The Carbon Cycle
Climate
Models
20th Century
Climate Change
Climate vs.
Weather
Sensitivity
Tipping
Points
Feedbacks
Greenhouse
Effect
What is a
Greenhouse Gas?
Buildup of
Greenhouse
Gases
Anthropogenic
Sources
Sea level
Ecosystems
Drought, Heat waves, Fires
Snow pack, Glaciers, Water supply
21st Century
Climate Projections
Global
Warming
Natural
Sources
Extreme
Events
Climate
Impacts
Ocean
Acidification
Hurricanes
Climate
Models
20th Century
Climate Change
Climate vs.
Weather
Sensitivity
Tipping
Points
Feedbacks
Greenhouse
Effect
What is a
Greenhouse Gas?
Buildup of
Greenhouse
Gases
Anthropogenic
Sources
Carbon
Cycle
Fossil fuel
reserves
Oil, Coal, Tar sands,
Natural Gas,
Methane hydrates
Sea level
Ecosystems
Drought, Heat waves, Fires
Snow pack, Glaciers, Water supply
21st Century
Climate Projections
Global
Warming
Natural
Sources
Extreme
Events
Climate
Impacts
Ocean
Acidification
Hurricanes
Components of the “Earth system”
anthrosphere (us)
atmosphere (air)
hydrosphere (water)
biosphere (life)
cryosphere (ice)
lithosphere (crust)
Lights are a fitting sumbol of the Anthrosphere
The Atmosphere shows up clearly in this view of the linb of the Earth.
This SeaWIfs image shows both the marie and terrestrial biosphere. On the ocean the green
areas are indicative of high chlorophyll content. They generally correspond to regions of
upwelling, which bring up nutrients into the sunlight where phyto- (plant) plankto can graze
on them. The distribution of upwelling, in turn, is determined by the winds. On land the
darker greens correspond to rich vegetation.
Materials in the Earth’s crust are continually being recycled on a time scale of tens to
hundreds of millions of years. The crust under the oceans is being subducted into the mantle
and being replaced by new crust expelled from the mantle by sea floor spreading. water
vapor, CO2, and other gases in the mantle are continuously being injected into the
atmosphere in volcanic activity.
Components of the “Earth system”
atmosphere
oceans
biosphere
cryosphere
crust
mantle
Atmosphere
Oceans
Biosphere
Crust
Water, carbon, and other substances are continuously being exchanged among the various
components of the Earth system.
The Carbon Cycle
short term organic (plants and animals)
long term organic (fossil fuels)
long term inorganic (limestone)
carbon reservoirs
implications for global warming
In this lecture we consider specifically the carbon cycle.
Short term organic carbon cycle
“The breathing of the biosphere”
Inhale: photosynthesis
Exhale: decay
Plants are responsible for all of the photosynthesis. Animals and humans eat a small fraction
of the plants. They give carbon back to the atmosphere both by breathing (respiration) and
decay. The time scale for carbon to go through this loop ranges from days for plankton
blooms, to a year for annual plants, to 3 years for the needles of conifers.
The summer “drawdown” of CO2
by the Northern Hemisphere
land biosphere
Photosynthesis in spring/summer
when sunlight is available...
Decay occurs year-round
Each year ~10% of the
atmospheric reservoir of CO2
is exchanged with the biosphere
Carbon stored in woody plants
may stay in the biosphere for
decades or even for centuries.
Deforestation puts carbon back in
the atmosphere; afforestation takes
it out.
Net primary productivity (photosynthesis)
June 2002
Net primary productivity (NPP) tends to be higher over land than over the oceans. Note the
maxima over the boreal (northern) forests. These peaks are visible only in late spring and
early summer. Note also the regions of relatively high productivity over the oceans, which
correspond up the same upwelling zones pointed out in Slide #7. Based on NASA MODIS
imagery which compares the reflected radiation at two wavelengths, one of which is in the
red part of the spectrum, where energy is absorbed by plants to produce photosynthesis.
This figure is for annual mean NPP based on NASA MODIS imagery as in the previous slide.
Note that the boreal forests aren’t as pronounced because they’re active only during late
spring and early summer.
World Resources Institute.
Reservoirs
800
Atmosphere
80 each
600year
160 Leaves
600
Tree trunks
1500
????
???? Marine
Biosphere
Soils and sediments
Organic carbon rocks
Simplified “box diagram” showing the major carbon reservoirs in the biosphere and the mass
of carbon stored in them in units of GtC. The thickness of the arrows provides a measure of
the relative rates of exchange of carbon between the reservoirs.
Residence
times
800
Atmosphere
80 each year
600
160 years
600
???? days, weeks
decades/centuries
1500
Biosphere
centuries, millennia
hundreds of millions of years
Orranic carbon in the deeper reservoirs have much longer residence times because the
reservoirs themselves are large and the rate of exchange of carbon is slow. For example,
organic carbon is stored in a plankton bloom for only a few days while it may reside in the
Earth’s crust for nundreds of millions of years.
Formation of coal
Two schematics illustrating how coal is formed. From sources on the web.
Fossil fuel deposits
coal
oil
gas
coal map from www.ldeo.columbia.edu/.../ U4735/lectures/12.html
Total known fossil fuel deposits
4500 Gt
Oil shale / Tar sands
Major deposits in Alberta (already under intensive development: above) and in Utah.
Short and long term organic carbon cycles in nature. Organic carbon in rocks is oxidized with
the rocks weather, returning the carbon to the atmosphere in the form of CO2.
The long term organic carbon cycle
The long term organic carbon cycle.
fossil fuel burning
Short circuit!
a few hundred million years
a few hundred years
Coal formation (left) has occurred over a time span of hudreds of millions of years. The
word’s coal reserves are being burned (right panel) on a time scale of hundreds of years.
Carbonate formation
The long term
inorganic
carbon cycle
The electron microscope image shows a tine shell-forming creature. These creatures play a
central role in the inorganic carbon cycle.
Carbonate formation
First CO2 dissolves in seawater, forming carbonic acid
This carbonic acid dissociates in the water, releasing hydrogen ions and bicarbonate:
Some of the bicarbonate ions combine with dissolved calcium ions that enter
the oceans through the weathering of rocks to form calcium carbonate
Calcium carbonate is limestone (chalk). It’s also the main ingredient in TUMS and
other antacids.
This reaction is what forms the skeletons of certain species of phyto (plant) plankton.
Carbonate formation removes carbon from the atmosphere-ocean reservoir and puts
it in long term storage. This is the process that will eventually remove the carbon that
we are putting into the combined atmosphere-ocean right now.
Carbonate formation is a slow process because it requires calcium ions that come
from weathering.
Limestone cliffs (sedimentary rocks)
Limestone versus marble
The long term inorganic carbon cycle
shells
limestone
dissolving
Part of the inorganic carbon cycle
The long term inorganic carbon cycle
also subduction,
metamorphosis,
and volcanism
shells
limestone
dissolving
The complete inorganic carbon cycle.
The long term inorganic carbon cycle
Calcium ions in the oceans are supplied by the weathering of carbonate-silicate rocks
brought to the surface in sea floor spreading. The calcium ions in these rocks has been
recycled. Calcium carbonate (limestone) deposits on the sea floor are subducted in the
mantle aling plate boundaries like the one a few hundred miles off the Washington coast. As
the limestone heats up it undergoes metamorphosis. The carbon is expelled in the form of
CO2 in volcanic eruptions. The calcium reacts with quartz to form calcium silicate. Eventually
the “metamorphic” calcium silicate rock returns to the surface in sea floor spreading and
makes its way onto land where it becomes subject to erosion. The calcium ios are carried by
rivers into the ocean where they become available to shell-forming phytoplankton, thereby
completing the cycle.
The carbon reservoirs
too dilute to be used
as fossil fuels
Capacities given in kilograms per square meter averaged over the surface of the Earth. Only
the reservoirs of organic carbon are listed here. The inorganic carbonate reservoir is 80,000
kg per sq. m.
What are the implications for global warming?
Residence time of CO2 in the atmosphere is difficult to interpret.
Roughly 10% of the carbon in the fossil fuel reservoir has been burned so far
and at the current rate it will take us 500 years to burn the rest of it.
Most of the carbon in the fossil fuel reservoir is in the form of coal.
Burning all known coal deposits would add to the atmosphere-ocean
reservoir a mass of carbon roughly equivalent to 5 x the carbon presently in
the atmosphere and 8 x the pre-industrial level.
The true size of the fossil fuel reservoir may be larger but it’s hard to say by
how much because we don’t know how much oil shale and methane
hydrates we will be able to utilize.
Were it not for the biosphere, and especially the marine biosphere, most of
the Earth’s carbon would be in the form of atmospheric CO2 as it is on
Venus.
From Chapter 7 of the IPCC Fourth Assessment Report (2007)