Global reconstruction of historical ocean heat storage and transport - PubMed (original) (raw)

Global reconstruction of historical ocean heat storage and transport

Laure Zanna et al. Proc Natl Acad Sci U S A. 2019.

Abstract

Most of the excess energy stored in the climate system due to anthropogenic greenhouse gas emissions has been taken up by the oceans, leading to thermal expansion and sea-level rise. The oceans thus have an important role in the Earth's energy imbalance. Observational constraints on future anthropogenic warming critically depend on accurate estimates of past ocean heat content (OHC) change. We present a reconstruction of OHC since 1871, with global coverage of the full ocean depth. Our estimates combine timeseries of observed sea surface temperatures with much longer historical coverage than those in the ocean interior together with a representation (a Green's function) of time-independent ocean transport processes. For 1955-2017, our estimates are comparable with direct estimates made by infilling the available 3D time-dependent ocean temperature observations. We find that the global ocean absorbed heat during this period at a rate of 0.30 ± 0.06 W/[Formula: see text] in the upper 2,000 m and 0.028 ± 0.026 W/[Formula: see text] below 2,000 m, with large decadal fluctuations. The total OHC change since 1871 is estimated at 436 ± 91 [Formula: see text] J, with an increase during 1921-1946 (145 ± 62 [Formula: see text] J) that is as large as during 1990-2015. By comparing with direct estimates, we also infer that, during 1955-2017, up to one-half of the Atlantic Ocean warming and thermosteric sea-level rise at low latitudes to midlatitudes emerged due to heat convergence from changes in ocean transport.

Keywords: Earth’s energy imbalance; climate change; ocean heat content; ocean processes; sea-level rise.

Copyright © 2019 the Author(s). Published by PNAS.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Fig. 1.

Global and Atlantic OHC timeseries and trends for GF and observational estimates relative to 2006–2015. Timeseries of global (A–C) and Atlantic (D–E) OHC changes in zetajoules (1 ZJ = 1021 J): (A and D) top 700 m, (B and E) top 2,000 m, and (C and F) below 2,000 m. The OHC timeseries include the reconstruction based on GFs (orange) and direct measurements from the NCEI (2) (black), the IAP (1) (green), Ishii et al. (20) (blue), and Domingues et al. (updated from refs. and 22) (brown). The latitudinal range for all products used here is 80° S to 80° N, except for the product from Domingues et al. (21), which uses 65° S to 65° N. The shading represents the uncertainty associated with each estimate (Materials and Methods). Insets above each panel represent the linear trends and associated error (zetajoules per year) over different periods for each best estimate available (see text). For the global ocean (A–C), we include trends from the ECCO-GODAE solution (red) and for the deep ocean (C) the updated estimates from refs. , , and (cyan).

Fig. 2.

Fig. 2.

Cumulative heat uptake from 1871 to 2017 (joules per year) shown for each patch (numbered here and shown in

SI Appendix, Fig. S1

), contributing to the integrated passive heat storage (A) globally and (B) in the Atlantic Ocean. Note the different scales for the two panels.

Fig. 3.

Fig. 3.

OHC and sea-level trends in the Atlantic Ocean as a function of latitude. Atlantic OHC linear trends calculated over 1955–2017 (ZJ per degree latitude per decade) as a function of latitude for GF (orange) and observational estimates (black) and for different depth ranges: (A) top 700 m, (B) top 2,000 m, (C) 700–2,000 m, and (D) below 2,000 m. The average uncertainty (shading) is calculated using the signal to noise ratio from the different datasets, thereby partially including both the departure of the signal from the linear trend over a decade and the uncertainty in the trends from the different observational products. E and F show the difference in sea level (centimeters per degree latitude) estimated using the upper 2,000-m OHC during the periods 1955–1970 and 1971–2016, respectively. The difference is estimated using an average of the first and last 5 y in each period.

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