This chapter summarizes how climate is changing, why it is changing, and what is projected for the future. While the focus is on changes in the United States, the need to provide context sometimes requires a broader geographical perspective. Additional geographic detail is presented in the regional chapters of this report. Further details on the topics covered by this chapter are provided in the Climate Science Supplement and Frequently Asked Questions Appendices.

The chapter presents 12 key messages about our changing climate, together with supporting evidence for those messages. The discussion of each key message begins with a summary of recent variations or trends, followed by projections of the corresponding changes for the future.

Key Message 12: Ocean Acidification

The oceans are currently absorbing about a quarter of the carbon dioxide emitted to the atmosphere annually and are becoming more acidic as a result, leading to concerns about intensifying impacts on marine ecosystems.

Supporting Evidence

Process for Developing Key Messages

Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment.

Key message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.⁷ The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.⁸ The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.⁹ The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.¹⁰ The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.⁷

The Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection.

Description of evidence base

The key message and supporting text summarize extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.

The oceans currently absorb a quarter of the CO₂ the caused by human activities.¹ Publications have shown that this absorption causes the ocean to become more acidic (for example, Doney et al. 2009²). Recent publications demonstrate the adverse effects further acidification will have on marine life.³^,⁴^,⁵^,⁶

New information and remaining uncertainties

Absorption of CO₂ of human origin, reduced pH, and lower calcium carbonate (CaCO₃) saturation in surface waters, where the bulk of oceanic production occurs, are well verified from models, hydrographic surveys, and time series data.³ The key issue (uncertainty) is how future levels of ocean acidity will affect marine ecosystems.

Assessment of confidence based on evidence

Given the evidence base and uncertainties, confidence is very high that oceans are absorbing about a quarter of emitted CO₂.

Very high for trend of ocean acidification; low-to-medium for intensifying impacts on marine ecosystems. Our present understanding of projected ocean acidification impacts on marine organisms stems largely from short-term laboratory and mesocosm experiments, although there are also examples based on actual ocean observations; consequently, the response of individual organisms, populations, and communities of species to more realistic, gradual changes still has large uncertainties.

Confidence Level

Very High

Strong evidence (established theory, multiple sources, consistent results, well documented and accepted methods, etc.), high consensus

High

Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models incomplete, methods emerging, etc.), competing schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.), disagreement or lack of opinions among experts

Ocean Acidification

As human-induced emissions of carbon dioxide (CO₂) build up in the atmosphere, excess CO₂ is dissolving into the oceans where it reacts with seawater to form carbonic acid, lowering ocean pH levels (“acidification”) and threatening a number of marine ecosystems.² Currently, the oceans absorbs about a quarter of the CO₂ humans produce every year.¹ Over the last 250 years, the oceans have absorbed 560 billion tons of CO₂, increasing the acidity of surface waters by 30%.¹³^,¹¹^,³ Although the average oceanic pH can vary on interglacial timescales,¹³ the current observed rate of change is roughly 50 times faster than known historical change.¹⁴^,¹⁵ Regional factors such as coastal upwelling,¹⁶ changes in discharge rates from rivers and glaciers,¹⁷ sea ice loss,¹⁸ and urbanization¹⁹ have created “ocean acidification hotspots” where changes are occurring at even faster rates.

Figure 2.30: As Oceans Absorb CO₂, They Become More Acidic

As Oceans Absorb CO₂, They Become More Acidic

Mauna Loa Atmospheric CO₂ (ppm)Aloha Ocean pCO₂ in situ (µatm)Aloha Ocean pH (in situ)

Figure 2.30: The correlation between rising levels of CO₂ in the atmosphere (red) at Mauna Loa and rising CO₂ levels (blue) and falling pH (green) in the nearby ocean at Station Aloha. As CO₂ accumulates in the ocean, the water becomes more acidic (the pH declines). (Figure source: modified from Feely et al. 2009¹¹).

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The acidification of the oceans has already caused a suppression of carbonate ion concentrations that are critical for marine calcifying animals such as corals, zooplankton, and shellfish. Many of these animals form the foundation of the marine food web. Today, more than a billion people worldwide rely on food from the ocean as their primary source of protein. Ocean acidification puts this important resource at risk.

Observations have shown that the northeastern Pacific Ocean, including the Arctic and sub-Arctic seas, is particularly susceptible to significant shifts in pH and calcium carbonate saturation levels. Recent analyses show that large areas of the oceans along the U.S. west coast,¹¹^,⁴ the Bering Sea, and the western Arctic Ocean³^,²⁰ will become difficult for calcifying animals within the next 50 years. In particular, animals that form calcium carbonate shells, including corals, crabs, clams, oysters, and tiny free-swimming snails called pteropods, could be particularly vulnerable, especially during the larval stage.²¹^,¹²^,⁵^,⁶

Figure 2.31: Shells Dissolve in Acidified Ocean Water

Shells Dissolve in Acidified Ocean Water

Figure 2.31: Pteropods, or “sea butterflies,” are free-swimming sea snails about the size of a small pea. Pteropods are eaten by marine species ranging in size from tiny krill to whales and are an important source of food for North Pacific juvenile salmon. The photos above show what happens to a pteropod’s shell in seawater that is too acidic. The left panel shows a shell collected from a live pteropod from a region in the Southern Ocean where acidity is not too high. The shell on the right is from a pteropod collected in a region where the water is more acidic (Photo credits: (left) Bednaršek et al. 2012;¹² (right) Nina Bednaršek).

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Projections indicate that in higher emissions pathways, such as SRES A2 or RCP 8.5, current pH could be reduced from the current level of 8.1 to as low as 7.8 by the end of the century.³ Such large changes in ocean pH have probably not been experienced on the planet for the past 100 million years, and it is unclear whether and how quickly ocean life could adapt to such rapid acidification.¹⁴

References

Barton, A., B. Hales, G. G. Waldbusser, C. Langdon, and R. A. Feely, 2012: The Pacific oyster, Crassostrea gigas, shows negative correlation to naturally elevated carbon dioxide levels: Implications for near-term ocean acidification effects. Limnology and Oceanography, 57, 698-710, doi:10.4319/lo.2012.57.3.0698. | Detail ↩
Bednaršek, N., G. A. Tarling, D. C. E. Bakker, S. Fielding, E. M. Jones, H. J. Venables, P. Ward, A. Kuzirian, B. Lézé, R. A. Feely, and E. J. Murphy, 2012: Extensive dissolution of live pteropods in the Southern Ocean. Nature Geoscience, 5, 881-885, doi:10.1038/ngeo1635. | Detail ↩
Caldeira, K., and M. E. Wickett, 2003: Oceanography: Anthropogenic carbon and ocean pH. Nature, 425, 365, doi:10.1038/425365a. | Detail ↩
Doney, S. C., V. J. Fabry, R. A. Feely, and J. A. Kleypas, 2009: Ocean acidification: The other CO₂ problem. Annual Review of Marine Science, 1, 169-192, doi:10.1146/annurev.marine.010908.163834. URL | Detail ↩
Doney, S. C., M. Ruckelshaus, J. E. Duffy, J. P. Barry, F. Chan, C. A. English, H. M. Galindo, J. M. Grebmeier, A. B. Hollowed, N. Knowlton, J. Polovina, N. N. Rabalais, W. J. Sydeman, and L. D. Talley, 2012: Climate change impacts on marine ecosystems. Annual Review of Marine Science, 4, 11-37, doi:10.1146/annurev-marine-041911-111611. URL | Detail ↩
Fabry, V. J., J. B. McClintock, J. T. Mathis, and J. M. Grebmeier, 2009: Ocean acidification at high latitudes: The bellwether. Oceanography, 22, 160-171, doi:10.5670/oceanog.2009.105. URL | Detail ↩
Feely, R. A., S. C. Doney, and S. R. Cooley, 2009: Ocean acidification: Present conditions and future changes in a high-CO₂ world. Oceanography, 22, 36-47, doi:10.5670/oceanog.2009.95. URL | Detail ↩
Feely, R. A., C. L. Sabine, J. M. Hernandez-Ayon, D. Ianson, and B. Hales, 2008: Evidence for upwelling of corrosive “acidified” water onto the continental shelf. Science, 320, 1490-1492, doi:10.1126/science.1155676. URL | Detail ↩
Feely, R. A., S. R. Alin, J. Newton, C. L. Sabine, M. Warner, A. Devol, C. Krembs, and C. Maloy, 2010: The combined effects of ocean acidification, mixing, and respiration on pH and carbonate saturation in an urbanized estuary. Estuarine, Coastal and Shelf Science, 88, 442-449, doi:10.1016/j.ecss.2010.05.004. | Detail ↩
Gruber, N., C. Hauri, Z. Lachkar, D. Loher, T. L. Frölicher, and G. K. Plattner, 2012: Rapid progression of ocean acidification in the California Current System. Science, 337, 220-223, doi:10.1126/science.1216773. URL | Detail ↩
Hönisch, B. et al., 2012: The geological record of ocean acidification. Science, 335, 1058-1063, doi:10.1126/science.1208277. | Detail ↩
Kunkel, K. E. et al., 2013: Monitoring and understanding trends in extreme storms: State of knowledge. Bulletin of the American Meteorological Society, 94, doi:10.1175/BAMS-D-11-00262.1. URL | Detail ↩
Le Quéré, C. et al., 2009: Trends in the sources and sinks of carbon dioxide. Nature Geoscience, 2, 831-836, doi:10.1038/ngeo689. URL | Detail ↩
Mathis, J. T., J. N. Cross, and N. R. Bates, 2011: Coupling primary production and terrestrial runoff to ocean acidification and carbonate mineral suppression in the eastern Bering Sea. Journal of Geophysical Research, 116, C02030, doi:10.1029/2010JC006453. URL | Detail ↩
Orr, J. C., 2011: Recent and future changes in ocean carbonate chemistry. Ocean Acidification, G.J.-P. & Ha L., Ed., Oxford University Press, 41-66. | Detail ↩
Orr, J. C. et al., 2005: Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437, 681-686, doi:10.1038/nature04095. | Detail ↩
Peterson, T. C. et al., 2013: Monitoring and understanding changes in heat waves, cold waves, floods and droughts in the United States: State of knowledge. Bulletin of the American Meteorological Society, 94, 821-834, doi:10.1175/BAMS-D-12-00066.1. URL | Detail ↩
Steinacher, M., F. Joos, T. L. Frölicher, G. - K. Plattner, and S. C. Doney, 2009: Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model. Biogeosciences, 6, 515-533, doi:10.5194/bg-6-515-2009. URL | Detail ↩
Vose, R. S. et al., 2013: Monitoring and understanding changes in extremes: Extratropical storms, winds, and waves. Bulletin of the American Meteorological Society, in press, doi:10.1175/BAMS-D-12-00162.1. URL | Detail ↩
Wuebbles, D. J., G. Meehl, K. Hayhoe, T. R. Karl, K. Kunkel, B. Santer, M. Wehner, B. Colle, E. M. Fischer, R. Fu, A. Goodman, E. Janssen, H. Lee, W. Li, L. N. Long, S. Olsen, A. J. Sheffield, and L. Sun, 2013: CMIP5 climate model analyses: Climate extremes in the United States. Bulletin of the American Meteorological Society, in press, doi:10.1175/BAMS-D-12-00172.1. URL | Detail ↩
Yamamoto-Kawai, M., F. A. McLaughlin, E. C. Carmack, S. Nishino, and K. Shimada, 2009: Aragonite undersaturation in the Arctic ocean: Effects of ocean acidification and sea ice melt. Science, 326, 1098-1100, doi:10.1126/science.1174190. | Detail ↩

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Welcome to the National Climate Assessment

Ocean Acidification

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Ocean Acidification

Convening Lead Authors

Lead Authors

Contributing Authors

Introduction

Key Message 12: Ocean Acidification

Supporting Evidence

Process for Developing Key Messages

Description of evidence base

New information and remaining uncertainties

Assessment of confidence based on evidence

Confidence Level

Very High

High

Medium

Low

Ocean Acidification

Figure 2.30: As Oceans Absorb CO₂, They Become More Acidic

As Oceans Absorb CO₂, They Become More Acidic

Figure 2.31: Shells Dissolve in Acidified Ocean Water

Shells Dissolve in Acidified Ocean Water

References

Welcome to the National Climate Assessment

Ocean Acidification

Menu

Ocean Acidification

Convening Lead Authors

Lead Authors

Contributing Authors

Introduction

Key Message 12: Ocean Acidification

Supporting Evidence

Process for Developing Key Messages

Description of evidence base

New information and remaining uncertainties

Assessment of confidence based on evidence

Confidence Level

Very High

High

Medium

Low

Ocean Acidification

Figure 2.30: As Oceans Absorb CO2, They Become More Acidic

As Oceans Absorb CO2, They Become More Acidic

Figure 2.31: Shells Dissolve in Acidified Ocean Water

Shells Dissolve in Acidified Ocean Water

References

National Climate Assessment

Figure 2.30: As Oceans Absorb CO₂, They Become More Acidic

As Oceans Absorb CO₂, They Become More Acidic