The fatal consequences of heat
Old role, new attention to detailFor many reasons the world’s oceans are and will remain key regulators of climate on Earth. In the summer they store the sun’s energy in the form of heat, and in the winter they release it into the atmosphere. At the same time, ocean currents are continuously transporting heat from the tropics to the high latitudes, and in this way distribute it around the globe. Both of these processes act to moderate the climate. Moreover, the oceans remove the greenhouse gas carbon dioxide from the atmosphere. A portion of this is stored in the deep sea, which helps to buffer global warming. They also feed the global water cycle through the evaporation of large amounts of water from their surfaces.
Scientists have known about these large-scale relationships for a long time, but what is new is the level of detail at which researchers now comprehend the complex interactions between the ocean, the atmosphere, the sun, ice and snow, and the land surface. The foundations for this knowledge are provided by modern observation systems, which are deployed today in space, in the air, on land and in many regions of the world’s oceans. Satellites record the growth and shrinking of ice sheets and glaciers. They measure the surface temperatures of the ocean, changes in sea level, the area and thickness of sea ice in the Arctic and Antarctic regions, and can also document the salinity, colour and chlorophyll content of surface waters. Sensors attached to ships’ hulls, along with submersible vehicles, buoys and moorings, record seasonal and long-term changes in key water parameters such as temperature, salinity, pH values, oxygen, nutrient concentrations and chlorophyll content. A good example is the Argo network of independently operating profiling drifters, comprising more than 3700 measuring devices. These robots measure the water temperature and salinity, and in some cases even pH values, oxygen and nitrogen content, down to depths of 2000 metres.
- 2.1 > With this one-metre-long and 23-centimetre-diameter submersible vehicle called Icefin, scientists have succeeded for the first time in penetrating beneath the floating ice tongue of the Thwaites Glacier in West Antarctica to study on a large scale how warm the water is on the underside of the ice.
- In addition, there are ultra-modern submersible vehicles that are either propeller-driven or glide through the ocean for months at a time, allowing scientists to steadily advance into previously inaccessible ocean areas. In West Antarctica, for example, British and American researchers were able for the first time, in the winter of 2019/2020, to obtain measurements in the water masses beneath the floating ice tongue of the Thwaites Glacier using an underwater robot. The scientists drilled a 40-centimetre hole through more than 600 metres of ice shelf and lowered the torpedo-shaped measuring tool down on a rope. Upon reaching the underside of the ice, the vehicle, called Icefin, began an hours-long exploratory tour documenting the temperature and conductivity of the water, among other properties. The data revealed that the water was two degrees warmer than the melting point of the glacier ice, which explains why the Thwaites Glacier is losing ice so rapidly.
Also contributing to our better understanding of the role of the ocean in the climate system, however, are a plethora of historical, mostly handwritten weather records (ships logs, marine weather reports, etc.) that have now been digitized and fill some of the gaps in long-term observation series. Progress has also been made in deciphering past weather and climate data extracted from coral reefs, ice cores, lake and marine sediments, fossils and other natural climate archives.
- 2.2 > The Thwaites Glacier is one of the fastest flowing ice streams in West Antarctica. It transports ice from a region as large as the US state of Florida toward the sea. Its melting ice masses alone are responsible for four per cent of the current sea-level rise.
- Furthermore, climate research now has the benefit of high-performance computers with much greater storage and calculating capacities. These supercomputers are enabling researchers to develop new generations of climate and Earth System models that either have a much higher spatial resolution than previous generations, or that take into account many more components (for example, ocean, ice, snow, vegetation) and interactions in their calculations, and can thus better represent the complexity of climate.
For example, the ocean component of the latest model generation is capable of representing ocean eddies with diameters only slightly larger than 100 kilometres. In addition, the fast-moving ocean-margin currents are more realistically simulated. Because of these two advances, the heat transport in the ocean can be much better represented. The resolution of earlier models was not sufficient to reproduce transport processes on such small scales and to account for them in climate simulations. The same is true for biogeochemical processes in the ocean or the depiction of overlying clouds. Their existence, as well as their many associated interactions can now be modelled in much greater detail.
Based on this abundance of new observational data and climate simulations, science today is much better able to describe how the Earth’s climate has changed over the past 800,000 years, and especially since the beginning of the industrial era about 150 years ago (1850 to 1900). There is also greater certainty about the causes of these changes, how climate change is affecting the oceans and seas, and what predictions can be made for the future and their degree of confidence, both on global and regional scales.
- 2.3 > The physical and chemical properties of the world’s oceans are changing. Modelling conducted for the IPCC’s Sixth Assessment Report simulates future development trajectories for a set of possible Shared Socioeconomic Pathways (SSPs) – green shows the scenario for a world with very low greenhouse gas emissions (SSP1-1.9), blue a world with low emissions (SSP1-2.6), yellow intermediate (SSP2-4.5), red high (SSP3-7.0) and maroon very high (SSP5-8.5) emissions.
Beyond all doubtThe most important finding of climate research is that the world is warmer today than at any other time in the past 2000 years, and probably far beyond that. Since the period of 1850 to 1900, the average global temperature of our planet has risen by 1.1 degrees Celsius, whereby the warming over the continents has been significantly greater than over the oceans.
The greatest warming trend over land as documented by researchers has been in the Arctic region. The temperatures in recent decades have increased three times as fast in the northern polar region than in the rest of the world, with average temperatures varying widely from year to year. The differences are sometimes more than one degree Celsius, which is a lot. This fluctuation range, or temperature variability as meteorologists call it, makes it difficult for scientists to clearly distinguish the signal of climate change from the natural fluctuations of climate, which are referred to as climate noise.
- 2.4 > Although temperatures have risen more in northern America than in the tropics, the northern temperature curve still drops regularly into the former range of variations. In the tropics, on the other hand, it has long since departed from the previous levels.
Argo profiling drifters are measurement platforms that, when deployed in the sea, sink to a depth of 2000 metres and record the most important parameters of the surrounding water. Every ten days they return to the sea surface and transmit their data via satellite. The data is made freely available to the public within a few hours. Research institutions representing more than 40 countries are currently involved in the Argo observation network.
- To date, the smallest temperature increases over land have been observed in the tropics. This knowledge alone, however, is not a cause for optimism because, unlike in the Arctic region, the interannual differences here are much smaller. This means that the temperature is rising more slowly, or in smaller steps, but it is then remaining permanently above the former upper limits.
Looking closely at the curves from the equatorial regions, we see that temperatures have been above the former range of fluctuations since the 1980s. Those regions have effectively moved into a new, higher temperature regime. In other words, people living in the tropics now experience a hotter climate than their ancestors did 100 years ago. Climate researchers thus conclude that global warming is particularly evident in the tropics even though the temperature increase expressed in pure numbers is actually lower there than in the Arctic region.
Globally rising surface temperatures, however, are not the only evidence of the Earth’s changing climate. Researchers are now observing numerous indicators. The air masses in the troposphere, the lowest layer of the atmosphere, are warming and therefore able to store more water vapour, which is leading to more precipitation in many parts of the world. Smaller temperature differences between the poles and the tropics are causing changes in air-mass flow and thus a shift of the important wind belts in the temperate latitudes. At the same time, the subtropical arid zones are expanding, and in the Arctic region the area of Arctic sea ice has shrunk by 40 per cent over the past 40 years.
Humans are primarily responsible for these changes. This statement can be made beyond any doubt today. Important natural climate factors such as the brightness of the sun or the cooling effects of large volcanic eruptions fade into relative insignificance in view of the effects of human activity on Earth. By burning coal, oil and natural gas, humankind is releasing such great quantities of greenhouse gases such as carbon dioxide, methane and nitrous oxide (laughing gas) every year that their concentrations in the atmosphere are increasing and the greenhouse effect is intensifying.
- For the regularly published IPCC Assessment Report, researchers repeatedly produce new evidence and construct increasingly better climate models to calculate the extent to which the world would have warmed with and without human activity. For some time now, the results have been telling a clear story. If the models only take into account natural climate drivers such as the sun, volcanoes, vegetation, the ocean and others, they are not able to account for the amount of warming since the beginning of the industrial era. A realistic simulation of the current climate situation can only be obtained if the researchers include the data related to human greenhouse gas emissions.
Over the past 170 years humans have released an estimated 2430 billion tonnes of carbon dioxide into the atmosphere. Of this amount, 70 per cent was produced by burning coal, oil and natural gas. The remaining 30 per cent was due to changes in land use, which include deforestation and the draining of wetlands and marshes, but also the intensification of agriculture.
- 2.5 > Energy produced by burning coal, oil and natural gas is an important factor worldwide that is continuing to drive climate change.
- Furthermore, it is notable that, throughout this time period, increasing amounts of carbon dioxide have been released by human activity with every advancing decade. In the past, financial and economic crises have at best only led to lower rates of increase or, as in the exceptional case of the financial crisis of 2008, only to a short-term decline in emissions. Afterwards, the global economy has always recovered and carbon dioxide emissions have increased again, so that the overall long-term increase has continued. It is therefore not surprising that until corona year 2020 statisticians have been reporting new record levels of emissions with each subsequent year. The current highest value, from the year 2019, was a global total of 43.1 gigatonnes (billion tonnes) of carbon dioxide emitted. In the pandemic year 2020, emissions from fossil fuel combustion decreased by seven per cent compared to the previous year.
Of the amount of emitted carbon dioxide meanwhile 46 per cent remains in the atmosphere. The ocean absorbs 23 per cent, and another 31 per cent is absorbed from the atmosphere by land plants during their growth.
- 2.6 > Around 40 to 45 per cent of global warming is caused by short-lived climate pollutants. Unlike carbon dioxide, these only remain in the atmosphere for a short time, from a few days (particulates, soot) to a few years or decades (e.g. methane and hydrofluorocarbons).
The world ocean as a heat repositoryFor the year 2020, due to global emissions, the annual average carbon dioxide concentration in the Earth’s atmosphere rose to a value of 413.9 ppm (parts per million). For comparison, in the year 1750, two decades before the Scotsman James Watt laid the foundation for the industrial era by optimizing the steam engine, the carbon dioxide concentration is estimated to have been 277 ppm. The more carbon dioxide there is in the Earth’s atmospheric shell, the more impenetrable it becomes to the heat energy that our planet is constantly radiating outward again due to the accumulation of incoming solar radiation. Instead of allowing the heat to escape into space, the greenhouse gases trap it in the atmosphere, so to speak, thus causing temperatures on the Earth to rise.
Thanks, first and foremost, to the world ocean, average global warming has so far been limited to a value of 1.1 degrees Celsius. Since the 1970s, the oceans have absorbed more than 90 per cent of the excess heat trapped in the Earth System due to human activities.
The enormous amount of energy involved becomes evident when one considers that, during the period from 2018 to 2019 alone, the oceans removed around 44 times more energy from the atmosphere in the form of heat than all of humanity had used in the same time period for transportation, industry, heating and in their households. The oceans are clearly the most effective component of the Earth’s climate system for storing heat.
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- The fact that the ocean can extract so much heat from the atmosphere is primarily due to the extremely high heat capacity of water. This means that, compared to other substances, a comparatively large amount of thermal energy is needed to heat water by even one degree Celsius. To put it another way, the sea is capable of absorbing large amounts of heat without becoming significantly warmer itself. Conversely, however, this also means that the oceans are able to store a great deal of heat, a property that becomes particularly important considering that they will also release that heat energy back into the atmosphere when the water masses cool down.
Because the heat capacity of water is four times greater per kilogram than that of air, the oceans are able to store over 1000 times more heat than the Earth’s atmospheric shell. The absorption of heat occurs at the sea surface. Winds, tides and ocean currents act to mix the water masses and keep them in constant motion, so that the heat is transported vertically to substantial depths as well as horizontally from the warmer regions toward the poles.
Heat absorbed by the sea, however, does not simply disappear. It is only stored temporarily. The ocean can therefore be compared to a gigantic thermal battery that, by the emission of greenhouse gases, we humans have been constantly charging with heat since the beginning of industrialization, thereby forcing climate change.
- 2.7 > The oceans absorb heat at their surface. Currents then transport it to greater depths. This pattern is observed in all oceans, as illustrated here by the changing temperatures to a depth of 2000 metres.
- The heat energy stored in the sea eventually has an impact on climate again by contributing to the melting of sea ice or floating glacier tongues in the Arctic and Antarctic regions, by enhancing the evaporation of seawater, or by warming the air directly above the sea surface.
In this case, the ocean releases its heat energy back into the atmosphere and causes air temperatures to rise, especially in the temperate and higher latitudes. The time frame in which this occurs is difficult to predict. Once it is absorbed, the heat in the ocean can significantly influence the Earth’s climate for decades. In order to make scientific climate predictions, therefore, it is crucial to know the heat content of the world ocean as accurately as possible.
The ocean temperature curve through time also serves scientists as an important monitoring tool. Data that describe changes in the heat content of the oceans are the best indicators of how global warming is progressing – whether it is abating (stable or declining water temperatures) or advancing (rising water temperatures). Data from the air are actually not particularly useful for such analyses because they are influenced by too many different factors. Nevertheless, they are still often used to make assertions about the development of global warming.
Temperature increases at great depthsAs the ocean continues to absorb heat, the increase in water temperatures becomes more evident, initially at the sea surface, but eventually also to greater depths. Since the beginnings of the 20th century, the mean surface temperature of the oceans has increased by 0.88 degrees Celsius.
However, scientists are also now observing substantial changes in the deep sea as well. Looking at the heat distribution at various depths, up to the year 2020 about 40.3 per cent of the absorbed heat had remained in the upper 300 metres. 21.6 per cent had reached depths of 300 to 700 metres, and another 29.2 per cent had accumulated in the water layers between 700 and 2000 metres. The remaining 8.9 per cent of the heat was transported to depths below 2000 metres, especially in the Atlantic and Southern Oceans.
- 2.8 > Sea levels do not rise uniformly as they do in a bathtub. Satellite observations from 1993 to 2017 reveal significant regional differences.
- Climate researchers therefore conclude that the large-scale warming of the ocean is one of the most convincing signs of global climate change. The oceans are warmer today than they have been at any time since the beginning of continuous observations. All signs point to a continued increase in water temperatures throughout the 21st century, even if humankind succeeds in reducing its greenhouse gas emissions.
Life-threatening consequencesAs a result of warming there are changes in several key physical properties of seawater. Some of these changes have a direct impact on climate, while others have a more pronounced impact on life in the sea and near the coasts.
The most important effects are:
- the rise in sea level,
- increased stratification of the water masses and the associated decrease in ventilation and oxygen content of the inner ocean,
- intensified evaporation of seawater,
- amplified danger of extreme weather events such as storms, and
- increased occurrence of heatwaves in the ocean.
Rising water level with no end in sightWater expands with warming. This basic law of nature also applies to the oceans, of course, and in recent decades this process has contributed to a mean global sea level that was 20 centimetres higher in 2018 than it was in the year 1900. And, according to predictions, the level will continue to rise by another 18 to 23 centimetres by 2050. Until the beginning of the 21st century, the increasing ocean temperatures and accompanying expansion of water masses were the main reason for the long-term rise of global mean sea level. From 1901 to 1990 the rise averaged 1.4 millimetres per year.
Since then, however, the rise in sea level has been accelerating noticeably. From 1971 to 2018 the global level rose by an average of 2.3 millimetres per year, whereby values as high as 3.7 millimetres were measured during the second half of this time period (2006 to 2018). This means that the rate of rise has more than doubled when compared to the past century.
This acceleration, however, cannot be attributed to ocean warming alone, even though the share due to density changes in the water now stands at 1.4 millimetres per year. Rising sea levels can be caused by a variety of processes. In the past two decades the amounts contributed by the worldwide melting of glaciers and of the ice sheets in Greenland and Antarctica have increased drastically. The constant influx of new meltwater means that there is actually more water circulating in the ocean, and this increase in mass is also contributing to sea-level rise. According to the Intergovernmental Panel on Climate Change (IPCC), over the past 15 years ice loss from glaciers and ice sheets accounted for 1.62 millimetres of rise per year. This is around 44 per cent of the total rate of increase.
- Furthermore, changes in water-usage practices on land also have a measurable influence on global sea levels. For example, if streams and rivers in numerous regions of the world are dammed to form reservoirs, it can actually have the effect of lowering sea level. The reverse effect occurs when large amounts of water on land are removed from groundwater sources or lakes, and this water is then discharged into the ocean through sewers, streams and rivers after its use.
At this point, it should be noted that the water levels in the world’s oceans do not rise uniformly like the water in a bathtub. The surface is also not level, as one might first think when looking out to sea from the beach. Satellite observations confirm that there are significant regional differences in sea level, as well as in the rise of water levels over time. These can be attributed, for example, to the influences of ocean currents and winds, or the variability of water-mass expansion due to heat. Rising or falling water levels, however, can also be affected by uplift or subsidence of the land areas in coastal regions that were covered by huge glaciers during the last glacial period. Expressed quantitatively, these differences can account for as much as plus/minus 30 per cent of the present global increase.
For this reason, scientists commonly refer to sea levels in the plural sense. In addition, experts frequently point out that when assessing the risk of local flooding, it is not the global trend alone that matters, but that local conditions in particular must be taken into account. A striking example of this is seen in the water-level trends along the coasts of North America. While sea levels along the west coast have remained almost unchanged or have even fallen in recent years, they are still rising for the most part on the east coast.
- Rising water levels are one of the most impactful effects brought about by climate change. They threaten countless atolls and small island nations as well as extensive, often densely populated coastal regions around the world. But it is still extremely difficult for scientists to make precise predictions about the future development of regional and local sea levels. This is because of the great uncertainties connected with the crucial influencing factors. For example, accurate future melting rates of the ice sheets in Greenland and Antarctica are still difficult to predict, and it is not certain whether they will eventually reach a tipping point, beyond which their collapse will be unstoppable and irreversible.
In its Sixth Assessment Report, published in 2021, the Intergovernmental Panel on Climate Change projected that the rise in global sea level will continue to accelerate, even if the international community is successful in reducing greenhouse gas emissions to the levels agreed to in the Paris Climate Agreement of 2015. According to present predictions, the mean global sea level will rise by 38 to 77 centimetres by the year 2100, depending on the amount of greenhouse gases humankind continues to emit.
As water levels rise, the danger of flooding becomes greater, especially in coastal regions that are less than ten metres above sea level. Statistically, many of these, in the past, were hit by exceptional flooding events related to storm and spring tides or extremely high waves only once every 100 years. According to the Intergovernmental Panel on Climate Change, by the year 2050 these events will be occurring every two to 50 years in the high and temperate latitudes. In the lower latitudes, the coastal areas of the tropics and subtropics, the experts expect several extreme high-water events every year. This means that cities with millions of inhabitants, such as Calcutta, could be regularly inundated by the sea in the future.
- 2.10 > Rising sea levels are threatening the world’s sandy beaches. For all coastal countries, these two maps show the calculated percentage of sandy beach coastline length with a loss in breadth of more than 100 metres by the year 2100. Above (a) shows a world that warms by 2.5 degrees Celsius by 2100; below (b), a world that is around 4.3 degrees Celsius warmer by the same time.
- Flooding will be particularly severe when a generally high sea level is combined with spring tides in an area of storms, where winds pile up seawater off the coast. Under these conditions, high waves are able to penetrate especially far inland and flood large coastal areas. Coastal protection experts estimate that the danger of severe flooding increases by a factor of about three with every decimetre of sea-level rise. This steep increase is mainly due to the fact that the coastal zones in many regions of the world are only slightly higher than the current local sea levels. So, if the regional level rises by about ten centimetres, the high-water line shifts landward by around 30 to 40 metres, depending on the amount of slope. During storms, the waves roll in much further over the coastal area unless steep cliffs or structures such as coastal protection walls block their path.
Discounting all high-water protection measures and considering only the land elevation in the coastal regions, around 360 million people presently live in low-lying regions that would be regularly flooded by the year 2100, even if the two-degree climate target were to be met. Most of these people are in Asia. In Vietnam, for example, almost one-fourth of the population would be affected under these conditions. In Bangkok, the capital of Thailand, large portions of the city would be permanently under water, and a similar situation would be seen in Shanghai.
This number, however, is only one among many, because scientists have proposed many quite different definitions of the conditions under which coastal populations are considered to be threatened by sea-level rise. More accurate prognoses are also difficult because the future population growth in coastal regions can only be approximated, and due to the fact that many coastal metropolitan regions are subsiding as a result of large quantities of groundwater being pumped out of the subsurface aquifers. Where local sea level is rising at the same time, the flooding risk is greatly multiplied.
But researchers are in full agreement in their overall assessment of the threat that the global rise in sea level presents. They leave no doubt that minimizing the impacts on coastal populations is one of the greatest societal challenges of our time.
Oceans running out of oxygenThe warming ocean water is not only expanding, it is also losing oxygen, which is vital to marine life. Between 1960 and 2010, the world’s oceans lost more than two per cent of their oxygen content (around 77 billion tonnes of O2). One reason for this was eutrophication, a process that mainly affects coastal waters. Another factor, which scientists can now clearly demonstrate is responsible for most of the oxygen loss, is ocean warming due to climate change.
Oxygen enters the ocean in two ways: either through gas exchange processes between the atmosphere and sea at the water surface, or as a by-product of photosynthesis, which is carried out by algae and aquatic plants in the upper part of the water column penetrated by light. Still, on average, a litre of seawater contains about 30 times less oxygen than a litre of air, which is why breathing underwater is such a difficult task. To obtain one gram of oxygen, fish, mussels, starfish and other animals have to pump around 152 litres of water through their gills or respiratory organs. By contrast, land organisms have to inhale only 3.6 litres of air to get the same amount of oxygen.
Oxygen in the ocean is not only used by fish and other highly developed marine organisms. It is needed mainly by microbes and multi-celled organisms that break down plant and animal remains (organic material) at great depths, whereby oxygen is consumed. The more biomass that is produced in the zone penetrated by light and the more algae and animals that die and sink, the more organic material there is available for the microbes and, accordingly, the greater the amount of oxygen they consume. The case is similar for rising water temperatures. As the ocean becomes warmer, the large and small marine inhabitants require more oxygen to maintain all of their vital processes. From this knowledge, scientists can draw two main conclusions.
- 2.11 > Low or declining oxygen concentrations are a global problem that is present both in coastal waters and in the open ocean. This map shows coastal regions marked in purple whose waters contain less than two milligrams of oxygen per litre of water (< 63 micromoles per litre). The distribution of the oxygen minimum zone at a depth of 300 metres is shown in orange.
- Firstly: Changes in the basic chemical or physical conditions within a water layer – for example, the amount of nutrient input, the temperature, or the amount of incident light – influence biomass production and thus, in the long term, the amount of oxygen-consuming decomposition of the biomass that sinks into the depths.
Secondly: When ocean circulation transports oxygen-rich surface water to greater depths, its oxygen concentration is initially comparatively high. But the longer this water remains in the deep, the more time the microbes and other organisms have to break down the sinking particles and thereby consume the oxygen contained in the water. For this reason, the deep water, as a rule, is relatively oxygen-poor. But let us return to the sea surface. The amount of oxygen that the ocean can absorb from the air depends on the temperature and salinity of the surface water. These two factors significantly determine the solubility of gases in water. Less oxygen can be dissolved in a warmer and saltier ocean. If the temperature of the surface water increases from four to six degrees Celsius, for example, its oxygen content automatically decreases by five per cent.
Scientists have studied how great the respective influences of temperature and salinity changes have been on the oxygen content of the oceans through recent decades. They have concluded that the decrease of oxygen in the upper 1000 metres of the water column is primarily due to the increasing levels of heat and the consequential lower solubility of gases in the ocean. Changes in salinity, on the other hand, have been found to play only a minor role.
Calculated for the entire water column, however, the oxygen losses related to heat and solubility account for only 15 per cent. The remaining 85 per cent are caused by the fact that the ocean currents as well as the mixing depths of the surface water are changing.
- 2.12 > Oxygen depletion in the open ocean is caused primarily by rising water temperatures. These have the effect of inhibiting the dissolution of oxygen in the water and preventing adequate mixing between the surface and deep waters. In coastal areas, on the other hand, a high influx of nutrients enhances algae growth, and their degradation by microorganisms eventually consumes all of the oxygen.
- The water masses at the sea surface are aerated by direct gas exchange with the atmosphere. This process is so efficient that the surface water is practically always oxygen-saturated with respect to its temperature. This means that it has the maximum possible oxygen concentration and, in this regard, is in equilibrium with the atmosphere. The depth to which this condition exists depends on the wind as well as the air and water temperatures, both of which vary depending on season and latitude. In the summer, when the surface water is warmed strongly by the sun and by higher air temperatures, it expands and becomes significantly lighter than the underlying, mostly cooler water layers. Fundamentally, the colder and saltier a water mass is, the greater its density, and the deeper it lies within the stratigraphy of the ocean. As a result of this density contrast with the deep water, the warm surface water lies like a stable, warm blanket on top of the ocean, and even a strong wind is no longer able to mix the upper layer with the underlying water masses. The oxygen-rich water remains near the sea surface, and is not transported to the deeper layers.
- Scientists refer to the layering of water masses due to density differences as stratification. Because the ocean warms from the top down, stratification of the layers is intensifying as a direct consequence of ocean warming, and the amount of water exchange between the surface and the underlying layers is decreasing at ever greater rates. In some regions of the world, the temperature-related stratification of the upper water layer is further amplified. For example, in the polar regions, the snow cover, glaciers and ice sheets are melting at increasing rates, and their meltwater is freshening the ocean at the surface. Scientists are observing the same effect in those ocean and coastal regions where there is more precipitation as a result of climate change. Like the meltwater, rain is pure freshwater, which dilutes the surface water of the ocean, making it less saline and therefore lighter.
The thermohaline conveyor belt of ocean currents is responsible for ventilating the levels below the wind-mixed surface layer. It transports the water masses of the ocean like a kind of conveyor belt through all of the major ocean basins. This conveyor belt moves because of temperature and salinity differences between the water masses, which is why scientists refer to it as thermohaline circulation (thermo: driven by temperature differences; haline: driven by differences in salinity).
- 2.14 > The location, size and distribution of oxygen-poor zones are closely related to ocean currents. This map shows the wind-driven currents of the subtropical gyres and the Antarctic Circumpolar Current, as well as the density-driven conveyor belt of thermohaline circulation.
- However, its operation is impeded by climate change, because when the water masses at the sea surface become warmer and lighter, the overturning by thermohaline circulation proceeds more slowly. This process entails the cooling and sinking of enormous masses of water, both in the middle latitudes where the intermediate water originates, and in the Arctic and Antarctic regions where heavy, oxygen-rich deep water is formed. The latter flows from the polar regions back toward the equator, thus ventilating the deep ocean. The intermediate water, on the other hand, supplies the middle layers of the ocean with oxygen.
There is now evidence from many parts of the world that the conveyor belt of thermohaline circulation is slowing down as a result of climate change. It indicates not only that less oxygen-rich surface water is reaching greater depths, but that the individual water masses, on their journey through the oceans, are also spending more total time within the middle and deepest layers of the ocean. But in these two levels, microbes and other small organisms are continuing to decompose organic particles and consume oxygen, which is leading to a further depletion of the oxygen content of the intermediate and deep waters.
- 2.15 > ince 1960 the total oxygen content of the ocean has decreased by more than two per cent. This map shows the regions in which the oxygen concentrations have declined most strongly.
- German scientists, using climate-ocean models, have calculated how these processes will play out. Their results indicate that the deceleration of global ocean circulation due to warming will be responsible for half of all the oxygen loss in the upper 1000 metres of the water column in the future. And in the deeper ocean, below 1000 metres, as much as 98 per cent of the loss will be attributable to the slowdown of thermohaline circulation.
Over the past 50 years in the open ocean, the total area of the oxygen minimum zone, in which fish no longer have enough oxygen to breathe, has expanded by around 4.5 million square kilometres. This increase is roughly equivalent to the land area of the European Union. During the same time period, the amount of anoxic water, completely void of oxygen, has quadrupled. The ocean is literally running out of air because of climate change. The catastrophic aspect of this development, however, is that the heat-induced loss of oxygen in the ocean cannot simply be stopped and reversed. Even if humans were able to successfully reduce their greenhouse gas emissions by amounts that are in accordance with the Paris Climate Agreement and live with net-zero emission levels in the future, it would take several centuries for greenhouse gas concentrations to decline, for the atmosphere and the world ocean to cool down, and for the oxygen content of the oceans to return to pre-industrial levels.
- 2.16 > In the open ocean the oxygen content of the water decreases with increasing depth. This is due to oxygen consumption by microorganisms.
Fuel for hurricanes and heavy rainsNinety-seven per cent of all liquid water on Earth circulates in the oceans and their marginal seas, which makes them the most important reservoir in the global water cycle. An estimated 420,000 cubic kilometres of water evaporate above the oceans each year. Around 90 per cent of this moisture then returns directly to the sea in the form of rain or snow. The remaining ten per cent, however, drifts over the continents in the form of water vapour or clouds and precipitates there. On its way back to the sea, it often makes temporary stopovers – for example, in the form of water droplets that help a plant to grow, or to seep through the soil and help replenish a groundwater reservoir. But eventually, this water too returns to the sea.
The amount of water that evaporates above the ocean to take up this journey depends greatly on the air and water temperatures. The more the atmosphere warms, the more water vapour it can hold (seven per cent more moisture per one degree Celsius of warming). And the warmer the seawater is, the more readily it evaporates at the surface. As a result, significant patterns of water distribution within the hydrological cycle are changing in the wake of climate change. Higher evaporation rates, for example, amplify the intensity of heavy rainfall events that mostly build up over the ocean. This means that during this kind of extreme weather event much more rain will fall today than it would have in the past.
A good example of this was tropical storm Imelda, which struck the south-eastern region of the US state of Texas in mid-September 2019 and triggered large floods because of its unusually high amounts of rainfall. On the second and third days of the storm, up to 500 litres of rain per square metre fell in the storm centre, an amount of rainfall that is normally only seen in this coastal region every 50 years. Around 1000 people had to be evacuated, five people died, and more than 10,000 cars were damaged by the rainfall and flooding. A state of emergency was declared for 13 counties with a total population of 6.6 million.
- 2.17 > The warmer the atmosphere and ocean are, the more water evaporates and the greater the danger of heavy rainfall becomes. The map illustrates the increased probability of heavy rainfall events in a world that is three degrees Celsius warmer than in pre-industrial times.
- After this extreme event, climate researchers collected all of the available meteorological data from the region – current weather data as well as historical records going back at least 80 years. Using climate models, they then calculated the degree to which climate change had increased the probability of the storm and its intensity of precipitation. Their analysis showed that, compared to the year 1900, the risk of this kind of two-day heavy rainfall occurrence had risen by a factor of 1.6 to 2.6. The intensity of the rainfall had increased by nine to 17 per cent. Researchers concluded that the study was further evidence that climate change along the US Gulf Coast is leading to increasing amounts of rain during extreme weather events.
These kinds of studies, referred to as extreme event attribution research, belong to a still relatively young research field within climate science. For almost 20 years researchers have been trying to identify the proportion of the contribution of human-induced climate change to extreme events such as droughts, heatwaves, storms and floods. They often compare the observational data from an extreme event to two kinds of climate simulations – one in a world without greenhouse gas emissions from humans, and a second which realistically reflects our present climate.
- 2.19 > Atmospheric rivers are air currents that carry approximately as much moisture in the form of water vapour as some rivers do as liquid water – thus the terminology. The current shown here caused extreme rainfall in Great Britain in November 2009.
- Over 350 individual studies have now been reviewed by experts and published in professional journals. Most of them provide new indications that human activity increases the probability of occurrence or the intensity of extreme weather events. In an overview study published in 2020, experts showed that man-made climate change had increased the probability or intensity of 78 per cent of the extreme events studied. In most cases, the triggers were rising temperatures resulting from high greenhouse gas emissions. When considering only the studies on heavy rainfall events and flooding, the results were not quite as conclusive. For these cases, a clear link to climate change could be detected in only 54 per cent of the studies.
In its most recent report, the Intergovernmental Panel on Climate Change similarly anticipates that precipitation patterns will change in many regions of the world. Exceptional events such as heavy rainfall or prolonged drought will occur more frequently and will be more intense. Moreover, the seasonal differences in amounts of precipitation will increase. In some regions this will mean less frequent rainfall. But when precipitation does fall, the sky will open its floodgates and more water will rain down within a short time than the local population has been accustomed to. The danger of flooding is increasing because tropical and extra-tropical storms are carrying much more moisture.
This is also true for a phenomenon known as atmospheric rivers. These are long, usually 400 to 600 kilometre-wide bands of moisture-saturated air, that transport humidity (water vapour) from the tropics into the middle latitudes, over both the Pacific and Atlantic Oceans. Atmospheric rivers are responsible for a large portion of the normal, typical seasonal rainfall on the west coasts of North and South America, as well as in Greenland and on the British Isles. In the US state of California, they bring 25 to 50 per cent of the annual precipitation. Atmospheric rivers, however, can also cause extreme events, especially when their moisture-laden air masses collide with the mountains on the west coast of the USA and are forced to rise. When this happens, heavy rainfall and flooding frequently result. When the air masses of the atmospheric rivers become warmer, they are able to carry greater amounts of moisture. Researchers therefore assume that in the course of climate change the intensity of rainfall they bring will increase along with the risk of flooding.
- 2.20 > The tropical cyclone Imelda made landfall on the Gulf Coast of the US state of Texas on 17 September 2019. Soon afterward, it rained so heavily in parts of Texas that Imelda was ranked at number seven on the list of tropical cyclones with the most abundant precipitation in the USA.
- More intensive rainfall is one consequence of ocean warming, but there is also a second consequence. Researchers are now able to confirm that rising water temperatures at the ocean surface are increasing the destructive power of large tropical storms. The physical principle is quite simple: Hurricanes, cyclones and typhoons derive their energy from the heat in the ocean below them. The warmer the water is, the higher the wind speeds that the storm can develop, and the greater its destructive power is when it makes landfall. Climate models have demonstrated this correlation for a long time. But verification of the influence of climate change through direct observation was first possible in 2020.
To achieve this, US scientists analysed satellite images of hurricanes over the past 40 years, and were able to show that as the sea temperature increased, so also did the probability that an approaching hurricane would develop into a major destructive storm of category 3 or higher. The destructive power of major tropical storms is rated according to the Saffir-Simpson hurricane wind scale (SSHWS). This assesses the potential damage of a storm based on its wind speed and assigns it to one of five categories. Under this system, a storm with wind speeds greater than 178 kilometres per hour (category 3) is considered to be a major hurricane.
- 2.21 > Specialists use the term “heatwave” to refer to phases when the water temperature in a marine region is above a certain temperature threshold for at least five consecutive days. The threshold changes with the time of year and is calculated statistically.
- Heightened evaporation and precipitation also cause changes in the surface layer of the sea, especially with regard to salinity. In regions where more water evaporates in the future than is replenished by rainfall, the surface water will become saltier – for example, in the tropical areas of the Atlantic Ocean and in the Mediterranean Sea. But where the amount of precipitation is greater than evaporation, the surface water will be diluted, and the result will be a long-term decrease in salt content. According to climate projections, the latter case will be most prevalent in the Pacific and Arctic Oceans.
Marine heatwavesAnother kind of extreme event that is now occurring more frequently and is routinely setting new records is the marine heatwave. This is the term specialists use to refer to phases where the water in a certain marine region is unusually warm for at least five consecutive days. Over the past decade, scientists have been documenting such phases in the open ocean as well as in marginal seas and coastal regions. They occur in summer as well as in winter, because the determining factor is not a specific temperature level, but rather how many degrees Celsius warmer the water temperature is at a given location than the average value that would normally be measured there at the same time of year.
Heatwaves often make headlines because they have a long-term impact on the biological communities in the affected marine regions. Notable examples over the past decade include the heatwave along the western coast of Australia in 2011, the Mediterranean heatwaves of 2012 and 2015, and the heatwave in the North Pacific that lasted from 2014 to 2016 and became known worldwide as “the Blob”.
The triggers for such warming of water masses can vary greatly. Ocean currents that concentrate warm water at a certain site are often involved. However, marine heatwaves can also form as a result of intense solar radiation and high air temperatures. Under certain conditions winds can cause the water to heat up, but under other conditions air motions could even act even to suppress a heatwave. Moreover, it is now well known that large climate cycles such as the El Niño phenomenon significantly increase the probability of heatwaves in certain marine regions.
The general warming of the world’s oceans in the wake of climate change, however, is much more significant for the future equilibrium. It increases the probability of large heatwaves, which are very harmful, especially for marine organisms with low heat tolerances. These are being more frequently pushed to their temperature limits. This, in turn, strains their adaptive capacity and reduces their prospects of survival. Such species either migrate to other areas or they perish. There are no other options for them.
In the long run, this development drives fundamental changes in the biological communities of the sea, and thus also in the ecosystem functions of the oceans.
No longer a reliable constantClimate change is altering the world’s oceans today in a manner unprecedented in the history of humankind. As a result of global warming, water temperatures are rising continuously along with sea level. These are the two most visible indicators of global warming. At the same time, the ocean is losing oxygen down to ever greater depths, and is becoming increasingly acidic everywhere.
These physical and chemical changes are having a direct impact on a wide range of ocean ecosystem processes, including its function as a reliable weather regulator. Due to the shift of wind-driven ocean currents towards the poles, for example, the heat of the ocean is now transported much further to the north and south than it was earlier, and is altering the weather in those regions.
A trio of stressors – ocean warming, acidification an diminishing oxygen – is also changing the fundamental conditions for life in the ocean. It is reducing the ocean’s ability to produce biomass, and is amplifying the harmful effects of direct human intervention to such an extent that the survival of marine biological communities is at risk in many places.