Natalie Mahowald

Scientist II
TIIMES - CGD
BGS

Contact Information:
PO Box 3000, Boulder, CO 80307-3000
Office: ML-202c
Currently on leave at Cornell University
Telephone: 303-497-1719 | Cornell: 607-255-5166
Email: mahowald@ucar.edu | mahowald@cornell.edu
CGD Home Page | Cornell Home Page

Project Summary:

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Anthropocene changes in desert area: Sensitivity to climate model predictions

Natalie M. Mahowald
Accepted for publication in GRL.

Changes in desert area due to humans have important implications from a local, regional to global level. Here I focus on the latter in order to better understand estimated changes in desert dust aerosols and the associated iron deposition into oceans. Using 17 model simulations from the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 multi-model dataset and the BIOME4 equilibrium vegetation model, I estimate changes in desert dust source areas due to climate change and carbon dioxide fertilization. If I assume no carbon dioxide fertilization, the mean of the model predictions is that desert areas expand from the 1880s to the 2080s, due to increased aridity. If I allow for carbon dioxide fertilization, the desert areas become smaller. Thus better understanding carbon dioxide fertilization is important for predicting desert response to climate. There is substantial spread in the model simulation predictions for regional and global averages.

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Global trends in visibility: implications for dust sources

N. M. Mahowald, J. A. Ballantine, J. Feddema, and N. Ramankutty
Atmospheric Chemistry and Physics, 7, 3309-3337, 2007.

There is a large uncertainty in the relative roles of human land use, climate change and carbon dioxide fertilization in changing desert dust source strength over the past 100 years, and the overall sign of human impacts on dust is not known. We used visibility data from meteorological stations in dusty regions to assess the anthropogenic impact on long term trends in desert dust emissions. We did this by looking at time series of visibility derived variables and their correlations with precipitation, drought, winds, land use and grazing. Visibility data are available at thousands of stations globally from 1900 to the present, but we focused on 357 stations with more than 30 years of data in regions where mineral aerosols play a dominant role in visibility observations. We evaluated the 1974 to 2003 time period because most of these stations have reliable records only during this time. We first evaluated the visibility data against AERONET aerosol optical depth data, and found that only in dusty regions are the two moderately correlated. Correlation coefficients between visibility-derived variables and AERONET optical depths indicate a moderate correlation (0.47), consistent with capturing about 20% of the variability in optical depths. Two visibility-derived variables appear to compare the best with AERONET observations: the fraction of observations with visibility less than 5 km (VIS5) and the surface extinction (EXT). Regional trends show that in many dusty places, VIS5 and EXT are statistically significantly correlated with the Palmer drought severity index (based on precipitation and temperature) or surface wind speeds, consistent with dust temporal variability being largely driven by meteorology. This is especially true for North African and Chinese dust sources, but less true in the Middle East, Australia or South America, where there are not consistent patterns in the correlations. Climate indices such as El Nino or the North Atlantic Oscillation are not correlated with visibility-derived variables in this analysis. There are few stations where visibility measures are correlated with cultivation or grazing estimates on a temporal basis, although this may be a function of the very coarse temporal resolution of the land use datasets. On the other hand, spatial analysis of the visibility data suggests that natural topographic lows are not correlated with VIS5 or EXT, but land use is correlated at a moderate level. This analysis is consistent with land use being important in some regions, but meteorology driving interannual variability during 1974–2003.

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Modeled soluble iron deposition and fraction from combustion shown for scenario 1(a & b) (SOLFE_COMB_AP+SOLFE_DUST_AP), scenario 2 (c & d) (SOLFE_COMB_BC+SOLFE_DUST_EM) and scenario 3 (e & f) (sum of scenario 1 & 2).

Combustion Iron Distribution and Deposition

Chao Luo, N. Mahowald, T. Bond, P. Chuang, P. Artaxo,R. Siefert, Y. Chen, J. Schauer
Submitted to GBC.

Iron is hypothesized to be an important micronutrient for ocean biota, thus modulating carbon dioxide uptake by the ocean biological pump. Studies have assumed that atmospheric deposition of iron to the open ocean is predominantly from mineral aerosols. For the first time, we model the source, transport and deposition of iron from combustion sources. Iron is produced in small quantities during fossil fuel burning, incinerator use, and biomass burning. The sources of combustion iron are concentrated in the industrialized regions and biomass burning regions, largely in the tropics. Model results suggest that combustion iron can represent up to 50% of the total iron deposited, but over open ocean regions is usually less than 5% of the total iron, with the highest values (< 30%) close to the East Asian continent in the North Pacific. For ocean biogeochemistry the bioavailability of the iron is important, and this is often estimated by the fraction which is soluble (Fe(II)). Previous studies have argued that atmospheric processing of the relatively insoluble Fe(III) occurs to make it more soluble (Fe(II)). Modeled estimates of soluble iron amounts based solely on atmospheric processing as simulated here cannot match the variability in daily averaged in situ concentration measurements in Korea, which is located close to both combustion and dust sources. The best match to the observations is that there is substantial direct emissions of soluble iron from combustion processes. If we assume observed soluble Fe/black carbon (BC) ratios in Korea are representative of the whole globe, we obtain the result that deposition of soluble iron from combustion contribute 20-100% of the soluble iron deposition over many ocean regions. This implies that more work should be done refining the emissions and deposition of combustion sources of soluble iron globally.

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Atmospheric CO2 growth rate anomalies at 6 selected stations calculated by deseasonalizing and smoothing each time series, calculating the slope and removing the mean...more

Contribution of Ocean, Fossil Fuel, Land Biosphere and Biomass Burning Carbon Fluxes to Seasonal and Interannual Variability in Atmospheric CO2

Cynthia D. Nevison, Natalie M. Mahowald, Scott C. Doney, Ivan D. Lima, Guido R. van der Werf, James T. Randerson, David F. Baker, Prasad Kasibhatla and Galen A. McKinley
Accepted in Journal of Geophysical Research, Biogeochemistry

The seasonal and interannual variability in atmospheric carbon dioxide (CO2)  concentrations is simulated using best available model estimates of surface carbon fluxes  and a tracer transport model that incorporates interannual variability (IAV) in transport. The atmospheric CO2 variability resulting from these surface fluxes is compared to observations from 89 GLOBALVIEW monitoring stations. At northern hemisphere stations, the model is generally able to capture the observed seasonal cycle in atmospheric CO2, which is dominated by the land tracer. The ocean tracer has a seasonal amplitude only ~10%, on average, of and tends to be out of phase with the observed cycle at these stations. Model and observed CO2 growth anomalies are moderately well correlated in the northern hemisphere (R ~0.4-0.8), but the correlation is less significant in the southern hemisphere (R < 0.6). Land dominates IAV in the northern hemisphere, and biomass burning in particular can account for most of the strong positive CO2 growth anomaly observed during the 1997-1998 ENSO event. The signals in atmospheric CO2 from the terrestrial biosphere extend throughout the southern hemisphere, but oceanic fluxes also exert a strong influence there, accounting for roughly half of the variability at many extratropical stations. However, the modeled ocean tracer is generally uncorrelated  to observations from 1979-2004, even in the southern hemisphere, with one exception during the weak El Nino/post-Pinatubo period of the early 1990s. The model suggests that the ocean may have accounted for 20-25% of the slowdown in the atmospheric CO2 growth rate observed during that time.

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Seasonal cycles at APO^oc at 12 selected stations ...more

Variability in air-sea O2 and CO2 fluxes and its impact on atmospheric potential oxygen (APO) and the partitioning of land and ocean carbon sinks

Cynthia D. Nevison, Natalie M. Mahowald, Scott C. Doney, and Ivan D. Lima
Submitted to Biogeochemistry

A three dimensional, time-evolving field of atmospheric potential oxygen (APO ~ O2/N2
+ CO2) is estimated using surface O2, N2 and CO2 fluxes from the WHOI ocean ecosystem model to force the MATCH atmospheric transport model. Land and fossil carbon fluxes are also run in MATCH and translated into O2 tracers using assumed
O2:CO2 stoichiometries. The model seasonal cycles in APO agree well with the observed cycles at 13 global monitoring stations, with agreement helped by the inclusion of oceanic CO2 in the APO calculation. The model latitudinal gradient in APO is strongly influenced by seasonal rectifier effects in atmospheric transport, which appear at least partly unrealistic based on comparison to observations. An analysis of the APO vs. CO2 method for partitioning land and ocean carbon sinks is performed in the controlled context of the MATCH simulation, in which the true surface carbon and oxygen fluxes are known exactly. This analysis suggests uncertainty ranging up to ± 0.2 PgC in the inferred sinks due to transport-induced variability. It also shows that interannual variability in oceanic O2 fluxes can cause increasingly large error in the sink partitioning when the method is applied over increasingly short timescales. However, when decadal or longer averages are used, the variability in the oceanic O2 flux is relatively small, allowing carbon sinks to be partitioned to within a standard deviation of 0.1 Pg C/yr of the true values, provided one has an accurate estimate of long-term mean O2 outgassing.

Fellowship:

• Marie Tharp, Columbia University, August-December 2006.

Community Service:

• Co-Chair - Young Scholar's network, Analysis Integration and modeling of earth system
• Member - Analysis Integration and Modeling of earth system, Internation Geosphere-Biosphere Project
• Thesis Committee: John-Andrew Ballentine, , PhD, UCSB, Santa Barbara, CA USA

Presentations:

• Desert dust and North African Precipitation (invited at Spring AGU), Acapulco MEX, May 2007
• Desert dust and North African Precipitation, New York USA, March 2007
• Global trends in visibility: Implications for dust sources (invited at Spring AGU), Acapulco MEX, May 2007

TIIMES External Collaborators:

Paulo Artaxo, University of São Paulo
Alex Baker, University of East Anglia
Mark Battle, Bowdoin College
Rob Bryant, The University of Sheffield
Doug Capone, University of Southern California
Oliver Chadwick, University of California, Santa Barbara
Scott Doney, Woods Hole Oceanographic Institution (WHOI)
Jean-Louis Dufresne, Centre national de la recherche scientifique (CNRS) - Université de Jussieu
Inez Fung, University of California, Berkeley
Catherine Gautier, University of California, Santa Barbara
Jenny Hand, Colorado State University
Tim Jickells, University of East Anglia
Charles Jones, University of California, Santa Barbara
Daniel Kirk-Davidoff, University of Maryland
Tony Michaels, University of Southern California
Keith Moore, University of California, Irvine
Daniel Muhs, United States Geological Survey (USGS)
Greg Okin, University of Virginia
Thomas Painter, University of Colorado
Alan Plumb, Massachusetts Institute of Technology
James Randerson, University of California, Irvine
Dar Roberts, University of California, Santa Barbara
Dennis Savoie, University of Miami
Ron Siefert, University of Maryland
Dave Siegel, University of California, Santa Barbara
Omar Torres, Goddard Space Flight Center (GSFC) - NASA
Alan Townsend, University of Colorado
Ric Williams, University of Liverpool
Charlie Zender, University of California, Irvine

Publications:

Thornton, P. E., J. F. Lamarque, N. A. Rosenbloom, N. Mahowald, 2007: Inclusion of carbon-nitrogen feedback fundamentally changes response of land carbon model to CO2 fertilization and climate variability. Global Biogeochemical Cycles. (In Press)

Yoshioka, M., N. M. Mahowald, A. J. Conley, W. D. Collins, D. W. Fillmore, C. S. Zender, D. B. Coleman, 2007: Impact of desert dust radiative forcing on Sahel precipitation: Relative importance of dust compared to sea surface temperature variations, vegetation changes and greenhouse gas warming. J. Climate, 20, 1445-1467, doi: 10.1175/JCL14056.1.

Patra, P. K., J. K. Moore, N. Mahowald, M. Uematsu, S. C. Doney, T. Nakazawa, 2007: Exploring the sensitivity of interannual basin-scale air-sea CO2 fluxes to variability in atmospheric dust deposition using ocean carbon cycle models and atmospheric CO2 inversions. J. Geophys. Res., 112, G02012, doi: 10.1029/2006JG000236.

Patra, P. K., M. D. Kumar, N. Mahowald, V. S. Sarma, 2007: Atmospheric deposition and surface stratification as controls of contrasting chlorophyll abundance in the North Indian Ocean. J. Geophys. Res., 112, C05029, doi: 10.1029/2006JC003885.

Bryant, R., G. Bigg, N. Mahowald, F. Eckardt, S. Ross, 2007: Dust emission response to climate in southern Africa. J. Climate, 112, D09207, doi: 10.1029/2005JD007025.