‘The world ocean, in 2023, is now the hottest ever recorded, and sea levels are rising because heat causes water to expand and ice to melt,’...‘Ecosystems are also experiencing unprecedented heat stress, and the frequency and intensity of extreme weather events are changing rapidly, and the costs are enormous.’ [Scientia Professor Matthew England, co-author of the study from the UNSW Centre for Marine Science and Innovation, in the Echo, 6 November 2023]
Over the years science has made the general public increasingly aware that anthropomorphic global warming and subsequent climate change has been heating the world's oceans beyond their normal temperature range.
What we aren't always aware of is exactly which oceans are exhibiting the most persistent warming and the fastest temperature rises.
This recent study below highlights those particular oceans.
It seems that ocean waters from the 30th Parallel south (latitude: -30° 00' 0.00" S longitude: 0° 00' 0.00") are experiencing the most rapid increase in temperatures.
To place that in perspective. From a line running through Australian waters from a point roughly halfway between Red Rock on the Clarence Coast and Corindi Beach on the Coffs Coast (NSW), right down to Tasmania and on towards Antarctica, seawater is heating and expanding until at latest measurement the reading over time now stands at 75.3 ± 4.
While from around Cape Leeuwin to Antarctica the reading is 43.2 ± 4.4.
On the Australian west coast the 30th Parallel can be thought of as running on a latitude approximately halfway between Leeman and Green Head (WA).
This study appears to indicate that, sooner rather than later, the considerable impacts of climate change will increase for the Australian population.
Nature Communications, Article number: 6888 (2023), 28 October 2023, excerpts:
Recentacceleration in global ocean heat accumulation by mode andintermediate waters
Authors: Zhi Li, Matthew H. England & Sjoerd Groeskamp
The ocean directly impacts the Earth’s climate by absorbing and redistributing large amounts of heat, freshwater, and carbon, and by exchanging these properties with the atmosphere1. About 91% of the excess heat trapped by greenhouse gases and 31% of human emissions of carbon dioxide2 are stored in the ocean, shielding humans from even more rapid changes in climate. However, warmer oceans result in sea-level rise, ice-shelf melt, intensified storms, tropical cyclones, and marine heatwaves, as well as more severe marine species and ecosystem damage. These effects depend on the pattern of ocean warming; it is thus critical to quantify the dynamics and distribution of ocean warming to better understand its consequences and predict its implications.
The observed distribution of ocean warming is not uniform. About 90% of total ocean warming is found in the upper 2000 m, with over two-thirds concentrated in the upper 700 since the 1950s, and an increase of warming rates at both intermediate depths of 700–2000 m, and in the deeper ocean below 2000 m. The Southern Ocean south of 30°S has been estimated to account for 35–43% of global ocean warming from 1970 to 2017, and an even greater proportion in recent years, while Northern Hemisphere ocean warming appears to be concentrated in the Atlantic Ocean. Due to the accumulated excess heat in ocean basins, an acceleration of total ocean warming has become more evident from recent observational-based studies. While much past work has focused on the distribution of ocean warming as a function of depth and basin, relatively little analysis has been undertaken of the distribution as a function of water-mass layers and within specific water masses. This is the focus of the present study......
When evaluating the ocean heat uptake for each decade (“Methods”), analysis of the past three decades reveals that the ocean heat uptake during 2010–2020 has increased more than 25% relative to 2000–2010 and has nearly doubled relative to the 1990’s WOCE era, as seen in Fig. 1b, where we highlight the decadal ocean heat uptake since the 1960s. Note that there has been both increased ocean sampling and a shift of the observational network from a ship-based system to the Argo network since the initiation of the global Argo array (2001–2003)34. This may impact the estimated increase in global ocean warming over the past three decades (Fig. 1). However, the rate of global mean sea-level rise has also been increasing since 1993 based on an independent estimate from satellite altimeter data1,35, providing confidence in our results given that half of the global sea surface height increase is due to thermal expansion of the ocean since altimeter measurements began. Significant ocean warming and accelerating OHC changes are also consistent with the increase in net radiative energy absorbed by Earth detected in satellite observations, something that is likely to continue throughout the 21st century in the absence of substantial greenhouse gas emissions reductions.
The increased ocean warming is non-uniformly distributed across ocean basins. Overall, in each ocean basin, an increase in OHC is observed (values indicated in Fig. 2a, b), with stronger warming in the mid-latitude Atlantic Ocean and the Southern Ocean compared with other basins. Total warming in the Southern Ocean is estimated to account for ~31% of the global upper 2000-m OHC increase from 1980–2000 to 2000–2010 (Fig. 2a), and almost half of the global OHC increase from 2000–2010 to 2010–2020 (values indicated in parentheses of Fig. 2b). Hence the Southern Ocean has seen the largest increase in heat storage over the past two decades, holding almost the same excess anthropogenic heat as the Atlantic, Pacific, and Indian Oceans north of 30°S combined (Fig. 2d). The most striking warming in the Southern Ocean is concentrated on the northern flank of the Antarctic Circumpolar Current, the location of deep mixed layers and subduction hotspots for Subantarctic Mode Water and Antarctic Intermediate Water, as well as the location of subtropical mode waters formation further equatorward (Fig. 3). The well-ventilated regions near western boundary current extensions in the North Atlantic and North Pacific also reveal large warming over the past two decades. These hotspots of ocean warming are likely linked to enhanced uptake, subduction, and lateral spreading of heat associated with mode and intermediate waters that warrant further investigation.
Fig. 2: Regional intensification in ocean warming over the past two decades, 0–2000 m. Click on image to enlarge
 |
The ensemble mean of ocean heat content (OHC) changes averaged for years a 2000–2010 and b 2010–2020, relative to the 1980–2000 mean. Units of shadings in panels (a, b) are shown as 109 J m−2. The values over each basin indicate the OHC increase relative to the 1980–2000 mean over the Southern (S.O., south of 30°S, dark-red line), Atlantic (ATL), Pacific (PAC), and Indian (IND) Oceans, and are limited to 65°S–65°N. Units are shown as 1021. The values in parentheses in panel (b) indicate the basin-integrated OHC increase from 2000–2010 to 2010–2020. The basin mask used to distinguish ocean basins of the Southern, Atlantic, Pacific, and Indian Oceans is obtained from ref. Superimposed gray contours represent the positions of wintertime isopycnals 25, 26.45, 27.05, and 27.5 kg m−3 at 10 m depth from SIO RG-Argo. c, d Zonally integrated OHC change (1021 per degree latitude) versus latitude for the period 2000–2010 (blue line), and 2010–2020 (red line), relative to the 1980–2000 mean. Lines in panels (c) and (d) represent the ensemble mean, and shadings indicate the ±2 ensemble standard deviation uncertainty range (±2σ) of OHC changes.
[my yellow highlighting in the excerpts]
The full study can be read and downloaded at:
https://www.nature.com/articles/s41467-023-42468-z#ref-CR13
