Chapter 5: The carbon cycle
Atmospheric methane abundance pre-industrial revolution
- 0.8 ppm in 1750, compared to 1.83 ppm in 2014
A gigatonne
- 1 billion metric tons (1 metric ton is 1,000 kg or 2,200 lbs)
Atmospheric carbon dioxide abundance pre-industrial revolution
- 280ppm in 1750, compared to 400ppm in 2014
radiocarbon dating
- Archaeologists use this test to determine the date/age of organic artifacts -> look at proportion of nitrogen-14 to radioactive carbon-14 (nitrogen-14 is original atom before nucleus transformed by a cosmic ray hitting the nucleus of an atom of nitrogen-14 (eject proton, absorb neutron) carbon-14 over time will revert back to nitrogen-14
Atmosphere-ocean carbon exchange
- Carbon cycles easily between atmosphere and ocean - carbon dioxide readily dissolves in water, leading to ocean acidification CO2 + H2O -> H2CO3 -> as oceans become more acidic, the biology of the oceans can change. Could be problematic as humans depend on oceans for food. - carbonic acid can react further with water to convert into other forms of carbon. Ocean can absorb huge amounts of carbon dioxide. Then, carbon can return to atmosphere. H2CO3 -> CO2 + H2O -> two equations transfer about 80GtC between atmosphere and ocean every year Ocean split into two parts: First part - top 100m - exchanges carbon very rapidly with atmosphere - makes up only a few percent of the mass of the ocean - often referred to as the mixed layer (well mixed by winds and other weather) - contains 900 GtC - high-carbon water Second part - the deep ocean - 97% of the ocean - contains 40,000 GtC (most of the ocean's carbon or 47x more carbon than is in the atmosphere) - low-carbon water - exchanges carbon at a rate of about 100 GtC per year with the mixed layer through ocean currents or through the biological carbon pump (sinking organic material (i.e. dead organisms or fecal material) falls from mixed layer into deep ocean)
How are humans perturbing the carbon cycle?
- Carbon dioxide varies without any human activities, but humans can also affect the carbon cycle --- from the combustion of fossil fuels for energy Net reaction (combustion reaction) similar to respiration reaction: CHx + O2 -> CO2 + H2O + energy -> CHx is fossil fuels (primarily carbon with varying amounts of hydrogen - but can also contain other trace species like sulfur which can lead to environmental problems (i.e. acid rain) when released into environment) -> CO2 released into the atmosphere, creating an additional large pathway for carbon from rocks to the atmosphere. Released 8.3 GtC per year to the atmosphere from 2002-2011 (more than 80x natural flow rate of carbon from rock reservoir to atmosphere). Atmospheric carbon dioxide began rapidly increasing around 1800, when the industrial revolution and widespread burning of fossil fuels began. Increase in about 120ppm atmospheric carbon dioxide Rise in carbon dioxide is accelerating because of increasing fossil fuel combustion over the past half-century. --- from land-use changes (i.e. deforestation) Contributed to approx 0.9GtC per year to the atmosphere from 2002 to 2011 (1/10 of emissions from fossil fuel combustion)
Carbon in the land biosphere
- Contains 2,500 GtC stored in living plants and animals and in organic carbon in soils (i.e. decaying leaves) - Photosynthesis removes approx. 120 GtC from the atmosphere every year and respiration roughly balances this. But, not balanced at every point in time. -> Most of Earth's land area & thus plants are found in the northern hemisphere. -> During northern hemisphere's spring & summer (May-September) global photosynthesis exceeds respiration = net drawdown of carbon dioxide out of the atmosphere and into the land biosphere. -> During northern hemisphere's fall and winter (October - April), planet material produced during spring and summer decays releasing CO2 back into the atmosphere = global respiration exceeds photosynthesis and net transfer of carbon from biosphere into atmosphere -> Large amount of carbon stored in permafrost (ground that is frozen year-round). Dead organic plant matter frozen into permafrost does not decay releasing carbon until ground thaws. Permafrost is melting now, but contributes little to atmospheric carbon dioxide right now, but could change as permafrost mostly in Artic continues to melt.
Where does the oxygen in our atmosphere come from?
- From photosynthesis that is not balanced by respiration -> Plant grows through photosynthesis, but plant material is buried before it can be consumed via respiration. Means oxygen produced through photosynthesis is not consumed.
Why focus on carbon dioxide from fossil fuel combustion when plants and animals emit far more carbon dioxide to the atmosphere?
- Humans, animals, bacteria, and plants do emit enormous amounts of carbon dioxide to the atmosphere -> the land biosphere emits 120 GtC per year, compared with present-day emissions from human activities of about 9 GtC per year. -> But, when an animal exhales carbon dioxide, it is releasing back into the atmosphere carbon dioxide that was in the atmosphere just a few months before. Can lead to seasonal variations in carbon dioxide, but not long-term increases in carbon dioxide. In contrast, when burning fossil fuels, you release carbon dioxide into the atmosphere that had been in rocks for hundreds of millions of years (a net addition to the atmosphere).
Greenhouse gases unequal in ability to warm planet
- Methane 20x more powerful than CO2 on a per molecule basis -> takes 20 molecules or so of CO2 to equal warming from one molecule of methane - Halocarbons are most powerful greenhouse gases on per molecule basis -> takes several thousand carbon dioxide molecules to equal warming from one halocarbon -> But, 1/10,000th as abundant as carbon dioxide
Three constituents that make up 99% of the dry atmosphere
- N2 (78%), O2(21%), Argon atoms (1%) - Don't absorb infrared photons, so they don't warm the surface of the planet (aren't greenhouse gases)
How do we know that combustion of fossil fuels is responsible for the increase in carbon dioxide, rather than nonhuman sources such as volcanoes or plants?
- amount of carbon dioxide absorbed by plants during the year and balanced by plant decay (approximately 120 GtC per year) is much larger than human emissions (which averaged 9 GtC per year over 2002 - 2011). Same for ocean fluxes. Could be that carbon dioxide driven by slight excess of plant respiration or excess flux of carbon dioxide out of the ocean or enhanced volcanic activity. BUT... - increase of atmospheric carbon dioxide tracks human emissions of carbon dioxide so closely - carbon dioxide can be chemically "fingerprinted" (examining isotopes of carbon - proportion of carbon-12 (favored by plants (plants take up carbon-12 and fossil fuels made of carbon-12) to carbon-13) to show that it comes from fossil fuels. Can also use radiocarbon dating to understand when the increase in atmospheric carbon dioxide came from (plants of today or hundreds of millions of years ago). Fossil fuels contain essentially no carbon-14 (are radiocarbon dead). So when they are burned, the carbon dioxide has no carbon-14 in it. Scientists measuring isotopic composition of atmospheric carbon dioxide have found that carbon dioxide is radio-carbon dead, showing it is coming from long-dead plants (fossil fuels) not modern plants.
Methane
- another crucial carbon-containing gas - 20x more powerful as a greenhouse than CO2, but much small atmospheric abundance than CO2 on a per molecule basis - began rising about 1800 (same point at which CO2 began rising). Industrial revolution. - 1/4 of the contribution of carbon dioxide to global warming - removed from the atmosphere by oxidation (on average removed 10 years after it was emitted - much faster than CO2 which takes centuries or millenia) CH4 + 2O2 -> CO2 + 2H2O - emitted to the atmosphere from human and natural processes Human: -> 60% from agriculture and waste (livestock - cattle, goats and sheep produce methane in guts when digesting food released into atmosphere). -> Next largest source from bacterial processes in landfills and other waste repositories. -> 3rd is emissions from rice paddies (bacteria in warm and wet flooded rice field). -> 30% of human emissions from petrochemical industry = leakage of methane from natural gas wells & release of geologic methane from coal mines -> remaining 10% of human methane emissions = from burning of forest and other biomass. methane if combustion temperature is sufficiently low (smoldering fire) Natural: - 2/3 from natural wetlands (produce methane like flooded rice paddies) - from the ocean, freshwater lakes and rivers, and from wild animals (i.e. termites)
ozone (O3)
- another greenhouse gas - abundance varies widely across the atmosphere -> in unpolluted air near surface, 10-40 parts per billion (bad ozone - not good to breath ozone) -> can reach 10ppm in stratosphere (1,000 times higher) (good ozone) - absolutely necessary for life on our planet bc it absorbs high-energy (dangerous) ultraviolet photons emitted by the Sun before they reach the Earth's surface
Nitrous oxide (N2O)
- another important greenhouse gas - atmospheric abundance of about 0.32 ppm - emitted into the atmosphere from nitrogen-based fertilizer and industrial processes, as well as natural sources
Turnover time (also "lifetime" or "residence time")
- calculated for the atmosphere and land biosphere reservoirs to make sense of the input/output numbers related to the combined atmosphere-land biosphere-ocean system - the length of time that a carbon atom in one of the resevoirs (ex: the atmosphere) will remain there before being transferred into one of the other two reservoirs -> size of the reservoir/total flux out of the reservoir = average time in years -: about 4 years in the atmosphere -: about 21 years in the land biosphere -: about 5 years in the mixed layer of the ocean -: about 400 centuries for the deep ocean - atmosphere exchanges carbon rapidly (years to decades) with the land biosphere and mixed layer, and much more slowly (centuries) with the deep ocean
atmosphere rock exchange
- carbon atom will remain in atmosphere-land biosphere-ocean system for approx 460,000 years before transferred to the rock reservoir. Takes many millions of years for a carbon atom to travel through rock reservoir and remerge into the atmosphere (extremely slow) - most carbon in the world (millions of gigatons of carbon) stored in rocks (i.e. limestone (CaCO3)). -> carbon slowly exchanged with atmosphere-land biosphere-ocean system. -: volcanic eruptions transfer carbon dioxide directly from rocks into the atmosphere. releases average of 0.1 GtC per year. small compared with other fluxes, but significant over millions of years. during periods of extreme volcanism, atmospheric carbon dioxide will increase. Also movement of continents, changing patterns of rainfall, can expose new rock to the atmosphere changing the rate of chemical weathering (rate at which carbon dioxide removed from the atmosphere) -: emissions of CO2 from rock resevoirs balanced by chemical weathering (removes about an equal amount of carbon from atmosphere and transfers it back into rocks) : CaSiO3 + CO2 -> CaCO3 + SiO2 (not the exact chemical reaction, a general description) -> CO2 refers to acidic rain (CO2 from atmosphere dissolving into raindrops - H2CO3) -> CaCO3 (limestone) runs off with rain into the ocean. In the ocean, molecules of CaCO3 are deposited on the sea floor. Over many millions of years, plate tectonics carry calcium carbonate within the Earth where high temperature and pressure turn the rock into magma. Eventually, carbon transferred back to surface by volcanism, completing the cycle. -> SiO2 is the primary component of sand, quartz and glass : carbon to move into the rock reservoir when plants are rapidly buried in sediment before they can decay (production of oxygen). once buried and subjected to great heat and pressure within the Earth, dead plant material can be converted to fossil fuels burned for energy.
Fossil fuels
- created when plants that grew hundreds of millions of years ago were buried before the carbon in them could be released back into the atmosphere by respiration - over millions of years high pressure and heat converted carbon in plants into oil, coal and natural gas - if humans hadn't discovered fossil fuels, the natural carbon cycle would have slowly released this carbon back to atmosphere through geologic processes over many millions of years - humans will extract and burn most fossil fuels in just a few hundred years -
the Keeling Curve
- curve of observed atmospheric carbon dioxide abundances - measurements by Charles D. Keeling - show an increase in CO2 and decrease in O2 -> decrease in oxygen is the result of burning fossil fuels -> exact amount of oxygen lost tells us important information about how much carbon the land and ocean are absorbing because different pathways lead to different changes in oxygen. Shows us that about half of the carbon is going into the land biosphere and half into the ocean.
Atmosphere-land biosphere exchange
- have been directly monitoring abundance of carbon dioxide in the atmosphere since middle of 20th century -> amount of CO2 in atmosphere varies throughout the year (6ppm higher in May than September), reflecting the annual cycle of plant growth and decay
Carbon in the atmosphere
- have been directly monitoring abundance of carbon dioxide in the atmosphere since middle of 20th century -> amount of CO2 in atmosphere varies throughout the year (6ppm higher in May than September), reflecting the annual cycle of plant growth and decay - approx. 850 gigatonnes in 2014 (GtC) - mass expressed as the mass of carbon dioxide often (includes the mass of the two oxygen atoms) -> 1 GtC = 3.67 GtCO2
the carbon cycle
- how carbon moves between the atmosphere (containing 850 GtC), ocean (900 GtC in mixed layer and 40,000 GtC in deep ocean), land biosphere (2,500 GtC), and rocks (millions and millions of GtC) on the Earth (through primary reservoirs) - regulates carbon's atmospheric abundance -
The long-term fate of carbon dioxide
- how long does the carbon dioxide we release stay in the atmosphere? -> after a pulse of CO2. land biosphere and mixed-layer of the ocean rapidly take carbon dioxide out of the atmosphere (40% of carbon dioxide out in 20 years) -> removing additional carbon dioxide requires transport into he deep ocean (slower process). In 400 years, 25% of carbon dioxide pulse remains. Then, deep ocean in equilibrium with atmosphere and cannot absorb any more carbon. -> Reactions between carbon dissolved in the ocean and calcium carbonate (CaCO3) sea floor sediments to further remove carbon. Carbon to ocean sediments. Takes 10,000 years and then 15% of initial pulse of CO2 left. -> Last 15% of carbon removed by chemical weathering over next few hundred thousand years We know that it takes a long time for carbon dioxide to be removed from the environment from the Paleocene-Eocene Thermal Maximum (55 million years ago). Then, a huge pulse of carbon (several thousand GtC) was released into the atmosphere. It took several hundred thousand years for that carbon to be removed and for the warming to dissipate. -> actions we take in next few decade will determine the trajectory of the climate for thousands, if not tens of thousands of years
human activity compared to volcanoes
- humans emit 100x more co2 into atmosphere than volcanoes
parts per million (ppm)
- indicates how many molecules out of every million are the gas in question
Water vapor
- next biggest component of the atmosphere (0.95%) - abundance various widely from place to place -> decreases rapidly with altitude, and in stratosphere typically makes up 0.0005% of the atmosphere - most abundant and important greenhouse gas in our atmosphere - main source: evaporation from oceans. primarily removed: when it rains - emissions of water vapor from human activities contribute essentially nothing to its atmospheric abundance
greenhouse effect
- occurs because our atmosphere is mostly transparent to visible photons but absorbs infrared photons -> but really only a few of the components of our atmosphere actually absorb infrared photons (greenhouse gases)
halocarbons
- powerful greenhouse gases - deplete ozone - includes chlorofluorocarbons and hydrochlorofluorocarbons (synthetic industrial chemicals used as refrigerants (in air conditioners and refrigerators) and in various industrial applications - also includes natural molecules (i.e. methyl chloride) - concentration of a few parts per billion in atmosphere
Carbon Dioxide (CO2)
- the primary greenhouse gas emitted by human activities - policies to control climate change frequently focus on reducing emissions of this gas (because most important greenhouse gas for climate change) - 0.04% of the atmosphere (in 2014) -> or 400 parts per million (ppm), meaning that there are 400 molecules of carbon dioxide in every million molecules of air - the second most important greenhouse gas, behind water vapor (absorbs infrared photons)
methane (CH4)
- the third most important greenhouse gas in our atmosphere - atmospheric abundance of 1.83 ppm in 2014
What accounts for the "missing carbon" in our atmosphere?
- we have good records of exactly how much fossil fuel is extracted & burned each year, so we can calculate how much atmospheric carbon dioxide should have increased (2002-2011 human emissions of carbon to atmosphere averaged 9.2 GtC per year but increase in atmospheric carbon dioxide averaged only 4.3 GtC per year) - Carbon is dissolving into the ocean - Carbon is going into the land biosphere (land sink) -> don't know what part of the land biosphere is absorbing carbon) - Concern whether oceans and land biosphere can continue taking up as much carbon in the future as they presently are -> Will we reach a saturation point? Reservoirs slow down or cease uptake. Abundance of CO2 in atmosphere will grow more rapidly. Climate change may alter the climate cycle.
Deforestation
- when humans chop down large tracts of forest to use land for agriculture or grazing livestock - forest frequently removed by burning it or bulldozing it -> releases carbon stored in trees and other plants to the atmosphere - when forest is removed, much of the organic plant material in the soil containing large amounts of carbon, decomposes releasing carbon back into the atmosphere
radiocarbon dead
- when there is no carbon-14 left in something
Respiration
CH2O + O2 -> CO2 + H2O + energy - humans, animals and bacteria consume plant material to produce energy - releases CO2 back into the atmosphere - the reverse of the photosynthesis equation (means no net change in either carbon dioxide or oxygen) - represents the net of a large number of complex biochemical reactions that occur within the cells of organisms (not the actual eqN)
Photosynthesis
CO2 + H2O + sunlight -> CH2O + O2 - CH2O is a carbohydrate - Diatomic oxygen released into the atmosphere. Main source for the oxygen in our atmosphere. - represents the net of a large number of complex biochemical reactions that occur within the cells of organisms (not the actual eqN)
What is a reason for large variability in the year-to-year increase of atmospheric carbon dioxide?
Mainly due to variations in the climate due to El Nino events. Regional climate variations vary areas of rainfall and drought modifying uptake and emissions of carbon.
