GPHY 314 Chapter 5: Carbon Cycle

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29 Dec 2020
CHAPTER 5: The Carbon Cycle
Carbon dioxide the primary GHG emitted by human activities and policies to control modern
climate change frequently focus on reducing our emissions of this gas
Greenhouse Gases and out Atmosphere’s Composition
78% of dry atmosphere (excluding water vapor) made up of diatomic nitrogen (two nitrogen
atoms bound together, N2)
21% if dry atmosphere diatomic oxygen (O2) -> This is the part we need to breathe to survive
Argon atoms make up ~1% of atmosphere
^None absorb infrared photons so don’t warm planet surface
Next biggest component of atmosphere is water vapor (H2O)
o Abundance varies widely between places
o In warm tropics, can make up as much as 4% of atmosphere
o In cold polar regions, may be only 0.2%
o Abundance decreases rapidly with altitude, and in stratosphere it typically makes up
0.0005% of atmosphere
Main source is evaporation from oceans and is primarily removed from
atmosphere when water forms raindrops and these fall to the surface
Emissions of water vapor from human activities contribute essentially nothing to
its atmospheric abundance
Remaining 0.05% crucial to life on planet, carbon dioxide
o Made up 0.04% of atmosphere in 2014
o Second most important GHG behind water vapor
Next most important GHG in atmosphere is methane (CH4), despite its small abundance
Another important GHG nitrous oxide (N2O) aka laughing gas
o Emitted into atmosphere from nitrogen-based fertilizer and industrial processes as well as
several natural sources
Ozone (O3) another GHG whose abundance varies
o In unpolluted air near surface, about 10-40 parts per billion
o Can reach 10ppm in stratosphere (1,000 times higher)
o Absolutely necessary for life because it absorbs high-energy UV photons emitted by the
Sun before they reach Earth’s surface
These photons carry enough energy they could seriously damage living tissue,
leading to disease in humans i.e. skin cancer
o But also one of primary components of photochemical smog -> Breathing it can lead to
health problems in humans and animals, ground-level ozone can damage plants
Halocarbons (chlorofluorocarbons and hydrochlorofluorocarbons) are synthetic industrial
chemicals used as refrigerants and in various industrial applications
o Present in atmosphere at concentration of few parts per billion
o Main culprits behind ozone depletion
GHGs not equal in ability to warm plant -> Methane ~x20 more powerful than carbon dioxide on
a per molecule basis, most powerful on per molecule basis are halocarbons
Atmosphere-Land Biosphere-Ocean Carbon Exchange
Amount of carbon dioxide in atmosphere varies throughout year -> During maximum in May,
about 6ppm higher than September minimum
Annual cycle reflects annual cycle of plant growth and decay
o Plants absorb carbon dioxide from atmosphere and use it to produce more plant material
through photosynthesis (CO2 + H2O + sunlight -> CH2O + O2)
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At same time, humans, animals, bacteria consume plant material to produce energy through
respiration (CH2O + O2 -> CO2 + H2O + energy)
Atmosphere contained ~850 gigatonnes of carbon in 2014
The fact that photosynthesis and respiration are balanced over the year doesn’t mean they’re in
balance at every point in time
o Most of the earth’s land area are found in the northern hemisphere
o During spring and summer when plants growing and trees leafing, global photosynthesis
exceeds respiration and there is a net drawdown of carbon dioxide out of the atmosphere
and into the land biosphere
o During fall and winter, planet material produced during spring and summer decays,
releasing carbon dioxide back into the atmosphere
Global respiration exceeds photosynthesis and there is net transfer of carbon
from biosphere into atmosphere
o Also large amount of carbon stored in permafrost, which is ground frozen year-round
If permafrost thaws out, organic matter stored there will begin to decay, releasing
carbon into atmosphere
Given that much of this permafrost is in Arctic, which is expected to continue
warming rapidly, melting will continue
At some point, resulting release of carbon dioxide may contribute
significantly to atmospheric carbon dioxide
One of carbon dioxide’s most important properties that it readily dissolves in water
o Once dissolved, converted to carbonic acid (CO2 + H2O -> H2CO3)
Sometimes referred to as ocean acidification
As oceans become more acidic, biology of oceans can change and given human
reliance on oceans for food, this could lead to important impacts on humans
o Carbonic acid can further react with water to convert into other forms of carbon
o Carbon returned to atmosphere in reaction that is reverse of above equation, followed by
escape of carbon dioxide back into atmosphere
Thus, carbon cycles easily between atmosphere and ocean
o Need to think about ocean as being split in two parts
First is top 100m which exchanges carbon very rapidly with atmosphere, makes
up only few percent of mass of ocean and is often referred to as mixed layer
because it is well mixed by winds and other weather events
o Below this lies other 97% of ocean known as deep ocean
Contains most of ocean’s carbon (x47 more carbon than is in atmosphere)
Approximately 120 GtC per year continuously cycling between atmosphere and land biosphere as
plants absorb carbon dioxide as they grow and then release carbon dioxide when they die
o About 80 GtC per year from atmosphere is continuously dissolving into ocean’s mixed
layer, while same mass of carbon atoms is coming out of ocean and back into
atmosphere, cycling between atmosphere and ocean
o At same time, mixed-layer and deep ocean exchanging 100 GtC per year
Calculate turnover times -> Length of time that a carbon atom in atmosphere will remain there
before being transferred into one of the other two reservoirs (also lifetime or residence time)
o Carbon atom stays in atmosphere for only four years before transferred into land
biosphere or ocean
o Carbon atom in land biosphere will stay there for 21 years before being transferred into
atmosphere, so takes few decades for carbon atom to make round trip
o Takes few centuries for carbon atom to make round trip from atmosphere through mixed
layer, deep ocean, and back
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