Module 7

Part II The Carbon Cycle

Carbon on Earth

Chapter 8

The Chemistry of Carbon

Biotic carbon

Highly organized molecules within living things

Abiotic carbon

After life they become disorganized goo – called kerogen, or humic acids

All three planets had about the same amount of carbon:

Venus has the carbon content in it’s very dense atmosphere of carbon dioxide and

sulfuric acidEarth has the highest concentration of carbon

in limestone and rocksMars has it’s carbon locked up in the polar ice

caps that are carbon dioxide dry ice

Terrestrial PlanetsVenus, Earth, and Mars

The backbone of life

A means of storing energy

Photosynthesis, carbon dioxide, water, and sunlight produces plants that store energy as food

The early plants were converted to fossil fuels – more stored energy as fuel instead of food

Organic Carbon

Methane is totally reduced carbon, has an oxidation state of -4

To calculate oxidation states we assign the common states to hydrogen and oxygen, then realize that the molecule has to be neutral, so the leftover number is assigned to carbon

Hydrogen is +1, there are four of them in methane, so the carbon must be -4

This is fully reduced carbon Reduced carbon is easily oxidized CH4 + 2 O2 → CO2 + 2 H2O

Oxidation states, electron bookkeeping

CO2 is fully oxidized The oxidation number for carbon is +4 We calculate this by assigning -2 to each oxygen (Group 16 in the

periodic table, needs two more electrons) Oxygen is -4, so carbon must be +4 Oxidized carbon is stable, low energy, and the preferred state for

carbon Oxidized carbon will not become reduced carbon without a great

deal of effort In between is the carbohydrates, where carbon has a zero

oxidation state CH2O formaldehyde, is the simplest carbohydrate. O is -2, H is +1(x2) so C must be in the 0 oxidation state

Oxidized carbon

Photosynthesis uses oxidized carbon to reduce the carbon to carbohydrates

We use carbohydrates as fuel and oxidize the carbohydrate back to CO2 when we exhale during respiration

Animals are not the only organisms to breathe!

Carbon forms

The Land Breathes

The land inhales CO2 in the summertime growing season and exhales during the winter months

Reversed in the Southern Hemisphere where there is less land

The land breathes on an annual cycle

The Ocean Breathes

The carbon is inorganic, and stable, it involves the carbonate buffer system that we will study in chapter 10, this is called dissolved inorganic carbon

The ocean effects atmospheric CO2 on time scales of centuries

The glacial-interglacial cycles were amplified somehow by the ocean carbon cycle.

The Rocks Breathe

The sedimentary rock carbon pool is larger than the ocean, land or atmospheric pools

Carbon in the solid Earth exists as limestone CaCO3, and to a lesser extent, organic carbon

Most of the organic carbon in sedimentary rocks is kerogen

Kerogen is useless as a fossil fuel because it is too diluted

The solid Earth is the largest but slowest breathing of the carbon reservoirs

The Atmosphere is the Grand Central Station for the CO2 Cycles

The beat of the ice-age rhythm apparently originates from variation in the Earth’s orbit

around the sun

The orbit varies through three main cycles, and the orbital variations drive climate by changing the distribution of sunlight at the

Earth’s surface 1. Precession Cycle 2. Obliquity Cycle 3. Eccentricity Cycle

Glacial-Interglacial Cycles

The axis of rotation spins like a wobbly top Called the precession of season, or the

precession of the equinoxes Completes the entire circle in 20,000 years Solar heat influx variability comes from

precession Seasonal cycle in the North is weakened and

in the South it is intensified because the Earth is closest to the sun in the winter season in the northern hemisphere

Precession

Precession orbital cycle

The angle of the pole of rotation relative to the plane of Earth’s orbit

Varies between 22 and 25.5 degrees Angle of tilt is currently 23.5 degrees Cycle time is 41,000 years The impact of obliquity on solar heating

is strongest in the high latitudes

Obliquity

Obliquity of Earth’s Orbit

The third cycle involves how elliptical the orbit of the Earth is

The eccentricity of the orbit has cycles of 100,000 and 400,000 years

At present the orbit of Earth is nearly circular The orbital cycles affect climate by

redistributing the energy from one place to another and from one season to another

Eccentricity

Milankovitch cycles

At the cool surface of the Earth, oxidized carbon wants to be calcium carbonate – limestone

In the hot interior of the Earth, oxidized carbon wants to be free, as CO2

The CO2 thermostat regulates atmospheric CO2 and climate on geologic time scales of hundreds of thousands of years

It is possible to change the set point of the thermostat, creating a hot house world like that of the dinosaurs, or an icy world like today

The thermostats of Venus and Mars are broken

The CO2 Thermostat

1. the most stable form of carbon on Earth is oxidized. Photosynthesis stores energy from the sun by producing organic carbon, which is the backbone of life

2. There is less carbon in the atmosphere that any other carbon reservoir. These other reservoirs tug on atmospheric CO2 seasonally for the land, and on glacial interglacial 100,000 year time scales from the ocean

3. The weathering of igneous rocks on land controls the partial pressure of CO2 in the atmosphere on million year time scales. The thermostat is broken on Venus because no water, and on Mars because there is no volcanic activity left.

Take home points of chapter 8

Fossil Fuels and EnergyChapter 9

All energy comes from the stars,Mostly from our sun

Previous definition: watts = joules/second

terawatts = 1012 watts, written TW 1,000,000,000,000 watts

Energy

Wind (Denmark) Hydroelectric (2% globally) Solar Biomass energy

Energy sources

Energy sources

Renewable

Geothermal Solar Wind Wood Waste electric power

Non-renewable

Fossil fuels Radioactive

elements

“Only a small fraction of the buried organic

carbon is in a convenient form for fuel”

Fossil Fuels

Largest reservoir is coal: it was produced in swamps where the organic material was protected from the atmosphere by water

Freshwater has less sulfur, burns “cleaner”

Saltwater swamps contains sulfur, burns to forms aerosols and produce acid rain as sulfuric acid

Traditional fossil fuels

Begins as plant material (carbon based)

Carbon Peat Coal

By a pressure and temperature process that takes millions of years.

The oldest coal is the cleanest coal.

Coal

“Coal is the most abundant fossil fuel, and the future of the

Earth’s climate depends mostly on

what happens to that coal”

Coal fired power plants are established

They produce cheap energy

Would be very expensive to replace with a cleaner fuel source until the necessity arises

Coal in power plants

“Oil is probably the most convenient but the least abundant

of the fossil fuels, so it is the most expensive.”

Organic rich sediments buried 2-5 km 50 – 150 ° C Temperature and pressure converts some of

the organics to oil Higher temperatures produce natural gas,

mostly methane Only a tiny fraction of the oil and gas

produced can be harvested

Source of oil

Oil fuels the transportation industry More energy per weight than any

battery (so far) Convenient liquid form as opposed to:

Coal, not used in transportation since the steam engine

Natural gas which must be a pressurized container

Oil is the most expensive

Traditional: Oil fields – pumped from under ground or

water largest fields in Saudi Arabia, and in KuwaitNon-traditional: Oil shales – low grade fuel for power plants,

Estonia produces about 70% Tar sands – requires steam (Canada)

Sources

We have differing opinions here: The oil industry has been saying forty years for a long time but new sources

and initiatives keep adding time.

“There is enough oil to keep pumping for decades, but the

peak rate of oil extraction could be happening right now.”

How long will it last?

Coal – solid Oil – liquid Natural gas – gas usually in the form of methane CH4

Natural gas

“Methane carries more energy per carbon that the others because methane is the most chemically

reduced form of carbon.”

Reduced form + oxygen → oxidized form + waterAlong with a release of energy (the ability to do work).

Energy of methane

Global sources of Energy in 2001

China India Brazil U.S. France Denmark Japan

Biggest users of energy

Energy consumption per dollar GPD (Gross Domestic Productivity).

U.S. Japan France Denmark Brazil China India

Energy Consumption per person

U.S. petroleum, gas, coal Japan petroleum, gas, coal France petroleum, gas, coal Denmark petroleum, gas, coal Brazil petroleum, gas, coal

Chinacoal, petroleum, gas India coal, petroleum, gas

Source?

China and India are building new coal fired plants at an alarming

rate.

New coal plants

http://ingienous.com/?page_id=8399

“Coal is the form of fossil fuel with the potential of increasing

the temperature past the turning point of 2° C. The future of the earth depends most on what happens to that coal.”

Bottom Line

Ultimately, the energy available to humankind includes instantaneous solar energy, which is abundant but spread out; stored solar energy is in the form of fossil fuels; and stored solar energy from stellar explosions in the form of radioactive elements.

Of the fossil fuels, coal is the most abundant. Oil may run out in the coming decades, and the peak rate of oil extraction may be upon us even now.

Take home points, Chapter 9

We can project energy demand in the future as the product of population, economic growth, and energy efficiency.

continued….

The Perturbed Carbon Cycle

Chapter 10

The atmosphere ain’t what it used to be!

Three oxygen atoms Very reactive O2 bonds break with UV-c,

forming O free radical, recombines with an O2 to form O3

Stratospheric O3 absorbs (filters) UV-b radiation, forming O2

Ozone

Phased out production and release of chlorofluorocarbons because it breaks down stratospheric ozone (Freon, aerosol propellants, refrigerants)

Asthma and allergy suffers feel it, plant leaves get burned and scarred

Montreal Protocol 1987

Is a Good thing CO2 in the stratosphere sheds

heat as IR to space The ozone depletion causes

cooling in the stratosphere Result: the stratosphere is cooling

Stratospheric Ozone

Tropospheric ozone comes from several sources. Biomass burning and industrial activity produce carbon monoxide (CO) and volatile organic compounds (VOCs) which are oxidized to form ozone. Nitrogen oxides (NOx) from industrial processes, biomass burning, automobile exhaust and lightning also form tropospheric ozone. A small amount of tropospheric ozone also comes from the stratospheric ozone layer.

http://earthobservatory.nasa.gov/Features/Aura/Images/TroposphericOzone_HiRes.jpg

Surface/tropospheric Ozone

Ozone

Ozone hole located over Antarctica is a different problem than the ozone as a greenhouse gas

HNO3 acid clouds react with chlorine, which in turn, consumes the ozone

Satellite was programmed to throw out data that violated common sense, so the hole was a surprise

Revisiting discarded satellite data revealed that the hole had been growing for some time

Ozone Hole

Methane

Natural Sources

Wetland degradation Termites Organic carbon in

freshwater swamps

Human Sources

Energy emissions Landfills – “swamp

gas” Ruminant animals Rice agriculture Biomass burning

http://www.youtube.com/watch?v=U46XOoU0DrM

Overall human impact has doubled since pre-human levels

CH4 is responsible for 25% of anthropogenic greenhouse heat trapping

Methane Clathrates – Fire Ice

Methane is transient, but CO2 accumulates

Background levels were around 280 ppm until ~ 1750, coinciding with the New World, “pioneer effect”

Deforestation for agriculture and development is one source of atmospheric CO2

The second source is fossil fuel combustion

Carbon Dioxide

CO2 and CH4, 1000 years

CO2 is complicated, and the atmosphere is the exchange place for the three remaining carbon reservoirs

Land cycles annually Oceans cycle by centuries or more Rock cycles by millennia or more

Atmospheric CO2

Deforestation releases about 1.5 Gtons C /year

Fossil fuels release about 8.5 Gtons C /year Release is about 10 Gtons C /year Atmospheric levels are rising by about 4

Gtons C /year Mathematically we are missing about 6

Gtons C /year

There is a missing carbon sink – about 6 Gtons /year

The Missing Sink

The measurements are variable The research indicates that the land is

taking up the missing carbon Studies conclude that the “missing

sink” is located in the high latitudes of the northern hemisphere

Terrestrial Carbon Sink

Higher concentrations of CO2 encourages plants to grow faster (greenhouses)

The growth is an initial spurt, and tends to level off

Higher CO2 concentrations fro plants means less water loss when plants open the stomata to take in the CO2

CO2 Fertilization

As organic carbon is oxidized to CO2, the soil releases the CO2

Warmer soils decompose faster Tropical soils contain very little

carbon The permafrost is full of carbon As soils warm, the CO2 emissions

get higher

Respiration in Soils

Ultimately the fossil fuel CO2 will be cleaned up by the oceans

60 years ago, scientists thought it would be a quick process

50x more CO2 in the ocean 70% of the Earths surface,

average 4 km deep

Ocean uptake CO2

The surface of the ocean limits the contact between the atmosphere and the deep ocean

The ocean uptake of fossil fuel carbon depends on circulation

Ocean ventilation – at high latitudes the cold water sinks and takes gases with it – it takes centuries to make the loop

But…

The thermocline is a few hundred meters deep, and the ventilation to the atmosphere is a few decades

The surface ocean mixed layer (driven by the wind) is about 100 meters deep and ventilation to the atmosphere is annually

Also…

In seawater, freshwater lakes, rivers, reservoirs, swimming pools and human blood

The major ions in seawater are Na+, Mg2+, Ca2+, K+, Sr2+, Cl-, SO4

2- (sulfate), HCO3

- (bicarbonate), Br-, B(OH)3 (boric acid), and F-. Together, they account for almost all of the salt in seawater.

Buffer chemistry of inorganic carbon

Atmospheric CO2 dissolves in seawater and is hydrated to form carbonic acid, H2CO3. Carbonic acid is diprotic; that is, it can undergo two de-protonation reactions to form bicarbonate (HCO3

-), and carbonate (CO32-). The co-

existence of these species in seawater creates a chemical buffer system, regulating the pH and the pCO2 of the oceans. Most of the inorganic carbon in the ocean exists as bicarbonate (~88%), with the concentrations of carbonate ion and CO2 comprising about 11% and 1%, respectively.

http://oceancolor.gsfc.nasa.gov/SeaWiFS/TEACHERS/CHEMISTRY/

Carbonate/bicarbonate buffer

pH reactions, CO2 reacts with H2O to form carbonic acid (carbonated soda drinks)

CO2 + H2O H2CO3 Carbonic acid loses a hydrogen, forms an acidic

proton and bicarbonate (hydrogen carbonate)H2CO3 H+ + HCO3-

Hydrogen carbonate loses the second acidic proton and forms more acid and the carbonate ion

HCO3- H+ + Co32-

What does that mean?

We can ignore the tiny input of the Hydrogen ion and recombine the equations to show and easier illustration of le Châtelier’s principal

CO2 + CO32- + H2O 2 HCO3-

1% 11% 88% Any additional CO2 is reacted with the

carbonate ion to produce the hydrogen carbonate ion

Lets Assume

A bucket of seawater can absorb or release more CO2 because of the pH chemistry

The buffer stabilizes the pH and the concentrations of the CO2

The amount of CO2 that can be absorbed depends on the concentration of the carbonate

It is about 11% and CO2 is about 1%, so it works well

This buffer system also keeps your blood pH in balance

pH Chemistry

If you perturb, stress, or change the system, it will

react in such a way to relieve the perturbation, stress, or

change in the system – it will reach a new equilibrium

Perturbation

Le Châtelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

In other words, look at the equation, if you add products, it will shift to reactants

If you take away reactants, it will shift to reactants

It will shift to overcome the stress

le Châtelier’s Principle

The relative concentrations of carbon dioxide and carbonate ion in seawater determine its pH

Fossil fuel CO2 makes seawater more acidic The buffer helps resist the change in pH Life forms in the ocean that make their shells

out of CaCO3 will suffer at lower pH Think of putting baking soda (sodium

bicarbonate) into vinegar (a weak acid) and watch the CO2 fizz out

Seawater pH

Eventually after a long period of time, the CO2 will spread out among the carbon reservoirs of the atmosphere, ocean and land surface

Models indicate that the atmospheric levels of CO2 will be higher than before the CO2 was released

Eventually the budget for dissolved CaCO3 in the ocean has to balance

As the buffer chemistry recovers, atmospheric CO2 drops

Equilibrium Models

The climate cycle will ultimately recover from the fossil fuel era when the carbon returns to the

solid Earth as a result of the silicate weathering CO2

thermostat from Chapter 8.

Recovery

The longevity of the global warming climate event

stretches out into time scales of glacial – interglacial cycles,

time scales that are longer than the age of human civilization.

How long? First we have to stop adding CO2 to the atmosphere.

The ozone hole problem is not the same as global warming. They are different issues.

Methane has about a 10 year lifetime in the atmosphere, so its concentration reaches an equilibrium after about this long.

The land surface and the ocean are absorbing some of our fossil fuel CO2, but this could slow or reserve in a changing climate.

Releasing fossil CO2 to the atmosphere will affect climate for hundreds of thousands of years – as far as we are concerned, forever.

Take home points, chapter 10