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Lin Xiaohui loves to dive into the boundless ocean and "roam" in it, where she encounters a magical world.

"Pink, bubble-like eggs rise simultaneously from thousands of coral cups and float along the current like a sky full of stars. It is one of the most magical moments of nature," she said, describing the coral spawning process.

Staghorn coral spawns. Photo courtesy of Lin Xiaohui

Since 2021, a private marine conservation organization co-founded by Lin Xiaohui has begun live-broadcasting the coral spawning process to the public. For nearly eight years, her team has been observing and recording coral communities in Shenzhen waters.

"Coral spawning is always in sync, and even scientists can't explain this synchronicity." She still feels emotional when she recalls the scene, "At that moment you will firmly feel a mission: I want to protect it."

In order to do a good job of live broadcasting, the team members conduct a careful survey in the sea every year to find the most suitable community samples. However, when Lin Xiaohui went into the water this year, she found that many coral reefs that were once vibrant now looked obviously unhealthy. "To exaggerate a little, some areas are like 'mass graves'."

In contrast, Luo Jie, an assistant researcher at the Kunming Institute of Zoology of the Chinese Academy of Sciences, saw more severe coral bleaching in Indonesia. It was around 2015, when the Pacific Ocean experienced the strongest El Nino event in 30 years. He and his teammates went underwater for scientific research, and the corals in the entire field of vision were all white. "Without exception, there was a feeling of no end in sight," he recalled.

Greater Bay Area Coral Community

As the "tropical rainforest" of the seabed, coral reefs provide homes for about 25% of marine life and are one of the most biodiverse ecosystems. However, in the past 30 years, humans have lost 50% of the world's corals, and the creatures in them have also faced a threat to their survival.

When the sea temperature is too high, the symbiotic algae in the coral will be expelled or die, and the coral will lose its original rich colors. This process is called bleaching. This is not a new phenomenon. Overexploitation of the ocean and pollution can cause coral bleaching, and the increasing global warming has exacerbated this trend.

According to the Australian Broadcasting Corporation, the water temperature around the Great Barrier Reef has been the highest in 400 years in the past decade. This year's January to March season was the warmest in more than four centuries. The three regions of the Great Barrier Reef, the north, middle and south, have all experienced extreme bleaching (i.e., bleaching rates exceeding 90%), and 32% of the coral reefs have experienced bleaching rates of more than 60%.

Coral bleaching is not irreversible, but if it lasts too long, coral reefs will die due to lack of energy, and the fish, turtles, seabirds and other creatures that depend on them will also be affected. In the sea of ​​Indonesia, Roger clearly felt that "the fish are very few, and many species of creatures may have disappeared."

Greater Bay Area Coral Community

In addition to the biosphere, the atmospheric environment and human society will also be affected. "The reduction of algae in the ocean will have a negative impact on the oxygen level in the atmosphere; when the protective effect of coral reefs on coastlines is weakened, the land area of ​​some Pacific island countries is shrinking - these are real and intuitive changes." He explained.

As Earth's largest carbon sink, the ocean absorbs excess heat and energy released by humanity's growing greenhouse gas emissions. Today, the ocean has absorbed about 90% of the heat generated by increased emissions. As excess heat and energy warm the ocean, the temperature changes have led to an unprecedented cascade of effects, including melting ice and snow, rising sea levels, marine heatwaves and ocean acidification.

Jacques Cousteau, an oceanographer known as the "father of underwater exploration," described the relationship between the ocean and humans as follows: "The water cycle and the life cycle are one." We can no longer stay out of the changes happening in the ocean - after all, this area, which occupies 71% of the Earth's surface and accommodates countless lives, is the only known ocean composed of liquid water in the universe and the origin of all known life.

Humans have changed the ocean, and ultimately the ocean will change the world.

Marine life under threat

2023 was the year with the warmest ocean surface temperature on record. Due to the combined effects of global warming and the El Niño phenomenon, the high temperature trend in the Earth's oceans continued in 2024.

The National Oceanic and Atmospheric Administration (NOAA) reports that Earth is currently experiencing a global coral bleaching event, the fourth on record and the second in the past decade. From the Caribbean to Australia’s Great Barrier Reef to coastal areas of China, corals are dying around the world.

This is just one of the many consequences of ocean warming. In fact, rising sea temperatures are changing the entire marine ecosystem. Some species are migrating to cooler waters to escape unsuitable high temperatures, while temperature-sensitive species that cannot migrate, such as corals, shellfish and seagrasses, may die directly due to high temperatures.

These changes not only trigger oscillations within the ocean, but also trigger chain reactions in human society. Fisheries are the source of livelihoods for many coastal communities, but as marine heatwaves cause changes in the habitats of fish and other seafood, catches are declining significantly. According to research, the amount of seafood that humans can sustainably obtain from the ocean decreased by 4.1% between 1930 and 2010, equivalent to a reduction of 1.4 million tons of fish.

Looking around the world, no place can take chances in this change: the main fishery near the New England continental shelf in the northeastern United States has gradually changed from lobster to short-fin squid. The former is moving offshore because it prefers cold environments; Japan's saury catch in 2020 was 90% less than 10 years ago, and salmon was 60% less; snow crabs in the Gulf of Alaska in the southern United States were "almost wiped out", and a fishery worth $270 million had to be completely closed for two years.

"The ocean is indeed changing, and fish resources and production are changing with it," Ray Hilborn, professor of aquatic and fishery sciences at the University of Washington, told The Paper (www.thepaper.cn). "I believe that fisheries will continue to provide important food and employment opportunities for people in the future, but the distribution and production of related species will change, which requires us to monitor the distribution, quantity and production of fish resources more closely."

Ryukyu flat brain coral

In 2020, Roger was working on coral reef restoration in Indonesia. The method they adopted was artificial transplantation, transplanting laboratory-grown corals onto reefs in the sea. In about two years, the coral coverage rate was increased from 10% to about 50%. "It is certainly effective within a restoration cycle," Roger explained, "but this method still has great uncertainty. Because the root cause of coral bleaching and death is climate, if the seawater environment is no longer suitable for coral survival, or if a large-scale El Niño event occurs in the future, the restoration work of those years will be in vain."

When discussing ocean warming, we cannot just discuss the ocean itself. In the vast ecosystem of the Earth, whether it is the remaining 30% of land or the sky above the sea, in fact, it is impossible to "stay out of the ocean's influence."

A breeding ground for hurricanes

This year's Atlantic hurricane season started unusually early. At the end of June, Hurricane Beryl became the first Atlantic hurricane in recorded history to form in June. A week later, Beryl, upgraded to a Category 5 hurricane, made landfall in Houston, the United States, causing more than 2.7 million households in southeastern Texas to lose power for nearly a week.

On July 25, after sweeping across the Philippine Islands, Typhoon Gemi was upgraded from a strong typhoon to a super typhoon, making landfall in Taiwan, China, and made a second landfall along the coast of Fujian Province on the 26th, bringing record-breaking heavy rain and strong winds to inland areas along the way.

Hurricanes and typhoons are the names of tropical cyclones in different regions. Rising sea surface temperatures provide more energy to these cyclones. Scientists believe that abnormally warm sea surface temperatures not only enhance the intensity of hurricanes, but also bring storm surges earlier. As Allison Wing, an associate professor in the Department of Earth, Ocean and Atmospheric Sciences at Florida State University, said: "It's not that these warm temperatures directly lead to the formation of storms, but once a storm is formed, it can take advantage of these extreme high temperatures and develop into a strong storm."

Research by the European Geosciences Union shows that since the 1980s, as sea surface temperatures have risen, the likelihood of extremely active hurricane seasons has tripled, and the number of high-intensity hurricanes has increased significantly, which means that in the context of global warming, extreme tropical cyclones are gradually becoming the "new normal" in coastal areas.

With global sea surface temperatures reaching their highest levels on record for 13 consecutive months, the Atlantic Basin will have an unusually active hurricane season this year. According to forecasts from various research institutions, the total number of storms this year will be around 20 to 30, an increase of almost 50% compared to the historical average of 14 named storms and 3 hurricanes per season in previous years.

It is worth noting that tropical cyclones not only rely on ocean heat to form, but also in turn affect ocean temperature and ocean current characteristics.

A study published last year in the Proceedings of the National Academy of Sciences further revealed that when a hurricane passes, warm surface water mixes downward, and the heat is carried into the deep sea, traveling thousands of kilometers with large-scale ocean currents. Once this heat is brought back to the surface by upwelling, it may bring about climate change in local areas. This means that a hurricane that crosses the western Pacific and lands in the Philippines may warm the coast of Ecuador a few years later.

Building on this, scientists are trying to answer a more global question: What impact will ocean warming have on global climate, given the interaction between the ocean and the atmosphere?

Before the critical point

In 2023, a study in the journal Nature predicted that as global warming intensifies, the Atlantic Meridional Overturning Circulation (AMOC) may collapse in the near future.

This is a warning that cannot be ignored. As a crucial part of the Earth's climate system, the AMOC regulates global climate by transporting heat from the tropics to high latitudes, especially in the North Atlantic region. Its collapse would reorganize the Earth's heat transport pattern and cause climate disorder.

"This will mainly make northern Europe colder," Peter Ditlevsen, the author of the study and a professor at the University of Copenhagen, told The Paper. "Climate model simulations show that winter temperatures will drop by 10 degrees Celsius and summer temperatures will drop by 5 degrees Celsius. This will have serious consequences for European agriculture, and agricultural production in Ireland and the UK will be significantly reduced."

Another way to estimate its impact is by analogy: Europe borders the Atlantic Ocean, just as the west coast of North America borders the Pacific Ocean. However, because the salinity is lower than that of the Atlantic Ocean, there is no similar meridional overturning circulation in the Pacific Ocean, so if the AMOC collapses, the climate in northern Europe may become as cold as northern Canada and Alaska at the same latitude.

AMOC is not the only ocean circulation process under threat. Another study predicts that in the Southern Ocean (also known as the Antarctic Ocean), as melting glaciers continue to mix fresh water into the ocean, the saltiness and density of the seawater decrease, and the deep water formation mechanism driven by salinity differences may be difficult to maintain in a few decades. This mechanism can bring carbon dioxide and heat from the surface into the deep sea, thereby helping to mitigate global warming. But since 2000, the Earth's polar ice has been losing ground due to global warming.

It should be noted that, from a statistical point of view, the limitations of existing climate models are likely to lead to errors in the results. But this does not mean that we can ignore its risks. In fact, one thing is clear - "It depends on whether (climate change) crosses the critical point." Ditlevson said.

The Intergovernmental Panel on Climate Change (IPCC) of the United Nations defines "climate tipping points" as "the critical threshold at which the global or regional climate changes from one stable state to another." Take the water cycle process as an example. The sea temperature rises, the circulation weakens, the glaciers melt... Each link is accumulating quantitative changes at a slow rate, like a string that is gradually tightened. If no action is taken, it may break due to being too tight.

Therefore, to deal with the crisis in the ocean, we must return to dealing with climate change. Establishing marine ranches, setting fishing bans, and researching artificial restoration technologies are certainly effective ways, but as Roger said, in the final analysis, "ocean protection is a global issue."

"The only viable path forward is to get our net emissions down to zero," said Australian oceanographer Matthew England. "We know for sure that unless we take drastic action to reduce emissions, global average ocean temperatures will continue to rise over the coming decades."

It is not enough to just propose a "net zero" goal. In Ditlevsen's view, humanity needs to complete the transformation "faster than imagined." "Western wealthy countries and major developing countries such as China must take the lead in helping less wealthy countries achieve sustainable development, helping the poor escape poverty, and giving them the opportunity to adapt and protect themselves."

"We are a community with a shared future." This is what Lin Xiaohui felt most when she was at the bottom of the sea. There was silence except for the sound of bubbles. People floated gently in the water, schools of fish floated above them, and yellow, blue or olive green corals grew quietly around them. Life did not interfere with each other, but was connected to each other by the same sea water.