Rate of Ocean Warming Quadrupled Since Late 20th Century, Study Reveals
By Roz Pidcock
The buildup of heat-trapping greenhouse gases is warming the upper ocean four times faster than during the period 1960-1990, according to new research.
The paper, published March 10 in the journal Science Advances, is the latest effort to piece together current and historical measurements from ships, self-propelled floats, satellites and even seals to get a global picture of how the oceans are faring under rising temperatures.
Since the 1990s, more heat is finding its way to the deep ocean and there has been no change of pace in ocean warming since 1998, compared with the previous decade, the paper notes.
The study marks a step forward, but the authors said they are concerned about the future of ocean science, given the political climate in the U.S. Dr. John Abraham, professor of thermal sciences at the University of St. Thomas in Minnesota and co-author on the paper, told Carbon Brief:
"We are seeing dramatic cuts planned for climate science. There is every reason to expect these cuts will include ocean-sensing systems."
Big Role to Play
Approximately 93 percent of the heat captured by greenhouse gases in the atmosphere ends up in the ocean. The remaining 7 percent heats the atmosphere and the land and causes ice to melt.
This means it is only by measuring the oceans that scientists can tell how fast the planet is heating and how much it will heat in the future, Abraham told Carbon Brief:
"If you want to know about global warming, you really have to understand ocean warming."
Rosamund Pearce / Carbon Brief
Completing the Picture
But accurately working out how quickly the oceans are warming is a difficult task, partly because of gaps in the historical data. Abraham told Carbon Brief:
"Measuring the oceans is challenging because you need enough sensors, in enough locations, for a long enough time to get a picture of the climate."
Early observations were typically done by commercial and scientific research ships, which means they were limited to developed countries and along shipping routes, the paper explains.
In the 1990s, the World Ocean Circulation Experiment greatly expanded the coverage of ocean temperature records. Since 1992, satellites have helped to infer ocean warming from changes in the height of the sea surface, since seawater expands as it warms.
The quality of ocean measurements stepped up a gear after 2005, with the introduction of ARGO floats, a network of nearly 4,000 free-floating buoys measuring temperature in the top 2,000m of the world's oceans.
Map showing the global coverage of the ARGO network and positions of floats that have delivered data within the last 30 days.ARGO
To get a global picture of what's happening across the huge expanse of global ocean, scientists combine measurements from the ARGO network, ship-based observation systems, buoys tethered to the seafloor and even temperature sensors attached to the heads of seals.
While there is a wealth of data post-2005, there are gaps before then that need filling in.
Scientists typically do this by making a best guess based on measurements taken at other locations nearby. The new study updates this approach, said Dr. Lijing Cheng, associate professor at the Chinese Academy of Sciences and lead author on the paper. He told Carbon Brief:
"Our study offers a new method to solve these problems."
Instead of including stations within 800km of the missing data point, as other studies have, the authors extends this to 2,500km to account for the often greater distance between historical stations. This left fewer gaps unfilled and more ocean accounted for, the paper explains.
The authors were also able to correct for past studies that may have underestimated warming by comparing good data from the ARGO era with sparse observations for the same region taken in the 1960s or 1970s. Prof. Kevin Trenberth, a senior scientist at the U.S. National Center for Atmospheric Research and co-author on the paper, told Carbon Brief:
"It has been amazing how much we have been able to recover in terms of information about ocean heat content from sparse past data."
The new method left the scientists with better-than-90 percent coverage of the oceans from the late 1950s to 2015 and from the sea surface all the way down to a depth of 2,000m.
The study shows the oceans warmed relatively slowly before 1980 and faster since then. The black line in the graph below from the paper shows how the pace picks up after 1980.
The authors say in the paper that their new estimate is "somewhat greater" than previous reports, including the latest report from the Intergovernmental Panel on Climate Change (IPCC).
"The paper represents an important refinement to our estimates of ocean heat content change … While the study does not alter our basic understanding, it does suggest that IPCC AR5 reported rates of ocean warming were underestimated by about 10-15%."
Increasingly sophisticated ways to keep tabs on the oceans allow scientists to pinpoint, not only how much heat enters the oceans, but also where it goes when it gets there.
The study shows the speed of warming in the upper ocean, between the surface and 700m, has quadrupled between 1960-1991 and 1992-2015 (slope of blue line, below). More strikingly, perhaps, the deep ocean (700-2000m) is warming nine times faster than in the 60s, 70s and 80s (red line).
Global ocean heat content from 1955 to 2015 for the upper ocean (blue), deep ocean (red) and both together (black). All figures are relative to the 1997-2005 average. Cheng et al., (2017)
Despite what the study calls a "surge" in research into whether global temperature at the Earth's surface slowed down temporarily in the late 90s and 2000s, the paper is clear that no such change of pace has happened in the oceans. It says:
"Our studies show that there has been no slowdown in global ocean heat content change since 1998 compared with the previous decade."
Looking at solely surface temperature over a decade or so is not a reliable way of to track the rate of global climate change, said Palmer:
"Since ocean heat content continued to increase at a time when the rate of global surface temperature rise slowed down, we can infer that a large part of the 'slowdown' must have originated from ocean heat re-arrangement."
All ocean basins experienced significant warming since 1998, with the greatest warming in the southern oceans, the tropical/subtropical Pacific Ocean and the tropical/subtropical Atlantic Ocean. As the paper puts it:
"The Atlantic Ocean and the southern oceans are the major new heat reservoirs (59%) even though their total area is just 48% that of the global ocean."
Overall, about 32 percent of the extra heat absorbed by the ocean between 1960 and 2005 found its way to the deep ocean. Over the past decade, however, this proportion jumped to 49 percent, indicating that the deep ocean is playing an increasingly important role in ocean warming.
Finally, the authors combined their estimate of heat taken up by the oceans between 0-2,000m with a previous estimate of warming below 2,000m.
They found a good match with how much heat they would expect to find in the oceans as a whole, based on the amount of solar radiation entering and leaving the atmosphere since 1985. (Compare the yellow line in the graph below with all of the different shades of blue combined).
This means the mystery of the "missing heat" posed by Trenberth a while back is now effectively solved, suggested Palmer.
Proportion of the top of atmosphere (TOA) radiative imbalance expected to enter the ocean (yellow) with estimates of the ocean heat content at different depth levels (blue shading). Cheng et al., (2017)
Despite the fact that ARGO floats are largely to thank for having such detailed information about the oceans, the scientists behind the study fear for the future of the network. Abraham told Carbon Brief:
"I am not confident that we will have continued coverage. The current system uses state-of-the-art sensors that are spread out across the oceans … It is not clear that there will be continued funding for this absolutely crucial system in the current political climate in the U.S."
The Oceanic and Atmospheric Research program at the National Oceanic and Atmospheric Administration (NOAA) is in line for a 26 percent cut and ocean observation and ARGO come out of that budget, Trenbeth told Carbon Brief. What's more, many other countries provide their contributions as a percentage of the NOAA contribution, he added:
"If NOAA cuts back so do others—and it all goes downhill … Argo is still a research enterprise, not an operational one. It is vulnerable. Such a cutback would be a travesty."
Reposted with permission from our media associate Carbon Brief.
By Melissa Gaskill
Two decades ago scientists and volunteers along the Virginia coast started tossing seagrass seeds into barren seaside lagoons. Disease and an intense hurricane had wiped out the plants in the 1930s, and no nearby meadows could serve as a naturally dispersing source of seeds to bring them back.
Restored seagrass beds in Virginia now provide habitat for hundreds of thousands of scallops. Bob Orth, Virginia Institute of Marine Science / CC BY 2.0<p>The paper is part of a growing trend of evidence suggesting seagrass meadows can be easier to restore than other coastal habitats.</p><p>Successful seagrass-restoration methods include <a href="https://www.sciencedirect.com/science/article/abs/pii/S0304377099000078?via%3Dihub" target="_blank">transplanting shoots</a>, <a href="https://onlinelibrary.wiley.com/doi/10.1111/j.1061-2971.2004.00314.x" target="_blank" rel="noopener noreferrer">mechanized planting</a> and, more recently, <a href="https://www.nature.com/articles/s41467-020-17438-4" target="_blank" rel="noopener noreferrer">biodegradable mats</a>. Removing threats, proximity to donor seagrass beds, planting techniques, project size and site selection all play roles in a restoration effort's success.</p><p>Human assistance isn't always necessary, though. In areas where some beds remain, seagrass can even recover on its own when stressors are reduced or removed. For example, seagrass began to recover when Tampa Bay improved its water quality by reducing nitrogen loads from runoff by roughly 90%.</p><p>But more and more, seagrass meadows struggle to hang on.</p><p>The marine flowering plants have declined globally since the 1930s and currently disappear at a rate equivalent to a football field every 30 minutes, according to the <a href="https://www.unep.org/resources/report/out-blue-value-seagrasses-environment-and-people" target="_blank" rel="noopener noreferrer">United Nations Environment Programme</a>. And research published in 2018 found the rate of decline is <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GB005941" target="_blank" rel="noopener noreferrer">accelerating</a> in many regions.</p><p>The causes of decline vary and overlap, depending on the region. They include thermal stress from climate change; human activities such as dredging, anchoring and coastal infrastructure; and intentional removal in tourist areas. In addition, increased runoff from land carries sediment that clouds the water, blocking sunlight the plants need for photosynthesis. Runoff can also carry contaminants and nutrients from fertilizer that disrupt habitats and cause algal blooms.</p><p>All that damage comes with a cost.</p>
The Value of Seagrass<p>As with ecosystems like rainforests and <a href="https://therevelator.org/mangroves-climate-change/" target="_blank">mangroves</a>, loss of seagrass increases carbon dioxide emissions. And that spells trouble not just for certain habitats but for the whole planet.</p><p>Although seagrass covers at most 0.2% of the seabed, it <a href="https://www.unenvironment.org/news-and-stories/story/seagrass-secret-weapon-fight-against-global-heating" target="_blank">accounts for 10%</a> of the ocean's capacity to store carbon and soils, and these meadows store carbon dioxide an estimated 30 times faster than most terrestrial forests. Slow decomposition rates in seagrass sediments contribute to their <a href="https://www.researchgate.net/publication/238506081_Assessing_the_capacity_of_seagrass_meadows_for_carbon_burial_Current_limitations_and_future_strategies" target="_blank" rel="noopener noreferrer">high carbon burial rates</a>. In Australia, according to <a href="https://onlinelibrary.wiley.com/doi/10.1111/gcb.15204" target="_blank" rel="noopener noreferrer">research</a> by scientists at Edith Cowan University, loss of seagrass meadows since the 1950s has increased carbon dioxide emissions by an amount equivalent to 5 million cars a year. The United Nations Environment Programme reports that a 29% decline in seagrass in Chesapeake Bay between 1991 and 2006 resulted in an estimated loss of up to 1.8 million tons of carbon.</p>
Eelgrass in the river delta at Prince William Sound, Alaska. Alaska ShoreZone Program NOAA / NMFS / AKFSC; Courtesy of Mandy Lindeberg / NOAA / NMFS / AKFSC<p>Seagrasses also protect costal habitats. A healthy meadow slows wave energy, reduces erosion and lowers the risk of flooding. In Morro Bay, California, a 90% decline in the seagrass species known as eelgrass caused extensive erosion, according to a <a href="https://www.sciencedirect.com/science/article/abs/pii/S0272771420303528?via%3Dihub" target="_blank" rel="noopener noreferrer">paper</a> from researchers at California Polytechnic State University.</p><p>"Right away, we noticed big patterns in sediment loss or erosion," said lead author Ryan Walter. "Many studies have shown this on individual eelgrass beds, but very few studies looked at it on a systemwide scale."</p><p>In the tropics, seagrass's natural protection can reduce the need for expensive and often-environmentally unfriendly <a href="https://www.nioz.nl/en/news/zeegras-spaart-stranden-en-geld" target="_blank" rel="noopener noreferrer">beach nourishments</a> regularly conducted in tourism areas.</p><p>Seagrass ecosystems improve water quality and clarity, filtering particles out of the water column and preventing resuspension of sediment. This role could be even more important in the future. By producing oxygen through photosynthesis, meadows could help offset decreased oxygen levels caused by warmer water temperatures (oxygen is less soluble in warm than in cold water).</p><p>The meadows also provide vital habitat for a wide variety of marine life, including fish, sea turtles, birds, marine mammals such as manatees, invertebrates and algae. They provide nursery habitat for <a href="https://wedocs.unep.org/bitstream/handle/20.500.11822/32636/seagrass.pdf?sequence=1&isAllowed=y" target="_blank" rel="noopener noreferrer">roughly 20%</a> of the world's largest fisheries — an <a href="https://www.floridamuseum.ufl.edu/science/seagrass-meadows-harbor-wildlife-for-centuries/" target="_blank" rel="noopener noreferrer">estimated 70%</a> of fish habitats in Florida alone.</p><p>Conversely, their disappearance can contribute to die-offs of marine life. The loss of more than 20 square miles of seagrass in Florida's Biscayne Bay may have helped set the stage for a widespread <a href="https://www.wlrn.org/2020-08-14/the-seagrass-died-that-may-have-triggered-a-widespread-fish-kill-in-biscayne-bay" target="_blank">fish kill</a> in summer 2020. Lack of grasses to produce oxygen left the basin more vulnerable when temperatures rose and oxygen levels dropped as a result, says Florida International University professor Piero Gardinali.</p>
Damaged Systems, a Changing Climate<p>Governments and conservationists around the world have already put a lot of effort into coastal restoration efforts. And that's helped some seagrass populations.</p><p>Where stressors remain, though, restoration grows more complicated. <a href="https://www.rug.nl/research/portal/en/publications/the-future-of-seagrass-ecosystem-services-in-a-changing-world(3a8c56db-7bed-4c9e-ac7f-c72453e2a102).html" target="_blank">Research</a> published this September found that only 37% of seagrass restorations have survived. Newly restored meadows remain vulnerable to the original stressors that depleted them, as well as to storms — and <a href="https://www.ecowatch.com/tag/climate-crisis">climate change</a>.</p>
Seagrass in Dry Tortugas National Park, Florida. Alicia Wellman / Florida Fish and Wildlife / CC BY-NC-ND 2.0<p>In Chesapeake Bay a cold-water species of seagrass is currently hitting its heat limit, especially in summer, according to Alexander Challen Hyman of University of Florida's School of Natural Resources and Environment. As waters continue to warm due to climate change, the species likely will disappear there.</p><p>Climate-driven sea-level rise complicates the problem as well. Seagrasses thrive at specific depths — too shallow and they dry out or are eaten, too deep and there isn't enough light for photosynthesis.</p>
But There’s Good News, Too<p>Luckily, left to its own devices, a seagrass meadow can flourish for hundreds of years, according to a <a href="https://royalsocietypublishing.org/doi/10.1098/rspb.2019.1861" target="_blank" rel="noopener noreferrer">paper</a> published last year by Hyman and other researchers from the University of Florida. The researchers arrived at their conclusion by looking at shells of living mollusks and fossil shells to estimate the ages of meadows in Florida's Big Bend region on the Gulf Coast.</p><p>That area has extensive, relatively pristine seagrass meadows. "Our motivation was to understand the past history of these systems, and shells store a lot of history," said co-author Michal Kowalewski.</p><p>A high degree of similarity between living and dead shells indicates a stable area, while a mismatch suggests an area shifted from seagrass to barren sand. The researchers found that long-term accumulations of shells resembled living ones, suggesting that the seagrass habitats have been stable over time.</p><p>That stability allows biodiversity to thrive, creating conditions where specialist species can survive and flourish, according to Hyman.</p><p>Discovering the long-term stability of seagrass meadows has implications for choosing restoration sites, Kowalewski notes.</p><p>"There must be reasons they thrive in one place, while a mile away they don't and fossil data says they probably never did," he said. "If we remove a seagrass patch, we cannot hope to plant it somewhere else. It's not just the seagrass that is special. The location at which it's found is special, too."</p><p>A better approach is conserving these habitats in the first place, but we're not doing enough of that right now. The UN reports that marine protected areas safeguard just 26% of recorded seagrass meadows, compared with 40% of coral reefs and 43% of mangroves.</p>
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