Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition

Abstract

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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RETRACTED ARTICLE
LETTER https://doi.org/10.1038/s41586-018-0651-8
Quantification of ocean heat uptake from changes in
atmospheric O2 and CO2 composition
L. Resplandy1*, R. F. Keeling2, Y. Eddebbar2, M. K. Brooks2, R. Wang3, L. Bopp4, M. C. Long5, J. P. Dunne6, W. Koeve7 & A. Oschlies7
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
2007
2,3
. However, these estimates all use the same imperfect ocean
dataset and share additional uncertainties resulting from sparse
coverage, especially before 2007
4,5
. Here we provide an independent
estimate by using measurements of atmospheric oxygen (O
2
) 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×10
22
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.
As shown in Fig.1, recent temperature-based hydrographic esti-
mates of ocean warming
9–12
show good agreement for the years 2007–
2016 (1.09±0.10×1022 to 1.16±0.20×1022Jyr−1), but a larger
spread when extending back to include the sparser data of the 1990s
(0.90±0.09×1022 to 1.36±0.10×1022Jyr−1 for 1993–2015). The
spread is mostly caused by gap-filling methods and systematic errors
5,9
,
which together introduce uncertainties of up to 25%–50% in warming
trends
4
. Because temperature-based estimates also use the same upper-
ocean observations and linear warming trend for depths below 2,000m
(ref.
11
), they may share additional unknown systematic errors
12
. An
alternative method based on the top of the atmosphere energy balance
13
is also not truly independent, because it is subject to large systematic
errors when estimating long-term trends and therefore depends on the
same hydrographic measurements for calibration13–15. Here we intro-
duce a third method, based on changes in the abundances of gases in
the atmosphere, which respond to whole-ocean warming through the
temperature dependence of gas solubility in sea water. This method is
not limited by data sparseness, because fast mixing in the atmosphere
efficiently integrates the global ocean signal.
Changes in ocean heat content on seasonal16 and glacial–interglacial17
timescales have been reconstructed using measurements of noble
gases in modern or ancient air. Our method is similar, but instead of
relying on noble gases (for example, ratios of argon to nitrogen), which
lack sufficient accuracy as yet
16
, we rely on measurements of atmos-
pheric O
2
and CO
2
, which can be summed to yield a tracer ‘atmospheric
potential oxygen’ (APO) that responds to warming similarly to a noble
gas
18
. When the ocean warms, the solubility of O
2
and CO
2
drops, and
the amount of gas lost by the ocean can be quantified with the com-
plementary change observed in the atmosphere. Precise atmospheric
O
2
measurements began in 1991 (CO
2
in 1958), enabling APO-based
reconstructions of ocean heat content that span nearly three decades
6
.
APO (O
2
+1.1×CO
2
) is computed using observed atmospheric
O2/N2 molar ratios and CO2 molar fractions (seeMethods)6,19. By
design, APO is insensitive to exchanges with land ecosystems, which
produce changes in O
2
and CO
2
that largely cancel in APO owing to
their approximate 1.1 O
2
/C oxidative ratio. Time-series measurements
at remote sites show a global long-term decline in APO, with ΔAPO
OBS
being −243.70±10.10 per meg (units defined in theMethods) between
1991 and 2016. ΔAPO
OBS
is driven by four primary contributors, illus-
trated in Fig.2:
Δ=Δ+Δ+Δ+ΔAPOAPO APOAPO APO(1)
OBSFFCantAtmDClimate
where ΔAPOFF is the decrease in APO caused by industrial processes
(fossil-fuel burning and cement production), which in aggregate
consume more than 1.1 moles of O2 for each mole of CO2 released;
ΔAPOCant accounts for the oceanic uptake of excess anthropogenic
atmospheric CO2; ΔAPOAtmD accounts for air–sea exchanges driven by
ocean fertilization from anthropogenic aerosol deposition (increased
fertilization leads to increased photosynthesis, with a concomitant
release of O2and uptakeof CO2); and ΔAPOClimate accounts for air–sea
fluxes of O2, CO2 and N2 driven by ocean processes, including warming-
induced changes in solubility, in ocean circulation, and in photosyn-
thesis and respiration (N2 influences O2/N2 ratios). Here, we derive
ΔAPOClimate from equation (1) and show that it tracks ocean warming.
We estimate ΔAPOFF using fossil-fuel and cement inventories20,
finding ΔAPO
FF
=−119.70±4.00per meg (Fig.3). ΔAPO
Cant
is con-
trolled by the increase in atmospheric CO
2
and by ocean mixing, which
is quantified by the distribution of transient tracers including chloro-
fluorocarbons (CFCs)
21
; we find that ΔAPO
Cant
=−154.30±4.20per
meg. ΔAPO
Cant
is relatively precise because it excludes the effects of
changing ocean biology and circulation on natural carbon fluxes
that are included in ΔAPOClimate. ΔAPOAtmD is derived from ocean
model simulations with and without aerosol fertilization (phosphate,
iron and nitrogen; Extended Data Fig.1)
22
. ΔAPO
AtmD
is uncertain,
owing in part to uncertainties in iron availability to photosynthetic
organisms, but is relatively small compared with the other terms:
ΔAPO
AtmD
=7.00±3.50per meg. From equation (1), we thereby find
that ΔAPO
Climate
=23.20±12.20per meg, corresponding to a least-
squares linear trend of +1.16±0.15per meg per year—larger than the
trends expected from 26-year natural variations alone in four Earth-
system models (the Community Earth System Model (CESM) and the
Geophysical Fluid Dynamics Laboratory (GFDL), Institut Pierre Simon
Laplace (IPSL) and University of Victoria (UVic) models). As shown in
Fig.3, a clear increase in ΔAPOClimate emerges over the period January
1991 to the end of December 2016.
A starting point for understanding ΔAPO
Climate
is to imagine that
O2 and CO2 behave like inert gases, such that the air–sea fluxes are
dominated by temperature-driven solubility changes. In this case,
1Department of Geosciences and Princeton Environmental Institute, Princeton University, Princeton, NJ, USA. 2Scripps Institution of Oceanography, University of California San Diego, La Jolla,
CA, USA. 3Department of Environmental Science and Engineering, Fudan University, Shanghai, China. 4LMD/IPSL, ENS, PSL Research University, École Polytechnique, Sorbonne Université, CNRS,
Paris, France. 5National Center for Atmospheric Research, Boulder, CO, USA. 6NOAA, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA. 7GEOMAR Helmholtz Centre for Ocean Research
Kiel, Kiel, Germany. *e-mail: laurer@princeton.edu
Retracted: Retraction Note
1 NOVEMBER 2018 | VOL 563 | NATURE | 105
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