Climate Change Indicators in the United States

• Measurements made over the last few decades have demonstrated that ocean carbon dioxide levels have risen in response to increased carbon dioxide in the atmosphere, leading to an increase in acidity (that is, a decrease in pH) (see Figure 1).
• Historical modeling suggests that since the 1880s, increased carbon dioxide has led to lower aragonite saturation levels (less availability of minerals) in the oceans around the world (see Figure 2).
• The largest decreases in aragonite saturation have occurred in tropical waters (see Figure 2). However, decreases in cold areas may be of greater concern because colder waters typically have lower aragonite levels to begin with. ³

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. Because of the slow mixing time between surface waters and deeper waters, it can take hundreds to thousands of years to establish this balance. Over the past 250 years, oceans have absorbed approximately 40 percent of the carbon dioxide produced by human activities. ¹

Although the ocean's ability to take up carbon dioxide prevents atmospheric levels from climbing even higher, rising levels of carbon dioxide dissolved in the ocean can have a negative effect on marine life. Carbon dioxide reacts with sea water to produce carbonic acid. The resulting increase in acidity (measured by lower pH values) reduces the availability of minerals such as aragonite, which is a form of calcium carbonate that corals, some types of plankton, and other creatures rely on to produce their hard skeletons and shells. Declining pH and reduced availability of minerals can make it more difficult for these animals to thrive. This can lead to broader changes in the overall structure of ocean and coastal ecosystems, and can ultimately affect fish populations and the people who depend on them. ²

While changes in ocean pH and mineral availability caused by the uptake of atmospheric carbon dioxide generally occur over many decades, these properties can fluctuate over shorter periods, especially in coastal and surface waters. For example, increased photosynthesis and respiration during the day and during the summer leads to natural fluctuations in pH. Acidity also varies with water temperature.

This indicator describes trends in pH and related properties of ocean water, based on a combination of direct observations, calculations, and modeling.

Figure 1 shows pH values and levels of dissolved carbon dioxide at three locations that have collected measurements consistently over the last few decades. These data have been either measured directly or calculated from related measurements such as dissolved inorganic carbon and alkalinity. Data come from two stations in the Atlantic Ocean (Bermuda and the Canary Islands) and one in the Pacific (Hawaii).

The global map in Figure 2 shows changes over time in the amount of aragonite dissolved in ocean water, which is called aragonite saturation. This map was created by comparing average conditions during the 1880s with average conditions during the most recent 10 years (2003-2012). Aragonite saturation has only been measured at selected locations during the last few decades, but it can be calculated reliably for different times and locations based on the relationships scientists have observed among aragonite saturation, pH, dissolved carbon, water temperature, concentrations of carbon dioxide in the atmosphere, and other factors that can be measured. Thus, while Figure 2 was created using a computer model, it is indirectly based on actual measurements.

This indicator focuses on surface waters, which absorb carbon dioxide from the atmosphere within a few months. ⁹ It can take much longer for changes in pH and mineral saturation to spread to deeper waters, so the full effect of increased atmospheric carbon dioxide concentrations on ocean acidity may not be seen for many decades, if not centuries. Studies suggest that the impacts of ocean acidification may be greater at depth, because the availability of minerals like aragonite is naturally lower in deeper waters. ¹⁰

Ocean chemistry is not uniform around the world, so local conditions can cause pH or aragonite saturation measurements to differ from the global average. For example, carbon dioxide dissolves more readily in cold water than in warm water, so colder regions could experience greater impacts from acidity than warmer regions.

Data for Figure 1 came from three studies: the Bermuda Atlantic Time-Series Study, the European Station for Time-Series in the Ocean (Canary Islands), and the Hawaii Ocean Time-Series. Bermuda data were analyzed by Bates et al. (2012) ¹¹ and are available at: http://bats.bios.edu/. Canary Islands data were analyzed by González-Dávila et al. (2010) ¹² and are available at: www.eurosites.info/estoc/ data.php. Hawaii data were analyzed by Dore et al. (2009) ¹³ and are available at: http://hahana.soest.hawaii
.edu/hot/products/products
.html.

The map in Figure 2 was created by the National Oceanic and Atmospheric Administration and the Woods Hole Oceanographic Institution using Community Earth System Model data. Related information can be found at: http://sos.noaa.gov/Datasets
/list.php?category=Ocean.

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