Urgently sought – ways out of the climate crisis
WOR 8 The Ocean – A Climate Champion? How to Boost Marine Carbon Dioxide Uptake | 2024

Code red for people and nature

Code red for people and nature fig. 1.4: Annie Spratt/unsplash

Code red for people and nature

> Climate change is now a daily reality. At least half the world’s population is already suffering the direct effects of global warming. Wells are drying up, heat levels are becoming unbearable, storms and flood waters are sweeping away goods and property, and already ravaged ecosystems are increasingly failing to deliver their services. The climate and nature make no compromises. For humanity, therefore, everything is at stake, for the change that we ourselves have set in motion is proving to be a potentially lethal risk multiplier.

Our future is at stake

We have known for decades that the Earth’s climate is warming, and that this is caused by our greenhouse gas emissions. However, the magnitude of the global climatic changes that have already occurred and the critical situation now facing life on Earth have rarely been described with such urgency as in the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).
On behalf of the IPCC, more than 750 climate scientists from around the world regularly review the current state of knowledge of global climate change. They analyse the findings of research on the causes and effects of climate change, collate information on the extent to which people and nature have the capacity to adapt to the new climatic conditions, and describe measures that may be effective in mitigating climate risks and limiting global warming.
The core message from the three volumes of the IPCC’s Sixth Assessment Report is very clear: with its persistently high levels of greenhouse gas emissions, humankind is gambling away the prospects of a liveable future for present and future generations.
1.1 > Smoke rises from the chimneys of a Chinese steelworks in Inner Mongolia. Meanwhile, ore is smelted illegally by workers at a nearby camp. China is the world’s largest emitter of carbon dioxide (accounting for around 30 per cent of global emissions in 2022), partly because coal is still the country’s main energy source.
fig. 1.1 © Kevin Frayer/Getty Images

Rapid warming and its effects on the Earth’s climate

According to data from the IPCC, the global surface ­temperature during the period from 2010 to 2022 was approximately 1.15 degrees Celsius higher than the reference figure for 1850 to 1900. There were much larger increases over land than over the ocean: mean temperatures over the continents rose by 1.65 degrees Celsius, while air masses over the ocean warmed by 0.93 degrees Celsius. Well-informed readers may wonder at these statistics, as the figure for global warming mentioned by other organizations and well-known news outlets since 2020 is 1.2 degrees Celsius. In light of this, it seems reasonable to ask: is the IPCC working with obsolete data? By no means.
Global climate reports such as those produced by the IPCC or the analyses published regularly by the World Meteorological Organization (WMO) focus on long-term changes in climate parameters. In order to deter-mine global surface temperature, therefore, they do not simply analyse the temperature data for a specific year, as these figures may be influenced by short-term natural temperature fluctuations. Instead, the IPCC authors use monitoring data from the previous 20 years as baseline figures. They are thus able to detect the real long-term trend.
And the fact is that global warming is accelerating: in the past 50 years (1970 to 2020), global surface temperature has increased faster than in any other 50-year period over the last 2000 years. A detailed look at the last four decades (1980 to 2020) reveals that each one of these four decades has been successively warmer than the decade that preceded it.
This development means that many of the Earth’s climate system components are changing at a speed not experienced by our planet for many hundreds or even thousands of years. However, the magnitude of these changes is not uniform everywhere. Some regions are more severely affected than others. What’s more, with every additional tenth of a degree of warming, the changes under way are amplified. This means that the magnitude and extreme speed of the changes, but also the associated risks, will increase with each additional increment of warming, no matter how small. This applies particularly to ocean warming, acidification and deoxygenation; the continued rise in hot extremes over land and in the oceans; the melting of the ice sheets; sea-level rise; and shifts in the Earth’s water cycle.

The oceans and seas – more warming, acidification and oxygen depletio

Current status: The oceans and seas are our planet’s largest storehouse for heat. This storehouse is being recharged continuously by climate change and the associated warming of the atmosphere. Over the last 60 years, around 90 per cent of the excess heat retained in the Earth’s atmosphere due to the greenhouse effect has been absorbed by the oceans and seas and stored in their depths. As a result, ocean heat content has increased significantly and water temperatures are rising more rapidly than at any time since the last glacial period. Sea surface temperature alone has risen by an average of 0.93 degrees Celsius in the period from 1850 to 1900 to 2022. Researchers describe the increase in ocean temperatures as the clearest indicator of human-induced climate change – firstly, because the ocean absorbs the largest proportion of the excess heat, and secondly, because its surface temperatures are subject to less year-to-year fluctuation than the atmosphere, for example. The warming trend is therefore easier to detect.
As the ocean has warmed, stratification of the water masses in the upper 200 metres of the water column has increased. Concurrently, due to increased evapotranspiration from the sea surface, the surface water at evapotranspiration sites, which already has a higher salt content, has become even more saline.
By contrast, in areas of the sea with heavy precipitation or high meltwater inflow, freshwater influx has increased, further reducing the already low levels of near-surface salinity here.
fig. 1.2 © NASA Physical Oceanography Distributed Active Archive Center


1.2 > In some marine regions of the North Pacific (shown here in dark red), the water temperature in May 2015 was up to 3 degrees Celsius higher than average. The marine heatwave – now known as “the Blob” – which caused this rise in temperature lasted for more than 250 days and killed thousands of fish, seabirds and marine mammals.
Both these trends – increased stratification of the water masses, and changes in salinity – have, since the 1950s, reduced the density-dependent mixing of surface water with the underlying water masses, thereby amplifying ocean deoxygenation. Oxygen depletion is particularly noticeable in the oxygen minimum zones which are forming in the Western Pacific, the Indian Ocean and off the west coast of Southern Africa below the surface layer, i.e. in water depths between 100 and 200 metres.
In these zones, the seawater contains less than 70 micromoles of oxygen per kilogram (μmol/kg), which means that marine fauna such as sharks and tuna, which rely on a plentiful supply of oxygen, have no chance of survival here.
The oceans and seas do not just absorb heat, how-ever; they also take up around a quarter of the carbon dioxide generated by human activity. But unlike oxygen, carbon dioxide does not simply dissolve in seawater: it undergoes a chemical chain reaction which increases the water’s acidity. This process of ocean acidification has highly detrimental effects on the habitat conditions of many marine organisms. Experts refer to a reduction in pH as the measure of the ocean’s acidity. According to the IPCC, in the last 40 years, ocean surface pH has decreased in almost all areas of the sea – to such an extent that oceanic acidity is now at its highest level for at least 26,000 years. What’s more, ocean acidification appears to be taking place with record-breaking speed at present. Making matters worse, acidification is no longer affecting surface water alone; in the last 30 years, it has been detected with increasing frequency in the deeper ocean as well.
Outlook: The temperature of the oceans and seas will continue to increase even if humankind succeeds in limiting global warming to 1.5 degrees Celsius. This can be explained by the inertia of the ocean system: key processes take place so slowly that the effects of any initiated changes are felt over hundreds, if not thousands of years and take just as long to reverse. Even so, we have the solution in our hands: the rate of ocean warming from 2050 will depend entirely on whether we can curb climate change. And it is the future water temperature which will determine how much oxygen the oceans will still contain. The warmer the sea, the less oxygen can be dissolved.

An increase in hot extremes in all regions of the world

Current status: Meteorologists have observed an increase in the frequency and intensity of hot extremes since the 1950s, as well as an increase in heatwave intensity and duration over land. What is new is that these ­weather extremes are now reaching record temperatures which would have been impossible without human-induced climate change. The extreme heatwave which struck north-west areas of the USA and Canada in late June 2021 is an example: in some localities, temperatures rose to 49.6 degrees Celsius, with highs at some weather stations breaking historically observed heat records by as much as 4.6 degrees Celsius. The research now confirms that temperatures during this heatwave would have been around two degrees Celsius lower without humaninduced climate change. In a world with global warming of two degrees Celsius, however, maximum temperatures during this heatwave would have exceeded 50 degrees Celsius. Global warming further increases not only the scale but also the likelihood of another similar heatwave in the North American west. The probability of such a heatwave in June 2021 was estimated at around one in 1000 years, but in a world with two degrees Celsius of global warming, this type of extreme event would occur every five to ten years.
The frequency, intensity and duration of marine heatwaves are also increasing. Marine heatwaves have approximately doubled in frequency since the 1980s and cause major damage to marine biological communities. Here too, researchers can now clearly identify human influence as a factor. Without climate change, the marine heatwave which devastated life in the Northeast Pacific in the years from 2013 to 2015 and has gone down in history as “the Blob” would, in all probability, not have occurred. Among other things, the heatwave caused mass die-offs of the common murre (Uria aalge), with as many as one million of these seabirds dying of starvation because the unusually warm water temperatures greatly reduced populations of their prey species compared with normal levels. As a result, there was a thousand-fold increase in the common murre’s mortality rate.
Outlook: The intensity and duration of heatwaves over land will continue to increase even if humankind succeeds in limiting global warming to 1.5 degrees Celsius. Marine heatwaves will also occur more frequently. If the world warms by an average of 1.8 degrees Celsius by the year 2100, there will be a two- to ninefold increase in the number of marine heatwaves over the next 60 to 80 years. If the global mean temperature rises by around 4.4 degrees Celsius relative to preindustrial levels, there will be a three- to 15-fold increase in the frequency of marine heatwaves in the final two decades of this century relative to 1995 to 2014, with the greatest changes predicted for tropical waters and the Arctic Ocean.
1.3 > Weakened by the heat: members of the public seek refuge in the air-conditioned rooms of a convention centre in Portland in the US state of Oregon. The rooms were opened to the public during an extreme heatwave in early summer 2021, providing an opportunity to rest and cool down.
fig. 1.3 Kathryn Elsesser/AFP via Getty Images

Global mountain glacier retreat

Current status: The world’s glaciers currently contain less ice than at any time in the last 2000 years. Global retreat of mountain glaciers has been accelerating since the 1990s because air temperatures are increasing at higher altitudes as well. Due to the temperature increase, less snow survives on the glacier’s surface until the end of summer, which means that there is less snow available for conversion into ice in subsequent years. Surface melting of mountain glaciers is also increasing. Their meltwater has contributed around 6.72 centimetres of mean sea-level rise in the last 120 years.
Outlook: A further decrease is projected in snow cover and glacial ice mass in the world’s mountain ranges in the coming decades, along with permafrost thawing in many high mountain regions. As there will be more heavy rainfall instead of continuous snowfall at the same time, researchers are predicting a growing risk of floods and landslides for many mountain regions. The loss of glacier ice will also adversely affect the vital water resources of millions of people who live along rivers that are fed from glacial meltwater.

A clear decline in Arctic sea ice

Current status: On average, the Arctic has been warming at least twice as fast as anywhere else on Earth in recent years. As a result, the Arctic minimum sea ice extent – when the ice shrinks to its minimum size at the end of summer – has decreased by around 40 per cent since the satellite record began in 1979. The remaining ice is also noticeably thinner than before, which means that it drifts across the Arctic Ocean more rapidly and rarely survives for more than two years before melting.
Outlook: Arctic sea ice will melt at an accelerated rate in summer, while less ice will form in winter. Both these developments mean that the Arctic Ocean will be ice-free at the end of summer at least once by 2050, apart from small residual areas of ice, totalling less than a million square kilometres, in sheltered bays and fjords.

Continued ice-mass loss for the Greenland and Antarctic Ice Sheets

Current status: From 1992 to 2020, an estimated 4890 gigatonnes of ice were lost from the Greenland Ice Sheet; the resulting meltwater added 1.35 centimetres to global sea-level rise. The Antarctic Ice Sheet lost 2670 gigatonnes of ice during the same period, with West Antarctica suffering the most significant ice loss. Both here and on the Antarctic Peninsula, glacier flow velocity has clearly increased in the last two decades. This means that relative to 2000, the glaciers are transporting far more ice from land into the sea today.
Outlook: With further warming, the world’s two major ice sheets will lose more ice and their contributions to global sea-level rise will increase. If the world warms by more than two degrees Celsius, the West Antarctic Ice Sheet will very likely collapse and its ice masses will slide into the sea. However, the timing, speed and magnitude of this potential collapse are very difficult to predict with any degree of certainty.

Accelerated sea-level rise

Current status: Between 1901 and 2018, global mean sea level rose by 20 centimetres; moreover, the rate of global mean sea-level rise has increased continuously since the 1960s. In other words, sea-level rise is accelerating. Between 2006 and 2018, sea levels were already rising by 3.7 millimetres per year, and according to the WMO, the figure for 2013 to 2022 reached 4.62 millimetres. This means that global mean sea level has risen faster than at any time in at least the last 3000 years. However, levels may have risen even more sharply in some localities and regions. This can be due to the simultaneous occurrence of coastal subsidence or because the action of wind and ocean currents has caused a localized build-up of water along the coast.
Outlook: The development of the global sea level is determined by two factors: seawater temperature (the warmer the water, the more it expands and takes up more space); and changes in water storage by terrestrial water systems (ice masses, groundwater, rivers, lakes). If more terrestrial water enters the ocean, sea levels will rise. A further aspect of relevance to every local stretch of coastline is whether the coastal area itself has undergone any changes in elevation; this can occur, for example, if large quantities of groundwater are extracted, resulting in underground subsidence, or if geological processes cause the land surface to rise or sink. Localized sea-level changes may therefore be significantly higher or, indeed, lower than the global trend.
According to the IPCC’s projections, global mean sea level will continue to rise even if humankind succeeds in reducing its greenhouse gas emissions to zero within a short time frame. Possible scenarios range between an additional 18 centimetres and 23 centimetres by 2050. A 38 to 77 centimetre rise is expected by the end of the century.

fig. 1.4: Annie Spratt/unsplash


1.4 > This iceberg has broken off the Greenland Ice Sheet. Since 1996, the Greenland Ice Sheet has lost more ice due to surface melting and iceberg calving than can be formed through the compression of fresh snow.

Changes in the water cycle

Current status: Global warming is increasing evapotranspiration from both land and sea worldwide. This in turn increases the amount of water vapour in the atmosphere, making it more likely that rain droplets will form. A further effect of evapotranspiration is loss of soil moisture, which is vital for plant growth. These two physical processes cause permanent changes in the weather and climate: an increase in the frequency and intensity of heavy precipitation events has been observed since the 1950s, at least in those regions of the world with continuous weather records. Concurrently, climate change heightens the risk of drought in some regions due to a lack of precipitation, especially during the driest months, although there may be heavy rainfall at other times of the year in such torrential amounts that it causes surface runoff, with very little water penetrating the soil. Reduced snow cover is also a significant problem. Winter snowfall has become a less common occurrence since the 1950s, with the result that snow cover is no longer forming in many localities. In the past, meltwater from snow provided a water supply for people and nature in spring, but is now largely unavailable in many areas, particularly in mountain regions and the tundra.
Outlook: Heavy precipitation is projected to intensify and be more frequent in many localities. As a result, the high-water and flood risk will also increase. There will also be a higher risk of drought, with more regions affected by drought more frequently and for longer periods in future. Snow cover will continue to decrease, mainly in the northern hemisphere, with earlier temperature-related onset of spring snowmelt and potentially smaller volumes of water in rivers and streams.

Extra Info Climate change in figures – the WMO’s seven Global Climate Indicators open Extra Info

More typhoons and hurricanes

Current status: The global proportion of tropical cyclone occurrence in Category 3 to 5 on the Saffir-Simpson Hurricane Wind Scale (wind speeds from 178 kilometres per hour) has increased in the last four decades, as has the frequency with which a fairly weak storm rapidly develops to hurricane strength. In the West Atlantic, tropical cyclones are now moving more slowly landward from the open sea; when they make landfall, they linger for longer, often resulting in increased damage. In the North Pacific, extratropical cyclones have shifted their tracks northward and now make landfall at different locations relative to 40 years ago.
Outlook: Researchers are predicting very little change in the number of cyclones overall. In the tropical regions, however, the proportion of very strong – and therefore destructive – storms will continue to increase.
1.5 > Many climate system components react swiftly to global warming – and the higher the temperature increase, the greater the changes. Other climate impacts have a slower onset but become locked in and cannot be reversed in the short term once they have begun. Sea-level rise is the most striking example.
fig. 1.5 after IPCC, 2021, Climate Change 2021: The Physical Science Basis. doi:10.1017/9781009157896, Infographic TS.1

When extremes collide

As well as warming up the world, climate change is exposing people and nature much more frequently to weather and climate extremes. They include heatwaves, heavy rainfall, severe storms, droughts and floods, as well as storm surge due to sea-level rise, which can cause extensive flooding in coastal areas. The frequency with which two or three weather extremes occur concurrently is also increasing. In the last 100 years, for example, more heatwaves have been observed in regions already affected by drought.
When such extremes collide, the climate impacts on people and nature are amplified. For example, a heatwave combined with drought causes far more extensive damage than would result from just one of these extreme events. This applies not only to heatwave-drought compounds but also to cases in which coastal regions are affected by severe storms involving both storm surge (marine flooding) and heavy rainfall (flooding on land, river floods). In combination, a storm, storm surge and heavy rainfall cause flooding on a much wider scale than would be induced by just one of these weather extremes.
The risk that extreme events will occur concurrently and that their respective impacts will be amplified is increasing as a result of climate change. Low-lying coastal regions which are regularly affected by cyclones are especially at risk.
1.6 > Climate change is not uniform in all parts of the world. Instead, there are regional differences which will become more apparent as global warming continues. For example, precipitation will increase in high latitudes, the tropics and monsoon regions and decrease in the subtropics.
fig. 1.6 after IPCC, 2021, Climate Change 2021: The Physical Science Basis. doi:10.1017/9781009157896, FAQ 4.3 Figure 1

The impacts – extensive damage to people and nature

The physical climate parameters determine the broad framework within which life on Earth can exist. Any change in these parameters affects the survival conditions not only for people and nature, but also for our built environment. Buildings, roads, power grids, bridges and other key infrastructures are, after all, designed to withstand specific environmental conditions. Global warming of 1.15 degrees Celsius has already led to wide-scale loss and damage for people and nature, and every additional tenth of a degree of warming will further increase the risk of harm.
The IPCC’s conclusions on the observed and future impacts of climate change on the various forms of life on Earth can be summarized as follows:

Reorganization of natural biological communities

Global warming is causing drastic and ever-increasing changes in the natural world. These changes affect species composition in natural biological communities on land and in lakes, rivers and seas, weakening their functionality and resilience. Slow onset changes (sea-level rise, ocean acidification) are as problematical as the increased frequency and intensity of extreme events.
In all regions of the world, rising temperatures and weather extremes such as droughts, heatwaves, storms, heavy rainfall and floods are creating climatic conditions that animal and plant species have not experienced for thousands of years. In many cases, the record-breaking temperatures measured already exceed living organisms’ tolerance limits. Furthermore, weather extremes are now occurring so frequently that ecosystems have little or no time to recover from one heat shock before the next one follows.
For example, the time needed for tropical coral reefs to recover from temperature-induced coral bleaching is at least ten years. However, in Australia, the Great Barrier Reef has experienced a total of six mass bleaching events since 2000, four of which occurred between 2016 and 2022. It is important to note that the coral bleaching event during the Australian summer of 2021/2022 was the first to occur under La Niña conditions, when cooler water temperatures would normally be expected off the east coast of Australia. And yet 91 per cent of corals on the Great Barrier Reef showed signs of significant heat stress.
1.7 > The increasing frequency and intensity of extreme events pose a genuine threat to plants and animals. The more frequently an individual species or entire ecosystem is affected by an extreme event and the less time organisms have to recover from the shock, the greater the risk that they will die out locally.
fig. 1.7 nach IPCC, 2022, Climate Change 2022: Impacts, Adaptation and Vulnerability, doi:10.1017/9781009325844, Figure Cross-Chapter Box EXTREMES.1
Approximately 14 per cent of the world’s corals – equal to 11,700 square kilometres of reef – has been lost since 2009, mainly as a result of marine heatwaves. However, scientists are also documenting mass mortality of trees, e.g. in boreal forests and mixed forests in western regions of North America. Stressed by drought and heat, they succumb to diseases or pests, fall victim to forest fires or dry out.
In light of recent studies on the impacts of climate change, combined with a better understanding of natural processes, the IPCC also concludes that the extent and magnitude of climate change impacts on nature are far greater than previously assumed. Most of the climateinduced changes that we are already seeing today are occurring more rapidly than was predicted 20 years ago. They also cause far more damage and affect much larger areas.
For example, as a result of climate change, many biotic communities’ biological clocks are changing, disrupting the synchronization of once finely coordinated events or processes. In the ocean, algal blooms are now occurring earlier, before the fish larvae which feed on them start to hatch. By the time the juvenile fish have developed to the stage where they are able to forage for food, the algal blooms have long gone. On land, hibernating animals are waking too early from their winter sleep, only to search in vain for food. Trees and flowers are coming into bloom before any pollinators appear, and when hungry chicks open their beaks for food, parent birds struggle to find enough insects with which to feed them.
1.8 > A coral colony before bleaching (right) and afterwards (left). If the water is too warm, the corals expel the symbiotic algae that supply them with food, consequently losing their colour. If these conditions persist, coral starvation occurs.
fig. 1.8 The Ocean Agency/XL Catlin Seaview Survey/Coral Reef Image Bank
In order to escape the rising temperatures, flora and fauna around the world are abandoning their established habitats or dying out locally. Around half of the many thousands of species assessed appear to be reacting in this way. Marine species are shifting polewards or into greater depths in search of the ambient temperatures to which they are habituated. The current rate of their habitat shift averages around 59 kilometres per decade. However, ocean warming is not the only stress factor affecting flora and fauna. Habitat conditions are also worsening due to increasing ocean acidification and oxygen depletion. Collectively, all three factors have resulted in a reorganization of life in the ocean, particularly near the surface, in the last 50 years.
Terrestrial organisms are also shifting polewards or migrating to higher elevations. Organisms which are only able to move slowly, if at all, run the risk of extinction, at least at local scale. This applies to terrestrial and marine biological communities alike. The prospects of survival are particularly bleak for organisms which live in geographically restricted habitats such as ponds and lakes, meaning that they have no chance of migrating, and for species which are adapted to cold habitat conditions in polar and mountain regions. Very few suitable refuge areas will be available globally for these cold-climate specialists in future.

Mass extinction
A mass extinction is defined by scientists as an event in which more than 75 per cent of species of flora and fauna die out, usually within a time span of less than two million years, and their roles in the ecosystem are not filled soon afterwards by new or different species. There is evidence that this has already occurred five times in the last 540 million years; however, these individual events took place over timespans up to several million years.

Making matters worse, the impacts of climate change on nature and species diversity are compounded by other human-induced stress factors – first and foremost the wide-scale destruction of natural habitats through deforestation, drainage of wetlands, coastal construction and development, overfishing of the seas, and resource extraction. Environmental pollution also plays a major role, as do uncontrolled soil sealing and the spread of invasive species. Wherever these stress factors overlap, their effects are mutually reinforcing, weakening the resilience of natural ecosystems. For many biological communities, climate change is thus a risk multiplier – and as warming continues, it will become a lethal threat for many. One fact stands out: with each tenth of a degree of warming, the impacts and climate risks for terrestrial and marine ecosystems will increase.
The world’s oceans, for example, face the prospect of a mass extinction due to the combined effects of climate change and human overexploitation of the marine environment. This would be the sixth mass extinction in Earth’s recent history. New research shows that if atmospheric and ocean temperatures continue to rise, the loss of marine species due to heat stress and oxygen depletion over the next 75 years will equal the losses from overfishing, pollution and habitat destruction. In sum, global warming of up to 4.9 degrees Celsius by the end of this century would cause so many marine species to die out that this would qualify for definition as mass extinction.
The extinction rate would be particularly high in the polar regions, where cold-climate specialists are struggling to adapt due to the speed of the changes. However, the greatest decline in diversity would be observed in the currently still species-rich tropics, where biological communities have already reached their maximum temperature tolerance limit. But the research also shows that if global warming can be held below two degrees Celsius, the risk of a mass extinction decreases significantly.

Climate change – a risk multiplier

The upheavals in the natural world have far-reaching implications for humankind. One by one, ecosystems are denying us their vital services. Cereals, fruit trees and other crops are no longer being adequately pollinated; grazing for livestock – cattle, sheep and goats – is proving increasingly difficult to find; more and more often, coastal fisherfolk are pulling in empty nets, particularly in the warm, tropical regions, because fish populations are migrating to cooler waters. There is less air and water purification, less effective protection of coasts from erosion, and popular holiday destinations are losing their main attractions – forests, snow-capped mountains and coral reefs.
In parallel, many people who enjoy woodland walks or relaxing by the sea are finding that their mental health is suffering. In short, the more the ecosystems change, the more we lose our vital natural resources.

Water – too much or too little

Climate change also directly affects human communities and our built environment. For example, more frequent heavy rainfall increases the risk of river floods in some regions of the world. The potential damage induced by this type of natural disaster is estimated to be four to five times greater in a world with four degrees Celsius of warming than if global warming were limited to 1.5 degrees Celsius. However, even with warming of 1.5 degrees Celsius, more people will lose their lives and property to floods than at present. In Colombia, Brazil and Argentina, for example, the number of people affected by river floods would increase by 100 to 200 per cent, with an increase of 300 per cent in Ecuador and even 400 per cent in Peru.
Rising spring and winter temperatures, in turn, cause earlier snowmelt at high altitudes, resulting in changes in average water levels in mountain streams and rivers. For human communities, this development means that the rivers may carry a large volume of water during periods when it is scarcely needed, while later in the year, water levels are too low to allow extraction of the required quantities.
Already, more than half the global population lives under conditions of severe water scarcity, at least partly induced by climate change, for at least one month of the year; this may be due to extreme aridity, but also to floods, storms and heavy rainfall events whose impacts also put the drinking water supply at risk in many localities. The effects are felt particularly by cities, municipalities and villages whose residents rely on meltwater from the shrinking mountain glaciers, as well as by people living in areas without a central water supply. If rivers burst their banks here or if a natural spring dries up due to drought, many thousands of people are often left without access to clean water.
Alongside agriculture, which is the world’s largest consumer of freshwater, the energy sector is also affected by water scarcity: since the 1980s, the amount of power generated in hydroelectric plants has decreased by four to five per cent worldwide due to falling water levels and reduced flow rates. Indeed, in some localities, hydropower plants are threatened with closure due to water scarcity. Conditions at Lake Powell, the second largest artificial reservoir in the USA, illustrate the gravity of the situation until the winter of 2022/2023. The Lake is located on the border between Utah and Arizona. It is fed by the Colorado River and together with the Lake Mead reservoir further downstream, supplies around 40 million people with drinking water. Farms along the length of the river also extract water to irrigate their fields and crops. After 22 years of drought in the western USA and persistently excessive water extraction from the Colorado River, the reservoir was filled to just 24 per cent of its capacity at the end of March 2022. Between 2019 and 2022 alone, the water level dropped by more than 30 metres, coming close to the critical threshold below which the lake’s hydroelectric dam is unable to generate power. The federal agency responsible for the reservoir therefore decided to release less water than usual from the lake during the rest of the year and to open the dam gates of another reservoir further upstream in order to provide an additional water inflow into Lake Powell. The drought in the American West has stretched over 22 years (2000 to 2022/2023) and is now classed as the driest period in 800 years.

Food – hard times for arable farming, livestock husbandry and aquaculture

Wherever there is too much rainfall, or it rains at the wrong time of the year, arable and livestock farming becomes more challenging. According to the IPCC, farmers and foresters, fishers and aquaculturists around the world are already adversely affected by climate change to such an extent that they are no longer able to produce sufficient staple crops and timber to meet the global population’s needs.
With higher temperatures and increased aridity, cereals and fodder crops wilt in the fields and diseases spread. Due to ocean acidification, rising water temperatures and multiple algal blooms (eutrophication), fish farmers are finding it increasingly difficult to bring mussels and other shellfish to maturity. Global warming also increases the complexities – and therefore the costs – of transporting, storing and selling perishable foods such as fruit and vegetables safely so that once purchased, they stay fresh for a few days at home. Climate change thus affects not only the producers but the entire supply chain up to and including the consumer, posing a threat to food security throughout the world.
The losses are particularly severe when regions are affected by extreme events such as droughts, floods and heatwaves. The frequency of these sudden harvest or production losses on land and in the sea has steadily increased since the 1950s and often has a domino effect. For farming families, crop failure means the loss of their food supply and livelihoods. Concurrently, the availability of basic foodstuffs is reduced, pushing up prices and making staple foods unaffordable, especially for low-income families. The resulting hunger and malnutrition have particularly negative effects on child health. These developments can be observed in Asia, Central America, the sub-Saharan regions, the Arctic, the small island states and elsewhere – and once again, it is the smallholder farmers and artisanal fisherfolk who are impacted most severely by climate change.
The situation will worsen as warming continues – in part because higher temperatures mean that more water is lost through evapotranspiration from foliage and soil. Water demand from agriculture will therefore increase. This situation will be compounded by the substantially reduced availability of water during the growing season in many regions, multiplying the risks. To take just three examples:
  • With global warming of two degrees Celsius by 2100, the probability of extreme droughts occurring across wide areas in Northern South America, the Mediterranean region, Western China and at high latitudes in Europe and North America will increase by 150 to 200 per cent.
  • Whereas around 40 per cent of total croplands (approximately 3.8 million square kilometres) experienced water scarcity in the period from 1981 to 2005, a new study shows that agricultural water scarcity will intensify in more than 80 per cent of global croplands by 2050 – even if the world warms by just 1.6 degrees Celsius by 2050 relative to preindustrial levels.
  • During the same period, the impacts of climate change alone will mean that an estimated eight to 80 million people in South Asia, Central America and sub-Saharan Africa will no longer have access to adequate food and will therefore suffer from hunger. The precise figure will depend on the degree of warming and hence the magnitude of future climate change.

Health – the limit of human tolerance

Climate change adversely affects both the physical and the mental health of people in all regions of the world. Severe mental health challenges are reported mainly by people who have been exposed to extreme weather events or by rescue workers deployed during such events, and by people who have suffered loss of livelihoods or even their homes, communities or culture as a result of climate change. Physical health is adversely affected primarily by extreme heat. Rising air temperatures and longer and more intense heatwaves have increased the occurrence of diseases and led to higher mortality worldwide, including in the middle latitudes. The elderly, people with medical conditions and outdoor workers are particularly impacted. Additionally, for this latter group, warming is often associated with loss of earnings if extreme heat makes outdoor labour in fields or on construction sites impossible.
Extreme heat is particularly hazardous when it is compounded by very high humidity. If the air is so humid that water and therefore also sweat cannot evaporate, the human body’s cooling mechanism begins to fail. As a result, the body steadily overheats, ultimately causing circulatory collapse and – in extreme cases – fatal heat stroke.
The human heat tolerance limit can be determined using the cooling limit temperature. This captures both ambient temperature and humidity. Until recently, it was assumed that a healthy individual cannot survive a cooling limit temperature of 35 degrees Celsius for more than around six hours. This limit is derived from the combination of temperature and humidity and corresponds to 35 degrees Celsius at 100 per cent humidity or 46 degrees Celsius when humidity is 50 per cent.
When researchers at Pennsylvania State University in the USA tested this assumption for the first time in heat stress experiments, they found that the theoretical threshold was far too high. In climate chambers with a high level of humidity, an ambient temperature of 30 to 31 degrees Celsius was enough to induce dangerously elevated core temperature in healthy young human test subjects. Contrary to all expectations, a slight decrease in humidity did not increase the test subjects’ heat tolerance. Instead, the critical cooling limit temperature under these conditions was just 25 to 28 degrees Celsius – almost ten degrees Celsius lower than scientists had previously assumed. The explanation offered by the research team is that despite the reduction in humidity, the test subjects’ sweat production did not increase above a certain temperature.
In the face of continued climate change, these research findings give cause for concern. They show that the heat risk to human health has been underestimated and that with accelerated global warming, more regions will be periodically affected by a level of heat stress that will make it impossible to survive without additional cooling.
1.9 > When extreme heat is compounded by high humidity, the human body can quickly overheat – a potentially life-threatening situation. This figure from the IPCC shows the various regions of the world where people will be exposed to the risk of overheating (hyperthermia) in future and for how many days a year. The core message: the sooner climate change is curbed, the fewer people will be exposed to this threat to life.
fig. 1.9 after IPCC, 2022, Climate Change 2022: Impacts, Adaptation and Vulnerability, doi:10.1017/9781009325844, Figure 6.3
In the long term, the situation is likely to be particularly challenging for the many millions of people living in mega-cities in the tropics and subtropics. Firstly, air temperature and humidity here are consistently high for most of the year, and secondly, the heat island effect also comes into play. This is a term used to describe the observation that conurbations reach higher daytime temperatures than less built-up outlying areas. Urban areas also cool down more slowly at night. It may be concluded from this that in mega-cities in the tropics and subtropics, it would only take a comparatively small amount of warming to push inner-city air temperature to such a high level that many people’s heat tolerance limit is exceeded.
There is much evidence to suggest that city dwellers are generally exposed to much higher temperatures than those reported for a wider region. For example, during the severe heatwave in India and Pakistan in May 2022, when daytime temperatures climbed as high as 51 degrees Celsius, temperatures remained high overnight, at 35 to 39 degrees Celsius, in the Indian capital New Delhi and neighbouring towns, whereas the air cooled to a tolerable 15 degrees Celsius in nearby fields and forests. Subsequent analyses by an international research team found that human-induced climate change had made this record-breaking heatwave 30 times more likely.
Climate change also increases the occurrence of many infectious diseases. Droughts, for example, heighten the risk that wells will dry up, while heavy precipitation can cause contamination or flooding of wells. In both cases, if communities then extract their drinking water from contaminated sources, their risk of contracting bacterial infections such as cholera increases. Higher temperatures enable Aedes mosquitoes to expand their habitat range north- and southwards from the tropics. These insects carry the dengue fever and yellow fever viruses, among others. The risk of contracting dengue fever is already increasing worldwide. Due to the more frequent occurrence of forest fires across larger areas, the risk of respiratory diseases is also increasing in affected regions.
1.10 > Aedes mosquitoes are also known as yellow fever or dengue mosquitoes as they are vectors of both these diseases. As a result of climate change, their range is expanding. Originally found only in the tropics and subtropics, they are now spreading further north and south.
fig. 1.10 Wellcome Collection no. 41477i

Sea-level rise – land under water!

As a consequence of sea-level rise, the climate risks to people, nature and built assets in the world’s coastal areas will increase at least tenfold by 2100, mainly due to the greater frequency of extreme floods. Sea-level rise poses a particular threat to the many millions of people living in low-lying coastal areas and on small islands. Higher tidal floods destroy the species-rich ecosystems in the tidal range, cause salinization of groundwater reservoirs and inundate large areas of land, affecting coastal forests and croplands, as well as coastal districts of large metropolitan areas. Due in part to these areas’ uncontrolled growth, ongoing sea-level rise will put increasing numbers of people at risk over time. In Africa, for example, some 108 to 116 million people will be living in high floodrisk areas by 2030, compared with just 54 million in 2000.
Globally, the numbers affected are very much higher: according to figures from the IPCC, more than a billion people in coastal cities and conurbations worldwide will be living with a high flood risk in 2050. The threats they face include recurrent storm surge, as well as the prospect of permanent flooding of their villages and districts.
fig. 1.11 “A Borrowed Planet – Inherited from our ancestors. On loan from our children“ by Alisa Singer. © 2022 All rights reserved. Source: IPCC


1.11 > The eye-catching collage on the title page of the IPCC’s report Impacts, Adaptation and Vulnerability, published in February 2022. Its key message: humanity knows what needs to be done to mitigate the impacts of climate change. What is lacking is resolute global action.

Extra Info Climate justice – the heaviest burden falls on the poor open Extra Info

fig. 1.11 “A Borrowed Planet – Inherited from our ancestors. On loan from our children“ by Alisa Singer. © 2022 All rights reserved. Source: IPCC


Climate change adaptation – the world is unprepared

In order to mitigate the impacts and risks of climate change, people and nature must adapt to the new environmental conditions. For us humans, this primarily entails taking measures to protect our lives, goods and property against high temperatures, weather extremes and sea-level rise. This can be achieved if we relocate from at-risk regions or make local lifestyle changes – for example, by greening our settlements and cities in order to minimize the heat island effect, or by conserving water so that we have sufficient reserves available during droughts.
The list of potential solutions is long. Nevertheless, the IPCC concludes that globally, there is a substantial gap between current adaptation planning and implementation and the levels needed to provide effective and sustainable protection for everyone. What is certain, however, is that there is now a greater awareness of the growing risks. More than 170 countries and many cities are now including adaptation in their climate policies and planning processes. Private sector and civil society actors are also engaging for more adaptation. Pilot projects are being implemented in various sectors, although in many cases, they simply aim to minimize the local storm, flood, heat or drought risk and therefore result in only minor changes with regional and time-limited impact.
In order to mitigate the impending climate risks on a long-term basis, holistic policies and fundamental adjustments to our lifestyles are required, which must include how we work, how we produce our food and treat the natural environment, and how we plan and construct our cities and settlements. The IPCC concludes that at present, humanity is completely unprepared for all the challenges that lie ahead as a result of climate change – particularly if the world warms by more than 1.5 degrees Celsius. Scientists refer in this context to an adaptation gap.
This gap is particularly large in regions where people are poor and highly exposed to climate risks. Furthermore, if the adaptation measures currently being planned are compared with the climate impacts predicted by scientists, it is already clear that this adaptation gap will widen steadily.
fig. 1.13 Elliot Ross/www.elliotstudio.com


1.13 > A vivid comparison: In the US city of Los Angeles, an adequate number of roadside trees to provide shade exists only in districts where residents have sufficient resources to pay for the trees’ upkeep (above). Trees are absent in poorer districts, partly because the city government does not invest in roadside trees. As a result, there is no cooling shade for local residents when temperatures soar.
fig. 1.13 Elliot Ross/www.elliotstudio.com


The limits to adaptation

Also new is the clarity with which the IPCC now describes the limits to human adaptation to climate change. In doing so, it differentiates between hard and soft limits. Hard limits are those where adaptive actions are no longer possible. For example, if an atoll is inundated by waves due to sea-level rise, resulting in the complete salinization of all the drinking water reserves, the island dwellers’ only long-term option is to leave. The same applies to flora and fauna that have already reached their upper temperature limit. If their habitats continue to warm, they are forced to migrate.
Soft limits, by contrast, are ones where options for adaptive action may exist. However, this requires political commitment, sufficient financial resources, scientific knowledge and local know-how. If all four factors are in place, it may be possible, for example, for farmers in drought-affected regions to cultivate new species that are resistant to aridity and to install modern irrigation systems in order to reduce their demands upon lakes, rivers and groundwater resources.
It is already clear, however, that many species of flora and fauna have already reached or are about to reach their hard adaptation limits. If they were to die out locally, this would destroy the livelihoods of the many millions of ­farming, fishing and pastoralist families who depend on these species. With global warming of 1.5 degrees Celsius or more, the ongoing decrease in snowfall and glacier retreat will mean that communities whose water supply depends on meltwater no longer have access to adequate water resources. And with warming of two degrees Cel­sius or more, it will become far more difficult to make a success of arable farming in many of the world’s cereal-growing areas.
As these few examples of adaptation limits show, the more quickly humankind acts to curb climate change, the more opportunities there will be to adapt to the new conditions and the more effective these options will be. Actions which will work with warming of 1.5 degrees Celsius may prove to be completely ineffective once warming reaches two degrees Celsius. For that reason, the effectiveness of all adaptation actions must be continuously monitored and the effects of the various measures regularly reviewed.

Climate, people and nature can only be winners together

The IPCC’s Sixth Assessment Report also highlights the scientific community’s new understanding of the close interconnections and interactions between nature, people and the climate. For example, if humans impair species diversity by destroying natural habitats and exploiting their resources, they deprive themselves of their most important partner in the fight against climate change. Yet at the same time, humanity is forcing the climate-related decline of natural ecosystems with its persistently high greenhouse gas emissions.
Breaking out of this conflict spiral and reversing past mistakes must henceforth be the goal of all human action. Among other things, this means thinking holistically about people, nature and the climate – in our daily lives and in all our decision-making, whether at local, national or international level. Only then will it be possible to identify solutions that benefit all three systems in the long term and guarantee a liveable future on Earth for present and future generations. Textende