Estimates from the IPCC Fourth Assessment Report
The IPCC Fourth Assessment Report found that warming in all three of the major ocean basins has occurred over the past few decades (Bindoff et al., 2007). Further, thermal expansion of the global ocean (thermosteric sea-level rise) exhibits significant decadal and interannual variations. Thermosteric sea-level rise was estimated to account for approximately one-quarter of the observed rate of global sea-level rise from 1961 to 2003, contributing 0.32 ± 0.12 mm yr-1 down to 700 m depth and 0.42 ± 0.12 mm yr-1 down to 3000 m depth. For the last 10 years of that period (1993–2003), the contribution of thermal expansion was estimated to have increased to 1.5 ± 0.5 mm yr-1 above 700 m and 1.6 ± 0.5 mm yr-1 above 3,000 m, about half of the observed rate of global sea-level rise.
At about the time the IPCC Fourth Assessment Report was published, systematic efforts began to be made to correct for biases in the expendable bathythermograph (XBT) and mechanical bathythermograph (MBT) data, which constitute the majority of ocean temperature observations prior to 2002, and in Argo data (Box 3.1). These biases affected the temperature inferred from measurements and thus the calculated rate of thermosteric sea-level rise. Thermosteric sea-level trends have recently been reanalyzed using bias-corrected temperature data, and the record has been extended by new observations. In addition, a few new estimates of the thermosteric fraction of sea level have been made using data assimilation products and satellite data.
Bathythermograph and Argo Measurements
Bathythermographs are dropped from ships and transmit temperature via a thin wire as they sink through the water column. Mechanical bathythermographs (MBTs) record temperature at 5 m depth intervals down to approximately 285 m. Thus, they are useful only for studying the thermal structure of the upper ocean. The successor expendable bathythermographs (XBTs) can provide temperature profiles to depths of approximately 760 m (standard instruments) or 1,830 m (special instruments). Data from MBTs and/or XBTs are available since 1948.
Ocean profiling floats are deployed under the multi-national Argo program and by individual countries. Argo profiling floats began measuring the temperature and salinity of the upper 1,000–2,000 m of the ocean in 2000. The Argo array currently comprises more than 3,000 ocean profiling floats distributed around the world (see Figure). Data from these floats are collected via satellite.
FIGURE Distribution of Argo profiling drifters on February 24, 2012. These floats measure salinity and temperature over the upper 1,000–2,000 m of the ocean. SOURCE: These data were collected and made freely available by the International Argo Program and the national programs that contribute to it (<http://www.argo.ucsd.edu>, <http://argo.jcommops.org>).