the carbon cycle - uw oceanography€¦ · 29 human perturbation to the carbon cycle human...
TRANSCRIPT
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The Carbon Cycle
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Questions I hope to address:
● What's so special about Carbon anyway?● Why do we care about the carbon cycle?● Where does the Earth keep all it's carbon? ● What factors determine atmospheric carbon dioxide
concentrations?● Where does the carbon dioxide we emit end up?● What can we do to remove our excess carbon dioxide
from the air?
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What's so special about Carbon anyway?
● carbon has a unique chemistry that makes it disproportionately abundant in living organisms relative to it's abundance on Earth.
● softest to hardest compounds on earth (graphite—i.e. pencil lead to diamond) are made of carbon
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Carbon is abundant in living organisms disproportionately to it's abundance on
earth...● Carbon only makes up 0.094 % of Earth's crust, and only
0.038 % of the atmosphere● Humans are 18% composed of Carbon.
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Carbon forms Polymers
● A polymer is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. The term is derived from the Greek words: polys meaning many, and meros meaning parts[1]
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Hypothetical siliconbased life
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Silicon?
● Silicon makes up 28% of the Earth's crust! (As opposed to carbon which is only 0.094%).
● Of meteorites and space dust, 84 carbon compounds but only 8 silicon, and 4 of those
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Why not silicon?● Silanes (SiH2) compounds are highly reactive with water.
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Why not silicon?● Silanes (SiH2) compounds are highly reactive with water.
● Carbon can form double and triple bonds, which silicon can't as easily do.
● Combustion of CH2 compounds produces carbon dioxide and water (both gases). Combustion of SiH2 compounds produces SiO2, which is a solid! You'd have to breathe out bricks to use silicon.
● Silicon might be more important on extremely hot planets
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Why do we care about the carbon cycle?
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The other molecules are less likely to absorb infrared heat.
● Nitrogen 78.084%● Oxygen 20.946%
● (oxygen and ozone absorb UV, visible and a little bit of IR)
● Argon 0.934%● Carbon dioxide 0.038%
● (absorbs IR)
● Water vapor 1% ● (absorbs IR)
● Other 0.002%
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What Factors control carbon dioxide in our atmosphere right now?
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The Annual Cycle of Photosynthesis and Respiration
● Notice how the biologically driven annual cycle is a about a factor 10 greater than the anthropogenic increase over 1 year
● Known as the Keeling curve
from Mauna Loa Observatory
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Carbon Exchange With Plants
• 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), so growing plants are net C sinks
• Decomposition– Respiration by bacteria that consumes organic matter
• Total exchange of CO2 ~1000 times faster than geologic exchanges
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Oceanic Carbon
● Oceans store about 50 times more CO2 than the atmosphere and 19 times more than terrestrial biosphere
CO2
CO2
CO2 + H2O ⇔ H+ + HCO3
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Oceanic CO2 Pressure
● Difference between water pCO2 and atmospheric pCO2
● Negative values (blue) mean the ocean takes up CO2
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Oceanic Biological Pump
● Phytoplankton absorb CO2
● Zooplankton consume phytoplanton– source of oceanic food web
● Respiration returns most of CO2 to the ocean● Some organic matter sinks to ocean bed and
provides a net (essentially permanent) uptake of CO2 into the ocean
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Ocean Uptake Timescale● Preindustrial:
– 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
– 8085% in oceans– 1520% in the atmosphere
The problem is the rate of emission of CO2
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Human Perturbation to the Carbon CycleHuman Perturbations to the Global Carbon Budget
CO2 sources Flux (Gt C/yr)
Fossil fuel combustion and cement production 5.5 ± 0.5
Tropical deforestation 1.6 ± 1.0
Total anthropogenic emissions 7.1 ± 1.1
CO2sinks
Storage in the atmosphere 3.3 ± 0.2
Uptake by the ocean 2.0 ± 0.8
Northern hemisphere forest regrowth 0.5 ± 0.5
Other terrestrial sinks (CO2 fertilization, nitrogen fertilization, climatic effects) 1.3 ± 1.5Source: Climate Change 1995, published by the IPCC
While we understand the Ccycle in principle, the budget is not yet well quantified
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Increasing emissions from the developing world
• Over the next few decades, 90 percent of the world’s population growth will take place in the developing countries, some of which are undergoing rapid economic development
• Per capita energy use in the developing countries, which is currently only 1/10 to 1/20 of the US level, will also increase
• If current trends continue, the developing countries will account for more than half of total global carbon dioxide emissions by 2035
• China, which is currently the second largest source, is expected to displace the US as the largest emitter by 2015
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Kyoto Accord – a quick review
• The Kyoto Accord calls for the 38 industrialized countries by 2012 to reduce their combined annual gas emissions to 5.2 percent below levels measured in 1990.
• It set a different negotiated target for each country (e.g., USA, 93% of baseline year – 1.348.2 MtC
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What can we do to reduce the rate of CO2 increase in the atmosphere?
● Don't put it in the atmosphere to begin with! ● Pump it into the ground or the deep ocean● Grow Trees● Sequester it with carbon dioxide absorbing minerals.
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Carbon footprint calculator
● http://www.carbonfootprint.com/USA/calculator.html
● Other ecological Footprint calculators● http://www.ecobusinesslinks.com/ecological_footprint_calculator.htm
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Pump it into the ground?
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Some sinks:
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Pump it into the Deep ocean?
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Pump it into the deep ocean
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Store it in minerals?
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Geological timescale carbon cycle
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Olivine could absorb CO2
• The mineral olivine (also called chrysolite and, when gemquality, peridot) is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. It is one of the most common minerals on Earth, and has also been identified on the Moon, Mars, and comet Wild 2.
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Serpentinite reactionsReaction 1MgOlivine + Water + Carbon dioxide Serpentine + Magnesite + Silica→
Reaction 2FeOlivine + Water + Carbonic acid Serpentine + Magnetite + Magnesite + Silica→
Reaction 3Serpentine + carbon dioxide Magnesite + silica + water→
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Store it in Trees?
● Trees are the main storage of terrestrial carbon
● First step is to reduce current forest clearance rates● Do existing trees grow faster in a CO2rich
environment?● What about new plantations?
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Carbon and Land Use1 Pg = 1 Peta gram = 1015 g = a thousand million million grams
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Using Forests to Sequester Carbon• Total fossil fuel reserves can’t be mopped up by trees
– 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
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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
– 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 nonstem Carbon accumulates at rates of about 2.5 tonnes/hectare/year
– total soil and nonstem 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 ‘capture’ 65,000 tonnes of CO2 over the 20 year period, or an average of 3,270 tonnes CO2/year
Question: What fraction of the world’s land area would you need to cover in such trees in order to mop up 10 Pg CO2 emission from fossil fuels per year?
A hectare (symbol ha) is a unit of area, equal to 10,000 square metres
"Surface Area: Land area, about 148,300,000 sq km, or about 30% of total surface area; water area, about 361,800,000 sq km, or about 70% of total surface area."
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The Carbon Budget
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Carbon Exchanges Between Reservoirs● Geologic carbon
exchanges on time scale of millions of years
● Biological/physical carbon exchanges on the time scale of days to ~1000 years