Carbon reservoirs • Geologic carbon • Carbon - e

The Carbon Cycle
Cambiamenti Climatici - Basi scientifiche
• Carbon reservoirs
• Geologic carbon
• Carbon exchanges with terrestrial biosphere and ocean
• Carbon budget
• Response of biosphere to increased CO2
• Reading for this lecture: An essay on the carbon cycle
http://www.gcrio.org/CONSEQUENCES/vol4no1/carbcycle.html
martedì 27 marzo 2012
The Carbon Cycle
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The exchange of carbon in all its forms between different
storage ʻreservoirsʼ through natural processes
• Carbon dioxide is the principal anthropogenic greenhouse gas
• We have significantly altered the CO2 balance of the atmosphere
• The rate of increase of CO2 in the atmosphere and the measures
necessary to slow the increase depend on us knowing the natural Ccycle
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Cambiamenti Climatici - Basi scientifiche
Where the carbon stored in the Earth?
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Carbon makes up about 0.032% of the lithosphere (compared with
45% for oxygen and 29% for Silicon)
The Carbon Cycle
Cambiamenti Climatici - Basi scientifiche
Simply based on the size of the reservoirs, what would happen if
we burnt all the known fossil fuels?
• Atmospheric CO2 would rise by about a factor of eight compared to its current
value: i.e., ten times more CO2 than in preindustrial times (2700 ppm)
• Each doubling of CO2 should produce an increase of 1.5 to 5° C mean surface
temperature change, so three doublings could drive the temperature 4.5 to 15° C
higher than what it is today.
• During the warmest time interval of the past 200 million years-- the MidCretaceous Period, when dinosaurs dominated a far different and more tropical
Earth, the mean temperature is thought to have been from 6 to 9° C above that of
today
• Oversimplification:
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– burning all fossil fuels at present rates would require several hundred years
– natural processes could remove part of the added CO2
– BUT, removal mechanisms become less efficient at high CO2
The Geologic Carbon
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• In the Archean Eon (4.0-2.5 billion years ago) the atmosphere was
probably mainly CO2 and N2
• Presence of liquid water allowed the formation of carbonic acid
CO2 + H2O
H2CO3 [H+ + HCO3-]
• Weathering of silicate rocks produced carbonates
CaSiO3 + H2CO3
CaCO3 + SiO2 + H2O
The Weak Sun Paradox
• Observations of other stars suggests that our Sun was 20-30%
weaker 3 billion years ago
• This would imply a surface T on Earth about 20 oC colder than today
• Presence of water (known from geologic records) implies Earth was
not colder
• Explanation may lie in CO2 as a greenhouse gas
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Cambiamenti Climatici - Basi scientifiche
The Carbon Exchenge Reservoirs
The global carbon cycle for the 1990s, showing the main annual fluxes in GtC yr–1: preindustrial ‘natural’ fluxes in black and ‘anthropogenic’ fluxes in red (modified from
Sarmiento and Gruber, 2006, with changes in pool sizes from Sabine et al., 2004).
• Geologic carbon exchanges on time scale of millions of years
• Biological/physical carbon exchanges on the time scale of days to
~1000 years
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Cambiamenti Climatici - Basi scientifiche
The Carbon Cycle flux
The global carbon budget (GtC yr–1); errors represent ±1 standard deviation uncertainty estimates and not
interannual variability, which is larger. The atmospheric increase (first line) results from fluxes to and from the
atmosphere: positive fluxes are inputs to the atmosphere (emissions); negative fluxes are losses from the
atmosphere (sinks); and numbers in parentheses are ranges. Note that the total sink of anthropogenic CO2 is well
constrained. Thus, the ocean-to-atmosphere and land-to-atmosphere fluxes are negatively correlated: if one is
larger, the other must be smaller to match the total sink, and vice versa.
Notes:
a TAR values revised according to an ocean heat content correction for ocean oxygen fluxes (Bopp et al., 2002) and using the Fourth Assessment Report
(AR4) best estimate for the land use change flux given in Table 7.2.
b Determined from atmospheric CO2 measurements (Keeling and Whorf, 2005, updated by S. Piper until 2006) at Mauna Loa (19°N) and South Pole (90°S)
stations, consistent with the data shown in Figure 7.4, using a conversion factor of 2.12 GtC yr-1 = 1 ppm.
c Fossil fuel and cement emission data are available only until 2003 (Marland et al., 2006). Mean emissions for 2004 and 2005 were extrapolated from energy
use data with a trend of 0.2 GtC yr-1.
d For the 1980s, the ocean-to-atmosphere and land-to-atmosphere fluxes were estimated using atmospheric O2:N2 and CO2 trends, as in the TAR. For the
1990s, the ocean-to-atmosphere flux alone is estimated using ocean observations and model results (see Section 7.3.2.2.1), giving results identical to the
atmospheric O2:N2 method (Manning and Keeling, 2006), but with less certainty. The net land-to-atmosphere flux then is obtained by subtracting the ocean-toatmosphere flux from the total sink (and its errors estimated by propagation). For 2000 to 2005, the change in ocean-to-atmosphere flux was modelled (Le
Quéré et al., 2005) and added to the mean ocean-to-atmosphere flux of the 1990s. The error was estimated based on the quadratic sum of the error of the
mean ocean flux during the 1990s and the root mean square of the five-year variability from three inversions and one ocean model presented in Le Quéré et
al. (2003).
e Balance of emissions due to land use change and a residual land sink. These two terms cannot be separated based on current o observations.
martedì 27 marzo 2012
The Carbon exchenge with plants
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• Photosynthesis:
- carbon dioxide + water + light => carbohydrate + oxygen
CO2 + H2O + light => CH2O + O2
• Respiration:
– ~half of the carbohydrates used to produce energy for metabolism
O2 + CH2O => energy + H2O + CO2
– ~half used to form new plant tissue (biomass)
• Decomposition
– Respiration by bacteria that consumes organic matter
Total exchange of CO2 ~1000 times faster than geologic exchanges
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Cambiamenti Climatici - Basi scientifiche
Annual cycle of photosynthesis and respiration
• Notice how the biologically driven annual cycle is a about a vfactor 10 greater than the
anthropogenic increase over 1 year
• Known as the Keeling curve
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The Global Distribution Carbon Dioxide
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The ocean carbon
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Oceans store about 50 times more CO2 than the atmosphere and 19
times more than terrestrial biosphere
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The CO2 oceanic pressure
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Difference between water pCO2 and atmospheric pCO2
Negative values (blue) mean the ocean takes up CO2
The biologica pump in ocean
• Phytoplankton absorb CO2
• Zooplankton consume phytoplanton
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– source of oceanic food web
• Respiration returns most of CO2 to the ocean
• Some organic matter sinks to ocean bed and provides a net uptake of CO2
into the ocean
martedì 27 marzo 2012
The ocean uptake timescale
Cambiamenti Climatici - Basi scientifiche
• Pre-industrial:
– 98.1% CO2 in oceans
– 1.9% in atmosphere
• For 100 molecules CO2 emitted today
– 6 dissolve in 1 year
– 29 in 10 years
– 59 in 60 years
– 84 in 360 years
• Currently 42% of CO2 emitted since 1800 has dissolved in the ocean
• At the time when atmospheric CO2 has doubled
– 80-85% in oceans
– 15-20% in the atmosphere
The problem is the rate of emission of CO2
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The Carbon Cycle
Change in pH due to human activity from
1770s to 1990s. From the Global Ocean
Data Analysis Project.
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The Global Carbon Cycle
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The Carbon Cycle - Human perturbation
While we understand the C-cycle in principle, the budget is not yet
well quantified
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The Carbon Cycle
What can we do to reduce the rate of CO2 increase in the atmosphere?
Cambiamenti Climatici - Basi scientifiche
Land-use Change
• Trees are the main storage of terrestrial carbon
• First step is to reduce current forest clearance rates
• Do existing trees grow faster in a CO2-rich environment?
• What about new plantations?
MtC
GtC
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One ton of carbon equals 44/12 = 11/3 = 3.67 tons of carbon dioxide
Using forest for carbon sequestration
• Total fossil fuel reserves canʼt be mopped up by trees
Cambiamenti Climatici - Basi scientifiche
– look back at the Reservoir Table
• What happens to biomass growth rate with increased atmospheric
CO2?
– In experiments: Faster growth of forest pine trees for a few years, then return to normal since
trees need other nutrients, such as nitrogen
– Other experiments show unabated increased growth rates
– Drought or other stress increases CO2 emissions
• Open issue, but trees unlikely to be a reliable CO2 remover
martedì 27 marzo 2012
New plantations
• A conservative estimate for the amount of CO2 sequested by a 100 hectare Blue gum
(Euc.globulus) plantation grown in Australia for 20 years can be worked out as follows:
• Growth rate of plantation
Cambiamenti Climatici - Basi scientifiche
– 20 cubic metres Stemwood/hectare/year
• Total stem wood volume at age 20
– 40,000 cubic metres
• One cubic metre of wood equals 0.32 tonnes of Carbon
– Total wood volume=12,800 tonnes of Carbon
• Soil and non-stem Carbon accumulates at rates of about 2.5 tonnes/ hectare/year
– total soil and non-stem Carbon accumulated at age 20 = 5,000 tonnes of Carbon
• Total Carbon (wood, non stem and soil) at age 20
– 17,800 tonnes of Carbon
• One tonne of Carbon = 3.67 tonnes CO2
– 100ha plantation would ʻcaptureʼ 65,000 tonnes of CO2 over the 20 year period, or an average of
3,270 tonnes CO2/year
Question: What fraction of the worldʼs land area would you need to cover in such
trees in order to mop up 10 Gt CO2 emission from fossil fuels per year?
One ton of carbon equals 44/12 = 11/3 = 3.67 tons of carbon dioxide
martedì 27 marzo 2012