Updated on 31 May 2013
Global Environment and Marine Department
Values of pH in surface seawater have shown a clear long-term trend of decrease from 3°N and 34°N along the repeat hydrographic line at 137°E in winter (Jan. - Feb.) since 1984. The rate of decrease is approximately 0.01 - 0.02 per decade at each latitude.
Long-term trends of pH at 10, 20 and 30°N in winter (left) and JMA’s repeat hydrographic line at 137°E (right).
The Numbers in the figure on the left indicate rates of change at each latitude. The '±' symbols indicate a 95% confidence interval.
Oceans are the earth’s largest sinks for carbon dioxide (CO2) emitted as a result of human activity, and the chemical properties of surface seawater change due to anthropogenic CO2 uptake. Ocean acidification, which stems from reduced seawater pH (hydrogen ion exponent), is a particular issue of concern because it accelerates global warming by reducing the capacity of oceans to take in CO2 from the atmosphere and affects marine ecosystems by disturbing plankton growth.
According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2007), it is estimated that average global surface seawater pH has decreased by 0.1 due to oceanic uptake of atmospheric CO2 emitted as a result of human activity since pre-industrial times (1750). Moreover, the outcomes of numerical model experiments based on future CO2 emission estimates indicate that surface seawater pH will further decease by 0.14 - 0.35 in the 21st century.
To monitor the long-term variability of ocean-related changes such as global warming and ocean acidification, the Japan Meteorological Agency (JMA) has long conducted oceanographic observations in the western North Pacific. This work has included monitoring to highlight long-term trends of pH in surface seawater from 3°N to 34°N along JMA’s repeat hydrographic line at 137°E in winter since 1984 using data on CO2 concentration in surface seawater and related variables. The results show a clear trend of in-situ pH decrease at each latitude at a rate of 0.013-0.020 per decade.
Time-latitude distribution of pH from 1984 to 2013 along the 137°E line in winter.