Q: How much of global warming is in the ocean?
A: 90%
Q: How long will it take for the ocean temperature to return to normal?
A: Hundreds of years
“The irreversibility of this ocean warming on centennial timescales creates additional requirements for climate policy, particularly considering the widespread impacts.”
Q: What is my greatest concern?
A: The rate of warming is accelerating… and the warmer the ocean, the less carbon the ocean absorbs… creating a nasty feedback loop.
“Increasing anthropogenic greenhouse gas (GHG) concentrations cause a positive Earth energy imbalance (EEI), with resulting surplus heat in the climate system increasing ocean heat content (OHC). Unprecedented oceanic warming has been observed since at least the 1950s, reaching record values from 2012–2021. This oceanic warming has been pervasive, spreading from the surface to the abyssal layers, and with the long-term OHC trend accelerating.”
These were the finding reported in the Nature Reviews Earth & Environment report Past and Future Ocean Warming
Impacts and consequences of ocean warming
Observed and projected ocean warming has substantial impacts across major Earth system components and scales (Fig. 1). For example, ocean warming accounts for more than 1/3 of global-mean sea level rise through thermal expansion, and thus dominates regional sea level patterns82,202. Sea level rise, in turn, increases the risks for coastal infrastructures and coastal habitats from salt water intrusion, coastal erosion and flooding in low-lying regions232,233 (Fig. 1b). Ocean warming also decreases ocean density and increases upper-ocean stratification by 5.3% since 1960 (ref.234) (Fig. 1b), affecting the vertical and lateral exchanges of heat, carbon, oxygen, nutrients and other substances. The stratification increase, solubility reduction and circulation changes drive deoxygenation in the ocean interior by ~0.5–3.3% since the 1960s (refs.6,235). By changing sea water buoyancy, ocean warming impacts ocean currents, for example, accelerating the zonally averaged Southern Ocean zonal flow in the upper layer174. Warmer water also reduces the efficiency of oceanic carbon uptake and storage28,236 (Fig. 1b). The compounded effect of each of these impacts, especially following extreme events236,237,238, poses more substantial stress on the environment than their individual effects236 (Fig. 1b), driving, for example, changes in net primary and export production239,240 with socioeconomic impacts on marine fisheries and aquaculture systems241,242. Indeed, it is projected that, driven by multiple stressors, the maximum catch potential of tropical fish stocks in some tropical exclusive economic zones will decline by up to 40% by the 2050s under the RCP8.5242.
MHWs offer a strong example, whereby the relentless increase in OHC has direct implications for the frequency, intensity and extent of MHWs and other ‘hot spots’ within the ocean (Fig. 1b). With human-induced global warming and higher ocean heat content, it is inevitable that MHWs become more abundant, extensive and longer-lasting237. The highly anomalous ocean waters, including SSTs and upper OHC, often persist for more than a month243, resulting in large impacts on ocean ecosystems and marine life. Effects from thermal stress causes mass mortality of benthic communities, including coral bleaching, changes in phytoplankton blooms, adverse effects on kelp forests and sea grasses, toxic algal blooms, shifts in species composition and geographical distribution, and decline in fish and fisheries and seabirds105,238. As such, MHWs modify ecosystem assemblages, biodiversity, population extinctions and redistribution of habitat244,245.
One example is the prolonged MHW known as ‘the blob’ in the northeast Pacific and Gulf of Alaska from 2014–2016. This event greatly affected the ocean food web, shrinking phytoplankton blooms, which, in turn, diminished copepods, zooplankton and krill, and small fish, leading to the demise of ~1 million birds (notably murres), ~100 million cod and hundreds of humpback whales104,105. A similar unprecedented MHW in the south Tasman Sea in 2015–2016, where SSTs were up to 2.9 °C above normal owing to an ENSO-related alteration of the ITF and East Australian Current119, also had substantial impacts246. Ecosystem impacts ranged from new disease outbreaks in farmed shellfish (oysters, abalone) and salmon, to mass mortality of wild mollusks, and many out-of-range observations of several fish species. Thus, with continued warming, MHW events and their impacts are expected to worsen: climate models project that the frequency of MHW might increase 50 times by 2081–2100 relative to 1850–1900 under RCP8.5 (ref.247).
Ocean warming also intensifies tropical cyclones248, and associated changing ocean surface currents can indirectly affect pathways of storms139 (Fig. 1b). In August 2017, the Gulf of Mexico became the warmest on record to that point in the summertime. There was a strong link between upper OHC and record high rainfalls of over 60 inches (1,520 mm) over five days and extensive flooding in hurricane Harvey over parts of Texas249. Other processes at the air–sea interface affected by ocean warming include an increase of surface evaporation and rainfall12,13, and an increase in precipitation anomalies tied to ENSO, and associated extreme weather events250,251 (Fig. 1b).
Implications of ocean warming are also widespread across the Earth’s cryosphere195, and have in turn affected the ocean itself 252 (Fig. 1b). Examples include the thinning of floating ice shelves and marine terminating glaciers from basal ice melt21,253, and the retreat and speedup of ice-sheet outlet glaciers in Greenland and Antarctica254 and of tidewater glaciers in South America and in the Arctic255.
Other particular concerns are the potential abrupt changes associated with warming, such as ocean circulation (for example, AMOC)256 and ocean ecosystems6,248. Ocean warming is a key driving element for AMOC changes. The potential for abrupt AMOC collapse as a ‘low-probability, high-impact’ event cannot be ruled out in the future6,247. Each species of marine organisms has an optimal temperature window for functioning; most organisms are therefore vulnerable to warming257. It is projected that most tropical coral reefs will be threatened258.
Summary and future perspectives
In summary, OHC has changed substantially since the 1950s and is projected to continue to do so in the future. In the upper 2,000 m, net global increases of 351.4 ± 59.8 ZJ (0.36 ± 0.06 W m−2) have been observed from 1958 to 2019, with the rate of warming accelerating from ~0.0–0.3 W m−2 in the 1960s to ~0.5–0.7 W m−2 in the 2010s. The pattern of ocean warming has been non-uniform in this historical era, including strong warming in the Atlantic and southern oceans, and overall is dominated by the redistribution of ocean heat by currents. Relative to 2005–2019, future warming is projected to reach 1,030 [839–1,228] ZJ for SSP1-2.6 and 1,874 [1,637–2,109] ZJ for SSP5-8.5 at the end of this century, ~2–4 (SSP1-2.6) to ~4–6 times (SSP5-8.5) the observed 1958–2019 change. On these timescales, added heat has an important role for OHC projections. Moreover, low GHG emissions would be likely to lead to a detectable and lasting reduction in ocean warming rate, with noticeable reductions in climate-change impacts. Indeed, given that ocean warming has already led to pervasive impacts and consequences, monitoring, understanding, adapting to and mitigating ocean warming must continue to be a high priority. Nonetheless, several gaps remain in measuring, estimating and understanding ocean warming.
First, the current ocean observing system needs to be sustained and extended to monitor OHC change at various spatiotemporal scales. The critical question of ‘how adequate is the ocean observing system for monitoring the OHC changes at various timescales?’ is still not fully answered. The international community have cast their eyes toward the future to improve not only the Argo fleet but also other measurement methodologies259,260,261. The ongoing and planned efforts include the maintenance of the current GOOS, and shipboard in situ measurements for calibration, validation and quality control of the Argo array; a drive toward spatial completion, including polar sea-ice zones, marginal seas and complicated channels; increased resolution in critical areas such as boundary currents and coastal areas; incorporation of deeper measurements (for example below 2,000 m); and inclusion of biological and chemical signals, along with temperature and salinity (Deep Argo and BioGeoChemical Argo programmes). The scientific community and funding agencies will need to be mindful of ensuring a continuous and comprehensive measurement network for the world’s ocean — a network that incorporates new technologies as they are developed and retires old technologies that have outlived their usefulness, but with a good understanding of the handoff between technologies.
Second, uncertainty for OHC estimate needs to be better understood and quantified. In addition to data coverage, uncertainty also stems from mapping methods associated with data sampling, instrumental bias correction, choice of climatology, quality-control and other data processing procedures. These error sources are not independent of each other and are likely to lead to biases in current analyses. Thus, uncertainty in OHC estimate is method-dependent and producer-dependent. The contribution of each error source to the total OHC uncertainty is not fully understood, and the error range given by different datasets results in roughly 10-fold differences33. New approaches can be used to better quantify the uncertainty: for example, synthetic data for understanding and evaluating the mapping method, and exploiting ensembles (applying different techniques and forming an ensemble of OHC estimates)59,262,263,264.
Third, syntheses of multisource (direct and indirect) observations and models are recommended to improve OHC estimates and mechanistic understanding. Synthesis of in situ observations with satellite-based observations (sea level altimetry, ocean colour, surface wind stress) and full atmospheric analyses shows the most promise. The indirect datasets can either serve as a cross-evaluation tool or constrain direct estimates, such as closing energy, water and sea level budgets. Attempts show promising results. As a coupled system, the separate impacts of atmospheric, ocean and ice (and other components) dynamics cannot be easily isolated265, and identifying the coupling mechanism driving OHC patterns remains a high research priority. Capabilities for integrating different sources of Earth system observations and information (for example, model-based data assimilation and simulations) for a comprehensive quantification of the energy budgets should be built. For example, the integration of atmospheric and oceanic data leads to a quantification of MHT time series at all latitudes, capable of resolving interannual variability133.
Fourth, the current generation of climate and Earth system models still contain non-negligible uncertainties in representing past and future ocean warming trends202,205,212. For example, there is substantial uncertainty in CMIP6 future projections in the Antarctic margin region, due to biases in simulated stratification158, hydrography around the Antarctic shelf 266, and missing processes, including eddies, tides, ice-shelf cavities, and ocean–ice-shelf interactions. In the tropics, it is known that tropical cyclones mix the ocean through substantial depths. Moreover, they form in hot spots and cool the ocean, yet they are largely absent from global models. In addition, interannual to decadal variability such as ENSO and the IPO remains poorly represented in modern-day climate models, yet they strongly control the pattern and timing of OHC anomalies. As ocean and atmospheric circulation have a key role in shaping the climate response (including ocean warming pattern and OHU efficiency)127,203, evaluating the accuracy of the wind, atmosphere and ocean circulation projections should continue to be a research priority. A continuous process-based understanding of model performance is recommended including the understanding and identification of persistent biases in simulations, especially with respect to the diagnostics for Earth’s major system cycles.
Fifth, understanding of extreme OHC events, their compound effects and past changes in OHC should be strengthened. For example, MHWs have been identified from surface conditions, but subsurface components are also important, and OHC as an indicator offers a way to integrate these aspects. The simultaneous occurrence of ocean warming extremes with other extremes (deoxygenation, acidification) requires special attention236. A more complete understanding of OHC changes in the deep past before the widespread availability of instrumental records is also critical to put the current changes in the context of a longer timescale267. Research on past climate change also helps us to understand how natural drivers and human influence have changed the Earth’s climate system. The difficulty is a lack of full-depth temperature proxy data. Several methods have been developed to derive OHC change back to the past 20,000 years (Last Glacial Maximum), but the uncertainty is large95,268.
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