Ocean Salinity Trends Show Human Fingerprint

Changes in ocean salinity over the second half of the 20^th Century are consistent with the influence of human activities and inconsistent with natural climate variations, according to a new study led by researchers at Scripps Institution of Oceanography, UC San Diego.

Observed changes agree with computer models’ predictions about how salinity will change in a steadily warming world, said Scripps climate researcher David Pierce, the study’s lead author. Ocean salinity changes are driven by patterns of evaporation and rainfall, which themselves are changing. Observations over recent decades have found a general intensification of salinity differences in which salty ocean regions, most notably in the north Atlantic Ocean, experience even more evaporation of surface waters and relatively fresh regions are becoming even more diluted with precipitation.  These patterns are part of global changes in precipitation and evaporation that influence the amount of rainfall over land.

Pierce said the study provides an independent check of the effects of climate change on the water cycle using different instruments and techniques than weather station rainfall measurements.  Studies of rainfall over land are harder to measure and place in context because of changes to weather stations over the years and the episodic nature of storms.

“The salinity in the ocean averages out all that variability,” said Pierce.

The paper, “The fingerprint of human-induced changes in the ocean’s salinity and temperature fields,” recently appeared in the American Geophysical Union journal Geophysical Research Letters. Co-authors include Peter J. Gleckler, Benjamin Santer, and Paul Durack of the Lawrence Livermore National Laboratory in Livermore, Calif. and Tim Barnett of Scripps Oceanography.

The study builds on previous analyses conducted in the last decade by Barnett, Pierce, and others that demonstrated that rising temperatures in the upper 700 meters (2,000 feet) of the ocean also can be explained only by anthropogenic climate change. The observed global warming is caused mostly by an accumulation of carbon dioxide created by fossil fuel use. 

The new study complements the temperature analysis by considering salinity, the other main factor besides temperature that determines the density of ocean water. Ocean water density is a key factor determining how water moves in the oceans.

“By combining the analysis of salinity and temperature, this study brings our level of understanding global scale oceanic changes to a new level,” said Gleckler.

  The previous temperature studies and this analysis of ocean salinity use a technique known as detection and attribution. In this method, observed trends in ocean salinity are compared to the effects of various historical phenomena such as volcanic eruptions or solar fluctuations and to climate cycles such as El Niño. When the computer climate models were run, the influence of those phenomena do not replicate the salinity or temperature patterns that researchers have observed since 1955. Only when the warming trends associated with human activity were added could the observed salinity trends and temperature changes be explained.

The research performed in this study will likely contribute to the next report of the Intergovernmental Panel on Climate Change, scheduled to be released in phases beginning in 2013.

The U.S. Department of Energy and NOAA funded the research.