Atmospheric Carbon Cycle: 18 March 2008 & 01 Apr 2008:

• The atmospheric concentration of CO2 has risen over the past century from 280 ppm to 382 ppm. The current rate of increase is ~2 ppm per year, corresponding to a CO2accumulation rate in the atmosphere of 4.2 G tons C/yr.Note: 1 ppm CO2 = 2.1 Gtons Carbon

· The rise in CO2 is due to fossil fuel combustion (presently about 8.2 G tons C/yr) and to deforestation in the tropics (presently 1.6 G tons C/yr)). Thus the total rate of CO2 emission to the atmosphere is 9.8 G tons C/yr.

· Comparison of the total rate of CO2 emission to the atmosphere (9.8 Gtons C/yr) to the actual accumulation rate of CO2 in the atmosphere (4.2 Gtons C/yr) implies that 3.6 Gtons C/yr (or almost half of the CO2 emitted) is being removed from the atmosphere.

The next two lectures examine the major sinks for anthropogenic CO2:

1. Dissolution in the oceans (carbonate chemistry)

2. Uptake of CO2 by plants to make organic matter (photosynthesis, mostly by land plants).

Course structure: an update

Atmospheric physics: air motions, regional and global circulation; the "first basics" of climate

Atmospheric radiation: shortwave (solar) and longwave(terrestrial) radiation, planetary energy balance, the "greenhouse effect"; the "second basics" of climate.

Climate change: effects of "greenhouse gases" on the planetary energy balance; observations of the climate; the "third basics" of climate.

Topics in current climate science: hurricanes, floating ice, glaciers.

Lectures 1-10 (completed)

11-12Atmospheric composition: the controls on absorption and emission of radiation — and on heating and cooling of the atmosphere

13-14

6.3 - 7.3Total

1-2Tropical Deforestation

5.3Fossil Fuel+ cement

Global CO2 budget (PgC yr-1 ) 1980 – 1990 1990 – 2000

Sources

1-2"Missing Sink"

6.3 - 7.3Total

2.1Ocean uptake

3.2Atmospheric accumulation

Sinks

2.1 Pg C = 1 ppm atmospheric CO2 [source: Cias et al., Science 269, 1098, (1995)]Is this budget accurate? What is the scientific basis for these numbers? Why should mid-latitude terrestrial plants absorb anthropogenic CO2?

When did this uptake begin, can/will it continue?What are the implications of terrestrial uptake for

♦ Future CO2? ♦ US policy? ♦ Climate change?

6.5

.5-1

7-7.5

3.2

1.5-2

1.8-2.8

7-7.5

Processes removing CO2 from the atmosphere ("sinks"): uptake by plants (photosynthesis) and dissolution in the ocean. · The next slide illustrates the global cycling of carbon between its geochemical reservoirs for the natural ("pre-industrial") atmosphere. The carbon amounts in each reservoir, and the fluxes of carbon between reservoirs, are estimated with variable reliability. The total amount of CO2 in the atmosphere was 615 Gtons in 1800 (280 ppm, the pre-industrial value). The rate of transfer of CO2 to and from the oceans is 60 Gtons/yr, to and from the terrestrial biosphere, 62 Gtons/yr.

· The RESIDENCE TIME of carbon in any of its reservoirs is defined as the average time that an atom of carbon resides in that reservoir. It is often estimated from knowledge of the components of the reservoir, e.g. the lifetime of a tree. Mathematically, the residence time is calculated as the total amount of carbon in the reservoir divided by the flux out of the reservoir. From the figure above you can see that the atmospheric residence time of CO2 is 615/(60+62) = 5 years. CO2 is thus rapidly transferred to terrestrial vegetation and to the oceans, and back again!

· It is important to distinguish between the size (Gtons of carbon) of a reservoir and the flux (Gtons of carbon/yr) through the reservoir.

The global cycle of carbon

The carbon cycle can be viewed as a set of "reservoirs" or compartments, each characterizing a form of C (e.g. trees; rocks containing calcium carbonate [limestone]).

The cycle of C globally is then represented as a set of transfer rates between compartments.

The total amount of carbon in the atmosphere + ocean + rocks that exchange with the atmosphere/ocean is fixed by very long-term geophysical processes.

Human intervention may be regarded as manipulation of the rates of transfer between important reservoirs.

Composite model of the global C cycle(McElroy, 2001)

The largest fraction of all the carbon on or near the surface of the Earth is stored in the oceans, leaving aside deep sediments and rocks that don't enter into the carbon cycle on time scales of interest to us. Uptake of CO2 occurs at the surface of the ocean where there is contact between the atmosphere and the ocean. CO2 taken up in surface waters can then be transported to other oceanic reservoirs, such the intermediate ocean and the deep ocean. These large reservoirs, which are not in contact with theatmosphere, account for the long residence time of carbon in theocean.

The uptake of CO2 occurs through both biological and chemical processes. Organisms at the surface of the ocean take up CO2during photosynthesis. The organic carbon stored in these organisms is then transferred up the food chain. CO2 can also be taken up by the ocean through the dissolution of atmospheric CO2into the surface waters. The capacity for this chemical process to take up CO2 is determined by the pH of the ocean, and by the transport rates of water.

28 to 30 C

88 to 26 Cto 26 C

-2 to 6 C

http://www.liv.ac.uk/physocean/schematics/thc.gif

90 S Eq 90N

Thermohaline circulation of the oceans

4000m

1000m

100m0m

-2 8 17 26 35

May, 2001 MODIS image

Temperature variability: seasonal and interannual

(movies)

625 G tonsAtmosphere

796 Gton

warm surface water

46 Gton

cold surface water

9744 Gtons

Intermediate Ocean

26280 Gtons

Deep Ocean

90,000,000 Gtons

Sediments

55 73

30 8 22

40

162 205

<1

3

1

0.23

18

523

Carbon Cycle of the Ocean (after McElroy, 2001 )

Reservoir sizes are in GtonsC, transfer fluxes in GtonsC/ yr. Biological transport by sinking fecal pellets in the ocean. This hypothetical

h i f h b

Dissolution and chemical reaction of CO2 with seawater

Acidic and basic solutions

Water constantly dissociates into H+ and OH- ions. These ions in turn recombine rapidly to form H2O again:

H+ + OH- H2O Reactions in both directions proceed rapidly, leading to a balance in aqueous(water) solutions where

[H+] [OH-] = 10-14 (moles/liter)2 (I)

DEFINITION:An acidic solution has [H+] > [OH-]. (NOTE:[ ] denotes concentration.) A basic solution has [H+] < [OH-]. From Eq. (I), [H+] > 10-7

moles/liter in an acid, and [H+] < 10-7 moles/liter in a base, and vice versa for [OH-]. The ocean is a basic solution with pH ~ 8.2.

DEFINITION: pH = - log10[H+]. pH is convenient to describe [H+], which varies by many orders of magnitude in solutions that we encounter every day. Examples: orange juice, vinegar, and Coca Cola are very acidic (pH = 2 - 3), soap and seawater are basic (pH 8 - 9), and cleaning solution for contact lenses is carefully kept neutral (pH = 7).

↔

Chemical and biological transformations of CO2 in the oceans

I. Chemical transformationsThe first step in dissolving CO2 in the oceans is the simple uptake of gaseous CO2:

CO2 (g) CO2 (aq)Carbon dioxide (aqueous) reacts with liquid water in a reversible reaction (i.e., goes in both directions):

CO2 (aq) + H2O (liquid) H2CO3 (aq)(aqueous carbon dioxide + liquid water carbonic acid)

Carbonic acid is a weak acid that can liberate a hydrogen ion in a basic solution, producing a bicarbonate ion HCO3

-: H2CO3 (aq) HCO3

- (aq) + H+ (aq)

This is also a reversible reaction, as are those given below. However, in the present discussion we are considering the steps that take place when CO2 is added to the atmosphere, which drives the reactions as indicated by the arrows.

The HCO3- can also can liberate a hydrogen ion if the solution is sufficiently

basic, producing a carbonate ion CO3-2:

HCO3- (aq) CO3

-2 (aq) + H+ (aq)Seawater is basic, i.e. it has an excess of OH- ions over H+ ions, by a factor greater than 104. Almost all dissolved CO2 is in the form of HCO3

- with about 10% as CO3

-2. Almost all of the H+ ions liberated in these reactions will be removed by the vast excess of OH- (the pH changes only slightly), so the reactions of H2CO3 and HCO3

- that release H+ are followed immediately by

H+ + OH- H2O

from seawater (basic)

We can summarize this series of reactions as

CO2 + OH- HCO3

Adding CO2 to seawater neutralizes an OH- ion, i.e. an acid has been added to the basic solution. The OH- must be supplied from some other base, with concentration only ~ 10-6 moles/liter while gaseous CO2 is about 10-4 and HCO3

- is 10-3 moles/liter. The process that re-supplies OH- is the reaction of water with carbonate ion (the reverse reaction for formation of CO3

-2 above)CO3

-2 + H2O HCO3- + OH-

The net reaction of CO2 with seawater is therefore CO2 + CO3

2- + H2O 2 HCO3-

Summary of reactions of CO2 with ocean water

CO2 (g) CO2 (aq)

CO2 (aq) + H2O (liquid) H2CO3 (aq)H2CO3 (aq) HCO3

- (aq) + H+ (aq)HCO3

- (aq) CO3-2 (aq) + H+ (aq)

__________________________________________

CO2 + CO32- + H2O 2 HCO3

-

Net reaction, fossil fuel CO2 dissolving in ocean waterNote: carbonate ions (CO3

2-) are a limited resource!

How much CO2 is removed by dissolving in the oceans?

If you take equal volumes of air and seawater at an average temperature, add 15 molecules of CO2 to the air and wait for the process to go to completion, approximately 14 will end up in the seawater and only one will remain in the air. But the amount of CO2 removed each year in this way is limited by the quantity of seawater that comes in contact with the air.

The effective depth of the atmosphere is the scale height, H (7 km), and of the oceans, 4 km. If we wait a very long time, all of the ocean water eventually comes into contact with the atmosphere (about 500 years for one "stirring"), so after a very long time (1000-1500 years) only 7- 10% of fossil fuel CO2 will remain in the atmosphere. This is the ultimate limit on the amount of CO2 that can be removed from the atmosphere by dissolution in the oceans on time scales of hundreds of years.

Current estimates suggest that 2 Gtons C/yr of fossil fuel CO2dissolves in the oceans. Why is this so much smaller than the 90% that can be taken up by reaction of CO2 with CO3

-2?

Only a very small portion of the ocean is in intimate contact with the atmosphere in a year (the "mixed layer")-- of the 5-7 Gtons of carbon humans add to the atmosphere each year, about 30% dissolves in the ocean.

How will ocean uptake of CO2 be affected by rising levels of CO2 in the atmosphere?

Absorption of excess CO2from the atmosphere depletes the supply of CO3

-2!

Penetration of anthropogenic CO2 into the oceans

Anthropogenic CO2 (µmol kg-1) in the Atlantic (a), Pacific (b) and Indian (c) oceans. Heavy lines on each section give characteristic potential density contours for the near surface water and intermediate water. Much of the penetration of anthropogenic carbon into the ocean follow isopycnal surfaces. From Sabine et al. (2004). Total storage in intermediate waters (grey) is about 130 Pg or 62 ppm.

The ocean’s capacity to take up CO2 will diminish with time, as the pH of the ocean declines due to uptake of CO2. The ocean becomes acidified.

Uptake of CO2 by chemical dissolution is limited by the rate for exchange between deep ocean water and surface water, and eventually, by acidification of the oceans. Acidification of the ocean is likely to lead to major shifts in marine ecosystems.

Atmospheric release of CO2 from burning of fossil fuels will likely give rise to a marked increase in ocean acidity, as shown in this figure. (upper) Atmospheric CO2 emissions and concentrations, historical (—) and predicted (---), together with changes in ocean pH based on mean chemistry. The emission scenario is based on the mid-range IS92a emission scenario assuming that emissions continue until fossil fuel reserves decline.

10 0.1=25% 100.7 = 5 (!) increase in [H+].

II. Biological processesThe "biological pump"Biological processes can help to transfer carbon from the surface waters of the ocean to the deep ocean, helping to remove fossil fuel CO2 from the atmosphere. CO2 is initially removed by growing plants via photosynthesis:

CO2 + H2O + sunlight + nutrients "CH2O"Most of the growing plants in the ocean are small organisms (plankton that float near the surface), so that they can remain in waters where sunlight is available for photosynthesis. When animals (small ones like Daphnia, or large ones like whales) eat and excrete these organisms, they are packaged into fecal pellets large enough to sink into the deep. This provides a "rain of organic matter" that extracts organic carbon from the near-surface environment, where exchange with the atmosphere is rapid, to the deep ocean, where carbon is stored for centuries. Eventually almost all of it is oxidized to CO2, but it can't return to the atmosphere until the deep water exchanges with surface water, a process that takes hundreds of years.

A small fraction is not oxidized, but is incorporated into organic-rich sediments. A *very* small portion of that may be converted into the oil deposits for the enjoyment of intelligent insects, or whatever organisms inherit the earth from mammals millions of years from now.

Nutrient limitation on the "biological pump"

Unfortunately, the rate at which the biological pump can operate is limited by the supply of nutrients (N, P, Fe, essentially fertilizer) in the surface waters. These nutrients have to come from the deep waters, and when you bring them up, you bring up the CO2 that descended in past centuries! Deep waters always contain more CO2 than would be there in equilibrium with the atmosphere, and can serve to release additional CO2 to the atmosphere.

Recently attention has focused on iron as a limiting nutrient for the "biological pump" of CO2. Iron is a very common element, and it is essential for plants to make chlorophyll. Iron is also very insoluble in sea water, thus it is in short supply in parts of the ocean. Experiments in which iron has been added to the sea have shown increased growth of phytoplankton, and a shift in the type of organisms present to favor larger plankton. Some scientists have argued that we should consider adding iron to seawater to stimulate growth of phytoplankton and remove CO2 from the atmosphere. Others think this is a "risky scheme" and voice concerns about the impact of the species shifts or the possibility that carbon removed in this way may return to the atmosphere in a brief time.

This figure represents the global cycles of marine organic matter, written as [C:N:P] to emphasize that it has (approximately) a mean composition with molar ratios C:N:P=106:16:1. Organic matter that descends by gravitational settling (sedimentation) is oxidized in deep water, releasing CO2, NO3

- (nitrate) and PO4

3- (phosphate) in the same molar ratios. Growth of marine plants, followed by grazing and sedimentation, effectively strips the upper ocean of nutrients, which are re-supplied by upwelling of deep water to the surface. Since the deep CO2 comes along with the nutrients, this process keeps high concentrations of CO2 in the deep, but there is little effect on removing anthropogenic CO2 from the atmosphere. But…are marine plants stimulated by dust [Fe] or rising CO2?

nutrients+ CO2 ++ light→ [C:N:P]

[C:N:P] <decay> → nutrients+ CO2

upwellingdeep water formation (polar)

The oceanic biological pump for C and nutrients

sedimentation

Main points of Ocean Processes The oceans have a great capacity to take up CO2, due to both chemical and biological processes. The oceans currently take up about 1/3 of the CO2 entering the atmosphere each year due to human use of fossil fuels and to clearance of forests for agriculture.

Chemical processes involve reaction of atmospheric CO2 (a weak acid) with basic ions (OH-) in seawater. The ocean becomes slightly acidified, and the over-all reaction may be represented by

CO2 + CO3-2 2 HCO3

-.Removal of fossil fuel CO2 by this process is limited by the rate for exchange

of cold deep ocean water with surface water. The capacity of the oceans to take up CO2 each year by the chemical process may be expected to decline over time as the ocean becomes more acidic (pH declines) or as the oceans get warmer.

Biological processes involving sinking of organic matter from the surface. The rate for removal of fossil fuel CO2 by this process is limited by the rate of supply of nutrients to surface waters. Some people think that anthropogenic nutrients (N, P in sewage, N and S in combustion exhaust gases, Fe in dust) could be stimulating uptake of CO2 by the oceans today.

The capacity of the ocean to absorb anthropogenic CO2 is huge, leaving < 15% in the atmosphere, but the time scale for this process is many hundreds years. Also oceanic uptake is reversible! Just as the oceans "buffer" atmospheric levels of CO2 by storing > 90% of what is added, they will tend to keep the higher levels in place so that the perturbation persists for a very long time.

Global CO2 cycle

EVIDENCE FOR LAND UPTAKE OF CO2 FROM TRENDS IN O2,

1990-2000

actual

The terrestrial part of the global carbon cycle.

Harvard Forest, central Massachusetts: A "typical New England forest" … but it was not always so! (in fact, it was never so…)

Do (Why do) regenerating forests in the US remove CO2 from the atmosphere?

NH

% o

f lan

d ar

ea in

fore

sts

20

40

60

8

0

1

00

Year

1700 1800 1900 2000

MA

Fitzjarrald et al., 2001

A legacy: land use change in New England

after C. Barford et al., Science 294(5547): 1688-1691, 2001

Eddy Flux vsBiometric Data

The Cadillac in the back lot…

Uptake of CO2 at Harvard Forest

-5

-4

-3

-2

-1

0 NEE = -1.28 - 0.146 x (yr-1990); R2 = 0.337

Year

1992 1994 1996 1998 2000 2002 2004

10

12

14

16-1 x GEE

Resp

GEE = 11.1 + 0.363 x (yr-1990); R2 = 0.732

R = 9.82 + 0.217 x (yr-1990); R2 = 0.626

NEE

(Mg-

Cha-1

yr-1)

Mg-C

ha-1

yr-1

0

20

40

60

80

100

120

Abo

vegr

ound

woo

dy b

iom

ass

(MgC

ha-1

)

93 94 95 96 97 98 99 00 01 02 03 04 05

oakother spp

Year

Rates for growth and for carbon uptake are accelerating at Harvard Forest, an 80-year-old New England Forest…why is that? Will that continue? How big do North American trees grow?

Uptake of CO2 in the US (PgC yr-1) [Pacala et al., 2001]

Forest Trees

Dead wood

soil CWood Prods

Woody encro

achment

fire su

press

Ag s

oils

Net trade

RIver export

Category Low Hi

Forest Trees 0.11 0.15Other Organic Matter

In Forests 0.03 0.15

Domestic Wood Products 0.03 0.07

Woody Encroachment on Non-forested Lands x-fire

0.12 0.13

Agricultural Soils 0.00 0.04

Exports Minus Imports of Food and Wood Products

0.04 0.09

Sediment Burial, River Export 0.04 0.08

Apparent U.S. Sink Including Woody Encroachment

0.37 0.71

Sink (actual net) 0.30 0.58

US "forests": Net sink: 0.3-0.6 PgC yr-1

Emissions (1996): US 1.44Mexico 0.09Canada 0.11

Forests in the US – and many other places – are in middle to young age classes (25-75 years), due to changes in agriculture (intensification) and forest management (intensification).

The role of forests in removing fossil-fuel CO2 from the atmosphere: summary

• Forests currently remove ~ 25% of CO2 released from combustion of fossil fuel.

• This uptake accelerated between 1980 and 1995, and may have slowed down from 1996-2006.

• It is largely a legacy of prior land use and current management of timberlands, including fire suppression.

•“Fertilization” of forests by rising CO2 may also play a role.

• Uptake of CO2 by forests can continue for a long time if land is managed to optimize a set of resource values that includes maximizing uptake of CO2. Uptake by forests is unlikely to grow much larger than today, and could be much smaller (or reverse sign!) if wise management is not instituted. Forests can help, but can't solve, the CO2 problem.

Uneven canopy, mean height 40 m, with large gaps and emergent trees to 55m.

Uneven-aged stand, with small and large trees interspersed.

TAPAJÓS National Forest, Santarém, Brazil.

Measurements include comprehensive ecological parameters—tree growth, CWD, soil respiration.

Boggy forest or forested bog?

120-yr old black spruce trees, sphagnum moss

black spruce and feather moss at BOREAS NOBS site.

20 30 50 cm

Year

T (C

)

1900 1920 1940 1960 1980 2000

-35

-30

-25

-20

-15 January

Year

T (C

)

1900 1920 1940 1960 1980 2000

1214

1618

JulyRegional Composite TRegional SmoothedNOBS T

January

Decade

Mon

thly

mea

n S

now

(cm

)

1970 1975 1980 1985 1990 1995 2000

2030

4050

60 ThompsonLynnLake

April

Decade

Mon

thly

mea

n S

now

(cm

)

1970 1975 1980 1985 1990 1995 2000

510

1520

2530

Boreal Forest: Is there warming, is more CO2 coming out and more tree growth?

Snow cover

Temperature

PEAT

45% cover

P2 (mm/2yr)

NEE

(kgC

/ha/y

r)

85 90 95 100 105 110 115

-40

-20

020

4060

r2=.72 p<.0035 slope=-3.5

Precipitation (mm in 2 yr)

(gC

m-2

yr-1

)

Upt

ake

|em

issi

on

Annual NEP, 1994-2004

Thompson, MB

T : warmer

Precip: wetter

95 96 97 98 99 00 01 02 03

Year

73

40 4113 12

-11-26 -30

-49-50

0

50

-2

0

2

-15

0

15

1995 1996 1997 1998 1999 2000 2001 2002 2003

-15

0

15

-2

0

2

-50

0

50 Net CO2 exchange (gC m-2)

Annual T anomaly (oC)

Annual Precip Anom (mm)

NOBS got warmer and wetter, and less CO2 was emitted. Water table depth and hydrology are key factors controlling the accumulation or ablation of peat.

uptake

emission

ThompsonFlin FlonLynn Lake

Norway HouseGillamNOBS

1970 1980 1990 2000

-6

-4

-2

0

2

-15-10-5051015

Mea

n te

mpe

ratu

re, °

C

Pre

cipi

tatio

nan

omal

y, c

m

a

b