University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Publications, Agencies and Staff of the .SU . U.S. Department of Commerce Department of Commerce 2017 Ocean changes – warming, stratification, circulation, acidification, and deoxygenation Libby Jewett NOAA Ocean Acidification Program Anastasia Romanou Columbia University, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/usdeptcommercepub Jewett, Libby and Romanou, Anastasia, "Ocean changes – warming, stratification, circulation, acidification, and deoxygenation" (2017). Publications, Agencies and Staff of ht e U.S. Department of Commerce. 580. https://digitalcommons.unl.edu/usdeptcommercepub/580 This Article is brought to you for free and open access by the U.S. Department of Commerce at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Publications, Agencies and Staff of the .SU . Department of Commerce by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Ocean changes – warming, stratification, circulation, acidification, and deoxygenation Libby Jewett, NOAA Ocean Acidification Program Anastasia Romanou, Columbia University Citation: In: Climate Science Special Report: A Sustained Assessment Activity of the U.S. Global Change Research Program [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA (2017), pp. 540-583. Comments: U.S. Government work Abstract 1. The world’s oceans have absorbed about 93% of the excess heat caused by greenhouse gas warming since the mid-20th century, making them warmer and altering global and regional climate feedbacks. Ocean heat content has increased at all depths since the 1960s and surface waters have warmed by about 1.3° ± 0.1°F (0.7° ± 0.08°C) per century globally since 1900 to 2016. Under a high emissions scenario, a global increase in average sea surface temperature of 4.9° ± 1.3°F (2.7° ± 0.7°C) by 2100 is projected, with even higher changes in some U.S. coastal regions. (Very high confidence) 2. The potential slowing of the Atlantic Meridional Overturning Circulation (AMOC) (of which the Gulf Stream is one component)—as a result of increasing ocean heat content and freshwater driven buoyancy changes—could have dramatic climate feedbacks as the ocean absorbs less heat and CO2 from the atmosphere. This slowing would also affect the climates of North America and Europe. Any slowing documented to date cannot be directly tied to anthropogenic forcing primarily due to lack of adequate observational data and to challenges in modeling ocean circulation changes. Under a high emissions scenario (RCP8.5) in CMIP5 simulations, it is likely that the AMOC will weaken over the 21st century by 12% to 54%. (Low confidence) 3. The world’s oceans are currently absorbing more than a quarter of the CO2 emitted to the atmosphere annually from human activities, making them more acidic (very high confidence), with potential detrimental impacts to marine ecosystems. In particular, higher-latitude systems typically have a lower buffering capacity against pH change, exhibiting seasonally corrosive conditions sooner than low-latitude systems. Acidification is regionally increasing along U.S. coastal systems as a result of upwelling (for example, in the Pacific Northwest) (high confidence), changes in freshwater inputs (for example, in the Gulf of Maine) (medium confidence), and nutrient input (for example, in urbanized estuaries) (high confidence). The rate of acidification is unparalleled in at least the past 66 million years (medium confidence). Under RCP8.5, the global average surface ocean acidity is projected to increase by 100% to 150% (high confidence). 4. Increasing sea surface temperatures, rising sea levels, and changing patterns of precipitation, winds, nutrients, and ocean circulation are contributing to overall declining oxygen concentrations at intermediate depths in various ocean locations and in many coastal areas. Over the last half century, major oxygen losses have occurred in inland seas, estuaries, and in the coastal and open ocean (high confidence). Ocean oxygen levels are projected to decrease by as much as 3.5% under the RCP8.5 scenario by 2100 relative to preindustrial values (high confidence). CSSR 5OD: FINAL CLEARANCE Chapter 13 1 13. Ocean Acidification and Other Ocean Changes 2 KEY FINDINGS 3 1. The world’s oceans have absorbed about 93% of the excess heat caused by greenhouse 4 gas warming since the mid-20th century, making them warmer and altering global and 5 regional climate feedbacks. Ocean heat content has increased at all depths since the 1960s 6 and surface waters have warmed by about 1.3° ± 0.1°F (0.7° ± 0.08°C) per century 7 globally since 1900 to 2016. Under a high emissions scenario, a global increase in 8 average sea surface temperature of 4.9° ± 1.3°F (2.7° ± 0.7°C) by 2100 is projected, with 9 even higher changes in some U.S. coastal regions. (Very high confidence) 10 2. The potential slowing of the Atlantic Meridional Overturning Circulation (AMOC) (of 11 which the Gulf Stream is one component)—as a result of increasing ocean heat content 12 and freshwater driven buoyancy changes—could have dramatic climate feedbacks as the 13 ocean absorbs less heat and CO2 from the atmosphere. This slowing would also affect the 14 climates of North America and Europe. Any slowing documented to date cannot be 15 directly tied to anthropogenic forcing primarily due to lack of adequate observational data 16 and to challenges in modeling ocean circulation changes. Under a high emissions 17 scenario (RCP8.5) in CMIP5 simulations, it is likely that the AMOC will weaken over 18 the 21st century by 12% to 54%. (Low confidence) 19 3. The world’s oceans are currently absorbing more than a quarter of the CO2 emitted to the 20 atmosphere annually from human activities, making them more acidic (very high 21 confidence), with potential detrimental impacts to marine ecosystems. In particular, 22 higher-latitude systems typically have a lower buffering capacity against pH change, 23 exhibiting seasonally corrosive conditions sooner than low-latitude systems. 24 Acidification is regionally increasing along U.S. coastal systems as a result of upwelling 25 (for example, in the Pacific Northwest) (high confidence), changes in freshwater inputs 26 (for example, in the Gulf of Maine) (medium confidence), and nutrient input (for 27 example, in urbanized estuaries) (high confidence). The rate of acidification is 28 unparalleled in at least the past 66 million years (medium confidence). Under RCP8.5, the 29 global average surface ocean acidity is projected to increase by 100% to 150% (high 30 confidence). 31 4. Increasing sea surface temperatures, rising sea levels, and changing patterns of 32 precipitation, winds, nutrients, and ocean circulation are contributing to overall declining 33 oxygen concentrations at intermediate depths in various ocean locations and in many 34 coastal areas. Over the last half century, major oxygen losses have occurred in inland 35 seas, estuaries, and in the coastal and open ocean (high confidence). Ocean oxygen levels 36 are projected to decrease by as much as 3.5% under the RCP8.5 scenario by 2100 relative 37 to preindustrial values (high confidence). Subject to Final Copyedit 540 28 June 2017 CSSR 5OD: FINAL CLEARANCE Chapter 13 1 13.0 A Changing Ocean 2 Anthropogenic perturbations to the global Earth system have included important alterations in 3 the nutrient composition, temperature, and circulation of the oceans. Some of these changes will 4 be distinguishable from the background natural variability in nearly half of the global open ocean 5 within a decade, with important consequences for marine ecosystems and their services (Gattuso 6 et al. 2015). However, the timeframe for detection will vary depending on the parameter featured 7 (Henson et al 2010; Henson et al 2016). 8 13.1 Ocean Warming 9 13.1.1 General Background 10 Approximately 93% of excess heat energy trapped since the 1970s has been absorbed into the 11 oceans, lessening atmospheric warming and leading to a variety of changes in ocean conditions, 12 including sea level rise and ocean circulation (see Ch. 2: Physical Drivers of Climate Change, 13 Ch. 6: Temperature Change, and Ch. 12: Sea Level Rise in this report; Rhein et al. 2013; Gattuso 14 et al. 2015). This is the result of the high heat capacity of seawater relative to the atmosphere, the 15 relative area of the ocean compared to the land, and the ocean circulation that enables the 16 transport of heat into deep waters. This large heat absorption by the oceans moderates the effects 17 of increased anthropogenic greenhouse emissions on terrestrial climates while altering the 18 fundamental physical properties of the ocean and indirectly impacting chemical properties such 19 as the biological pump through increased stratification (Gattuso et al. 2015; Rossby 1959). 20 Although upper ocean temperature varies over short- and medium timescales (for example, 21 seasonal and regional patterns), there are clear long-term increases in surface temperature and 22 ocean heat content over the past 65 years (Cheng et al. 2017; Rhein et al. 2013; Levitus et al. 23 2012). 24 13.1.2 Ocean Heat Content 25 Ocean heat content (OHC) is an ideal variable to monitor changing climate as it is calculated 26 using the entire water column, so ocean warming can be documented and compared between 27 particular regions, ocean basins, and depths. However, for years prior to the 1970s, estimates of 28 ocean uptake are confined to the upper ocean due to sparse spatial and temporal coverage and 29 limited vertical capabilities of many of the instruments in use. Ocean heat content estimates are 30 improved for time periods after 1970 with increased sampling coverage and depth (Abraham et 31 al. 2013; Rhein et al.