Report calls for need to provide regular data on how ocean warming is evolving and its impacts; this is needed to support the decade challenge for healthy and resilient oceans
The oceans play a key role in regulating climate. Yet the world’s understanding is still insufficient to design solutions for multiple ocean crises and validate new technologies that aim to remove carbon dioxide from the atmosphere, according to the UNESCO State of Ocean Report.
“In this State of the Ocean Report 2024, the message remains that observations and research are falling short and hence there is a lack of adequate and aggregated data,” Vidar Helgesen, executive secretary of the Intergovernmental Oceanographic Commission of UNESCO, wrote in the report.
The upper 2,000 metres (m) of oceans warmed at a rate of 0.32 ± 0.03 watt per square metre (W/m2) from 1960 through 2023. It is expected to warm in the future as well, leading to irreversible changes in the centennial to millennial time scales.
Scientists are particularly concerned about the accelerated ocean warming in the past two decades, where the rate has doubled to 0.66 ± 0.10 W/m2.
The report calls for the need to provide regular data on how ocean warming is evolving and its impacts. This is needed to support the decade challenge for healthy and resilient oceans.
Another consequence of increased greenhouse gas emissions from human activities is increased uptake of the Earth energy imbalance (EEI) by oceans. EEI is the balance between incoming energy from the Sun and outgoing energy from the Earth.
Credit: State of the Ocean Report 2024
About 90 per cent of the EEI is being absorbed by oceans, resulting in a cumulative increase in ocean heat content (OHC) in the upper 2,000 m of the water column. OHC is the total amount of heat stored by oceans.
Scientists think that increased OHC prevents ocean layers from mixing, lowering the preformed oxygen content of near-surface high latitude waters reaching the deeper layers of oceans. The reduction is termed ‘deoxygenation’. This is a concern as it can have long-term negative impacts on the health of coastal and large marine ecosystems, a sustainable blue economy, and coastal communities that depend on oceans such as tourism, fisheries, aquaculture, and ecosystem services. Excess nutrients from coastal areas also cause deoxygenation.
The report highlights that it is still unclear whether deoxygenation is accelerating in response to increased OHC.
Another crisis highlighted in the report is the mean global increase in ocean acidification in all ocean basins and seas.
The open ocean has been experiencing a continuous decline in pH (increase in acidic levels), with an average global surface ocean pH decline of 0.017-0.027 pH units per decade since the late 1980s.
However, the report also highlights that this data comes from a limited set of long-term observations in the open ocean.
In 2024, 638 stations that record ocean pH levels exist. “The current coverage is inadequate, with time series not long enough to determine trends and data gaps due to the lack of observations found in all areas,” the report reads.
Coastal waters can turn acidic due to natural processes, such as freshwater influx, biological activity, temperature change and climate patterns like El Nino/Southern Oscillation (ENSO), the report noted.
Human activities like nutrient input from agricultural and industrial activities also influence the chemistry of coastal areas.
The report notes that longer-term data sets are needed for coastal areas than for the open ocean due to these natural variabilities. This can help determine the time of emergence of ocean acidification trends.
Sea levels continued to rise in 2023. The global mean sea level from 1993 to 2023 increased at a rate of 3.4 +/-0.3 mm/yr.
Going forward, the world will have to improve the space-based and in situ observing systems for monitoring sea level rise at global, regional, and coastal scales, the report stated.
The UNESCO report also takes stock of the recent developments in marine carbon dioxide removal (mCDR) technologies. This involves techniques that capture carbon dioxide from the air and store it durably.
Examples include altering the chemical composition of seawater so that oceans absorbs more carbon dioxide from the atmosphere or adding nutrients such as iron to encourage the growth of microscopic plankton that can sink to the seafloor and be stored for centuries or longer.
Since 2020, it noted that there has been a surge of interest in mCDR using wide-ranging methods, which may pose many technical, environmental, political, legal, and regulatory challenges, among others.
The increased interest is due to several scientific papers on mCDR, the growing number of start-ups developing mCDR techniques, and the significant funding for mCDR research announced by the United States and the European Union in 2023.
There are still many unknowns, including the potential of using mCDR to enhance the ocean carbon sink.
Other unknowns include how they will interact with the ocean carbon cycle and whether these interactions will cause unintended consequences.
Further, the coastal ocean is witnessing an increased interest in restoring or expanding coastal blue carbon habitats such as mangrove forests, seagrass meadows, and tidal saltmarshes to increase the sequestration of carbon. However, questions remain on their effectiveness, the report noted.
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