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# Another Record: Ocean Warming Continues through 2021 despite La Niña Conditions

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The increased concentration of greenhouse gases in the atmosphere from human activities traps heat within the climate system and increases ocean heat content (OHC). Here, we provide the first analysis of recent OHC changes through 2021 from two international groups. The world ocean, in 2021, was the hottest ever recorded by humans, and the 2021 annual OHC value is even higher than last year’s record value by 14 ± 11 ZJ (1 zetta J = 1021 J) using the IAP/CAS dataset and by 16 ± 10 ZJ using NCEI/NOAA dataset. The long-term ocean warming is larger in the Atlantic and Southern Oceans than in other regions and is mainly attributed, via climate model simulations, to an increase in anthropogenic greenhouse gas concentrations. The year-to-year variation of OHC is primarily tied to the El Niño-Southern Oscillation (ENSO). In the seven maritime domains of the Indian, Tropical Atlantic, North Atlantic, Northwest Pacific, North Pacific, Southern oceans, and the Mediterranean Sea, robust warming is observed but with distinct inter-annual to decadal variability. Four out of seven domains showed record-high heat content in 2021. The anomalous global and regional ocean warming established in this study should be incorporated into climate risk assessments, adaptation, and mitigation.
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Another Record: Ocean Warming Continues through 2021
despite La Niña Conditions
Lijing CHENG*1,2, John ABRAHAM3, Kevin E. TRENBERTH4, John FASULLO4, Tim BOYER5,
Michael E. MANN6, Jiang ZHU1,2, Fan WANG2,7, Ricardo LOCARNINI5, Yuanlong LI2,7, Bin ZHANG2,7,
Zhetao TAN1,2, Fujiang YU8, Liying WAN8, Xingrong CHEN8, Xiangzhou SONG9, Yulong LIU10,
Franco RESEGHETTI11, Simona SIMONCELLI12, Viktor GOURETSKI1, Gengxin CHEN13,
Alexey MISHONOV5,14, and Jim REAGAN5
1International Center for Climate and Environment Sciences, Institute of Atmospheric Physics,
Chinese Academy of Sciences, Beijing 100029, China
2Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
3University of St. Thomas, School of Engineering, Minnesota 55105, USA
4National Center for Atmospheric Research, Boulder, Colorado 80307, USA
5National Oceanic and Atmospheric Administration, National Centers for
Environmental Information, Silver Spring, Maryland 20910, USA
6Department of Meteorology &Atmospheric Science, The Pennsylvania
State University, University Park, Pennsylvania 16802, USA
7Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
8National Marine Environmental Forecasting Center, Ministry of
Natural Resources of China, Beijing 100081, China
9College of Oceanography, Hohai University, Nanjing 210098, China
10National Marine Data and Information Service, Tianjin 300171, China
11Italian National Agency for New Technologies, Energy and Sustainable Economic
Development, S. Teresa Research Center, Lerici 19032, Italy
12Istituto Nazionale di Geofisica e Vulcanologia, Sede di Bologna, Bologna 40128, Italy
13South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
14ESSIC/CISESS-MD, University of Maryland, College Park, MD 20742, USA
(Received 19 December 2021; revised 8 January 2022; accepted 10 January 2022)
ABSTRACT
The increased concentration of greenhouse gases in the atmosphere from human activities traps heat within the climate
system and increases ocean heat content (OHC). Here, we provide the first analysis of recent OHC changes through 2021
from two international groups. The world ocean, in 2021, was the hottest ever recorded by humans, and the 2021 annual
OHC value is even higher than last year’s record value by 14 ± 11 ZJ (1 zetta J = 1021 J) using the IAP/CAS dataset and by
16 ± 10 ZJ using NCEI/NOAA dataset. The long-term ocean warming is larger in the Atlantic and Southern Oceans than in
other regions and is mainly attributed, via climate model simulations, to an increase in anthropogenic greenhouse gas
concentrations. The year-to-year variation of OHC is primarily tied to the El Niño-Southern Oscillation (ENSO). In the
seven maritime domains of the Indian, Tropical Atlantic, North Atlantic, Northwest Pacific, North Pacific, Southern oceans,
and the Mediterranean Sea, robust warming is observed but with distinct inter-annual to decadal variability. Four out of
seven domains showed record-high heat content in 2021. The anomalous global and regional ocean warming established in
this study should be incorporated into climate risk assessments, adaptation, and mitigation.
Citation: Cheng, L. J, and Coauthors, 2022: Another record: Ocean warming continues through 2021 despite La Niña
* Corresponding author: Lijing CHENG
Email: chenglij@mail.iap.ac.cn
• Original Paper •
Article Highlights:
• The world ocean, in 2021, was the hottest ever recorded by humans.
• The warming pattern is mainly attributed to increased anthropogenic greenhouse gas concentrations, offset by the impact
of aerosols.
• Ocean warming has far-reaching consequences and should be incorporated into climate risk assessments, adaptation, and
mitigation.
1.Introduction
The increased concentrations of greenhouse gases in
the atmosphere from human activities trap heat within the cli-
mate system and result in massive changes in the climate sys-
tem. As a result, outgoing energy from the Earth system is
not balancing the incoming solar radiation, thus creating
Earth’s Energy Imbalance (EEI) in the climate system
(Trenberth et al., 2014; von Schuckmann et al., 2016a,
2020a; Wijffels et al., 2016; Johnson et al., 2018; Cheng et
al., 2019a). The oceans store over 90% of EEI, leading to an
increase of ocean heat content (OHC), which currently
provides the best estimate for EEI (Hansen et al., 2011;
IPCC, 2013; Rhein et al., 2013; Trenberth et al., 2016;
Abram et al., 2019). Ocean warming leads to increased
ocean vertical stratification, thermal expansion, and sea-
level rise. These processes provide a compelling means to
quantify climate change (Cheng et al., 2018).
This study provides the first analysis of recent OHC
changes through 2021. Two international data products
include those from the Institute of Atmospheric Physics
(IAP) at the Chinese Academy of Sciences (CAS) (Cheng et
al., 2017) and the National Centers for Environmental Inform-
ation (NCEI) of the National Oceanic and Atmospheric
Administration (NOAA) (Levitus et al., 2012). Both data-
sets corrected systematic errors and then used thorough map-
ping methods to convert discrete ocean measurements into a
comprehensive picture of the ocean. Both global and
regional analyses of OHC changes are provided in this
study.
2.Data and Methods
The IAP/CAS and NCEI/NOAA analyses are based on
available in situ observations from various measurement
devices held in the World Ocean Database (WOD) of the
NCEI/NOAA. Data from all instruments are used, includ-
ing eXpendable BathyThermographs (XBTs), profiling
floats from Argo, moorings, gliders, Conductivity/Temperat-
ure/Depth devices (CTDs), bottles, and instruments on mar-
ine mammals (Boyer et al., 2018). The XBT biases are correc-
ted according to Cheng et al. (2014) for IAP/CAS and Levi-
tus et al. (2009) for NCEI/NOAA. Model simulations guide
the mapping method from point measurements to the compre-
hensive grid in the IAP/CAS product. At the same time,
sampling errors are estimated by sub-sampling the Argo
data at the locations of the earlier observations (a full descrip-
tion of the method is in Cheng et al., 2017).
The Argo Program is part of the Global Ocean
Observing System. The Argo observing network achieved a
near-global upper-2000 m coverage since about 2005
(Argo, 2020). Argo data are made freely available by the
International Argo Program and the contributing national pro-
grams (http://www.argo.ucsd.edu; http://argo.jcommops.
org).
In addition to these observations, the Community Earth
System Model Version 1 (CESM1) Large Ensemble
(LENS; Kay et al., 2015) data are used to explore the influ-
ence of different forcings (aerosols, greenhouse gasses, indus-
trial aerosols, biomass aerosols, land use, and land cover)
on the formation of OHC patterns in a large ensemble simula-
tion of climate. Previous assessments indicate that CESM is
one of the most skilled models representing the global
energy budget, water cycles, and associated dynamics
(Fasullo, 2020).
3.Global ocean changes
The up-to-date data indicate that the OHC in the upper
2000 m layer of the world’s oceans has increased with a
mean rate of 5.7 ± 1.0 ZJ yr–1 for the 1958–2021 period
(IAP/CAS) and 4.7 ± 1.0 ZJ yr–1 for the 1958–2021 period
(NCEI/NOAA) (Fig. 1a). Both datasets show an unambigu-
ous increase in ocean warming since the late 1980s (Fig. 1a).
From 1986–2021, the average annual increase is 9.1 ± 0.3
ZJ yr–1 for IAP and 8.3 ± 0.7 ZJ yr–1 for NCEI/NOAA.
These warming rates represent a maximum of 8-fold
increase compared to 1958–85 (1.2 ± 0.6 ZJ yr–1 IAP/CAS,
1.5 ± 1.0 ZJ yr–1 for NCEI/NOAA). Moreover, each decade
since 1958 has been warmer than the preceding decades.
The 2021 annual OHC value is higher than the last
year’s value, by 14 ± 11 ZJ using the IAP/CAS data and by
16 ± 10 ZJ using NCEI/NOAA (95% confidence interval).
Both projections are the highest on record (Table 1). Differ-
ences between the OHC analyses reflect the uncertainties in
the calculation due to data quality, mapping differences, and
data coverage. Nevertheless, it is evident that according to
both IAP/CAS and NCEI/NOAA, the 2021 oceans were the
hottest ever recorded by humans.
There are notable inter-annual fluctuations in the OHC
record (Fig. 1b); Cheng et al. (2018) indicated that it
requires ~four years for the long-term trend signal to signific-
antly exceed the inter-annual variability at the 95% confid-
ence interval, noting that ENSO is the dominant driver of
year-to-year variations in OHC (i.e., Cheng et al., 2019b).
2RECORD OCEAN WARMING IN 2021
The Indo-Pacific basin transitioned from a weak El Niño
state in the first six months of 2020 toward a La Niña state
during the last half of 2020, and a La Niña state has since con-
tinued in 2021. There is a decreasing tendency in OHC dur-
ing the transition from El Niño to La Niña because of ocean
heat release associated with anomalous warming of sea sur-
face conditions in the tropical Pacific Ocean (Cheng et al.,
2019b) (Fig. 1b). During La Niña, the ocean absorbs heat,
which caused an increase in OHC from late-2020 to early
2021 (Fig. 1b). Similar OHC changes also have occurred dur-
ing the 2009, 2015, and 2017 El Niño events. The El Niño-
driven heat buildup and higher sea levels are most typical of
the tropical western Pacific region (see Fig. 2).
In addition to providing an update on the 2021 OHC,
we present an improved calculation for previous years. Both
IAP/CAS and NCEI/NOAA have re-calculated the 2020
OHC values using the most up-to-date observations. A not-
able revision of 0–2000 m OHC anomaly from 234 ZJ to
221 ZJ is found for IAP/CAS data. With this correction, the
IAP/CAS and NCEI/NOAA OHC time series become consist-
ent in 2020 and 2021 (Fig. 1b), increasing confidence in the
near real-time OHC calculation. By comparing the two ver-
sions of WOD in situ observational data, it was found that
data Quality-Control (QC) flags had not been assigned in
the previous dataset (i.e., there is no QC applied to the
Table 1.  Ranked order of the hottest five years of the global
ocean, since 1955. The OHC values are anomalies for the upper
2000 m in units of ZJ relative to the 1981–2010 average.
Rank Year IAP/CAS NCEI/NOAA
1 2021 235 227
2 2020 221 211
3 2019 214 210
4 2017 202 189
5 2018 195 196
Fig. 1. (upper) The global upper 2000 m OHC from 1958 through 2021. The histogram presents annual anomalies
relative to a 1981–2010 baseline, with positive anomalies shown as red bars and negative anomalies as blue. Units:
ZJ. (bottom) Detrended OHC time series from 2005 to 2021 compared with ONI index.
CHENG ET AL. 3
data), highlighting the importance of QC in OHC calcula-
tions. Considering the non-negligible impact of QC on the
OHC estimate, comprehensive quantification of QC uncer-
tainty continues to be a priority.
4.Regional patterns of ocean warming
The spatial pattern of upper OHC anomaly (0–2000 m)
(Fig. 2a) illustrates several key features, including an anom-
aly maxima near 40° in both hemispheres and broad anomal-
ous warming in the Atlantic Ocean relative to other ocean
basins, but a pronounced minimum in the northern North
Atlantic Ocean. The drivers of these features can be
explored by using so-called single-forcing large ensembles,
within which only a single external climate forcing agent is
held fixed over time in a multi-member ensemble experi-
ment (Deser et al., 2020). By averaging across ensemble mem-
bers and differencing with the fully-forced ensemble, the con-
tributions of internal variability are reduced to allow direct
estimation of the response to forcing. Important uncertain-
ties exist in inferring natural changes from such ensembles,
including uncertainties related to model structure and
imposed external forcing agents. Strategies for addressing
Here, we use the Community Earth System Version 1
(CESM1) Large Ensemble (CESM1-LE; Kay et al., 2015)
and Single-Forcing Large Ensemble (CESM1-SF; Deser et
al., 2020) to attribute the drivers of observed OHC trend pat-
terns from 1979 to 2020 (Fig. 3). A strong correlation pat-
tern exists between the fully-forced ensemble mean and
observed changes (Fig. 3a vs. Fig. 2a), thus providing eviden-
tiary support for both the fidelity of the CESM1-LE and the
emergence of forced changes in the presence of internal vari-
ability (i.e., Cheng et al., 2018; Fasullo and Nerem, 2018).
Many of the salient features apparent in observations are
also evident in the ensemble mean, suggesting a role for for-
cing in their development. Examples include the relative max-
ima near 40° in each hemisphere, the region of strong cool-
ing in the northern Atlantic southeast of Greenland, the
subtle changes in the Pacific Ocean equatorward of 30°, and
the relative warming of the Atlantic Ocean as compared to
other ocean basins (Fig. 3a). Variability in trends across
ensemble members is largest in the oceans north of 30°N
(Fig. 3b).
The influence of industrial aerosols (AER, Fig. 3c) is
strong and pervasive across much of the global ocean, exert-
ing a strong cooling influence between 30°N and 60°S and
significantly contributing to the regional cooling southeast
of Greenland. The influence of biomass aerosols (BMB,
Fig. 3d) is more regional yet important, with strong cooling
signals evident in all basins and a warming contribution to
the North Atlantic. The contribution of greenhouse gases
(GHG, Fig. 3e) is notable in nearly all regions, with substan-
tial warming, especially near 40°S and across the Atlantic
Ocean. A strong contribution to cooling southeast of Green-
land is also evident. Lastly, land use and land cover (LULC)
contributions are small, with significant areas spanning less
than 5% of the global ocean.
Together these results promote an understanding of the
observed pattern of ocean warming since 1980. Forcing
from greenhouse gases dominates many of the observed fea-
tures but contributions from AER and BMB are also import-
ant. Pervasive warming of the global ocean is driven by
GHG and offset somewhat by both AER and BMB (Fig. 3c,
d). The elevated warming of the Atlantic Ocean relative to
other basins results from both the elevated warming effects
of GHG and the reduced cooling effects of AER in the
Atlantic. Together these effects are offset somewhat by a
strong regional cooling driven by BMB. The region of
strong cooling southeast of Greenland is driven by both
GHG and AER influences (associated with AMOC
changes). It stands out as one of the few regions where
GHG and AER effects mutually reinforce each other.
Lastly, the relative minimum in the tropical Pacific Ocean
Fig. 2. (a) The ocean heat content anomaly in 2021 relative to
1981–2010 baseline. (b) The difference of OHC for upper
2000 m between 2021 and 2020. (c) As in (b) but for
NCEI/NOAA data. Units: 109 J m–2. [Data updated from
Cheng et al. (2017) in (a) and (b), from Levitus et al. (2012) in
(c)].
4RECORD OCEAN WARMING IN 2021
equatorward of 30° results from the combined influences of
relatively weak GHG-driven warming and the offsetting
effects of both AER and BMB.
This diagnosis of the drivers of OHC trend patterns
relies on the fidelity of the CESM1 and the estimates of
external forcing agents used in the CESM1-LE. Further
exploration with alternative models and forcings will play
an important role in establishing confidence in the interpreta-
tions based on the experiments used here. Because of space
limitations here, accounting for the uncertainties in forcings
(Fasullo et al., 2021; Fyfe et al., 2021; Smith and Foster,
2021) remains a topic for future work. Formal detection and
attribution analysis plus single forcing experiments with
more climate models are also being conducted.
Comparing the spatial OHC anomalies in 2021 versus
2020 shows an imprint of La Niña (Figs. 2b, c, Cheng et al.,
2019b). The NCEI/NOAA data (Fig. 2c) shows a consistent
spatial pattern compared with IAP/CAS (Fig. 2b), but the
anomalies are spottier and tied to the spatial covariance in
mapping strategy. The IAP/CAS product is smoother than
NCEI/NOAA because of the stronger effect of smoothing
(Cheng et al., 2017). There is a strong increase of OHC in
the western Pacific that extends into the South and North
Pacific regions to 20°S and 20°N, and a broad decrease of
OHC in the eastern Pacific, including coastal Peru and the
California oceans (Figs. 2b, c). A reverse zonal gradient
(warm in the east and cold in the west) is evident in the
Indian Ocean. This pattern in the Indo-Pacific Ocean mainly
reflects the thermocline changes during ENSO: the thermo-
cline is steeper during La Niña, with a deeper thermocline
in the western Pacific-Indonesian warm pool. Meanwhile,
the thermocline uplifts in the tropical eastern Pacific and west-
ern Indian oceans, leading to cooling anomalies. The trop-
ical Atlantic Ocean shows a small decrease of OHC
(20°S−20°N), likely due to two separate but reinforcing
factors: (1) the shift of the Walker circulation during La
Niña events increases evaporation in the tropical Atlantic
(Wang, 2019) and (2) those same La Niña conditions contrib-
uted to an especially active and energetic Atlantic hurricane
season as tropical storm-related mixing leads to substantial
Fig. 3. Ensemble mean trends in 0–2000 m OHC in (a) the CESM1-LE and CESM1-SF estimated contributions from (c)
industrial and (d) biomass aerosols, (e) greenhouse gasses, and (f) change in land use and land cover. Units: 109 J m–2.
Stippled and hatched regions correspond to areas where positive and negative trends exceed twice the standard ensemble
error, respectively. (b) The standard deviation of CESM1-LE members in 0–2000 m OHC trend.
CHENG ET AL. 5
poleward ocean heat export from the tropics (Sriver and
Huber, 2007).
5.Basin-wide changes and regional hot spots
A regional assessment of ocean warming is relevant to
community risk assessment and societal adaptation, even
more so than global metrics. Here, we briefly assess basin
OHC changes, including several regional hot spots.
5.1.The northwest Pacific Ocean
The Northwest Pacific Ocean is the most active basin
for tropical storms on the planet, accounting for one-third of
all tropical cyclones. Changes in its nature are directly relev-
ant to weather conditions and societies in East Asia. The
OHC shows strong inter-annual and decadal fluctuations in
this area (Fig. 4a), which are predominantly associated with
internal climate variabilities such as ENSO and Pacific
Decadal Variability (PDV) (Cheng et al., 2019b; Xiao et al.,
2020). The typhoon activities in the Northwest Pacific show
correlations with PDV and further reveal the remote
impacts of the tropical Indian Ocean and Western Pacific
(Yang et al., 2018). Nevertheless, the mean OHC over the
recent decade (2012–21) is higher than the 1981–2010 aver-
age, which could, in turn, impact the regional OHC (Wang
et al., 2014).
5.2.The Indian Ocean
There was no significant trend in the Indian OHC dur-
ing the late 20th century (Fig. 4b), owing to interdecadal
changes of the Indo-Pacific Walker circulation that drove an
upper-layer cooling of the tropical Indian Ocean and greatly
moderated the greenhouse-gas forced warming (Ummen-
hofer et al., 2021). Unprecedented rapid warming during the
2000s was mainly associated with an increased Indonesian
throughflow heat transport under a negative Interdecadal
Pacific Oscillation (IPO) condition (La Niña-like) of the trop-
ical Pacific (Lee et al., 2015; Li et al., 2018). The warming
trend has been delayed since ~2013, as the IPO shifted back
to its positive phase. Hence, the pronounced interannual fluc-
tuations in the past decade were primarily dictated by
ENSO. The thermocline of the southern tropical Indian
Ocean is particularly sensitive to teleconnection imprints of
ENSO (Xie et al., 2002), as evidenced by the thermocline
depression and OHC increase during the 2015−16 super El
Niño and subsequent thermocline shoaling and OHC
decrease during the 2017−18 La Niña (Li et al., 2020b;
Volkov et al., 2020) (Fig. 4b). The climate of the Southern
Ocean also modulates the subtropical sector of the southern
Indian Ocean. During past decades, the poleward migration
of westerly winds has led to a southward shift of the subtrop-
ical circulation gyre and thereby caused a multi-decadal
increasing trend in the OHC of the subtropical southern
Indian Ocean (Yang et al., 2020; Duan et al., 2021).
5.3.The tropical Atlantic Ocean
The tropical western North Atlantic Ocean 10°−30°N,
where hurricanes initiate and develop, has shown a continu-
ous increase of OHC since 1958 (Fig. 4c; Trenberth et al.,
2018). La Niña conditions typically are associated with
reduced tropical storm activity in the eastern tropical Pacific
and a greater risk of enhanced activity in the tropical North
Atlantic. In addition to the global warming signature from
continuous unabated warming since 1958 (Fig. 4c), this is
associated with high levels of Atlantic tropical storm activ-
ity during the past two northern summers (Emanuel, 2021a,
b). During 2021 there were 21 named storms, ~30% fewer
than the record 30 named storms of 2020. When atmo-
spheric conditions are favorable, the increased thermal
energy provided by high OHC fuels increased tropical
storm activity (in terms of both frequency, intensity, size
and/or lifetime) and also increases rainfall (Trenberth et al.,
2018).
5.4.The Mediterranean Sea
In 2021, the OHC in the Mediterranean Sea was the
highest on record since reliable records exist (Fig. 4d). A
marked temperature increase appeared in the last few dec-
ades, starting from the Eastern Basin where warmer Intermedi-
ate Waters (IW) formed and spread towards the Western
Basin on their way back to the North Atlantic (Pinardi et al.,
2015; Section 3.1 in von Schuckmann et al., 2016b; Simon-
celli et al., 2018, 2019; Storto et al., 2019). The Sicily Chan-
nel represents the chokepoint between the Eastern and West-
ern Mediterranean basins where a two-layer system brings
Atlantic Waters (0–150 m) eastwards and IW (150–450 m)
westwards (Schroeder et al., 2017). A temperature and salin-
ity time series collected since 1993 at 400 m depth at the
Sicily Channel show increasing trends for both parameters,
stronger than those observed at intermediate depths in differ-
ent regions of the global ocean (Ben Ismail et al., 2021).
The reported IW rate of temperature increase averages
0.026°C yr–1 with a maximum value of 0.034°C yr–1 after
2011. The IW enters the Tyrrhenian Sea after crossing the
Sicily Channel, where the temperature has been monitored
with XBT probes since 1999 along the MX04 Genova-
Palermo transect (Fig. 5d) as part of the Ship of Opportun-
ity program of GOOS. Figure 5 presents two Hovmöller
plots of mean temperature anomalies in the layer 0–800 m
layer computed along the transect considering different
baselines: one subtracting the monthly average computed
from the MX04 survey (Fig. 5a); the other subtracting the
IAP/CAS baseline 1981–2010 (Fig. 5b). Consistent and con-
tinuous warming of the IW appears from spring 2013 in the
150–450 m layer and propagates progressively deeper. The
mean temperature evolution in the 150–450 m layer (Fig. 5c)
indicates a steep increase between 2014 and 2017 and a lin-
ear trend over the time period 1999–2021 equal to 0.028°C
yr–1, consistent with the one observed in the Sicily Channel.
The overall mean temperature variation is approximately
0.6°C, with temperatures ranging from 13.8°C to 14.4°C.
The warm IW propagated northwards mainly from 38° to
42°N and, as in the Sicily Channel, the IW exhibited a slight
decrease in 2018-2019. The data gathered in September and
6RECORD OCEAN WARMING IN 2021
Fig. 4. Regional observed upper 2000 m OHC change from 1955 through 2021 relative to 1981–2010 baseline. The time series (black) are smoothed by LOWESS (locally weighted
scatterplot smoothing) with a span width of 24 months. The gray shadings are the 95% confidence interval. [Data updated from Cheng et al. (2017)].
CHENG ET AL. 7
December 2021 report mean values greater than or equal to
14.4°C, as in 2017, but the heat penetrates progressively
deeper and occupies a larger area.
5.5.The North Atlantic Ocean
The North Atlantic Ocean shows a warming trend for
the entire instrumental record, with an increasing trend com-
mencing around 2000 (Fig. 4e). The most dramatic warm-
ing occurs in the Gulf Stream region (Figs. 2, 3). Despite
the broad-scale warming, a key feature of the North Atlantic
Ocean is a cooling region south of Greenland (the so-called
“warming hole”) (Seidov et al., 2017). In single forcing exper-
iments, it is demonstrated that both GHG and AER forcings
can produce such a cooling signature (Fig. 3). Previous stud-
ies have indicated that this cooling trend reveals a slow-
down of the Atlantic meridional overturning circulation,
which leads to reduced oceanic heat transport into the warm-
ing hole region (Rahmstorf et al., 2015; Keil et al., 2020).
Other potential culprits include the changes to the Gulf
Stream extension region (Seidov et al., 2019; Piecuch,
2020; Boers, 2021), local atmospheric forcing (Li et al.,
2021), and Indian Ocean warming (Hu and Fedorov, 2020).
In 2021, the North Atlantic OHC was the highest on record
since 1958 (Fig. 4e).
5.6.The North Pacific Ocean
The North Pacific (30°−62°N) showed a dramatic
increase in OHC after 1990 and reached a record high level
in 2021 (Fig. 4f). The North Pacific Ocean warming is
broad and deep-reaching. Warming anomalies in 2021 (relat-
ive to the 1981–2010 baseline) are ~2°C near the surface
and 1oC at ~300 m in the middle North Pacific (Fig. 6). The
relentless increases in OHC have direct implications for the
frequency, intensity, and extent of marine heat waves
(MHWs) and other “hot spots within the ocean (Holbrook
et al., 2019). A series of MHWs have developed here within
the past decade. These are typically triggered by benign atmo-
spheric conditions with anticyclonic circulation, decreased
evaporative cooling, and lighter winds that lead to less
ocean mixing (Holbrook et al., 2019). In the North Pacific
winter, such conditions are favored by La Niña through an
atmospheric bridge (Trenberth et al., 1998), and this helped
set up the North Pacific MHW in 2013-14. Such events
impact marine life and propagate through the food web to lar-
ger marine creatures. “The Blob” was one such strong event
from 2014–16 and demonstrated that the entire ocean food
web can be severely impacted due to these extremes (Corn-
wall, 2019). The 2021 North Pacific warm “blob” was a con-
tinuation of the 2020 anomaly. Scannell et al. (2020) indic-
ate that the oceanic conditions (OHC, salinity, and stratifica-
tion) support the development of marine heat waves in the
North Pacific Ocean. The marine heat wave is also associ-
ated with an atmospheric circulation pattern. A “high-pres-
sure heat dome” developed in late June over the Pacific North-
west and southwest Canada, along with temperatures that
exceeded 40°C (a record-breaking major heatwave), which
resulted in widespread wildfires and associated destruction.
In September and October 2021, the MHW continued and
was strongest in the central North Pacific, consistent with
La Niña influences through the atmospheric bridge from the
Fig. 5. The temperature along the MX04 Genova-Palermo transect in the 1999–2021 time period recorded by XBT probes from
ships of opportunity: (a) Hovmöller plot of mean temperature anomalies computed subtracting the overall average 1999–2021
from each monitoring survey; (b) Hovmöller plot of mean temperature anomalies computed subtracting the IAP/CAS 1981–2010
baseline from each monitoring survey; (c) mean temperature values and relative linear trend in the layer 150–450 m; (d) data
distribution map in the Tyrrhenian and Ligurian Seas (Western Mediterranean).
8RECORD OCEAN WARMING IN 2021
tropics.
5.7.The Southern Ocean
According to model simulations, the Southern Ocean
(SO) served as a major anthropogenic heat sink in the past
century (Frölicher et al., 2015); observation-based estim-
ates suggest that the SO accounted for 30%–60% of the
global OHC increase since ~1970 (IPCC, 2019), contempo-
raneous with significant ocean warming (Fig. 4g; Gille,
2002). Mass subduction of water from the SO mixed layer
serves as a rapid pathway for anthropogenic warming signa-
tures to be communicated to the deep ocean (Marshall and
Speer, 2012). Based on CMIP5 model simulations, Silvy et
al. (2020) showed that the anthropogenic signal ‘emerges’
from the noise and overwhelms the natural variability much
more quickly in the well-ventilated SO water masses than in
Northern Hemisphere waters. The SO shows a clear warm-
ing peak on the northern flank of the Antarctic Circumpolar
Current (ACC) (Figs. 2 and 6) (Böning et al., 2008; Gouret-
ski et al., 2013; Roemmich et al., 2015), corresponding to
the rapid warming of the Subantarctic Mode Water
(SAMW) (Gao et al., 2018) and Antarctic Intermediate
Water (Schmidtko and Johnson, 2012) with widespread
impacts across the entire Southern Hemisphere. By 2021,
Fig. 6. 3-D fields of oceanic temperature changes at 2021 (January to September) relative to 1981–2010 baseline in the
North Pacific Ocean (30°−62°N) (upper), and in the Southern oceans (78°−30°S) (bottom). The IAP/CAS data are used, and
the illustration is modified from Seidov et al. (2021).
CHENG ET AL. 9
the warming exceeded 0.6°C in the major formation regions
of the SAMW, compared to the 1981–2010 baseline (Fig.
6b). The Antarctic continental shelf and adjacent seas also
show significant warming trends (Schmidtko et al., 2014)
linked to the detected warming of abyssal and bottom
waters over the globe (Purkey and Johnson, 2010, 2013).
The surface layer of the subpolar regions shows little warm-
ing and has slightly cooled over the past decades, owing to
wind-driven upwelling, increased northward heat export to
the northern flank of the ACC, and sea ice changes (Armour
et al., 2016). Purich et al. (2018) added that the freshening
of the SO — linked to the amplification of the global hydrolo-
gical cycle may also favor subpolar surface cooling by
reducing the entrainment of subsurface warm water. These
complex heat uptake and storage patterns are primarily attrib-
utable to greenhouse-gas forcing, with stratospheric ozone
depletion playing a secondary role (Swart et al., 2018).
6.Summary and implications for climate
actions
This study provides up-to-date estimates of OHC
through 2021 based on two international datasets. Single for-
cing simulations are used to analyze the contribution of differ-
ent forcings on the regional OHC pattern. The regional and
global changes both reveal a robust and significant ocean
warming since the late-1950s the entirety of the reliable
instrumental record. Natural variability and change in ocean
circulation play important roles locally, but the predomin-
ant changes result from human-related changes in atmo-
spheric composition. As oceans warm, the water expands,
and sea level rises. Preparing for sea level rise and its implica-
tions for coastal communities are particularly important.
Changes to engineering design, building codes, and modi-
fications to coastal development plans are recommended in
anticipation of increased sea levels and increases in extreme
precipitation events, which are already being observed (Abra-
ham et al., 2015, 2017; Scambos and Abraham, 2015). In addi-
tion, warmer oceans supercharge the weather systems, creat-
ing more powerful storms and hurricanes, and increased pre-
cipitation. Warmer oceans lead to a warmer and moister atmo-
sphere that promotes more intense rainfall in all storms, espe-
cially hurricanes, thereby increasing the risk of flooding
(Trenberth et al. 2003; IPCC, 2021). Warming ocean waters
threaten marine ecosystems and human livelihoods, for
example, coral reefs and fisheries (IPCC, 2019).
The United Nations has proposed 17 inter-linked goals
and 169 targets focused on maintaining the health and vital-
ity of human societies and natural ecosystems worldwide
(UN, 2021). The Sustainable Development Goals (SDGs)
were set up in 2015 by the United Nations General
Assembly and are intended to be achieved by 2030. Many
goals are closely related to the health of the world’s oceans,
and the ocean’s health is vital to the SDGs. For example,
the SDG13-Climate Actions; SDG14-Life Below Water;
SDG6-Clean Water and Sanitation; SDG2-Zero Hunger;
SDG3-Good Health and Well-Being (Abram et al., 2019;
von Schuckmann et al., 2020) are all either directly or indir-
ectly related to oceans. In 2021, the United Nations initi-
ated the Decade of Ocean Science for SDGs to increase the
awareness of many problems the ocean faces and to build
up capability for ocean monitoring and scientific research.
This study indicates that there are still uncertainties and know-
ledge gaps in monitoring ocean warming; for example, the
quantification of uncertainty from inter-annual to multi-
decadal scales and the impact of data QC and regional differ-
ences of OHCs revealed by different datasets. Thus, better
awareness and understanding of ocean dynamics are funda-
mental in combating climate change.
Acknowledgements.  The IAP/CAS analysis is supported by
the National Natural Science Foundation of China (Grant No.
42122046, 42076202), Strategic Priority Research Program of the
Chinese Academy of Sciences (Grant No. XDB42040402),
National Natural Science Foundation of China (Grant No.
42076202), National Key R&D Program of China (Grant No.
2017YFA0603202), and Key Deployment Project of Centre for
Ocean Mega-Research of Science, CAS (Grant Nos.
COMS2019Q01 and COMS2019Q07). NCAR is sponsored by the
US National Science Foundation. The efforts of Dr. Fasullo in this
work were supported by NASA Award 80NSSC17K0565, and by
the Regional and Global Model Analysis (RGMA) component of
the Earth and Environmental System Modeling Program of the
U.S. Department of Energy’s Office of Biological & Environ-
mental Research (BER) via National Science Foundation IA
1844590. The efforts of Dr. Mishonov and Mr. Reagan were par-
tially supported by NOAA (Grant NA14NES4320003 to CISESS-
MD at the University of Maryland). The IAP/CAS data are avail-
able at http://www.ocean.iap.ac.cn/ and https://msdc.qdio.ac.cn/.
The NCEI/NOAA data are available at https://www.ncei.noaa.
gov/products/climate-data-records/global-ocean-heat-content. The
historical XBT data along the MX04 line (Genova-Palermo) are
available through SeaDataNet - Pan-European infrastructure
(http://www.seadatanet.org) for ocean and marine data manage-
ment. Since 2021, XBT data have been collected in the frame-
work of the MACMAP project funded by the Istituto Nazionale di
Geofisica e Vulcanologia in agreement between INGV, ENEA,
and GNV SpA shipping company that provides hospitality on their
commercial vessels.
ing, adaptation, distribution and reproduction in any medium or
format, as long as you give appropriate credit to the original author (s)
and the source, provide a link to the Creative Commons licence,
and indicate if changes were made. The images or other third party
mons licence, unless indicated otherwise in a credit line to the mater-
ial. If material is not included in the article’s Creative Commons
licence and your intended use is not permitted by statutory regula-
tion or exceeds the permitted use, you will need to obtain permis-
sion directly from the copyright holder. To view a copy of this
10 RECORD OCEAN WARMING IN 2021
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CHENG ET AL. 13
... The burning of fossil fuels adds more carbon than what is taken up, leading to an accumulation and rise in the concentration of CO2 and other greenhouse gasses in the atmosphere and dissolved in water. As greenhouse gasses redirect heat that is radiating back into space, earth surface temperature not only increases, but the increased energy in the atmosphere and ocean, where most of the accumulated energy is stored (Cheng et al. 2022). ...
... The increased atmospheric CO2 concentration that is a main driver of climate change also may affect plants directly by allowing increased photosynthesis. Chen et al. (2022) quantified this CO2 fertilization effect (CFE) by combining observations from a globally distributed network of eddy covariance measurements with photosynthetic optimization theories, concluding with an enhancement of photosynthesis and that CFE has played a critical role in the global carbon cycle in recent decades. However, Launiainen et al. (2022) working with eddy-covariance measurements of net ecosystem production, evapotranspiration, light and water use efficiency from a boreal coniferous forest in Southern Finland found similar trends for CFE, but were able to look closer at the mechanism in their system. ...
... Most of the energy retained by increasing atmospheric greenhouse gas concentrations is absorbed by the world's oceans. The year 2021 set a new record for ocean heat content, meaning it was on average the warmest ocean temperatures recorded by humans (Cheng et al. 2022). The North Atlantic temperature plays an important role for regional temperature and precipitation in Norway, and is heavily influenced by the Gulf Stream. ...
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In this report, we summarize the current state of knowledge and best estimates of how climate change is expected to impact Norwegian forest ecosystems from now to the year 2100
... However, Piecuch & Quinn [109] showed that the steric component also contributes to the positive/negative GMSL anomalies during ENSO events. This was recently confirmed by Hamlington et al. [110,111], who demonstrated that both the steric and mass components equally contribute to the interannual variability of the GMSL, especially during El Niño and La Niña episodes (see also [112,113]). ...
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Sea-level variations spread over a very broad spectrum of spatial and temporal scales as a result of complex processes occurring in the Earth System in response to natural variability of the climate system, as well as to external forcing due to natural phenomena and anthropogenic factors. Here, we address contemporary sea-level changes, focusing on the satellite altimetry era (since the early 1990s), for which various observing systems from space and in situ allow precise monitoring of sea-level variations from global to local scales, as well as improved understanding of the components responsible for the observed variations. This overview presents the most recent results on observed global and regional sea-level changes and on associated causes, focusing on the interannual to decadal time scale. Recent progress in measuring sea level at the coast are presented. Finally, a summary of the most recent sea-level projections from the Intergovernmental Panel on Climate Change is also provided.
... Consequently, climate trends generally require long-duration consistent and calibrated measurements. The methods used to extract OHC changes are discussed and applied in ( Cheng et al., , 2022Abraham et al., 2013 ;Meyssignac et al., 2019 ). ...
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The ocean's thermal inertia is a major contributor to irreversible ocean changes exceeding time scales that matter to human society. This fact is a challenge to societies as they prepare for the consequences of climate change, especially with respect to the ocean. Here the authors review the requirements for human actions from the ocean's perspective. In the near term (∼2030), goals such as the United Nations Sustainable Development Goals (SDGs) will be critical. Over longer times (∼2050–2060 and beyond), global carbon neutrality targets may be met as countries continue to work toward reducing emissions. Both adaptation and mitigation plans need to be fully implemented in the interim, and the Global Ocean Observation System should be sustained so that changes can be continuously monitored. In the longer-term (after ∼2060), slow emerging changes such as deep ocean warming and sea level rise are committed to continue even in the scenario where net zero emissions are reached. Thus, climate actions have to extend to time scales of hundreds of years. At these time scales, preparation for “high impact, low probability” risks — such as an abrupt showdown of Atlantic Meridional Overturning Circulation, ecosystem change, or irreversible ice sheet loss — should be fully integrated into long-term planning. 摘要 在全球变化背景下, 海洋的很多变化在人类社会发展的时间尺度上 (百年至千年) 具有不可逆转性, 海洋巨大的热惯性是造成该不可逆性的主要原因. 这个特征为人类和生态系统应对海洋变化提出一系列挑战. 本文从海洋变化的角度总结了人类应对气候变化的要求, 提出需要进行多时间尺度的规划和统筹. 在近期 (到2030年) , 实现联合国可持续发展目标至关重要. 在中期 (2050–2060年前后) , 全球需要逐步减排并实现碳中和目标. 同时, 适应和减缓气候变化的行动和措施必须同步施行; 全球海洋观测系统需要得以维持并完善以持续监测海洋变化. 在远期 (在2060年之后) , 即使全球达到净零排放, 包括深海变暖和海平面上升在内的海洋变化都将持续, 因此应对全球变化的行动需持续数百年之久. 在该时间尺度, 应对“低概率, 高影响”气候风险 (即发生的可能性较低, 但一旦发生影响极大的事件带来的风险, 例如: 大西洋经圈反转环流突然减弱, 海洋生态系统跨过临界点, 无可挽回的冰盖质量损失等) 的准备应充分纳入长期规划.
... Our study implicates that more C. acicula outbreaks may occur in the warming ocean. The average global ocean surface temperature has been increasing over the past 100 years (Cheng et al., 2022;Huang et al., Fig. 6. Allocation of ingested food carbon by Creseis acicula at different temperatures. ...
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An outbreak of a macrozooplankton Creseis acicula occurred in the summer of 2020 nearby the Daya Bay Nuclear Power Plant located on the coast of the Daya Bay in the South China Sea. The outbreaks of C. acicula often threaten human health, the marine environment, and other human activities including the safe operation of coastal nuclear power plants. Seawater temperature has been suggested as an important factor influencing such outbreaks. However, the underlying mechanisms through which temperature influences C. acicula remains unknown. Here, we studied the effects of temperature on the ingestion and assimilation of algal food by feeding radiocarbon-labeled algae Chlorella sp. at simulated field temperatures (19–31 °C) to C. acicula collected during the outbreak in the Daya Bay. We also quantified the allocation of the food carbon to dissolved organic carbon (DOC), CO2, and fecal pellets. The results showed that the zooplankton during the same feeding time ingested doubled or tripled algal food at higher temperatures, and it produced and released significantly more DOC, CO2, and fecal pellets with more ingested food carbon. Meanwhile, the assimilation efficiency for the ingested food carbon slightly increased from 48% to 54% with rising temperature. As a result, higher assimilation rates indicating faster growth of C. acicula were observed at higher temperatures. In addition, the high activation energy of 0.908 eV indicated that the assimilation rate was very sensitive to temperature rising. Our results show that relatively rising temperature can enhance C. acicula’s ingestion and assimilation rates for algal food, benefit its growth and metabolism, and contribute to its outbreak. This study provides a mechanistic interpretation for the relationship between rising temperature and the outbreaks of C. acicula and suggests that such outbreaks may occur more frequently and widely in the warming ocean.
... The global ocean warming trend, with a 0.06 • C temperature increase per decade since 1900, is a direct consequence of anthropogenic CO 2 (Garcia-Soto et al., 2021;Bindoff et al., 2013). Ocean adsorbed almost 90% of excess heat from Earth system since the industrial revolution, reaching an unprecedented heat content in 2021, the hottest year ever recorded (Cheng et al., 2022). This long-term and gradual warming is superimposed to an increased frequency of marine heatwaves (MHWs), short-term extreme events with seawater temperature exceeding a seasonally varying threshold (usually the 90th percentile) for at least 5 consecutive days (Collins et al., 2019;Frölicher et al., 2019;Oliver et al., 2019;Hobday et al., 2018;Perkins et al., 2012). ...
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The increased frequency and intensity of short-term extreme warming phenomena have been associated to harsh biological and ecosystem outcomes (i.e., mass mortalities in marine organisms). Marine heatwaves (MHWs), occurring when seasonal temperature threshold is exceeded for at least 5 consecutive days, may reduce the tolerance of coastal species toward additional pressures, but interactions between such multiple stressors are virtually unexplored. The present study aimed to characterize in Mytilus galloprovincialis the influence of a simulated MHW scenario on the toxicological effects of the pharmaceutical carbamazepine (CBZ), ubiquitously detected in the marine environment and chosen as model compound for this relevant class of emerging contaminants. The bioaccumulation of CBZ and responsiveness of various biological parameters, including immune system, antioxidant status, lipid metabolism and cellular integrity, were analyzed in exposed mussels both during and after the end of the heatwave. MHW appeared to strongly modulate accumulation of CBZ, paralleled by weakened immunocompetence and onset of oxidative disturbance that finally evolved to cellular damages and lipid metabolism disorders. Elaboration of the overall results through a quantitative Weight of Evidence model, revealed the highest hazard in organisms exposed to both the stressors 10 days after the end of the heatwave, suggesting that MHWs could leave a footprint on the capability of mussels to counteract CBZ toxicity, thus affecting their vulnerability and predisposition to adverse effects toward multiple stressors.
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A well-known exception to rising sea surface temperatures (SST) across the globe is the subpolar North Atlantic, where SST has been declining at a rate of 0.39 ($$\pm$$ 0.23) K century−1 during the 1900–2017 period. This cold blob has been hypothesized to result from a slowdown of the Atlantic Meridional Overturning Circulation (AMOC). Here, observation-based evidence is used to suggest that local atmospheric forcing can also contribute to the century-long cooling trend. Specifically, a 100-year SST trend simulated by an idealized ocean model forced by historical atmospheric forcing over the cold blob region matches 92% ($$\pm$$ 77%) of the observed cooling trend. The data-driven simulations suggest that 54% ($$\pm$$ 77%) of the observed cooling trend is the direct result of increased heat loss from the ocean induced by the overlying atmosphere, while the remaining 38% is due to strengthened local convection. An analysis of surface wind eddy kinetic energy suggests that the atmosphere-induced cooling may be linked to a northward migration of the jet stream, which exposes the subpolar North Atlantic to intensified storminess.
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The subtropical oceans between 35°-20°S in the Southern Hemisphere (SH) have exhibited prevailingly rapid sea-level rise (SLR) rates since the mid-20 th century, amplifying damages of coastal hazards and exerting increasing threats to South America, Africa, and Australia. Yet, mechanisms of the observed SLR have not been firmly established, and its representation in climate models has not been examined. By analyzing observational sea-level estimates, ocean reanalysis products, and ocean model hindcasts, we show that the steric SLR of the SH subtropical oceans between 35°-20°S is faster than the global mean rate by 18.2%±9.9% during 1958-2014. However, present climate models—the fundamental bases for future climate projections—generally fail to reproduce this feature. Further analysis suggests that the rapid SLR in the SH subtropical oceans is primarily attributable to the persistent upward trend of the Southern Annular Mode (SAM). Physically, this trend in SAM leads to the strengthening of the SH subtropical highs, with the strongest signatures observed in the southern Indian Ocean. These changes in atmospheric circulation promote regional SLR in the SH subtropics by driving upper-ocean convergence. Climate models show systematic biases in the simulated structure and trend magnitude of SAM and significantly underestimate the enhancement of subtropical highs. These biases lead to the inability of models to correctly simulate the observed subtropical SLR. This work highlights the paramount necessity of reducing model biases to provide reliable regional sea-level projections.
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The Atlantic Meridional Overturning Circulation (AMOC), a major ocean current system transporting warm surface waters toward the northern Atlantic, has been suggested to exhibit two distinct modes of operation. A collapse from the currently attained strong to the weak mode would have severe impacts on the global climate system and further multi-stable Earth system components. Observations and recently suggested fingerprints of AMOC variability indicate a gradual weakening during the last decades, but estimates of the critical transition point remain uncertain. Here, a robust and general early-warning indicator for forthcoming critical transitions is introduced. Significant early-warning signals are found in eight independent AMOC indices, based on observational sea-surface temperature and salinity data from across the Atlantic Ocean basin. These results reveal spatially consistent empirical evidence that, in the course of the last century, the AMOC may have evolved from relatively stable conditions to a point close to a critical transition. The Atlantic Meridional Overturning Circulation (AMOC) is currently strong, but transition to its weak mode could see significant changes in the climate system. This work presents an observation-based early-warning system for such transitions and shows that the AMOC may be approaching a transition.
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Multiple 50-member ensemble simulations with the Community Earth System Model version 2 are performed to estimate the coupled climate responses to the 2019–2020 Australian wildfires and COVID-19 pandemic policies. The climate response to the pandemic is found to be weak generally, with global-mean net top-of-atmosphere radiative anomalies of +0.23 ± 0.14 W m⁻² driving a gradual global warming of 0.05 ± 0.04 K by the end of 2022. While regional anomalies are detectable in aerosol burdens and clear-sky radiation, few significant anomalies exist in other fields due to internal variability. In contrast, the simulated response to Australian wildfires is a strong and rapid cooling, peaking globally at −0.95 ± 0.15 W m⁻² in late 2019 with a global cooling of 0.06 ± 0.04 K by mid-2020. Transport of fire aerosols throughout the Southern Hemisphere increases albedo and drives a strong interhemispheric radiative contrast, with simulated responses that are consistent generally with those to a Southern Hemisphere volcanic eruption.
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The COVID-19 (coronavirus disease 2019) pandemic has resulted in a marked slowdown in greenhouse gas and aerosol emissions. Although the resulting emission reductions will continue to evolve, this will presumably be temporary. Here, we provide estimates of the potential effect of such short-term emission reductions on global and regional temperature and precipitation by analyzing the response of an Earth System Model to a range of idealized near-term emission pathways not considered in available model intercomparison projects. These estimates reveal the modest impact that temporary emission reductions associated with the COVID-19 pandemic will have on global and regional climate. Our simulations suggest that the impact of carbon dioxide and aerosol emission reductions is actually a temporary enhancement in warming rate. However, our results demonstrate that even large emission reductions applied for a short duration have only a small and likely undetectable impact.
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Across the Indo-Pacific region, rapid increases in surface temperatures, ocean heat content and concomitant hydrological changes have implications for sea level rise, ocean circulation and regional freshwater availability. In this Review, we synthesize evidence from multiple data sources to elucidate whether the observed heat and freshwater changes in the Indian Ocean represent an intensification of the hydrological cycle, as expected in a warming world. At the basin scale, twentieth century warming trends can be unequivocally attributed to human-induced climate change. Changes since 1980, however, appear dominated by multi-decadal variability associated with the Interdecadal Pacific oscillation, manifested as shifts in the Walker circulation and a corresponding reorganization of the Indo-Pacific heat and freshwater balance. Such variability, coupled with regional-scale trends, a short observational record and climate model uncertainties, makes it difficult to assess whether contemporary changes represent an anthropogenically forced transformation of the hydrological cycle. Future work must, therefore, focus on maintaining and expanding observing systems of remotely sensed and in situ observations, as well as extending and integrating coral proxy networks. Improved climate model simulations of the Maritime Continent region and its intricate exchange between the Pacific and Indian oceans are further necessary to quantify and attribute Indo-Pacific hydrological changes.