Climatic Feedbacks in the Global Carbon Cycle - ACS Publications

Chapter 21

Climatic Feedbacks in the Global Carbon Cycle 1

2

1

3

1

W. M. Post, F.Chavez, P. J.Mulholland, J. Pastor, T.-H. Peng, K. Prentice, and T. Webb III 4

5

Downloaded by FUDAN UNIV on February 1, 2017 | http://pubs.acs.org Publication Date: February 4, 1992 | doi: 10.1021/bk-1992-0483.ch021

1

Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6335 Monterey Bay Aquarium Research Institute, 160 Central Avenue, Pacific Grove, CA 93950-0020 Natural Resources Research Institute, University of Minnesota, Duluth, MN 55811 Institute for Space Studies, National Aeronautics and Space Administration Goddard Space Flight Center, 2880 Broadway, New York, NY 10025 Department of Geological Sciences, Brown University, Providence, RI 02912-1846

2

3

4

5

Increasing atmospheric CO is likely to produce chronic changes in global climate, as it may have done in the geologic past. Future CO -induced changes in temperature and precipitation distribution changes could equal or exceed the changes which have occurred over the past 160,000 years and have affected the global carbon cycle. We consider ocean and terrestrial processes that could involve large changes in carbon fluxes (>2 Pg C·yr ) or changes in storage in large carbon pools (>200 Pg C) resulting from CO -induced climate changes. These include (1) air-sea exchange of CO in response to changes in temperature and salinity; (2) climate-induced changes in ocean circulation; (3) changes in oceanic new production and regeneration of organic debris caused directly by climate change; (4) altered oceanic nutrient supply needed to support new production due to climate-induced alteration of ocean circulation and river discharge; (5) CaCO compensation in sea water; (6) altered river nutrient flux and effects on coastal organic matter production and sediment accumulation; (7) seasonal balance between G P P and decomposition-respiration in terrestrial ecosystems in response to changes in temperature and precipitation; (8) successional processes in terrestrial ecosystems and formation of new plant associations in response to climatic change; (9) effects on soil nutrient availability, which amplifies ecosystem responses to climate change; (10) and responses of northern forests, tundra, and peatlands which have, until recently, been a sink for CO . The potential effect of these processes 2

2

-1

2

2

3

2

0097-6156/92/0483-0392$06.25/0 © 1992 American Chemical Society Dunnette and O'Brien; The Science of Global Change ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

21. POST ET AL. Climatic Feedback in the Global Carbon Cycle

393

on the rate of atmospheric CO concentration changes are esti mated where possible, but not much quantitative information at a global scale is known, so uncertainty in these estimates is high. Each of these secondary feedbacks, however, has the potential of changing atmospheric CO concentration in magnitude similar to the effects of the direct human processes (fossil fuel burning and land clearing) responsible for the concern about global warming in the first place. It is therefore urgent that these uncertainties be resolved. Lines of research to accomplish this are suggested. 2


2

Over long time-scales (millions of years), geological processes determine the range of variation in rates of global carbon cycle fluxes (1). At shorter timescales (decades and centuries), the rates of rock formation and weathering are too slow, and the exchanges of CO2 between the relatively active reservoirs of carbon in the atmosphere and terrestrial ecosystems and ocean surface waters are responsible for fluctuations in atmospheric CO2 concentration. The flux rates of CO2 between the atmosphere, land, and ocean are determined by a complex and interactive array of chemical, biological, and physical processes (Figure 1). For at least the last 160,000 years, a balance between these faster carbon cycle processes may have been responsible for the atmospheric CO2 concentrations remaining in the relatively narrow range of 200 to 300 μL·L~ (Figure 2a). Fossil fuel burning is resulting in an unprecedented rate of increase in atmospheric CO2 (Figure 2b). This increase could shift the carbon cycle to a new mode of operation (8, 9). In the future, increasing atmospheric CO2 lev els may significantly reduce or eliminate some negative feedbacks (interactions between carbon cycle components that tend to restore or maintain atmospheric CO2 levels), enhance existing positive feedbacks (interactions that tend to am plify small deviations from existing CO2 levels), or introduce new positive or negative feedbacks in the global carbon cycle. Feedbacks may be affected directly by atmospheric CO2, as in the case of possible CO2 fertilization of terrestrial production, or indirectly through the effects of atmospheric CO2 on climate. Furthermore, feedbacks between the carbon cycle and other anthropogenically altered biogeochemical cycles (e.g., nitrogen, phosphorus, and sulfur) may affect atmospheric CO2. If the creation or alteration of feedbacks have strong effects on the magnitudes of carbon cy cle fluxes, then projections, made without consideration of these feedbacks and their potential for changing carbon cycle processes, will produce incorrect esti mates of future concentrations of atmospheric CO2. 1

Climate-Ocean C 0 Feedbacks 2

Presently, it is not known whether the feedback of oceanic CO2 in response to the C02-induced climatic warming will, on the whole, be positive or negative. The rate of exchange between the oceans and the atmosphere is regulated by the following processes: (a) exchange of CO2 gas between the atmosphere and the surface layers of the oceans, (b) exchange of water between the upper and the deep layers of the ocean (mixing, upwelling, and deep-water formation) and (c) photosynthetic utilization of CO2 and nutrients in the surface photic zone and the subsequent gravitational settling of the biogenic debris to the deep ocean. Each of these processes is affected by climatic changes, including temperature, evaporation, precipitation, wind, ice formation, and cloudiness. The feedback responses of each of these processes are discussed below. Climate Change and Air-Sea Exchange of C 0 . The air-sea exchange flux of CO2 is governed by the gas exchange rate and by the difference between 2

Dunnette and O'Brien; The Science of Global Change ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

THE SCIENCE OF GLOBAL CHANGE

394

the partial pressure of CO2 (PCO2) in the surface ocean water and that in the atmosphere, as expressed by F = E[(pC0 )air 2

— (pC0 2 ) seawater],

(1)


where F is the net CO2 flux from the air to the surface ocean water and Ε is the gas transfer coefficient for CO2. The value of Ε is insensitive to small changes in ocean temperature but is quite sensitive to wind speed over the sea surface (boundary layer thickness, wave action, and bubble formation are functions of wind speed). Therefore changes in surface wind speed accompanying a climate change could affect rates of air-sea CO2 exchange.

ATMOSPHERIC C 0 CONCENTRATION

G L O B A L CLIMATE PATTERN

2

VEGETATIVE PRODUCTION ORGANIC MATTER DECOMPOSITION SPECIES REPLACEMENT, SUCCESSION

CONTINENT MARGINS RIVER WATER AND NUTRIENT DISCHARGE COASTAL PRODUCTION SEDIMENT FORMATION

OCEAN

I

GAS EXCHANGE

1

O C E A N CIRCULATION BIOLOGICAL PRODUCTION C a C 0 COMPENSATION

1 1 1

3

Figure 1. Changes in global climate due to increased atmospheric CO2 will alter carbon cycle processes in land, continent margins, and oceans, which will in turn effect the atmospheric CO2concentration. Processes that may have effects large enough to alter future projections of atmospheric CO2 are listed under their geographic region.


21.

POST ET AL

Climatic Feedback in the Global Carbon Cycle

400

ι

V o s t o k Ice C o r e B y r d Ice C o r e

350

¥

395

300

250


*****

* ** *•*• *

8

*

200|

*

•

• **

** •

ο

ο

•* * **°

** I

1

1

1

1

1

1

-180,000

-150,000

-120,000

-90,000

-60,000

-30,000

0

1 5 0

YEARS BEFORE PRESENT Figure 2a. Carbon dioxide concentrations in the atmosphere have varied over the glacial cycles of the earth's history, with high values at of around 300 μL^L~ during the interglacial period approximately 130,000 years ago and reaching that level again at the end of the last glaciation. This graph shows CO2 measurement from air bubbles trapped in Antarctic ice sampled at Vostok and Byrd stations (2, 8). 1

400

Ο u

Q

200 -

1

150 1000

1

1

1200

1400

1

1

1600

1800

1

2000

YEAR Figure 2b. Since the end of last glaciation, atmospheric CO2 concentration has remained around 280 μL·L~ until it began rising in the 18th century. Direct measurements made at Mauna Loa since 1958 (4) indicate that the rate of increase in atmospheric CO2 is increasing. In 1988, the atmospheric carbon reservoir was estimated at 351 μL·L~ and larger than at any time during the past 160,000 years. The South Pole and Siple ice-core data are from (5-7). 1

1



396


The (pCO2)seawater is a function of temperature, salinity, total CO2, and alkalinity. Increase in seawater temperature will result in greater seawater pCC>2 hence reducing oceanic uptake of CO2. The seawater pCC>2, will increase by 4.3% as the temperature increases each degree Celsius. The effect of salinity on (DCO2)seawater is small. On the average, pC02 in seawater will increase by 0.94% for a 1% increase in salinity. The effect of total seawater CO2 on (PCO2)seawater is expressed as the Revelle factor, which is defined as a ratio of (pCÛ2)seawater to total seawater CO2 changes. The mean Revelle factor is 8 for warm surface water and 14 for cold surface water. A global mean value is ~10. This means that a 1% increase in total CO2 in the ocean surface water causes a 10% increase in seawater pC02 at equilibrium. The Revelle factor increases rapidly with increasing total CO2 in the ocean water that has a constant alkalinity (10). A 10% increase in total seawater CO2 would increase the Revelle factor from 10 to 18. Thus, seawater would take up less CO2 for a given increase in atmospheric pC02, weakening the negative feedback of oceans on atmospheric CO2 levels. In other words, the more CO2 the oceans take up, the slower they will become at taking up additional CO2. A n increase in seawater alkalinity (for example, by the dissolution of CaCOa) would decrease pC02 in seawater and decrease the Revelle factor (10). Thus C a C 0 3 dissolution would provide a a strong negative feedback in response to an increased level of CO2 in the atmosphere and ocean. However, the surface water of temperate and tropical oceans is supersaturated with respect to CaC03 by several fold. It is not likely that the dissolution of CaC03 would provide a negative feedback to the air-sea CO2 transfer process in the near future. Our existing knowledge of the relationships between pC02 in seawater and other parameters, such as temperature, salinity, total CO2, and alkalinity, allows us to compute precisely the pC02 of a given parcel of seawater. Climate change will have the following effects on surface seawater chemistry: colder or wetter climate will favor the uptake of more CO2 from the atmosphere by the oceans, whereas warmer or dryer climate will decrease the uptake of CO2 by the oceans. What is not currently known with enough certainty is how the gas exchange rate will vary with changes in surface winds under altered climate. Quantifying the relationship between wind speed and gas exchange rates, particularly at high wind speeds, is necessary for determining oceanic uptake of CO2 that accompanies C02-induced climate change. Climate Change and Ocean Circulation. The earth's climate is a nonlinear system and responds both gradually and abruptly to perturbations. The climate record obtained from Greenland ice cores reveals several brief regional climate oscillations during glacial time. The most recent of these oscillations, (the "Younger Dryas" cold period), also found in a number of continental pollen records, had a great impact in the climatic regions under the meteorological influence of the northern Atlantic. Atmospheric CO2 rose quickly by 80 μL·L~ following the transition from cold to warm conditions at the end of the Younger Dryas period 11,000 years ago. The only feasible mechanisms to explain rapid changes in the atmospheric CO2 content involve modifications in patterns and intensity of ocean circulation. Broecker et al. (11) suggest that a rapid shutdown of North Atlantic deep water formation thereby slowing general ocean circula tion during glacial time, could result in this dramatic change in atmospheric CO2 concentration. Broecker et al. (11) demonstrate that this Younger Dryas event occurred between two periods during which the glacial meltwater flooded down the Mississippi River to the Gulf of Mexico. They argue that between these two events, meltwater was diverted to the far north and upset the den sity stratification of the North Atlantic. Model studies show that if the ocean

1


21. POST ET A L

Climatic Feedbacks in the Global Carbon Cycle 397

circulation rate during glacial times was half as fast as the current circulation rate and if CaC03 compensation (see CaC03 compensation section), was effec tive, then the atmospheric CO2 concentration could increase from 219 / i L - L " to 281 μL·L~ when ocean circulation increased rapidly to present rates (12). Extrapolation of this model calculation to the future climate warming suggests that an amplification of the current rate of increase in atmospheric CO2 content could result from climate-induced changes in ocean circulation rates. Fairbanks (IS), using information provided by oxygen and carbon isotope concentrations recorded in coral reefs, showed that the input of glacial melt water input into the oceans was much reduced during the Younger Dryas event; thus he casts doubt on Broecker et al.'s (11) hypothesis. A full explanation of the Younger Dryas, therefore, remains a key research goal (14)· The cli mate period is an attractive one to study because rapid climate changes may be involved. A n understanding of the Younger Dryas may help in predicting how climate and CO2 may change during the "super-interglacial conditions" projected by current climate models for atmospheres with greater amounts of 1


1

C0 . 2

Studies in the areas of paleoceanography and paleoclimatology to under stand past modifications in ocean circulation patterns are needed to extrapolate future patterns of ocean circulation and levels of atmospheric CO2. Research in the reconstruction of glacial ocean circulation patterns on the basis of polar ice records and deep sea sediment records may shed some light on how such changes may take place and on the magnitude of the changes. Further develop ment of tracer-style ocean models (box models) and ocean general circulation models, especially ones coupled to atmospheric general circulation models will be essential for analyzing paleoclimatic data and in generating and testing hy potheses.

Climate Change and Ocean Biological Processes. The partitioning of

CO2 between the ocean and the atmosphere is affected by the pelagic food web of the upper ocean. Photosynthesis, carried out primarily by small microscopic plants (phytoplankton) in the shallow, well-lit layer of the ocean (the euphotic zone), converts dissolved inorganic carbon dioxide into organic matter. A ma jor portion of this primary production is recycled within the food web above the thermocline. The remaining fraction escapes from the upper ocean to the thermocline and below, where most of it is recycled and only a minor fraction is deposited in the sediments. It is this escaping fraction that effectively low ers the concentration of dissolved inorganic carbon in the upper ocean. Since this layer is in contact with the atmosphere the concentration of dissolved in organic carbon, and therefore carbon dioxide, has important consequences for the global carbon cycle and climate change. Ocean carbon cycle models show that changes in the quantity of organic carbon leaving the upper ocean to the thermocline and below coupled with changes in oceanic circulation, can have rapid and significant impact on atmospheric CO2 (15, 16). In the absence of ocean primary production, surface ocean total CO2 would be 20% higher, and at equilibrium with such a surface ocean, the atmosphere would have a CO2 concentration close to double present levels (IT). The quantity of primary production that is exported from the upper ocean is said to be equivalent to "new production" (18, 19). New primary production is that associated with allocthonous nutrients (i.e., those upwelled or mixed into the euphotic zone or input via rivers and rain). In order for steady state to be maintained, an equivalent flux out of the euphotic zone is required. Earlier studies (19) suggested that sediment-trap measurements of particulate organic carbon ( P O C ) flux were equivalent to new primary production; however, re cently it has become clear that these measurements probably represent only a


398


fraction of the primary production removed from the upper ocean. Small suspended particles (20) and dissolved organic matter (21) have been suggested as important agents in the removal of primary production from the upper ocean. There are several direct and indirect methods of estimating removal of organic carbon from the surface ocean. These include sediment-trap measurements (22), estimates from total production (19, 23), estimates from upwelling models (24), and estimates of recycling rates in the thermocline and below (25). Previous global estimates of new production have been on the order of 2-4 Pg C - y r " (19,26). Recent sediment trap estimates suggest that the P O C removed from the surface ocean (upper 100 m) is between 7-8 P g C - y r (22) and that new production could be as high as 15-22 Pg C-yr"" (27, 28). These estimates (Table I.) show that global new production is a large term in the ocean carbon cycle, and changes in this flux quantity may significantly alter current rates of accumulation of CO2 in the atmosphere. Since most of the new production in the ocean is supported, primarily, by upwelling of limiting nutrients from below, one way to estimate the upper limit of new production is to estimate the nutrient flux into the euphotic zone. There are well-defined regions where significant quantities of nutrients are upwelled or mixed into the euphotic zone. These include the equatorial divergences, regions of coastal upwelling, and high-latitude oceans. Although the combined total primary productivity of these regions is probably less than that of the dystrophic open ocean, their proportion of new to total production (the /-ratio) is much higher so that these areas contribute a disproportionate fraction of global new production relative to their area. Changes in the nutrient supply in the upwelling and polar regions can therefore significantly alter global new production. Using models to calculate nutrient supply requires knowledge of (a) the volume of water upwelled and of the concentration of nutrients in the upwelled water (29) or (b) mixed-layer depth and vertical distribution of nutrients (30). New production can then be estimated by using a Redfield ratio conversion from nutrient to carbon. In the following paragraphs we describe an example of a feedback loop between climate, ocean circulation, and ocean biology. We show how changes in climate and ocean circulation affect nutrient supply, which in turn alters new production, which in turn alters atmospheric CO2, and which in turn can alter climate, completing the loop. Rates of coastal and equatorial upwelling rates are directly dependent on ocean-atmosphere dynamics. Greenhouse effects will first be noticeable in the atmosphere and may significantly alter pressure fields. G C M simulation of the changes in global wind fields as a result of increasing CO2 levels can therefore be used to estimate changes in upwelling rates. For example, a simulated wind field could be used to drive a numerical model of the equatorial Pacific (31). Thus there is a theoretical framework for estimating equatorial new production for the present climate and for future climates with increased atmospheric CO2. If deep-water formation and high-latitude mixing can also be adequately modeled, the oceanic new production at high latitudes can also be estimated. A second feedback mechanism involving increasing atmospheric CO2 and new production can result from predicted changes in upper ocean temperatures and increased ice melting. High-latitude primary production may be temperature limited during the summer months but is enhanced by the increased water column stability provided by melting ice (32). Under the assumption that the rate of nutrient supply in high latitudes remains constant, then an increase in upper-ocean temperatures and increased ice melting could lead to an increased P O C flux at high latitudes. 1

- 1


1



685 1000

340 1000

Inshore waters

Neritic waters

2

1

482

200

Water of equatorial divergences and oceanic regions of polar zones

1

1

3.9 58

23

9.7

2

1

15.2

10.1

19.3

C&B

3.90

4.80

6.31

4.22

3.79

E&P

15

Annual Production for Each Type of Water (10 g C y r " ) % New Production

H I G H (50-80)

I N T E R M E D . (30)

H I G H (50-80?)

L O W (13-?)

L O W (6-14)

Estimates based on information summarized by Eppley and Peterson (19). Estimates of Chavez and Barber (24).

Total, all waters

336

140

2

Transitional waters between subtropical and subpolar zones; extremity of the areas of equatorial divergences

C&B 356

1

70

E&P

Oligotrophic waters of the central parts of subtropical halistatic areas

Type of Water

Primary Production (mg C - m - ^ d a y " )

Table I. Summary of ocean biological productivity (from 27)


15.0-22.1+

2.0-3.1

2.9

7.6-12.1

1.3-?

1.2-2.7

15

2

- 1

New Production (10 g C - y r )


400


A n important uncertainty associated with oceanic P O C fluxes is the rate of regeneration of P O C and its relationship to the regeneration of particulate organic Ν and P. Martin et al. (22) have found that 50% of the organic carbon removed from the surface is mineralized or regenerated at depths

Climatic Feedbacks in the Global Carbon Cycle - ACS Publications

Recommend Documents