A review of global ocean temperature observations: Implications for ocean heat content estimates and climate change

Abstract

[1] The evolution of ocean temperature measurement systems is presented with a focus on the development and accuracy of two critical devices in use today (expendable bathythermographs and conductivity-temperature-depth instruments used on Argo floats). A detailed discussion of the accuracy of these devices and a projection of the future of ocean temperature measurements are provided. The accuracy of ocean temperature measurements is discussed in detail in the context of ocean heat content, Earth's energy imbalance, and thermosteric sea level rise. Up-to-date estimates are provided for these three important quantities. The total energy imbalance at the top of atmosphere is best assessed by taking an inventory of changes in energy storage. The main storage is in the ocean, the latest values of which are presented. Furthermore, despite differences in measurement methods and analysis techniques, multiple studies show that there has been a multidecadal increase in the heat content of both the upper and deep ocean regions, which reflects the impact of anthropogenic warming. With respect to sea level rise, mutually reinforcing information from tide gauges and radar altimetry shows that presently, sea level is rising at approximately 3 mm yr À1 with contributions from both thermal expansion and mass accumulation from ice melt. The latest data for thermal expansion sea level rise are included here and analyzed.

Bidecadal Thermal Changes in the Abyssal Ocean

“…14 of Abraham et al (2013). Perhaps coincidentally, it is similar to the 135-yr 700-m depth ocean rate of 0.2 6 0.1 W m 22 of Roemmich et al (2012).…”

“…The specific estimate used is labeled version 4, release 1, and in contrast to earlier estimates includes a full sea ice model (Losch et al 2010;Fenty and Heimbach 2013) and extends to the North Pole [see Forget et al (2013, unpublished manuscript) for full details]. 4 Very recently, Abraham et al (2013) have published a useful discussion of the methods used both historically and today for direct ocean temperature measurements including, especially, the ongoing debates about systematic errors in the different datasets. The present state estimate uses all of the post-1991 data types they discuss, but combines them also with the continuous high-density altimetric height and other measurements as well as with the best initial estimate we could obtain of the air-sea heat transfers.…”

“…Although the upper ocean is not the focus here, an interesting and complex question is whether the observational network is capable of producing estimates of small changes with a useful accuracy. Abraham et al (2013) have described many of these calculations in detail and provide a list of references.…”

“…Historically, deep hydrographic measurements (below a few hundred or perhaps 1000 m) have been both difficult and expensive to acquire (see Abraham et al 2013). The consequence has been sampling by a few, rare (in a multidecadal or centennial context), fragmentary top-to-bottom hydrographic stations and sections.…”

Recent Progress in Understanding and Projecting Regional and Global Mean Sea Level Change

“…[40]), but not all (e.g., [41,42]), reported a sharp spike in ocean heat uptake in the early 2000s followed by a slowing of the rate of heat uptake (and thus ocean thermal expansion). Since then, Abraham et al [43] published a major review on the evolving observing system, the reduction of XBT biases, and estimates of heat content and thermosteric sea level trends over different periods, generally confirming the AR5 assessment of trends since 1971 and 1993. These estimates are dependent on a range of uncertainties, including ocean climatologies used, vertical resolution [44,45], and mapping techniques [46].…”