Download IPCC AR5: Chapter 3: Observations: Ocean

IPCC AR5: Chapter 3: Observations: Ocean
Abstract by Sherry Young and Gus Robertson
Fire & Water, The Evergreen St. College, Fall 2013, Zita & Chin-Leo
Global sea level is rising as the ocean continues to heat up. Most of the warming and expansion
is occurring in the upper ocean, which is in direct contact with the warming atmosphere and solar
radiation, while deeper waters are affected at a much slower rate. The ocean has also become
more acidic from the absorption of excess carbon dioxide produced by human activities. As
humans burn fossil fuels, the resultant greenhouse gases warm the ocean and atmosphere, acidify
the ocean, increase surface water evaporation and atmospheric water vapor, and strengthen the
hydrologic cycle, among other effects.
The concentration of salt in the world’s oceans is changing since the 1950s. In mid-latitude
regions surface ocean salinity has increased. In tropical and polar regions, salinity has
decreased. This may be due to increased transport of moisture in the atmosphere. It is likely that
increased fresh water and increased surface warming have contributed to both midlatitude
salinity increases, and decreases in salinity in tropical regions (e.g. from increased rainfall) and
polar regions (due to ice melt).
Wind stress data is the strongest in the southern oceans, where the average wind stress has
increased since 1980. Wind stress is important because it can affect mixing and tropical storm
strength. (Ch.14, Regional Climate Phenomena, discusses increasing tropical storm strength.)
The average wave height in the northern hemisphere has increased by 20 centimeters, above 45
degrees latitude.
Gradients in salinity and temperature are the main drivers of ocean circulation. The increasing
warming in the upper ocean is causing slight changes in circulation and transport of salinity and
heat. The warmer surface layer of the ocean is expanding significantly. This raises concern about
the ability of warmer waters to sink and form cold water currents, which could affect global
ocean circulation. Ocean circulation has impacts on heat transport and weather globally, and on
fisheries and other human activities. Changes in any of several aspects of oceans can have wideranging impacts.
Figure 3.1: a) Depth-averaged 0–700 m temperature trend for 1971–2010 (longitude vs. latitude, colors
and grey contours in C per decade). b) Zonally-averaged temperature trends (latitude versus depth,
colors and grey contours in C per decade) for 1971–2010, with zonally averaged mean temperature overplotted (black contours in °C). c) Globally-averaged temperature anomaly (time versus depth, colors and
grey contours in C) relative to the 1971–2010 mean. d) Globally-averaged temperature difference
between the ocean surface and 200 m depth (black: annual values, red: 5-year running mean). All panels
are constructed from an update of the annual analysis of Levitus et al. (2009).
Rhein, M., S.R. Rintoul, S. Aoki, E. Campos, D. Chambers, R.A. Feely, S. Gulev, G.C. Johnson, S.A. Josey, A. Kostianoy, C. Mauritzen, D. Roemmich, L.D. Talley and F. Wang, 2013: Observations: Ocean. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.‐K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.