‘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
