Outline

Announcements

What is a biogeochemical cycle and why do we care?

The carbon cycles

Human activities and biogeochemical cycles

Biogeochemical Cycles Reading, Chapter 16, can skip Biological Concentration of Elements, pp. 368-371; Biogeochemical cycles of a metal and a nonmetal

The Carbon Cycle

A biogeochemical cycle is the complete pathway that a chemical element follows through the Earth system -- from the biosphere to the atmosphere, to oceans, to sediments, soils and rocks, and from rocks back to the atmosphere, ocean, sediments, soils and biosphere

• Bio because they involve life

• Geo because they involve rock and soil

• Chemical because they involve chemical elements

We will focus on elements that are (1)essential to the biosphere and (2) whose cycles are strongly influenced by humans

Figure 16.2, Skinner et al., 1999

Biogeochemical Cycling

People are now playing major roles in many biogeochemical cycles, including carbon, nitrogen and phosphorous cycles

BUT most of the manipulation is being done by the developed nations, which represent a minority of the world’s population – so population is not the only story, it is resource use per capita

Elements Required for Life

About 2 dozen elements are required for life

• Macronutrients: required in large amounts by all life

• Micronutrients: required in small amounts by all life or in moderate amounts by some life and not others

The “Big Six:” carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur

• Carbon: the building block of organic compounds

• Carbon, hydrogen, oxygen: form carbohydrates

• Nitrogen: makes proteins

• Phosphorus: enables energy use by cells

Figure 16.3, Skinner et al., 1999

Carbon – a key element in the Carbon – a key element in the Earth SystemEarth System

Atmosphere - Carbon dioxide (gas) COAtmosphere - Carbon dioxide (gas) CO2; 2; methane (gas) CH4

Ocean -Dissolved Carbon (bicarbonate and carbonate ions)Land - Organic matter - Carbon is a constituent of all living Land - Organic matter - Carbon is a constituent of all living thingsthings

Fossil organic matter (coal, petroleum, natural gas)Fossil organic matter (coal, petroleum, natural gas)Limestone (solid) CaCO3

Graphite, diamond (solid) C Lithosphere

Carbon – a key element in the Carbon – a key element in the Earth SystemEarth System

Atmosphere - Carbon dioxide (gas) COAtmosphere - Carbon dioxide (gas) CO22; ; methane (gas) CH4

Ocean -dissolved ions (bicarbonate and carbonate)Land - Organic matter - Carbon is a constituent of all living Land - Organic matter - Carbon is a constituent of all living thingsthings

Fossil organic matter (coal, petroleum, natural gas)Fossil organic matter (coal, petroleum, natural gas)Limestone (solid) CaCO3

Graphite, diamond (solid) C Lithosphere

Keeling, C.D. and T.P. Whorf. 2005. Atmospheric CO2 records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

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1955 1965 1975 1985 1995 2005

Mauna Loa

South Pole

Carbon dioxide mixing ratio (parts per million)1 ppm = 1 liter CO2 in 1,000,000 liters air

Overall increase in CO2

Rate of increase changes with timeSeasonal cycleGradient between hemispheres

Northern hemisphereSouthern hemisphere

Reasons for differences in CO2 between hemispheres

Because of the ITCZ, air rises at the equator – but does not easily mix across it. It takes about 1 year to mix air from the northern to southern hemisphere, but only a few weeks to mix air within each hemisphere.If most sources of CO2 to the atmosphere are in the north, concentration will be higher there….

How do we know it is people who are causing the recent increase?

• Is there evidence of other such increases in the past? How much does CO2 concentration vary with time?

• Is there something unique about fossil fuel burning that we can use to tie atmospheric changes directly to its use?

History of Carbon Dioxide in the Atmosphere From Ice Cores and Direct Measurements supports the idea that humans are causing the increase

CO2 starts to rise between 1850 and 1950, before the start of large amounts of fossil fuel burning

Goude,1990Human impact on the natural environment, MIT Press

US Forest cover 1620 and 1920 AD

Net C flux to atmosphere in the past ~200yrs is ~180-200 PgC

Bonan (1997) Climatic Change eastern deforestation leads to significant summer cooling

Rise in CO2 concentrations tracks estimates from land use change and fossil fuel use

Attribution of CO2 rise to fossil fuels using its unique fingerprint…

• Parallel decline of the 14C/12C ratio of atmospheric CO2. Fossil fuels do not contain 14C precisely because they are millions of years old – the half-life of 14C is roughly 6000 years. A decline in the 14C of atmospheric CO2 in the early 1900s is recorded in tree rings.

• Parallel decline in the oxygen concentration of the atmosphere, which is the inescapable signature of burning (or respirsation, see below) of carbon.

Data from Ralph Keeling, Scripps Inst. Oceanography http://bluemoon.ucsd.edu/data.html

O2 in atmosphere

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Time of glacial advance (cold)

Carbon Dioxide has varied pre-industrially – so there must be natural variations as well; present rates of increase are as fast or faster than previous changes

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IPCC high scenario2100, 975 ppm

IPCC low scenario2100, 540 ppm

2004, 380 ppm

1959, 316 ppm

Why do we care about CO2 in the atmosphere? The greenhouse effect; more CO2 in the

atmosphere means less IR radiation escapes directly to space, atmosphere must warm

Carbon cycle questions

• What caused the change in CO2 between glacial and interglacial cycles?

• Only about half of the fossil fuel we emit stays in the atmosphere (see next slide) – where does the rest go? How long can we expect it to be taken up?

• Can we really predict future CO2 concentrations – what are the feedbacks involved?

To answer these questions, we need to know

• How much carbon is in each of the major reservoirs (atmosphere, ocean, land vegetation/soils, rocks)

• What processes might cause those reservoirs to change in size?

• What is the response time for C in these reservoirs (what reservoirs dominate the response on what time scales)

• What are the feedbacks that link carbon dioxide and climate

ATMOSPHERIC CO2

640 X1015 g C

LIVING BIOMASS

830 X1015 g C

DISSOLVED ORGANICS

1500 X1015 g C

ORGANIC CARBON IN SEDIMENTS AND SOILS

3500 X1015 g C

CO2 DISSOLVED IN OCEANS

38,000 X1015 g C

LIMESTONE AND SEDIMENT CARBONATES

18,000,000 X1015 g C

TRAPPED ORGANIC CARBON: NATURAL GAS, COAL PETROLEUM, BITUMEN, KEROGEN

25,000,000 X1015 g C

Distribution of Carbon;

1015 grams =

1 Petagram (Pg)

Dominate over years to centuries

Dominate over centuries to millennia

Dominate over tens of thousands to millions of years

Long-term controls: the rock/CO2 cycle

Continental crust

Mantle

Oceanic crust

1. Carbon dioxide released from volcanoes

2. CO2 dissolves in water, making a weak acid that will react with rocks in crust. CO2 also forms organic matter

3. Calcium carbonate (ocean salts) and organic matter accumulate in sediments on ocean floor

4. Subducted sediments are heated, which causes them to release carbon dioxide again – CO2 travels up to be released in volcanic gases

Thousands of years – the oceanEquilibrate with the atmosphere;CO2 in the atmosphere is like the headspace in your soda pop – CO2 controlled by the acidity of the oceans and the temperature of surface water

Sediments accumulate (calcium carbonate and organic matter)

Ocean mixing – Deep waters, isolated from the surface for thousands of years, dissolve carbonate and organic matter that fall from the surface. Upwelling waters high in CO2

River inputs

Sedimentation

Deep water formation

Upwelling

Photosynthesis

Respiration

High CO2

CO2 dissolves more readily in cold than warm water. So … it is taken up in polar regions, where water is

cooling, and released from the ocean to atmosphere in upwelling areas and areas of warming water

Bubbles form because CO2 is under pressure – when you remove the seal that holds the pressure, the CO2 comes out in bubbles. Warm beer will hold less CO2 than cold beer – and will go flat quickly

Timescales of decades: the living biosphere

• Important transfers here are:

Photosynthesis

CO2 + H2O + light energy CH2O + O2

Carbon dioxide + water + energy organic matter + oxygen gas

Respiration (also burning)

CH2O + O2 CO2 + H2O + energyorganic matter + oxygen gas carbon dioxide + water + energy

Photosynthesis

CO2

Carbon dioxide

H2OWater

Energy(sunlight) O2

Oxygen gas

CH2OGlucose(SUGAR)

CO2

Carbon dioxide

H2OWater

Energy – (food)

O2

Oxygen gas

CH2OGlucose(SUGAR)

Respiration (and fire)

What controlled glacial –interglacial CO2 Change?

• We still don’t know the details; but we DO know that a change in thermohaline circulation was involved (see more explanation below on how that impacts CO2)

• CO2 changed in response to temperature (ie was part of the feedback, not the forcing of climate change)

Why does deep water have more CO2 dissolved in it than surface water? Two reasons:

(1)deep water is cold, and cold water holds more CO2 than warm water

(2)The biological ‘pump’ – organic matter made in the surface ocean falls into the deep ocean where it gets decomposed and releases CO2

Ocean feedback mechanisms

River inputs

Sedimentation

Deep water formation

Upwelling

Photosynthesis

Respiration(organic C CO2

1. Slow down thermohaline circulation; warmer surface waters don’t cool enough for sea ice to form; less deep water formation, therefore less upwelling

How can that feed back on CO2?

Increase the time required to equilibrate deep sea water with the atmosphere

Organic carbon still falls and is decomposed in deep waters

C transferred from atmosphere to deep ocean reduces atmospheric CO2

Higher CO2

Only about half of the CO2 we emit to the atmosphere stays there – where does the rest go?

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1955 1965 1975 1985 1995 2005

Mauna LoaSouth Polefossil fuel added

CO2 concentration we expect if all of the C from burning fossil fuel stayed in the atmosphere

Observed rise in CO2

Slide from N. Gruber, UCLA

CO2 taken up in excess of what was there preindustrially

Ways the land surface may take up carbon from the atmosphere

(photosynthesis exceeds respiration)

1. Regrowth of forests that were cut in 1850-1900 in the northeastern US and Europe2. “woody encroachment” growth of trees in areas that were formerly grassland

3. CO2 ‘fertilization’ - some plants grow better with higher levels of CO2 in the atmosphere

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Sources of CO2 Where CO2 goes

Fossil fuel emission Increase in

atmospheric CO2

Release by tropicaldeforestation

Uptake by oceans

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Term we don’t understand (determine as residual)

Uptake byTerrestrialEcosystems

Role of Terrestrial EcosystemsIn perturbed global C cycle (1990s)

Terms we know well

Terms we kind of know

Carbon dioxide is not only the climate-active component of the Earth System that is changing - other greenhouse gases, as well as changes in aerosols

Nitrous oxide – fertilizer useMethane – rice and ruminant agricultureOzone – air pollutionAerosols – fossil fuel burning

What you need to know

• Definition of a biogeochemical cycle• Forms carbon takes in atmosphere, biosphere,

hydrosphere, lithosphere• Relative sizes of the reservoirs of C• Response times of the important reservoirs

(years, decades, centuries, millennia)• Know what processes causes changes in

atmospheric CO2 on seasonal-decadal, centuries-millennial, and million year timescales

Need to know -II

• In what two ways does the ocean influence atmospheric CO2?

• Know what photosynthesis and respiration are, and what the reactions include (energy needed or released, carbon dioxide, water, oxygen, and sugar/glucose)

• What evidence do we have that recent rise in CO2 is due to fossil fuel use and land use change?

• What fraction of the CO2 we put into the atmosphere accumulates there? Where does the rest go?