The ocean is running a fever: Will extreme weather bring us into an era of "boiling"?

The ocean is one of the most important parts of the Earth's cycle (even more important than the atmosphere). It transports heat, provides a buffer, and supports life. It plays a key role in the water cycle, carbon cycle, and nitrogen cycle.

However, the frequent occurrence of extreme weather seems to indicate to us that this huge circulation system is failing.

This year, the global sea surface temperature has shown a trend far exceeding previous years since March and April. As of August, the global average sea surface temperature exceeded 21°C. The World Meteorological Organization found that in August this year, the sea surface temperature in the Mediterranean was abnormally high, even exceeding 30°C in some areas.

Meanwhile, there are reports that in July this year, the sea surface temperature in Manatee Bay, Florida, soared to 38.4°C, making swimming in the sea like soaking in hot soup. The sea is not warming, but boiling.

Sea surface temperature change map | NOAA OISST V2., ClimateReanalyzer.org, Climate Change Institute, University of Maine[1]

What happened to the sea?

The ocean isn't warming, it's boiling

Changes in sea surface temperature require more attention than changes in air temperature.

The reason is simple: water has a higher heat capacity than air . To raise the temperature of the two by the same amount, water requires four times as much energy as air. Scientists usually use the energy absorbed by the upper ocean to determine the extent of global warming.

According to the latest data, the energy absorbed by the top 2,000 meters of the world's oceans has been increasing year by year since 1990. In the first seven months of this year alone, the increased energy has been at least 10^22 joules higher than the average in 2022[2].

What does this rising value mean?

If compared with China's annual power generation of 8.4 trillion kWh in 2022, the increase in ocean heat by July this year is more than 330 times the power generation . If compared with the energy released by a TNT explosion, it is equivalent to dropping nearly 2.4 trillion tons of TNT in the ocean. If used to boil water, this heat is enough to boil 600 million 1.5-liter pots of water every second for a year.

The ocean absorbs most of the heat generated by greenhouse gas emissions (more than 90%), and also absorbs a lot of heat from solar radiation. Because of the existence of the ocean, the surge in the Earth's atmospheric temperature is alleviated .

But the huge amount of energy trapped in the ocean can also have serious consequences, as it is how the world's most powerful and destructive storms are created, such as typhoons, hurricanes and tropical cyclones. One of the necessary conditions for these meteorological events to form is a sea surface temperature of at least 27°C.

The imbalance of ocean energy transport caused by rising sea temperatures, combined with other complex meteorological factors, will increase the uncertainty of cyclone generation and operation, leading to changes in precipitation intensity, frequency and region. This uncertainty also increases the difficulty of predicting the development of cyclones.

The World Meteorological Organization's 2022 Asia Climate Status Report released in July this year shows that Asia is particularly vulnerable to water-related disasters. Of the 81 major natural disasters that occurred last year, 83% were floods and storms[3]. In 2022, my country's economic losses due to floods exceeded US$5 billion[3].

This summer, many cities and regions in China suffered from severe floods due to record-breaking heavy rainfall brought by typhoons. I believe that friends in North China and the south still remember it clearly. Although the increase in the number and destructive power of typhoons is related to many factors, the increased energy from the rise in sea temperature provides favorable conditions for the formation of storms such as typhoons.

The amount of heat absorbed by the upper 2000 meters of the global ocean is increasing year by year｜NOAA/NESDIS/NCEIOcean Climate Laboratory. Updated from Levitus et al. 2012 [2]

As the ocean warms

What will the increase in sea temperature bring?

The most direct impact is sea level rise . Water expands slightly when it warms, and the increase in ocean heat content will also increase the volume of seawater, causing sea level rise.

At the same time, ocean currents transport warmer seawater to the North and South Poles, causing glaciers to melt from the bottom. The resulting glacier melting is even more severe than the melting caused by the rise in atmospheric temperature.

Because the impact of warm currents is ignored, the actual melting rate of glaciers is much faster than the model predicts｜Pixabay

According to the U.S. National Snow and Ice Data Center, in July 2023, the global sea ice area was about 1.2 million square kilometers less than the record low in July 2019[4]. As of July, the Antarctic sea ice cover ranked the lowest in history for the third consecutive month, about 2.59 million square kilometers below the 1991-2020 average[4], equivalent to a reduction of 1.2 Greenlands (about 2.16 million square kilometers).

In recent years, there have been more and more reports on the consequences of large-scale glacier melting, such as rising sea levels causing land to be flooded, the destruction of the ecological balance of polar animals, the reduction of freshwater resources, and even the release of unknown ancient microorganisms and pathogens. However, few people know that this melting is fueled by the warming of ocean waters.

Extreme high temperature , once again in the ocean

Extreme heat events on land have made each of us feel "burned", and similar events are also happening in the ocean. Marine heatwave events caused by the continued rise in sea temperature are occurring in some oceans on Earth.

A marine heatwave refers to an event in which the temperature in a specific sea area is abnormally high for a long period of time. If the sea temperature exceeds the 90th percentile threshold of the past climate average for five consecutive days or more, it is counted as a marine heatwave.

The sixth report of the Intergovernmental Panel on Climate Change (IPCC) released in 2021 shows that the frequency of marine heat waves doubled between 1982 and 2016, and that heat waves have become longer and more intense since the 1980s[5]. The National Oceanic and Atmospheric Administration of the United States has even warned that by September this year, marine heat waves may occur in half of the world's oceans[6].

The occurrence of marine heat waves directly threatens the survival of marine life. For coral reefs, rising sea temperatures will lead to a large loss of algae that coexist with them and provide food, causing coral reefs to "bleach" or even die. To make matters worse, because the ocean absorbs a large amount of carbon dioxide, the seawater acidification is serious, and the growth of coral reefs is further slowed down.

Healthy coral reefs are home to more than 25% of marine life, protect coastlines from storms and erosion, and provide economic income and employment opportunities (such as tourism) for local communities. More than a billion people around the world directly benefit from coral reefs, especially in many small island and atoll nations.

Although coral reefs have a relatively fast recovery capacity in an undisturbed environment, their overall number is declining year by year as global warming progresses. According to estimates by the United Nations Environment Programme, 25% to 50% of the world’s coral reefs have been destroyed. If greenhouse gas emissions are not significantly reduced, coral reefs in all oceans will die by 2100[7].

Bleaching coral reefs | BJ Warnick/Kyodo/Newscom/Alamy

In addition to coral reefs, marine heat waves can also cause the migration and extinction of some other marine species. The food chain in the ocean is linked together, and the extinction and migration of bottom-level organisms will have a huge impact on upper-level organisms. For example, seabirds starve to death due to lack of food, whales change their usual routes to find food and get entangled in fishing nets, and new toxic algae appear in some areas, affecting fisheries.

A future where the sea stops

What's the worst outcome of the continued warming of the ocean? Perhaps it's the destruction of all life in the sea.

Under climate change, ocean currents are likely to change.

Ocean currents are the steady flow of ocean water, acting much like conveyor belts, carrying warm water and precipitation from the equator to the poles and cold water from the poles back to the tropics. Ocean currents thus regulate global climate, helping to offset the uneven distribution of solar radiation reaching the Earth's surface. The oceans are closely connected to the atmosphere, and ocean currents are an important driver of weather patterns.

The Atlantic Meridional Overturning Circulation (AMOC) is the most important meridional heat transport belt in the global ocean. Changes in its intensity can directly affect the heat distribution between the northern and southern hemispheres.

There have been many predictions about the collapse of the AMOC in recent years. In its 2021 report, the IPCC pointed out that many large ocean currents will change in the 21st century, and the AMOC is likely to decline this century[5]. However, according to the latest forecasts in July this year, if current greenhouse gas emissions continue, the collapse of the AMOC may be brought forward to around the middle of this century (2025-2095, 95% probability)[8].

Without the ocean currents, temperatures in various regions would be more extreme, with super-high temperatures in the equatorial region and cold temperatures in the polar regions, and the land area suitable for habitation on Earth would be greatly reduced . Of course, the study also pointed out the uncertainty of the degree of collapse, that is, it is possible that there is a partial collapse. However, we should remain highly concerned about the possibility of sudden changes in the North Atlantic climate system.

Schematic diagram of the AMOC: red represents near-surface transport, blue represents deep return flow. Ocean currents are primarily driven by surface winds, but are also partly driven by temperature and salinity gradients, the Earth's rotation, and tides. │Met Office [9]

The ocean is still absorbing heat and carbon dioxide from the air, but rising temperatures are also reducing the ocean's ability to absorb carbon dioxide. With the onset of the El Niño event, the rise in ocean temperatures will become more severe, exacerbating the climate crisis.

The ocean absorbs and releases heat slowly, so the impact of ocean warming on the climate will be delayed. But we all know that the heat absorbed by the ocean will eventually flow back into the atmosphere in the next few centuries, and we cannot predict what will happen then.

References

[1] https://climatereanalyzer.org/clim/sst_daily/

[2] https://www.ncei.noaa.gov/access/global-ocean-heat-content/

[3] CLIMA, THER, and W. TE. "State of the Climate in Asia." (2023). https://public.wmo.int/en/our-mandate/climate/wmo-statement-state-of-global-climate/Asia-2022

[4] https://public.wmo.int/en/media/news/july-2023-confirmed-hottest-month-record

[5] Fox-Kemper, B., HT Hewitt, C. Xiao, G. Aðalgeirsdóttir, SS Drijfhout, TL Edwards, NR Golledge, M. Hemer, RE Kopp, G. Krinner, A. Mix, D. Notz, S. Nowicki, IS Nurhati, L. Ruiz, J.-B. Sallée, ABA Slangen, and Y. Yu, 2021: Ocean, Cryosphere and Sea Level Change. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, SL Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, MI Gomis, M. Huang, K. Leitzell, E. Lonnoy, JBR Matthews, TK Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1211–1362, doi: 10.1017/9781009157896.011.

[6] https://research.noaa.gov/2023/06/28/global-ocean-roiled-by-marine-heatwaves-with-more-on-the-way/

[7] https://www.unep.org/interactives/status-world-coral-reefs/

[8] Ditlevsen, P., Ditlevsen, S. Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Nat Commun 14, 4254 (2023). https://doi.org/10.1038/s41467-023-39810-w

[9] Met Office https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/research/climate-science/met-office-hadley-centre/risk-management-of-climate-thresholds-and-feedbacks---2-atlantic-meridional-overturning-circulation-amoc.pdf

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