Climate Change
Climate Change Indicators in the United States
This figure shows changes in ocean carbon dioxide levels (measured as a partial pressure) and acidity (measured as pH). The data come from two observation stations in the North Atlantic Ocean (Canary Islands and Bermuda) and one in the Pacific (Hawaii). Dots represent individual measurements, while the lines represent smoothed trends.
Data source: Bindoff et al., 2007 4
This figure shows changes in ocean pH levels around the world from pre-industrial times to the present based on modeled data.
Data source: Yool, 2007 5
Acidity is commonly measured using the pH scale. Pure water has a pH of about 7, which is considered neutral. A substance with a pH less than 7 is acidic, while a substance with a pH greater than 7 is basic or alkaline. The lower the pH, the more acidic the substance. The pH scale is based on powers of 10, which means a substance with a pH of 3 is 10 times more acidic than a substance with a pH of 4. For more information about pH, visit: www.epa.gov/acidrain/measure/ph.html.
Source: Environment Canada, 2008 1
Key Points
- Measurements made over the last few decades have demonstrated that ocean carbon dioxide levels have risen, accompanied by an increase in acidity (that is, a decrease in pH) (see Figure 1).
- Modeling suggests that over the last few centuries, ocean acidity has increased globally (meaning pH has decreased), most notably in the Atlantic (see Figure 2).
- Direct observations show that pH levels fluctuate more frequently in some areas of the ocean than in others. 3 More measurements are needed to better understand the links between these natural fluctuations and long-term changes in ocean acidity.
Background
The ocean plays an important role in regulating the amount of carbon dioxide in the atmosphere. As atmospheric concentrations of carbon dioxide rise (see the Atmospheric Concentrations of Greenhouse Gases indicator), the ocean absorbs more carbon dioxide to stay in balance. Because of the slow mixing time of the ocean compared with the atmosphere, it can take hundreds of years to establish a balance between the atmosphere and the ocean.
Although the ocean's ability to take up carbon dioxide is a positive attribute with respect to mitigating climate change, these reactions can have a negative effect on marine life. Carbon dioxide from the atmosphere reacts with sea water to produce carbonic acid. Increasing acidity (measured by lower pH values) reduces the availability of chemicals needed to make calcium carbonate, which corals, some types of plankton, and other creatures rely on to produce their hard skeletons and shells. The effect of declining pH on shell-producing ocean organisms can cause changes in overall ecosystem structure in coastal ecosystems. 2
While changes in ocean pH caused by the uptake of atmospheric carbon dioxide generally occur over long periods of time, some fluctuation in pH can occur over shorter periods, especially in coastal and surface waters. Increased photosynthesis during the day and during summer months, for example, leads to natural fluctuations in pH.
About the Indicator
This indicator presents ocean pH values based on direct observations and modeling. Scientists have only begun to directly measure ocean carbon dioxide and related variables (dissolved organic carbon, alkalinity, and pH) on a global scale during the last few decades.
While direct observations are important in monitoring recent ocean acidity changes, it is even more important to examine trends over longer time spans, given the slow rate at which sea water balances with atmospheric carbon dioxide. Because of the lack of historical observation data, modeling has been used to make comparisons between pre-industrial times and the present.
Indicator Confidence
Changes in ocean pH caused by the uptake of atmospheric carbon dioxide tend to occur slowly relative to natural fluctuations, so the full effect of atmospheric carbon dioxide concentrations on ocean pH may not be seen for many decades, if not centuries.
Ocean chemistry is not uniform throughout the world's oceans, so local conditions could cause a pH measurement to seem incorrect or abnormal in the context of the global data.
Data Sources
Data for Figure 1 came from three ocean time series studies: the Bermuda Atlantic Time-series Study, the Hawaii Ocean Time-series, and the European Station for Time-series in the Ocean (Canary Islands). Bermuda data were analyzed by Bates et al. (2002) 6 and Gruber et al. (2002). 7 Hawaii data were analyzed by Dore et al. (2003), 8 and Canary Islands data were analyzed by Gonzalez-Davila et al. (2003). 9 Bermuda and Hawaii data are available at: www1.whoi.edu. The map in Figure 2 was created using Global Ocean Data Analysis Project data, and the figure was provided by the Pacific Science Association Task Force on Ocean Acidification. This map and other information are available at: www.pacificscience.org/ tfoceanacidification.html.
Indicator Documentation
- Download related technical information PDF (6 pp, 62K)