Hot topics in a warming ocean – How acoustical oceanography can help monitor climate change
 
Gabriel R. Venegas – gvenegas@arlut.utexas.edu
 
Applied Research Laboratories, The University of Texas at Austin
 
10000 Burnet Rd
 
Austin, TX 78758
 
Popular version of paper 3pID3
 
Presented Wednesday afternoon, December 4, 2019. 1:45pm-2:05pm
 
178th ASA Meeting, San Diego, CA
 
Sound is an effective way to study the ocean by non-invasively and quickly surveying large areas, and acoustical oceanography has lent an extra pair of ears to help scientists monitor climate change. This talk will showcase the work of some of the many acoustical oceanographers in the Acoustical Society of America (ASA) that have made valuable contributions to aid in climate change related monitoring, in the hope of inspiring other members to think of new potential acoustic monitoring applications.
 
Heat
The planet is warming and so are its oceans. This warming causes the seawater to expand and large volumes of ice to break off from glaciers and melt in the ocean, ultimately resulting in sea level rise. An acoustic technique called passive acoustic thermometry1,2 takes the noise created by these calving events at the north and south poles to calculate the speed of sound averaged over path lengths as long as 132 km. Temperature can then be inferred from sound speed using a well-established formula relating the two quantities.
 
As the glaciers melt, they release tiny compressed air bubbles that make loud popping sounds underwater.3 If these popping sounds can be reasonably characterized at one or many glacial bays, at a safe distance, these sounds can be used to estimate the glacial melt rate.4,5
 
An increase in ocean temperature also causes methane hydrate, a material in ocean sediments that can store large amounts of methane, to turn from solid to greenhouse gas, which bubbles up from the seafloor and is ultimately released into the atmosphere. The sound of these bubbles has also been exploited to estimate the volume of methane released from hydrates and seeps.6–8
 
CO2
Global CO2 concentrations are higher than they have been over the last 800,000.9 A quarter of this gas is absorbed into the ocean and has caused the what is thought to be the fastest increase in ocean acidity in the last 60 million years.10 An increase in ocean temperature, actually decreases the ocean’s capacity to store CO2, causing it to be released back into the atmosphere. The relationship between ocean acidity and the absorption of sound is well understood. A passive acoustic technique using the sound of wind over the water is being investigated to estimate the absorption and thus ocean acidity.11
 
Ocean acidity also causes damage to many coastal ecosystems including valuable “blue carbon” stores such as mangroves, salt marshes, and seagrasses, which store 50% of the ocean’s organic carbon.12 The destruction of these carbon stores can also release CO2 back into the atmosphere. An ultrasonic sensor that will improve organic carbon estimates in these ecosystems is currently under development.13 These climate-altering feedback loops can cause rapid and catastrophic consequences for future generations, and should be the responsibility of all scientists, elected officials, and the general public, alike.
 
References
 
1K. F. Woolfe, S. Lani, K. G. Sabra, and W. A. Kuperman, “Monitoring deep-ocean temperatures using acoustic ambient noise,” Geophys. Res. Lett. 42, 2878-2884 (2015); https://doi.org/10.1002/2015GL063438
2K. G. Sabra, B. Cornuelle, W. A. Kuperman, “Sensing deep-ocean temperatures,” Physics Today 69, 32-38 (2016). https://doi.org/10.1063/PT.3.3080.
3R. J. Urick, “The noise of melting icebergs,” J. Acoust. Soc. Am. 50, 337-341, (1971); https://doi.org/10.1121/1.1912637
4E. C. Pettit, K. M. Lee, J. P. Brann, J. A. Nystuen, P. S. Wilson, S. O’Neel, “Unusually loud ambient noise in tidewater glacier fjords: A signal of ice melt,” Geophys. Res. Ltt. 42, 2309-2316 (2015); https://doi.org/10.1002/2014GL062950
5O. Glowacki, G. B. Deane, and M. Moskalik, “The intensity, directionality, and statistics of underwater noise from melting icebergs,” Geophys. Res. Ltt., 45, 4105–4113 (2018); https://doi.org/10.1029/2018GL077632
6C. A. Green, P. S. Wilson, “Laboratory investigation of a passive acoustic method for measurement of underwater gas seep ebullition,” J. Acoust. Soc. Am. 131, EL61 (2012); https://doi.org/10.1121/1.3670590
7T. G. Leighton and P. R. White, “Quantification of undersea gas leaks from carbon capture and storage facilities, from pipelines and from methane seeps, by their acoustic emissions,” Proc. R. Soc. A 468, 485-510 (2012); https://doi.org/10.1098/rspa.2011.0221
8T. C. Weber, L. Mayer, K. Jerram, J. Beaudoin, Y. Rzhanov, D. Lovalvo, “Acoustic estimates of methane gas flux from the seabed in a 6000 km2 region in the Northern Gulf of Mexico,” Geochem. Geophys. Geosys. 15, 1911-1925 (2014); https://doi.org/10.1002/2014GC005271j
9D. Lüthi, M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, U. Siegenthaler, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T. F. Stocker, “High-resolution carbon dioxide concentration record 650,000–800,000 years before present,” Nature 453, 379-382 (2008); https://doi.org/10.1038/nature06949
10C. Turley and J.-P. Gattuso, “Future biological and ecosystem impacts of ocean acidification and their socioeconomic-policy implications,” Curr. Opin. Environ. Sustain. 4, 278-286 (2012); https://doi.org/10.1016/j.cosust.2012.05.007
11D. R. Barclay and M. J. Buckingham, “A passive acoustic measurement of ocean acidity (A),” Conference & Exhibition Series on Underwater Acoustics, 5, 941 (2019).
12J. Howard, S. Hoyt, K. Isensee, E. Pidgeon, M. Telszewski (eds.). Coastal Blue Carbon: Methods for assessing carbon stocks and emissions factors in mangroves, tidal salt marshes, and seagrass meadows. Conservation International, Intergovernmental Oceanographic Commission of UNESCO, International Union for Conservation of Nature. Arlington, Virginia, USA. (2014).
13G. R. Venegas, A. F. Rahman, K. M. Lee, M. S. Ballard, P. S. Wilson, “Toward the Ultrasonic Sensing of Organic Carbon in Seagrass‐Bearing Sediments,” Geophys. Res. Ltt. 46, 5968-5977 (2019); https://doi.org/10.1029/2019GL082745
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