Satellite observations of coastal pCO2 and air-sea flux of carbon dioxide

Presenter: Steven E. Lohrenz

Department of Marine Science

The University of Southern Mississippi

Steven.Lohrenz@usm.edu

Contributors: Wei-Jun Cai, Wei-Jen Huang – UGA

John Lehrter, Mike Murrell, Jan Kurtz – EPA

ACE Workshop6 June 2011

Acknowledgements

• Support provided for this effort from NASA, NSF, NOAA, and EPA

• This work supports is aligned with goals and objectives of the Ocean Carbon and Biogeochemistry and North American Carbon Program

Overview

• The role of coastal margins in the global carbon budget

• Satellite algorithms for estimating pCO2 in coastal waters

• Applications of the approach in the Gulf of Mexico

• Conclusions and Recommendations

Importance of Coastal Margins Global Carbon Budgets

“The North American Carbon Program therefore requires marine observations and diagnostic models focused on understanding the role of coastal systems on adjacent ocean basins and on atmospheric CO2 distributions.” - Wofsy and Harris (2002)

Need for improved estimates of “North American coastal ocean and continental margin air-sea fluxes, land-ocean and coastal open ocean exchange, and biogeochemical cycling…in order to close the carbon budget over North America” - Doney (2004)

Importance of Coastal Margins in the NACP

• Specific coastal objectives included: – “better estimates of air-sea fluxes and

their impact on the CO2 concentrations of continental air masses,

– “estimates of carbon burial and export to the open ocean,

– “elucidation of factors controlling the efficiency of solubility and biological pumps in coastal environments,

– “quantification of the influence of margin biogeochemical processes on the chemical composition of open ocean surface waters, and

– “the development of coupled physical biogeochemical models for different types of continental margins.”

Denning, 2002

Current Efforts• Ocean Component of NASA Carbon

Monitoring System– “The quantity of carbon exchanged between

the land and ocean remains an unknown in the global carbon budget,” NASA Carbon Monitoring System Scoping Study Workshop Report, 13-14 July 2010

• Carbon Cycle Science Plan Update– understanding how natural processes and

human actions affect the carbon cycle, on land, in the atmosphere, and in the oceans

Synthesis Activities Addressing Coastal Carbon

Chavez et al. (SOCCR, 2007) Liu et al. (2010)Hales et al. (2008)

Crossland et al. (2005)

• Numerous other reviews:– Borges et al., 2005; – Chen and Borges, 2009; – Cai, 2011

Significance of Coastal Ecosystems to Global C Cycling

• Global riverine C exports to the ocean approach or exceed 0.9 Pg C a-1 (McKee et al., 2003; Cai, 2011)

• Roughly half of this is organic carbon

• This is a substantial flux between land and ocean ecosystems and represents a substantial fraction of the net ocean atmospheric uptake of carbon (-1.6 Pg C a-1 (Takahashi et al., 2009)

Significance of Coastal Ecosystems to Global C Cycling

• It has been argued that much of this carbon is respired in shelf environments

• However, recent syntheses (Borges et al., 2005; Liu et al., 2010; Cai, 2011) have shown that most shelves (except in low latitudes) act as a net sink for CO2

• Increasing levels of atmospheric CO2 may have contributed to the current state of continental shelves as net sinks for atmospheric carbon (Cai, 2011)

Significance of Coastal Ecosystems to Global C Cycling

• In contrast to shelf ecosystems, inner shelf and estuarine waters appear to be a net source of carbon (Borges et al., 2005; Chen and Borges, 2009; Cai, 2011)

• However, there is some debate as to whether these estuarine and systems are net atmospheric sinks given high productivity of salt marsh and mangrove ecosystems (Cai, 2011)

• It also seems likely that riverine carbon is largely transported beyond these near shore systems to shelf and ocean waters

Significance of Coastal Ecosystems to Global C Cycling

• Clearly, there remains large uncertainty in the contribution of coastal ecosystems to global carbon budgets and the exchanges and transformation of carbon along the land-water interface

• The high heterogeneity of these systems necessitates a combination of in situ and satellite approaches

• Coastal ecosystems also pose serious challenges to satellite based methods given their optical complexity

LandCarbon Management

Land Use and Land Cover ChangeAgriculture, FertilizerEnergy and BiofuelsPopulation Growth

Water Resource Management

Ocean Carbon Reservoir

Air-Sea ExchangeCarbon Sequestration?

Coastal MarginAir-Sea Exchange

Eutrophication and HypoxiaOcean Acidification

Sea Level Rise ImpactsCoastal Habitat Loss

Water QualityFisheries

Land-Ocean

Exchange

Coastal Ocean Exchange

• Coastal zones are subject to increasing variability and human impacts including rising CO2

Boundary Effects on Continental Air Masses

Overview

• The role of coastal margins in the global carbon budget

• Satellite algorithms for estimating pCO2 in coastal waters

• Applications of the approach in the Gulf of Mexico

• Conclusions and Recommendations

Satellite Algorithms for Estimation of pCO2 and Air-Sea Flux of Carbon

• Various approaches that use combinations of in situ and satellite observations:– Climatologies (e.g., Chavez et al., 2007; Takahashi et al.,

2009)– Simple regression (Olsen et al., 2004)– Multiple regression (Lefevre et al., 2002; Ono et al., 2004;

Sarma et al., 2006; Chierici et al., 2009) – PCA – regression (Lohrenz and Cai, 2006; Lohrenz et al.,

2010)– Neural net – SOM (Telszewski et al., 2009; Evans et al.,

2011)

• Satellite observations allow for the extrapolation of in situ observations in time and space

Overview

• The role of coastal margins in the global carbon budget

• Satellite algorithms for estimating pCO2 in coastal waters

• Applications of the approach in the Gulf of Mexico

• Conclusions and Recommendations

Applications in the Gulf of Mexico

• The Gulf of Mexico example:– Gulf of Mexico a net source?

• 0.014 Pg C a-1 (Chavez et al., 2007 – climatology)

– More recent observations (Wanninkhof et al., 2007; Lohrenz et al., 2010) suggest seasonally variable conditions and a possible weak net sink

• Compare to Lohrenz and Cai (2006) for June 2003:– Plume: -6.8- -7.7 mmol m-2 d-1 – Shelf: -4.9 - -5.5 mmol m-2 d-1

Lohrenz et al., 2010

• These earlier studies relied on regressions between in situ observations:

• An alternative approach is to relate directly to satellite observations:

• In situ surveys show seasonal patterns in northern Gulf (Cai et al.)

Seasonal patterns in fluxes in the northern Gulf of Mexico

Sea to air flux of CO2 (mmol C m-2 d-1)

Date Plume Shelf Entire Image

Jun 2006 -4.0 - -5.9 2.6 – 3.8 0.92 – 1.4

Sep 2006 -5.1 – -6.9 8.3 – 11.0 3.4 – 4.6

May 2007 -29 - -35 -2.9 - -3.5 -6.7 - -8.2

Aug 2007 -4.7 - -5.8 1.6 – 2.0 -0.58 - -0.71

Seasonal patterns in fluxes in the northern Gulf of Mexico

-40

-30

-20

-10

0

10

20

0 2 4 6 8 10 12

Sea

to A

ir C

O2

Flux

(m

mol

m-2

d-1

)

Month

Plume

Shelf

Region

Overview

• The role of coastal margins in the global carbon budget

• Satellite algorithms for estimating pCO2 in coastal waters

• Applications of the approach in the Gulf of Mexico

• Conclusions and Recommendations

Conclusions• The U.S. coastal margins are economically critical regions and

coastal regions are subject to extreme environmental variability and increasing human impact

• Coastal carbon processes may have a significant impact on the continental carbon budget, but large uncertainties remain in air-sea fluxes and land-ocean exchanges

• Satellite ocean color as well as other satellite-derived products (e.g., SST, winds) will be invaluable in helping to constrain temporal and spatial variability in air-sea flux of carbon dioxide; ACE will have advanced capabilities for coastal applications

• pCO2 and air-sea flux of carbon dioxide represent products directly relevant to oceanic uptake of carbon

• Such measurement approaches provide a context for understanding other aspects of ocean productivity and carbon cycling

Recommendations

• Strategies should be developed to combine field and satellite observations to generate improved estimates of air-sea fluxes using novel experimental designs that are part of an overall flight program

• Complementary field and satellite-based efforts are needed to understand biogeochemical factors influencing the relative importance of biological and solubility pumps and land-ocean-atmosphere fluxes

• The fate of riverine and estuarine carbon as well as should be a focus of efforts to constrain carbon budgets

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