This Week

• The atmosphere as part of the Earth System

• Global Biogeochemical Cycles (Box-Model Heaven)

•N2

•O2

•CO2

READING: Chapter 6 of text

Announcements Problem Set 1 due Fri Oct 12.Problem Set 2 due Tuesday Oct 16.

Why N2, O2, etc? (Mars and Venus aren’t)

Atmospheric Composition and Biogeochemical Cycles

Planetary Atmospheres

Planet Earth Venus Mars

Radius (km) 6400 6100 3400

Tavg (K) 250 700 200

Ps (atm) 1 91 6x10-3

N2 0.78 .03 0.027

O2 0.21 0.007 0.0015

CO24x10-6 0.96 0.95

Today: Earth System and N Cycle

Oxidizing Atmosphere

Earth System Surface Reservoirs

N2 Cycling—does it do anything?

The Atmosphere: An Oxidizing Medium

EARTHSURFACE

Emission

Reduced gasOxidized gas/aerosol

Oxidation

Uptake

Reduction

Gas phase radical chemistry

Cloud Chemistry

Deposition

Geological or Biological

Surface Reservoirs of the Earth System

Atmosphere

Biosphere

SoilsHydrosphere

Lithosphere

decay

assimilation

erosion

decayphoto-

synthesis

assimilationdecay

runoff

air-sea exchange

What are the time scales of exchange between the various reservoirs of the Earth System?

Oxidation States of Nitrogen

-3 0 +1 +2 +3 +4 +5NH3

--AmmoniaNH4

+

--AmmoniumR1N(R2)R3

--Organic N

N2 N2O

--Nitrousoxide

NO--Nitric oxide

HONO--Nitrous acidNO2

-

--Nitrite

NO2

--Nitrogen dioxide

HNO3

--Nitric acidNO3

-

--Nitrate

Decreasing oxidation number (reduction reactions)

Increasing oxidation number (oxidation reactions)

N has 5 electrons in valence shell 9 oxidation states from –3 to +5

Nitrogen Cycle: Major Processes

ATMOSPHEREN2 NO

HNO3

NH3/NH4+ NO3

-

orgN

BIOSPHERE

LITHOSPHERE

combustionlightning

oxidation

deposition

assimilation

decay

nitrification

denitri-fication

biofixation

burial weathering

Box Model of the Nitrogen Cycle

Inventories in Tg N, 1Tg = 1x1012 gFlows in Tg N yr-1

From Jaffe, 1992; Jacob text--modified

Atmospheric N2

3x109 Tropospheric Fixed N(non-N2O) 5

Land biota1x104

Soil1x105

Ocean biota1x103

Deep ocean1x106

Lithosphere2x109

Combustion, biomass burning, lightning

40

402530

2300

80 150

Agricult. biofixation

150

denitri-fication

10

80(NH3)

90 404030

biofixationdenitri-fication

rainrain

10

16401650

weathering

burial

1. If denitrification shuts off, while fixation continues, how long will it take for atmospheric N2 to be depleted?

2. How many times does an N atom cycle between atmospheric N2 and oceanic N before being transferred to the lithosphere?

3. Combustion and fertilizer use increase the rate of transfer of N2 from the atmosphere to the soil. Assume that these human activities have been in place and constant for the past 100 years, and prior to that they were negligible. By how much have humans increased the nitrogen contents of the total land reservoir (soil + land biota) and contributed to a global fertilization of the biosphere?

Questions

N2O

Very important byproduct of nitrification/denitrification• source of reactive nitrogen in stratosphere• greenhouse gas

IPCC[2001]

Fast Oxygen Cycle: Atmosphere--Biosphere

• Source of O2: photosynthesis

nCO2 + nH2O (CH2O)n + nO2

• Sink: respiration/decay(CH2O)n + nO2 nCO2 + nH2O

O2

CO2

orgC

orgClitter

Photosynthesis- respiration

decay

O2 lifetime: ~ 5000 years

Fast O2 Cycle: Atmosphere-Biosphere

(figure from DJJ)

Can photosynthesis/decay control O2 levels?I.e., if photosynthesis stopped, by how much would O2 decrease due to complete decay of all biomass?

Slow Oxygen Cycle: Atmosphere-Lithosphere

O2CO2

Compressionsubduction

Uplift

CONTINENTOCEAN

FeS2orgC

weathering

Fe2O3

H2SO4

runoff

O2CO2

Photosynthesisdecay

orgC

burial

SEDIMENTS

microbesFeS2orgC

CO2orgC: 1x107 Pg CFeS2: 5x106 Pg S

O2 in atmosphere: 1.2x106 Pg O 0.4 Pg O/yr

1. Does atmospheric oxygen have a seasonal cycle? If so, when would it maximize?

2. Do you think humans are increasing or decreasing atmospheric O2, why?

Question

Recent Growth in Atmospheric CO2

Arrows indicate El Nino events

Notice:• atmospheric increase is ~50% of fossil fuel emissions• large inter-annual variability

Where is rest of CO2 going?

IPCC 2001

Uptake of CO2 by Oceans

CO2(g)

CO2.H2O

CO2.H2O HCO3

- + H+

HCO3- CO3

2- + H+

KH = 3x10-2 M atm-1

K1 = 9x10-7 M

K2 = 7x10-10 M

pK 1Ocean p

H

pK 2

Net uptake:

CO2(g) + CO32- 2HCO3

--

CO2.H2O HCO3

- CO32-

OCEAN

ATMOSPHERE

Fcalc = 0.03 97% of CO2 resides in the oceans

This is definitely wrong! It greatly underestimates the fraction of CO2 that resides in atmosphere (Ftrue~ 70%)…Why? What’s wrong with this estimate?

Want to know fraction of atmospheric and oceanic CO2 that is in atmosphere at equilibrium

2

2 2

atmCO

atm oceanCO CO

nF

n n

2

2

COatmCO air

surf

Pn n

P

2 2 2

1 1 22

( ) ( )

1ocean HCO ocean CO CO

aq aq

K K Kn V K P

H H

Vocean = 1.4x1018 m3 pHocean = 8.2PCO2 = 375 x 10-6 atm

Equilibrium Partitioning of CO2

CO2 Uptake Limited by Ocean Mixing

Inventories in 1015 m3 waterFlows in 1015 m3 yr-1

Uptake by oceanic mixed layer only (VOC= 3.6x1016 m3) would give f = 94% of added CO2 remains in atmosphere…now estimate is too small…?!

CO2 Uptake also Limited By Ocean Alkalinity

Equilibrium calculation

pCO2 , ppm100 200 300 400 500

8.6

8.4

8.2

2

3

41.4

1.6

1.8

1.9

2.0

2.1

Ocean pH

[CO32-],

10-4 M

[HCO3-],

10-3M

[CO2.H2O]+[HCO3

-]+[CO3

2-], 10-3M

uptake of CO2 is limited by the existing supply of CO3

2-

To increase supply of CO32-, CaCO3 in

sediments/deep ocean must dissolve:

CaCO3 Ca2+ + CO32-

…which takes place over a time scale of thousands of years

1. Marine biota take in CO2 during photosynthesis to make OrgC. About 10% of this OrgC sinks to the ocean bottom (fecal matter, dead tissue, etc), and is buried into the sediments. How does this process affect the equilibrium partitioning of CO2 between the atmosphere and ocean?

2. Does the growth of corals/shells (Ca2+ + CO32- CaCO3) cause

atmospheric CO2 to increase or decrease?

3. A consequence of global warming is melting of the polar ice caps. This melting decreases deep water formation. Why? Would this effect reduce or amplify warming caused by anthropogenic CO2 emissions?

Questions

Evidence For Land Uptake of CO2

Trends in O2,1990-2000

Atmosphere--Terrestrial Biosphere C Cycle

Inventories in PgCFlows in PgC yr-1

Time scales are short: ~ 12 yrs w.r.t uptake; ~ 160 yrs w.r.t soil emission

790

From DJJ

2000

Global Preindustrial Carbon Cycle

Inventories in PgCFlows in PgC yr-1

When we burn fossil fuels, we take C from the sediments and put it into the atmosphere as CO2. How long-term is this perturbation to the carbon cycle?

(from DJJ)

A Long View of Fossil Fuel Perturbation

It takes a long time for fossil fuel CO2 to completely leave the atmosphere.

Future Atmospheric CO2

2000 23002100 2200

Using estimates about future population growth, energy needs, etc. project future CO2 emissions.

Using a climate model with a carbon cycle, predict CO2 based on projected emissions and sinks.

CO2 double pre-industrial value by ~ 2150

Stabilization Scenarios

2000 23002100 2200

To make CO2 growth rate 0, sources must balance sinks

These calculations show what our emissions can be for different CO2 levels.

Note that sinks are predicted to get smaller.

To stop CO2 increase now, we’d have to cut our emissions by 50%

Projected Trends in CO2 Sinks

IPCC [2001]

Questions

1. The Kyoto Protocol (heard of it?) aimed to cut emissions to be 6% lower than the 1990 values. Emissions would be only slightly less than 7 GtC/yr. Why was this even considered potentially useful?

2. To keep CO2 constant at its current value 380 ppm, we’d have to cut emissions by 50% to 4 GtC/yr. This would match the current sink rate. After a few hundred years, if we didn’t want CO2 to start increasing again, we’d have to cut our emissions even lower. Why might this be?

3. Fossil fuel abundance is estimated at ~ 5000 GtC. If we burn this much eventually, will the terrestrial biosphere be of much significance as a sink/storage of this carbon?