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How fast are the oceans warming?

January 2019
Science 363(6423):128-129

DOI:10.1126/science.aav7619

Authors:

Lijing Cheng

Institute of Atmospheric Physics

John Patrick Abraham

University of St. Thomas

Observational records of ocean heat content show that ocean warming is accelerating

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INSIGHTS |

PERSPECTIVES

sciencemag.org SCIENCE

PHOTO: ALICIA NAVIDAD/CSIRO

By Lijing Cheng1, John Abraham2,

Zeke Hausfather3, Kevin E. Trenberth4

Climate change from human activities

mainly results from the energy imbal-

ance in Earth’s climate system caused

by rising concentrations of heat-trap-

ping gases. About 93% of the energy

imbalance accumulates in the ocean

as increased ocean heat content (OHC). The

ocean record of this imbalance is much less

affected by internal variability and is thus

better suited for detecting and attributing

human influences (1) than more commonly

used surface temperature records. Recent

observation-based estimates show rapid

warming of Earth’s oceans over

the past few decades (see the

figure) (1, 2). This warming has

contributed to increases in rain-

fall intensity, rising sea levels,

the destruction of coral reefs,

declining ocean oxygen levels,

and declines in ice sheets; gla-

ciers; and ice caps in the polar

regions (3, 4). Recent estimates

of observed warming resemble

those seen in models, indicat-

ing that models reliably project

changes in OHC.

The Intergovernmental Panel

on Climate Change’s Fifth As-

sessment Report (AR5), pub-

lished in 2013 (4), featured five

different time series of histori-

cal global OHC for the upper

700 m of the ocean. These time

series are based on different

choices for data processing (see

the supplementary materials). Interpreta-

tion of the results is complicated by the

fact that there are large differences among

the series. Furthermore, the OHC changes

that they showed were smaller than those

projected by most climate models in the

Coupled Model Intercomparison Project 5

(CMIP5) (5) over the period from 1971 to

2010 (see the figure).

Since then, the research community has

made substantial progress in improving

long-term OHC records and has identified

several sources of uncertainty in prior mea-

surements and analyses (2, 6–8). In AR5, all

OHC time series were corrected for biases

in expendable bathythermograph (XBT)

data that had not been accounted for in

the previous report (AR4). But these cor-

rection methods relied on very different as-

sumptions of the error sources and led to

substantial differences among correction

schemes. Since AR5, the main factors influ-

encing the errors have been identified (2),

helping to better account for systematic er-

rors in XBT data and their analysis.

Several studies have attempted to im-

prove the methods used to account for

spatial and temporal gaps in ocean tem-

perature measurements. Many traditional

gap-filling strategies introduced a conser-

vative bias toward low-magnitude changes

(9). To reduce this bias, Domingues et al.

(10) used satellite altimeter observations to

complement the sparseness of in situ ocean

observations and update their global OHC

time series since 1970 for the upper 700 m.

Cheng et al. (2) proposed a new gap-filling

method that used multimodel simulations

to provide an improved prior estimate and

error covariance. This method allowed

propagation of information from data-rich

regions to the data gaps (data are available

for the upper 2000 m since 1940). Ishii et

al. (6) completed a major revision of their

estimate in 2017 to account for the previ-

ous underestimation and also extended the

analysis down to 2000 m and back to 1955.

Resplandy et al. (11) used ocean warming

outgassing of O2 and CO2, which can be

isolated from the direct effects of anthro-

pogenic emissions and CO2 sinks, to inde-

pendently estimate changes in OHC over

time after 1991.

These recent observation-based OHC esti-

mates show highly consistent changes since

the late 1950s (see the figure). The warm-

ing is larger over the 1971–2010 period than

reported in AR5. The OHC trend for the up-

per 2000 m in AR5 ranged from

0.24 to 0.36 W m−2 during this

period (4). The three more con-

temporary estimates that cover

the same time period suggest a

warming rate of 0.36 ± 0.05 (6),

0.37 ± 0.04 (10), and 0.39 ± 0.09

(2) W m−2. [Note that the analy-

sis in Domingues et al. (10) is

combined with that in Levitus

et al. (12) for 700 to 2000 m to

produce a 0 to 2000 m time se-

ries.] All four recent studies (2,

6, 10, 11) show that the rate of

ocean warming for the upper

2000 m has accelerated in the

decades after 1991 from 0.55 to

0.68 W m−2 (calculations pro-

vided in the supplementary

materials).

Multiple lines of evidence

from four independent groups

thus now suggest a stronger

observed OHC warming. Although climate

model results (see the supplementary ma-

terials) have been criticized during de-

bates about a “hiatus” or “slowdown” of

global mean surface temperature, it is in-

creasingly clear that the pause in surface

warming was at least in part due to the

redistribution of heat within the climate

system from Earth surface into the ocean

interiors (13). The recent OHC warming

estimates (2, 6, 10, 11) are quite similar

to the average of CMIP5 models, both for

the late 1950s until present and during the

1971–2010 period highlighted in AR5 (see

the figure). The ensemble average of the

models has a linear ocean warming trend

of 0.39 ± 0.07 W m−2 for the upper 2000 m

1International Center for Climate and Environment Sciences,

Institute of Atmospheric Physics, Chinese Academy of

Sciences, Beijing 100029, China. 2School of Engineering,

University of St. Thomas, 2115 Summit Avenue, St. Paul, MN,

USA. 3Energy and Resources Group, University of California,