Atmospheric carbon-14 as a tracer of the contemporary carbon cycle Nir Y Krakauer NOAA Climate and Global Change Postdoctoral Fellow Department of Earth and Planetary Science University of California at Berkeley

The atmospheric CO 2 mixing ratio Australian Bureau of Meteorology

Carbon cycle conundrums Consistently, the atmospheric CO 2 increase amounts to 55-60% of emissions from fossil-fuel burning What are the sinks that absorb over 40% of the CO 2 that we emit? –Land or ocean? Where? What processes? –Why the interannual variability? How will CO 2 sinks change?

In this presentation… How can measuring 14 C/ 12 C isotope ratios help us map where carbon is going today? a) Ocean carbon uptake b) Carbon emissions from fossil-fuel burning

Measuring Δ 14 C : a short excursus Challenge: The typical atmospheric 14 C/ 12 C ratio is 1.1 * 10 –12 Fluctuations are O(1%) of that

1) Liquid scintillation counting measures 14 C decay GMI

2) Accelerator mass spectrometry measures (ionized) 14 C atoms Oxford Radiocarbon Accelerator Unit

The 14 C cycle at steady state 14 C (λ 1/2 = 5730 years) is produced in the upper atmosphere at ~6 kg / year Notation: Δ 14 C = 14 C/ 12 C ratio relative to the preindustrial troposphere Stratosphere +80 ‰ 90 Pg C Troposphere 0‰ 500 Pg C Shallow ocean –50‰ 600 Pg C Deep ocean –170‰ Pg C Land biota –3‰ 1500 Pg C Sediments –1000‰ Pg C 14 N(n,p) 14 C Air-sea gas exchange

Steady-state Δ 14 C traces ocean circulation Orr (OCMIP report); NASA Inferred timescale: ~1000 y (~1/5 of 14 C half- life) PacificAtlantic

Ocean CO 2 uptake The ocean must be responding to the higher atmospheric pCO 2 ; estimated ~2 Pg C/ year net uptake Actual uptake pattern is inferred from measurements of sea- surface pCO 2 + estimates of the gas transfer velocity Air-sea CO 2 flux is the product of (1) the surface-water (minus atmopsheric) CO 2 concentration (reasonably easy to measure sea-surface and atmospheric CO 2 partial pressure) and (2) the gas transfer velocity (hard to measure) LDEO

Wu 1996; Pictures: WHOI Measured gas transfer velocities range widely… Gas transfer velocity, k w, usually plotted against windspeed (roughly correlates w/ surface turbulence) Many other variables known/theorized to be important: wave development, surfactants, rain, air-sea temperature gradient… Several measurement techniques have been used (tracer release, eddy covariance, …) – all imprecise, sometimes seem to give systematically different results What’s the effective mean transfer velocity in a particular region?

..as do parameterizations of k w versus windspeed Common parameterizations assume k w to increase with windspeed v (piecewise) linearly (Liss & Merlivat 1986), quadratically (Wanninkhof 1992) or cubically (Wanninkhof & McGillis 1999) Large differences in implied k w, particularly at high windspeeds (where there are few measurements) –My approach: Are these formulations consistent with ocean tracer ( 14 C) distribution? Feely et al 2001

Idea: Track where the ocean took up carbon-14 Massive production in nuclear tests ca (“bomb 14 C”) The ocean took up ~half of the bomb 14 C by the 1980s The 14 C taken up by the ocean stands up more against the natural background than the fossil-fuel CO 2 taken up, so is easier to measure; The gas transfer velocity would be the same for both processes! bomb spike data: Levin & Kromer 2004; Manning et al 1990; Druffel 1987; Druffel 1989; Druffel & Griffin 1995

Inferring gas transfer from measured ocean bomb 14 C (2) Gas transfer at the turbulent air-sea interface (3) Ocean circulation Hokusai via OceanWorld; Britannica 14 CO 2 [ 14 CO 2 ] aq (1) 14 C in the atmosphere from bombs (4) Measured elevated ocean Δ 14 C: Includes gas transfer + ocean circulation

Modus operandi Simulate ocean 14 C uptake with an ocean- circulation model (MITgcm-ECCO) assuming some gas transfer velocity for each region Compare with ocean Δ 14 C measurements from the 1970s-1990s Adjust gas transfer velocities until the simulated fields match observations (as well as possible!) Krakauer et al., Tellus, 58B(5), (2006)

Previous work on ocean bomb 14 C Broecker and Peng (1985; 1986; 1995) used 1970s (GEOSECS) measurements of 14 C in the ocean to estimate a global ocean inventory and thus the global mean gas transfer velocity (21±3 cm/hr) My work builds on this by –Adding measurements from more recent cruises –Using the spatial distribution of bomb 14 C in the ocean together with a model of transport of 14 C by ocean circulation to estimate not only the global mean transfer velocity but also how it varies regionally

Results: Gas transfer velocities by region Higher transfer velocities in tropical ocean regions than previously assumed (for each map region: top line: velocity estimated from relationship with windspeed bottom lines: inferred from 14 C uptake Less apparent dependence on regional wind speed (dashed line), usually taken to be the factor with the biggest influence on gas transfer velocity, than expected

Implications for ocean CO 2 uptake We can apply the new parameterization of gas exchange to pCO 2 maps: –uptake by the Southern Ocean [high windspeed] is lower than previously calculated (fitting inversion results better) –outgassing near the Equator [low windspeed] is higher (reducing the required tropical land source)

To be determined… Can 14 C uptake help test models based on boundary-layer turbulence theory of what processes control gas exchange beside wind speed? How might climate change affect these processes, and hence ocean uptake of CO 2 ?

Atmospheric Δ 14 C: Mapping CO 2 from fossil-fuel burning

“Inverse modeling” from atmospheric CO 2 patterns Transport +ocean uptake, plant growth Fossil-fuel burning CO 2 concentrations (Carbontracker) Infer emissions, sinks from concentrations?

Why doesn’t inverse modeling work (better)? Limited number of accurate CO 2 measurements Imperfect models of atmospheric transport Large seasonal and interannual variations in carbon flux As a result, the transport model and methodology used strongly affect one’s findings land vs. ocean sinks tropical vs. midlatitude land North America vs. Eurasia etc. “While attractive in principle, this approach is subject to several serious limitations.” -Martin Heimann

Can isotopes help by tracing processes? CO 2  13 C ∆ 14 C

The contemporary budget for atmospheric Δ 14 C Contribution (‰/yr) Biosphere +4 Fossil fuels−10 Cosmogenic +6 Ocean −6 Total −6

Modeled gradients in atmospheric Δ 14 C Fossil-fuel burning Ocean uptake (‰) Forest release For 2004; MATCH-NCEP, CASA biosphere

Hsueh et al., Regional patterns of radiocarbon and fossil fuel-derived CO 2 in surface air across North America, GRL (2007) UCI Growing-season Δ 14 C over the USA from corn leaves

HomeHome > News > Press Releases & Media Advisories > Press Release News Press Releases & Media Advisories Scientists map air pollution using corn grown in U.S. fields New method uses plants to monitor carbon dioxide levels from fossil fuels Irvine, Calif., January 22, 2007 Scientists at UC Irvine have mapped fossil fuel air pollution in the United States by analyzing corn collected from nearly 70 locations nationwide. …

How might Δ 14 C help carbon-flux inversions? basic idea: knowing what the fossil- fuel component is of an observed high CO 2 concentration, we don’t have to model it

1) Transport model testing: what’s the N-S gradient due to fossil-fuel burning? “The ratio of largest fossil fuel interhemispheric difference to smallest IHD is 1.5 …” (Gurney et al 2003) Ongoing analysis of Scripps air archive (Heather Graven) Combine with the time evolution of the N-S Δ 14 C gradient to fix fossil vs. ocean contributions Levin and Hesshaimer 2000

2) Vertical dispersion of fossil fuel CO 2 Comparison of actual with modeled vertical CO 2 profiles suggests that most current models keep too much fossil-fuel CO 2 near the surface, creating a bias in flux estimation from predominantly surface data so that spurious sinks are attributed to fossil- burning regions (Yang et al, GRL; Stephens et al, Science (2007)) Vertical profiles of Δ 14 C enable distinguishing plant from fossil CO 2 fluxes Turnbull et al, GRL (2006)

3) Verification of fossil-fuel burning figures Given transport errors, regional fossil-fuel burning can be estimated from Δ 14 C measurements only to within ~20%; is this ever useful? Time series of Δ 14 C depletion can reveal trends in fossil fuel use more accurately, though, with accuracy of up to perhaps ~5% Levin et al 2003

Conclusion Δ 14 C measurements have helped inform on many aspects of the carbon cycle, and promise to continue to be helpful Precise new air measurement programs will no doubt raise more questions e.g. can we explain the Δ 14 C seasonal cycle with current understanding of carbon sources and transport? ; Meijer et al, Radiocarbon (2006) Stratospheric air? NH fossil carbon? Respired bomb carbon? Fossil emissions/ transport?

Acknowledgements Advisors: Inez Fung, Jim Randerson, Tapio Schneider Δ 14 C measurements and interpretation: Stanley Tyler, Sue Trumbore, Xiaomei Xu, John Southon, Jess Adkins, Paul Wennberg, Yuk Yung, Heather Graven, François Primeau, Dimitris Menemenlis, Nicolas Gruber NOAA, NASA, and the Betty and Gordon Moore Foundation for fellowships

Extra slides

Physical models of air-sea gas exchange Stagnant film model: Air-sea gas exchange is limited by diffusion across a thin (<0.1 mm) water- side surface layer; more turbulent energy  thinner layer Surface renewal model: the surface layer is periodically replaced by new water from below; more turbulent energy  more frequent renewal The air-sea interface becomes complicated (sea spray, bubbles ), and physically poorly understood, in stormy seas Common parameterizations assume k w to increase with windspeed v (piecewise) linearly (Liss & Merlivat 1986), quadratically (Wanninkhof 1992) or cubically (Wanninkhof & McGillis 1999) Large differences in implied k w, particularly at high windspeeds (where there are few measurements) –My approach: Are these formulations consistent with ocean 14 C distribution? J. Boucher, Maine Maritime Academy 100 μm

Windspeed varies by latitude Climatological windspeed estimated from satellite measurements (SSM/I; Boutin and Etcheto 1996)

CO 2 isotope gradients are excellent tracers of air-sea gas exchange Because most (99%) of ocean carbon is ionic and doesn’t directly exchange, CO 2 air-sea gas exchange is slow to restore isotopic equilibrium Thus, the size of isotope disequilibria is uniquely sensitive to the gas transfer velocity k w Ocean carbonate speciation (Feely et al 2001) O2O2 14 d N2ON2O17 d CFC-1121 d CO d C isotopes2926 d Sample equilibration times with the atmosphere of a perturbation in tracer concentration for a 50-m mixed layer ppm μmol/kg

Ocean bomb 14 C uptake: previous work Broecker and Peng (1985; 1986; 1995) used 1970s (GEOSECS) measurements of 14 C in the ocean to estimate the global mean transfer velocity,, at 21±3 cm/hr This value of has been used in most subsequent parameterizations of k w (e.g. Wanninkhof 1992) and for modeling ocean CO 2 uptake Based on trying to add up the bomb 14 C budget, suggestions have been made (Hesshaimer et al 1994; Peacock 2004) are that Broecker and Peng overestimated the ocean bomb 14 C inventory, so that the actual value of might be lower by ~25%

Optimization scheme Assume that k w scales with some power of climatological windspeed u: k w = (u n / ) (Sc/660) -1/2, (where <> denotes a global average, and the Schmidt number Sc is included to normalize for differences in gas diffusivity) find the values of, the global mean gas transfer velocity and n, the windspeed dependence exponent that best fit carbon isotope measurements using transport models to relate measured concentrations to corresponding air-sea fluxes

Results: simulated vs. observed bomb 14 C by latitude – 1970s For a given, high n leads to more simulated uptake in the Southern Ocean, and less uptake near the Equator Observation-based inventories seem to favor low n (i.e. k w increases slowly with windspeed) 1990s (WOCE) observations are also most compatible with a low value of n Simulations for = 21 cm/h and n = 3, 2, 1 or 0 Observation-based estimates (solid lines) from Broecker et al 1995; Peacock 2004

Simulated-observed ocean 14 C misfit as a function of and n The minimum misfit between simulations and (1970s or 1990s) observations is obtained when is close to 21 cm/hr and n is low (1 or below) The exact optimum and n change depending on the misfit function formulation used (letters; cost function contours are for the A cases), but a weak dependence on windspeed (low n) is consistently found

Simulated mid-1970s ocean bomb 14 C inventory vs. and n The total amount taken up depends only weakly on n, so is a good way to estimate The simulated amount at the optimal (square and error bars) supports the inventory estimated by Broecker and Peng (dashed line and gray shading)

Other evidence: atmospheric Δ 14 C I estimated latitudinal differences in atmospheric Δ 14 C for the 1990s, using observed sea-surface Δ 14 C, biosphere C residence times (CASA), and the atmospheric transport model MATCH The Δ 14 C difference between the tropics and the Southern Ocean reflects the effective windpseed dependence (n) of the gas transfer velocity °N°N

Observation vs. modeling The latitudinal gradient in atmospheric Δ 14 C (dashed line) with the inferred and n, though there are substantial uncertainties in the data and models (gray shading) More data? (UCI measurements) Similar results for preindustrial atmospheric Δ 14 C (from tree rings) Also found that total ocean 14 C uptake preindustrially and in the 1990s is consistent with the inferred n (cm/hr) ( ‰ difference, 9°N – 54°S)

Acknowledgements Advisors and collaborators: Jim Randerson, Tapio Schneider, François Primeau, Dimitris Menemenlis, Nicolas Gruber Δ 14 C measurements and interpretation: Stanley Tyler, Sue Trumbore, Xiaomei Xu, John Southon, Jess Adkins, Paul Wennberg, Yuk Yung Inez Fung and the Keck Hydrowatch team NOAA, NASA, and the Betty and Gordon Moore Foundation for fellowships The Earth System Modeling Facility for computing support

Warming is underway Dragons Flight (Wikipedia) Data: Hadley Centre

Global Observing Network

CO 2 Inversions: (1) Forward Step Premise: Atm CO 2 = linear combination of response to each source or sink Divide surface into “basis regions” Specify unitary source (e.g. 1 PgC/month) each month from each region Simulate atm CO 2 “basis” response with atm general circulation model