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How fast are the oceans warming?



Observational records of ocean heat content show that ocean warming is accelerating
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, 68). 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
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,
Berkeley, 310 Barrows Hall, Berkeley, CA 94720, USA. 4National
Center for Atmospheric Research, Post Office Box 3000,
Boulder, CO 80307, USA. Email:
How fast are the oceans warming?
Observational records of ocean heat content show that ocean warming is accelerating
128 11 JANUARY 2019 • VOL 363 ISSUE 6423
Scientists deploy an Argo float. For over a decade, more than 3000 floats have
provided near-global data coverage for the upper 2000 m of the ocean.
DA_0111PerspectivesR2.indd 128 3/1/19 11:18 AM
Published by AAAS
Erratum 8 March 2019. See Erratum.
on March 7, 2019 from
from 1971–2010 compared with recent ob-
servations ranging from 0.36 to 0.39 W m−2
(see the figure).
The relatively short period after the
deployment of the Argo network (see the
photo) in the early 2000s has resulted
in superior observational coverage and
reduced uncertainties compared to earlier
times. Over this period (2005–2017) for
the top 2000 m, the linear warming rate
for the ensemble mean of the CMIP5
models is 0.68 ± 0.02 W m−2, whereas ob-
servations give rates of 0.54 ± 0.02 (2),
0.64 ± 0.02 (10), and 0.68 ± 0.60 (11) W
m−2. These new estimates suggest that
models as a whole are reliably projecting
OHC changes.
However, some uncertainties remain,
particularly for deep and coastal ocean re-
gions and in the period before the deploy-
ment of the Argo network. It is important to
establish a deep ocean observation system
to monitor changes below 2000 m (14). It is
also essential to improve the historical re-
cord, for example, by recovering undigitized
OHC observations.
Simulations of future climate use a set
of scenarios or plausible radiative forcing
pathways based on assumptions about de-
mographic and socioeconomic development
and technological changes (5). Two scenarios
shown in the figure project a substantial
warming in the 21st century. For the Rep-
resentative Concentration Pathways (RCP)
2.6 scenario, the models project an ocean
warming (0 to 2000 m) of 1037 zettajoules
(ZJ) (~0.40 K) at the end of the 21st century
(mean of 2081–2100 relative to 1991–2005);
this pathway is close to the Paris Agreement
goal of limiting global warming to well be-
low 2°C. For the RCP8.5 scenario, a business-
as-usual scenario with high greenhouse gas
emissions, the models project a warming
of 2020 ZJ (~0.78 K). This level of warming
would have major impacts on ocean ecosys-
tems and sea level rise through thermal ex-
pansion; 0.78 K warming at 2100 is roughly
equal to a sea level rise of 30 cm. This is in
addition to increased sea level rise caused by
land ice melt.
The fairly steady rise in OHC shows that
the planet is clearly warming. The pros-
pects for much higher OHC, sea level, and
sea-surface temperatures should be of con-
cern given the abundant evidence of effects
on storms, hurricanes, and the hydrologi-
cal cycle, including extreme precipitation
events (3, 15). There is a clear need to con-
tinue to improve the ocean observation and
analysis system to provide better estimates
of OHC, because it will enable more refined
regional projections of the future. In ad-
dition, the need to slow or stop the rates
of climate change and prepare for the ex-
pected impacts is increasingly evident. j
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Am. Meteorol. Soc. 84, 1205 (2003).
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Press, 2 013), pp . 215–3 15.
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Soc. 93, 485 (2012).
6. M. Ishii et al., Sci. Online Lett. Atmos. 13, 163 (2017).
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Chang. 4, 999 (2014).
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13. M. A. Ba lmase da, K. E. Trenbe rth, E. K ällé n, Geophys. Res.
Lett. 40, 1754 (2013).
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Tec hn ol . 32, 2187 (2015).
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Earth’s Future 6, 730 (2018).
This study is supported by the National Key R&D Program of
China (2017YFA0603202). The National Center for Atmospheric
Research (NCAR) is sponsored by the National Science
Foundation. We thank the climate modeling groups (listed in
table S1) for producing and making available their model output,
and we acknowledge J. Fasullo for calculating OHC in CMIP5
models and making the data available to the authors. Institute
of Atmospheric Physics data and the CMIP5 time series are
available at
Updated OHC estimates compared
with AR5. The three most recent observation
-based OHC analyses give ocean warming
rates (depths from 0 to 2000 m) for
1971–2010 that are closer to model results
than those reported in AR5. This increases
condence in the model projections (see
supplementary materials for more detail).
The error bars are 90% condence intervals.
1960 1970 1980 2010 2020 2040 2060 2080
Ocean heat content anomaly (0–2000m)
Baseline: 1991–2005
RCP 8.5
Domingues et al. + Levitus et al. (10, 12)
Cheng et al. (2)
Ishii et al. (6)
Resplandy et al. (11)
21001990 2000
1990 1995 2000 2005 2010 2015
Cheng et al.
(monthly, Jan. 1990 to Sep. 2018)
Mean OHC
RCP 2.6
Warming rate (W m )
AR5 (4)
AR5 period (1971-2010), 0-2000m
CMIP5 (5)(10, 12)(6)(2)
11 JANUARY 2019 • VOL 363 ISSUE 6423 129
Past and future ocean heat content changes
Annual observational OHC changes are consistent with each other and consistent with the ensemble means of
the CMIP5 models for historical simulations pre-2005 and projections from 2005–2017, giving confidence in
future projections to 2100 (RCP2.6 and RCP8.5) (see the supplementary materials). The mean projected OHC
changes and their 90% confidence intervals between 2081 and 2100 are shown in bars at the right. The inset
depicts the detailed OHC changes after January 1990, using the monthly OHC changes updated to September
2018 [Cheng et al. (2)], along with the other annual observed values superposed.
DA_0111PerspectivesR2.indd 129 3/1/19 11:18 AM
Published by AAAS
Erratum 8 March 2019. See Erratum.
on March 7, 2019 from
How fast are the oceans warming?
Lijing Cheng, John Abraham, Zeke Hausfather and Kevin E. Trenberth
originally published online January 10, 2019DOI: 10.1126/science.aav7619
(6423), 128-129.363Science
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... In agreement with Cheng et al. (2019) and Gulev et al. (2021), our results confirm a continuous increase in ocean warming over the entire study period (Fig. 2). Moreover, rates of global ocean warming have increased over the three different study periods, i.e., historical up to the recent decadal change. ...
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... The ocean plays a crucial role in the Earth's energy balance, with its extensive volume and inherent thermal capacity making its contribution greater than that of any other component (continents, atmosphere, glaciers). In fact, the oceans absorb more than 90 % of the thermal excess, leading to an increase in global ocean heat content and temperatures (Cheng et al., 2019). This internal energy imbalance in the ocean has direct consequences on the properties of the system, its dynamics, its contribution to the water cycle, and its unique biotope. ...
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The mid-depth ocean circulation is critically linked to actual changes in the long-term global climate system. However, in the past few decades, predictions based on ocean circulation models highlight the lack of data, knowledge, and long-term implications in climate change assessment. Here, using 842,421 observations produced by Argo floats from 2001-2020, and Lagrangian simulations, we show that only 3.8% of the mid-depth oceans, including part of the equatorial Pacific Ocean and the Antarctic Circumpolar Current, can be regarded as accurately modelled, while other regions exhibit significant underestimations in mean current velocity. Knowledge of ocean circulation is generally more complete in the low-latitude oceans but is especially poor in high latitude regions. Accordingly, we propose improvements in forecasting, model representation of stochasticity, and enhancement of observations of ocean currents. The study demonstrates that knowledge and model representations of global circulation are substantially compromised by inaccuracies of significant magnitude and direction, with important implications for modelled predictions of currents, temperature, carbon dioxide sequestration, and sea-level rise trends.
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As a major component of Earth’s energy budget, ocean heat content (OHC) plays a vital role in buffering climate change. The annual cycle is the most prominent change in OHC but has always been removed to study variations and changes in the Earth’s energy budget. Here we investigate the annual cycle of the upper 2000 m OHC at regional to global scales and assess the robustness of the signals using the spread of multiple observational products. The potential drivers are also investigated by comparing the annual OHC signal with the corresponding change in top-of-atmosphere radiation, surface heat flux, ocean heat divergence and meridional heat transport. Results show that the robust signal of annual OHC change is significant down to 1000 m depth globally and can reach down to 1500 m in some areas such as the tropical ocean. The global OHC (0-1500 m) changes from positive anomalies within September-February to negative anomalies within March-August mainly because of the larger ocean area in the southern hemisphere and the seasonal migration of solar irradiance. Owing to the huge ocean heat capacity, the annual cycle of OHC dominates that of the global energy budget. The difference among the OHC annual cycle in the three major ocean basins is mainly attributed to ocean heat transport, especially in the tropics. In the upper 1500 m at mid and high latitudes and in the upper 50 m of the tropics, the net sea surface heat flux dominates the OHC annual cycle, while in the tropics below 50 m, wind-driven Ekman heat transport is the main driver.
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The ocean is the main source of thermal inertia in the climate system¹. During recent decades, ocean heat uptake has been quantified by using hydrographic temperature measurements and data from the Argo float program, which expanded its coverage after 20072,3. However, these estimates all use the same imperfect ocean dataset and share additional uncertainties resulting from sparse coverage, especially before 20074,5. Here we provide an independent estimate by using measurements of atmospheric oxygen (O2) and carbon dioxide (CO2)—levels of which increase as the ocean warms and releases gases—as a whole-ocean thermometer. We show that the ocean gained 1.33 ± 0.20 × 10²² joules of heat per year between 1991 and 2016, equivalent to a planetary energy imbalance of 0.83 ± 0.11 watts per square metre of Earth’s surface. We also find that the ocean-warming effect that led to the outgassing of O2 and CO2 can be isolated from the direct effects of anthropogenic emissions and CO2 sinks. Our result—which relies on high-precision O2 measurements dating back to 1991⁶—suggests that ocean warming is at the high end of previous estimates, with implications for policy-relevant measurements of the Earth response to climate change, such as climate sensitivity to greenhouse gases⁷ and the thermal component of sea-level rise⁸.
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While hurricanes occur naturally, human-caused climate change is supercharging them and exacerbating the risk of major damage. Here using ocean and atmosphere observations, we demonstrate links between increased upper ocean heat content due to global warming with the extreme rainfalls from recent hurricanes. Hurricane Harvey provides an excellent case study as it was isolated in space and time. We show that prior to the beginning of northern summer of 2017, ocean heat content was the highest on record both globally and in the Gulf of Mexico, but the latter sharply decreased with hurricane Harvey via ocean evaporative cooling. The lost ocean heat was realized in the atmosphere as moisture, and then as latent heat in record-breaking heavy rainfalls. Accordingly, record high ocean heat values not only increased the fuel available to sustain and intensify Harvey but also increased its flooding rains on land. Harvey could not have produced so much rain without human-induced climate change. Results have implications for the role of hurricanes in climate. Proactive planning for the consequences of human-caused climate change is not happening in many vulnerable areas, making the disasters much worse.
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Earth’s energy imbalance (EEI) drives the ongoing global warming and can best be assessed across the historical record (that is, since 1960) from ocean heat content (OHC) changes. An accurate assessment of OHC is a challenge, mainly because of insufficient and irregular data coverage. We provide updated OHC estimates with the goal of minimizing associated sampling error. We performed a subsample test, in which subsets of data during the data-rich Argo era are colocated with locations of earlier ocean observations, to quantify this error. Our results provide a new OHC estimate with an unbiased mean sampling error and with variability on decadal and multidecadal time scales (signal) that can be reliably distinguished from sampling error (noise) with signal-to-noise ratios higher than 3. The inferred integrated EEI is greater than that reported in previous assessments and is consistent with a reconstruction of the radiative imbalance at the top of atmosphere starting in 1985. We found that changes in OHC are relatively small before about 1980; since then, OHC has increased fairly steadily and, since 1990, has increasingly involved deeper layers of the ocean. In addition, OHC changes in six major oceans are reliable on decadal time scales. All ocean basins examined have experienced significant warming since 1998, with the greatest warming in the southern oceans, the tropical/subtropical Pacific Ocean, and the tropical/subtropical Atlantic Ocean. This new look at OHC and EEI changes over time provides greater confidence than previously possible, and the data sets produced are a valuable resource for further study.
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Ocean warming accounts for the majority of the earth’s recent energy imbalance. Historic ocean heat content (OHC) changes are important for understanding changing climate. Calculations of OHC anomalies (OHCA) from in situ measurements provide estimates of these changes. Uncertainties in OHCA estimates arise from calculating global fields from temporally and spatially irregular data (mapping method), instrument bias corrections, and the definitions of a baseline climatology from which anomalies are calculated. To investigate sensitivity of OHCA estimates for the upper 700m to these different factors, the same qualitycontrolled dataset is used by seven groups and comparisons are made. Two time periods (1970–2008 and 1993–2008) are examined. Uncertainty due to the mapping method is 16.5 ZJ for 1970–2008 and 17.1 ZJ for 1993–2008 (1 ZJ 5 1 3 1021 J). Uncertainty due to instrument bias correction varied from 8.0 to 17.9 ZJ for 1970–2008 and from 10.9 to 22.4 ZJ for 1993–2008, depending on mapping method. Uncertainty due to baseline mean varied from 3.5 to 14.5 ZJ for 1970–2008 and from 2.7 to 9.8 ZJ for 1993–2008, depending on mapping method and offsets. On average mapping method is the largest source of uncertainty. The linear trend varied from 1.3 to 5.0 ZJ yr21 (0.08–0.31Wm22) for 1970–2008 and from 1.5 to 9.4 ZJ yr21 (0.09– 0.58Wm22) for 1993–2008, depending on method, instrument bias correction, and baseline mean. Despite these complications, a statistically robust upper-ocean warming was found in all cases for the full time period.
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The global ocean stores more than 90% of the heat associated with observed greenhouse-gas-attributed global warming. Using satellite altimetry observations and a large suite of climate models, we conclude that observed estimates of 0–700 dbar global ocean warming since 1970 are likely biased low. This underestimation is attributed to poor sampling of the Southern Hemisphere, and limitations of the analysis methods that conservatively estimate temperature changes in data-sparse regions. We find that the partitioning of northern and southern hemispheric simulated sea surface height changes are consistent with precise altimeter observations, whereas the hemispheric partitioning of simulated upper-ocean warming is inconsistent with observed in-situ-based ocean heat content estimates. Relying on the close correspondence between hemispheric-scale ocean heat content and steric changes, we adjust the poorly constrained Southern Hemisphere observed warming estimates so that hemispheric ratios are consistent with the broad range of modelled results. These adjustments yield large increases (2.2–7.1 × 1022 J 35 yr−1) to current global upper-ocean heat content change estimates, and have important implications for sea level, the planetary energy budget and climate sensitivity assessments.
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We provide updated estimates of the change of ocean heat content and the thermosteric component of sea level change of the 0–700 and 0–2000 m layers of the World Ocean for 1955–2010. Our estimates are based on historical data not previously available, additional modern data, and bathythermograph data corrected for instrumental biases. We have also used Argo data corrected by the Argo DAC if available and used uncorrected Argo data if no corrections were available at the time we downloaded the Argo data. The heat content of the World Ocean for the 0–2000 m layer increased by 24.0 � 1.9 � 1022 J (�2S.E.) corresponding to a rate of 0.39Wm�2 (per unit area of the WorldOcean) and a volume mean warming of 0.09�C. This warming corresponds to a rate of 0.27 W m�2 per unit area of earth’s surface. The heat content of the World Ocean for the 0–700 m layer increased by 16.7 � 1.6 � 1022 J corresponding to a rate of 0.27 W m�2 (per unit area of the World Ocean) and a volume mean warming of 0.18�C. The World Ocean accounts for approximately 93% of the warming of the earth system that has occurred since 1955. The 700–2000 m ocean layer accounted for approximately one-third of the warming of the 0–2000 m layer of the World Ocean. The thermosteric component of sea level trend was 0.54 � .05 mm yr�1 for the 0–2000 m layer and 0.41 � .04 mm yr�1 for the 0–700 m layer of the World Ocean for 1955–2010.
Data from full-depth closely sampled hydrographic sections and Argo floats are analyzed to inform the design of a future Deep Argo array. Here standard errors of local decadal temperature trends and global decadal trends of ocean heat content and thermosteric sea level anomalies integrated from 2000 to 6000 dbar are estimated for a hypothetical 5 degrees latitude x 5 degrees longitude x 15-day cycle Deep Argo array. These estimates are made using temperature variances from closely spaced full-depth CTD profiles taken during hydrographic sections. The temperature data along each section are high passed laterally at a 500-km scale, and the resulting variances are averaged in 5 degrees x 5 degrees bins to assess temperature noise levels as a function of pressure and geographic location. A mean global decorrelation time scale of 62 days is estimated using temperature time series at 1800 dbar from Argo floats. The hypothetical Deep Argo array would be capable of resolving, at one standard error, local trends from <1 m degrees C decade(-1) in the quiescent abyssal North Pacific to about 26 m degrees C decade(-1) below 2000 dbar along 50 degrees S in the energetic Southern Ocean. Larger decadal temperature trends have been reported previously in these regions using repeat hydrographic section data, but those very sparse data required substantial spatial averaging to obtain statistically significant results. Furthermore, the array would provide decadal global ocean heat content trend estimates from 2000 to 6000 dbar with a standard error of +/- 3 TW, compared to a trend standard error of +/- 17 TW from a previous analysis of repeat hydrographic data.
The elusive nature of the post-2004 upper ocean warming has exposed uncertainties in the ocean's role in the Earth's energy budget and transient climate sensitivity. Here we present the time evolution of the global ocean heat content for 1958 through 2009 from a new observation-based reanalysis of the ocean. Volcanic eruptions and El Niño events are identified as sharp cooling events punctuating a long-term ocean warming trend, while heating continues during the recent upper-ocean-warming hiatus, but the heat is absorbed in the deeper ocean. In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend. The warming below 700 m remains even when the Argo observing system is withdrawn although the trends are reduced. Sensitivity experiments illustrate that surface wind variability is largely responsible for the changing ocean heat vertical distribution.