Changes in ocean salinity over the second half of the 20th 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 suggestions about salinity trends in a steadily warming world, said Scripps climate researcher David Pierce, the study's lead author. Ocean salinity changes are driven by the world's 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 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 rainfall or the lack of it over land.
Pierce said the significance of the study is that it 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,"appears Nov. 2 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 only be explained by anthropogenic climate change. The observed global warming is caused mostly by an accumulation of carbon dioxide created by fossil fuel use.
This research complements the temperature analysis by considering salinity, the other main factor 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.
Salinity and temperature trends corresponded closely to signals related to anthropogenic global warming but not to natural climate influences alone
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.
Scripps Institution of Oceanography at the University of California, San Diego, is one of the oldest, largest, and most important centers for global science research and education in the world. Now in its second century of discovery, the scientific scope of the institution has grown to include biological, physical, chemical, geological, geophysical, and atmospheric studies of the earth as a system. Hundreds of research programs covering a wide range of scientific areas are under way today on every continent and in every ocean. The institution has a staff of more than 1,400 and annual expenditures of approximately $195 million from federal, state, and private sources. Scripps operates robotic networks and one of the largest U.S. academic fleets. Birch Aquarium at Scripps serves as the interpretive center of the institution and showcases Scripps research and a diverse array of marine life through exhibits and programming for more than 430,000 visitors each year. Learn more at scripps.ucsd.edu.
About UC San Diego
The University of California, San Diego is a student-centered, research-focused, service-oriented public institution that provides opportunity for all. Recognized as one of the top 15 research universities worldwide and born of a culture of collaboration, UC San Diego sparks discoveries that advance society, drive economic growth and positively impact the world. Our students, who learn from Nobel laureates, MacArthur Fellows and National Academy members, are committed to public service. For the sixth consecutive year, UC San Diego has been ranked first in the nation based on research, civic engagement and social mobility. We are one campus with multiple pillars of excellence, a top ten public university that is transforming lives, shaping new disciplines and advancing the frontiers of knowledge. Learn more at www.ucsd.edu.