You may not have noticed, but the ocean seems to be about to "explode"

Produced by: Science Popularization China

Author: Half Lazy (Institute of Atmospheric Physics, Chinese Academy of Sciences)

Producer: China Science Expo

Imagine that when you are in the vast universe and look at the earth from afar, what you see is a vast expanse of tranquil and beautiful blue ocean. This area, which accounts for 71% of the earth's surface, has created the origin of life with its vastness and selflessly nurtures these lives.

However, in response to the call for global warming, the ocean, a major carrier that absorbs more than 93% of the earth's system energy income, has finally lived up to expectations and "caught a fever".

NOAA releases first image from GOES-18 (Geostationary Operational Environmental Satellite)

Image credit: NOAA

As early as 1965, people had proposed that the increase of atmospheric greenhouse gases would cause ocean warming. In 2000, S. Levitus, a researcher from the National Oceanic and Atmospheric Administration (NOAA), wrote in Science that the heat content (average temperature) of the world's oceans from the surface to a depth of 3,000 meters increased widely between 1948 and 1998, but how much the global oceans have warmed has always been a controversial issue.

Why is it so difficult to “measure the temperature” of the ocean?

A large amount of observational data confirms that global warming is an indisputable fact, so why is ocean warming still controversial?

The controversy over how much the ocean is warming stems from the lack of quality and quantity of ocean observation data in the past.

In order to understand the climate changes in the historical period, scientists can find traces of historical climate changes from historical documents, fossils, sediments (such as oceans, loess, stalagmites, etc.), glaciers, tree rings, etc. In modern times, satellites and dense global observation stations have become one of the best weapons for measuring atmospheric temperature. Currently, there are about 11,000 stations on land that are conducting atmospheric temperature observations. However, facing the vast ocean, these methods seem to be out of reach.

Zhu Kezhen reproduced the temperature curve of China in the past 5,000 years based on historical documents

Image source: "A Preliminary Study on Climate Change in China over the Past Five Thousand Years"

The complexity of the ocean lies in that it is not only wide (360 million square kilometers), but also deep (average depth of 3,680 meters, with the deepest trench reaching 10,911 meters below sea level). The surface temperature varies in different regions, and the oceans in different regions certainly have their own "tempers."

It is precisely because the exploration of the ocean is a three-dimensional process that makes it particularly difficult to clarify the changes in the ocean's heat capacity. Even now, human exploration of the ocean is still a drop in the bucket. The story of "Twenty Thousand Leagues Under the Sea" is still just science fiction! At present, human exploration of ocean temperature is only concentrated in the upper 2,000 meters of the ocean.

Between 1970 and 2001, the main temperature observations of the ocean subsurface were made using disposable thermometers (XBTs) dropped by ships along the waterway, accounting for 41% of the total data. However, due to the limitations of the technology at the time, its results had systematic biases.

If we want to know the long-term changes in sea temperature, we must find ways to correct these data. Therefore, since 2008, scientists around the world have begun to correct the systematic bias of historical XBT data, and at the same time, by combining data from the Argo global real-time ocean observation network officially implemented in 2000, they have tried to quantify the long-term changes in ocean temperature. However, due to different methods of correcting data bias, inconsistent conclusions about ocean warming are often obtained.

Global distribution of Argo floats

Image source: https://argo.ucsd.edu/about/status/

Breaking the record again! Ocean: I surpass myself

In 2014, the ocean data bias correction scheme proposed by the research team of the Institute of Atmospheric Physics of the Chinese Academy of Sciences (IAP) has become the best correction scheme recommended internationally, effectively reducing the error in sea temperature estimates in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.

In 2023, the 2022 Global Ocean Environment Change Research Report led by the Institute of Atmospheric Physics showed that 2022 was the warmest year for the ocean since modern ocean observation records.

In 2022, the heat stored in the upper 2,000 meters of the global ocean increased by 10.9±8.3ZJ (1 ZJ=10 to the 21st power joule) compared to 2021. This heat can boil 700 billion kettles of 1.5 liters of water at the same time, which is about 325 times the total annual power generation of China in 2021 and 100 times the total power generation of the world in 2021. Since the 1990s, scientists have also observed warming signals in the deep sea below 2,000 meters.

Global ocean heat content anomaly above 2000 m, 1958-2022

Image source: Reference 3

Among regional warming, the sea temperature records in seven oceans in 2022 ranked among the top ten in historical records, and four of them (the North Atlantic, the Mediterranean, the North Pacific (30°-62°N) and the South Pacific, and the Southern Ocean) even set the hottest records in history.

Regional observed changes in heat content in the upper 2000 m from 1958 to 2022 relative to the 1981–2010 baseline.

Image source: Reference 3

What's so scary about ocean warming? Very scary!

It would seem that marine life would be the most vulnerable to warming oceans: a 2018 study showed that rising ocean temperatures and increased stratification, which lead to a reduction in dissolved oxygen in seawater, are responsible for most recorded extinctions of marine species.

Schematic diagram of temperature-dependent anoxia, the driver of the end-Permian marine mass extinction

Image source: Reference 4

In 2022, scientists' research further showed that if humans do not take effective actions to reduce greenhouse gas emissions, the Earth will be on the path of a large-scale marine species extinction at the end of the Permian period (an event known as the "mass extinction", which caused the disappearance of 96% of marine species and 70% of terrestrial species) in 2300. Among them, polar species are at the highest risk of extinction, but the biodiversity in tropical regions has declined even more.

Only by reversing the trend of greenhouse gas emissions on a large scale can we reduce the risk of species extinction by more than 70% and protect the marine biodiversity accumulated over the past 50 million years of evolution.

The past and future of marine biodiversity

Image source: Reference 5

In addition, as land experiences increasingly frequent heat waves, marine heatwaves (extreme high temperature events occurring in the ocean, defined as sea surface temperatures exceeding the 90th percentile threshold of the climate average for at least five consecutive days) are also becoming the new normal.

Marine heatwave

Image credit: NOAA

The number of marine heatwave days doubled between 1982 and 2016. If global warming is limited to 1.5°C, the number of marine heatwave days is expected to increase 16 times, however, according to the current nationally determined contributions (non-binding commitments made by each country on how to reduce greenhouse gas emissions and the extent of the reduction), the world will warm by about 3.5°C by the end of the 21st century, and the number of marine heatwave days may increase 41 times.

Change in the probability of marine heatwave days exceeding the 99th percentile above pre-industrial levels for global warming levels of 1°C (a), 2°C (b), and 3.5°C (C)

Image source: Reference 6

Just like facing high temperature heat waves, fisheries and even human socio-economics are highly vulnerable to this. The South China Sea marine heat wave event has led to the continuous occurrence of coral bleaching in the South China Sea since 2010, and serious coral bleaching events have occurred in my country's coastal waters. The extreme heat wave in Canada in 2021 directly roasted mussels and other aquatic shelled organisms on the beach.

Marine heat waves not only affect marine life, but also extreme weather on land through atmospheric teleconnections, thus affecting our lives.

Extreme heat frying shellfish on western Canadian beaches

Image source: CNN

In 2021, nearly 60% of the world’s ocean surface experienced at least one marine heatwave (WMO).

The United Nations Environment Programme has suggested that if the ocean continues to warm, every coral reef in the world could be bleached by the end of the century. Ocean warming will also lead to a sharp decline in marine life, the proliferation of toxic algae, and the death of aquatic life on the coastline. These changes will ultimately affect about 680 million people living in low-lying coastal areas, nearly 2 billion people living in the world's coastal cities, and nearly 60 million people working in fisheries and aquaculture.

Coral bleaching

Image credit: Kelsey Roberts/USGS

It's not just global warming, ocean crisis SOS!

The crisis facing the ocean is of course not just a “fever.” As the ocean warms, temperature changes are causing a chain reaction like dominoes.

Heat stored in the ocean causes it to expand and could account for one-third to one-half of global sea level rise, putting coastal and low-lying areas at increasing risk.

Sea levels have more than doubled in the past 30 years

Image credit: WMO

Changes in the ocean will also intensify extreme weather disasters such as storm surges and floods. Studies have shown that under a mild climate change scenario, continued warming of the upper ocean in the low-latitudes of the northwest Pacific will further increase the average typhoon intensity by 14% in 2100.

In addition, because the ocean has a very high heat storage capacity, it acts as a "buffer" for global warming. From the beginning of the Industrial Revolution in 1800 to 1994 alone, the global ocean absorbed 118 billion metric tons of carbon.

But there is always a critical value behind the ocean's unlimited blaming for emissions reduction.

The study found that while CO2 entering the ocean is increasing, the amount of emissions absorbed by the ocean (about 31%) remains relatively stable compared to the first global ocean carbon survey in 2004. If the ocean cannot absorb the continued increase in CO2, or if the ocean's carbon absorption efficiency decreases as the ocean warms and the ocean stratification intensifies, the human-emitted CO2 will have to remain in the atmosphere, which will further exacerbate global warming.

1800Global CO2 budget in 1994 and 1994-2007

Image source: Reference 8

In addition, the ocean's continued absorption of CO2 is also causing the ocean to continue to acidify, which is undoubtedly a disaster for shellfish and other marine organisms that use the mineral calcium carbonate to form their shells and exoskeletons.

Can carbon neutrality put a pause on the ocean crisis?

Over the past 80 years, each decade has been warmer than the previous one. Research shows that historic ocean warming is irreversible this century. By 2100, warming in the upper 2,000 meters of the ocean is expected to be 2 to 6 times greater than the warming observed to date. The Pacific Ocean will become the largest heat reservoir, and the regional average warming in the Atlantic and Southern Oceans will continue to be the strongest.

So, if the carbon neutrality goal is achieved, carbon emissions and carbon absorption are balanced, and the temperature no longer rises, can the sea level stop rising or even fall?

The lifetime of CO2 in the atmosphere is determined by how quickly it is absorbed by carbon sinks. Although about half of emissions are absorbed relatively quickly, some of the CO2 we do have accumulated in the atmosphere will remain for tens of thousands of years. Even if global greenhouse gas emissions were at a steady state, sea level would still rise for hundreds of years due to the thermal expansion of ocean water. The melting of glaciers and ice sheets would also contribute to further sea level rise.

Figure 1 shows the global sea level rise anomaly and sea temperature anomaly (difference from the average) due to thermal expansion. a, global annual mean sea level rise anomaly (relative to 1986-2005); b, global annual mean sea temperature anomaly at 300 meters (relative to 1986-2005). c, global annual mean sea temperature anomaly at 3000 meters (relative to 1986-2005).

Image source: Reference 10

But even in a zero-carbon emissions world, sea levels will still rise over the next few hundred years, with at least 80 centimeters of additional sea level rise "certain."

Conclusion

Yesterday was World Meteorological Day. We told the story of the ocean to warn everyone that the greenhouse gas problem is a fundamental issue for our lives and survival. The ocean crisis may be unavoidable, but emission reductions can greatly curb the rise in sea levels. We need to be more proactive in slowing down warming and preparing adaptation measures.

Countless scientists have shown the same possible future we may face: the Earth will become even less suitable for human survival. What we may ultimately need is not just net zero emissions, but more stringent global net negative emissions, in order to prevent the world from becoming unbearable after the temperature rises.

"Weather, climate, and water, from generation to generation, into the future." Late at night, when we close our eyes, there are more than 10,000 weather stations, 7,000 ships, 1,000 drifting buoys, hundreds of weather radars, about 30 weather satellites, 200 research satellites, and other equipment that are constantly observing the land and ocean. When we have enough data to equip our knowledge, the next step is to take concrete action.

References:

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【12】WMO Provisional State of the Global Climate 2022

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