria in the soil.
- lighting breaking N3 bonds in the atmosphere. - Anthropogenic nitrous oxide:
- produced as a byproduct of agriculture: nitrogen fertilizer used for crops feed bacteria, which produce more nitrous oxide.
Removed from the atmosphere by bacteria, breakdown by UV radiation.
