"We are on course for 4°C of heating. “A 4°C future is incompatible with an organised global community, is likely to go beyond ‘adaptation’, is devastating to the majority of ecosystems and has a high probability of not being stable”
- Prof Kevin Anderson
We live in times of accelerating and escalating crises.
From a natural science perspective, we can define specific global thresholds of Earth system processes. Their transgression puts us outside the safe operating space for humanity. We are currently breaking six out of these nine so-called planetary boundaries, which include:
- Climate change. We passed the boundary of 350 ppm of atmospheric CO2 in 1988, and are currently at above 420 ppm. Increase in atmospheric CO2 is caused primarily by the burning of fossil fuels.
- The biogeochemical flows of nitrogen and phosphorus. We are vastly overshooting these flows due to our excessive use of fertilizers in agriculture.
- Land system change. With a third of all forests lost, again mostly due to agricultural practices, in particular animal agriculture.
- Biosphere integrity. The current extinction rate of species is at least 100 times higher than the background extinction rate.
- The increasing production and release of novel entities (chemicals, including plastics) with diverse risk patterns that exceed societies’ capacity of assessment and monitoring.
- Researchers recently proposed a green water (terrestrial precipitation, evaporation and soil moisture) planetary boundary, which we are also transgressing. This is a general pattern: The more we learn about our perturbation of the Earth system, the more reason for concern we find.
Fig 1. The nine planetary boundaries.
Researchers recently proposed a green water planetary boundary, which we are also transgressing. This is a general pattern: The more we learn about our perturbation of the Earth system, the more reason for concern we find.
While the climate crisis is only one element of our multiple crises, and even though these crises are interconnected, it is arguably the most urgent one. We therefore focus on it in the following sections.
General Principles of Climate Science
Every object in the universe radiates energy (heat). When you put on a blanket, your body emits energy until the rate at which heat escapes from the blanket is equal to the rate at which your body adds heat into the space between you and the blanket. When there is no net flow of thermal energy between two physical systems (in this case, your body and the space between it and the blanket), they are in thermal equilibrium.
Radiation from the Sun warms the Earth’s surface. That energy is re-radiated into space, but the Earth’s atmosphere traps heat like a blanket. It has been known since the 1850s that some atmospheric gases trap more heat than others – these are called greenhouse gases. As human activity adds greenhouse gases (such as CO2 and methane) to the atmosphere, the total heat energy trapped between the atmosphere and the Earth increases, and so the Earth warms up. The thicker the blanket, the warmer it will ultimately get. The last time there was this much CO2 in the Earth’s atmosphere, the Earth was 1-3°C warmer than it is today.
Some processes thicken the Earth’s blanket (burning fossil fuels, emissions from animals such as methane), while others reduce its thickness (like some aerosols). Some CO2 dissolves into the oceans; some is sequestered by photosynthesising organisms (plants, algae, and some bacteria), which extract carbon from the atmosphere and store it in their bodies. Some of this carbon ends up trapped in forests, soils, and on ocean floors, and so life can slow climate change. Ecological destruction effectively cuts the brakes, accelerating Earth’s heating, and rapidly transforming our world into a new state that is alien to humans, while threatening the ecosystems which supply us with food, water, and some protection from natural disasters.
Tipping elements are large-scale subsystems of the climate system that can exhibit a threshold behaviour: once a tipping point is crossed, self-amplifying feedbacks kick-in and push the system into a different stable state. One example is the Greenland ice sheet, which is characterised by surface elevation feedback: as the ice sheet gets smaller due to melting, more of its area is at lower and warmer altitude, leading to further melting, and so forth. Once a critical temperature is crossed, this feedback will lead to runaway melting, and it eventually will lead to the total collapse of the Greenland ice sheet, even if global temperatures are stabilised.
Fig 3. Tipping elements and their temperature thresholds. From MacKay et al. (2022).
The most recent assessment of tipping elements notes that we might have already crossed five tipping points. These include the melting of the Greenland and West Antarctic ice sheets, which combined would eventually lead to a roughly 10 metres rise in sea levels; the large-scale die-off of low-latitude coral reef, on which over half a billion people depend on for food, income, and coastal protection; abrupt thaw in the Boreal permafrost, which would amplify global heating; and the collapse of the North Atlantic subpolar gyre, which among other things would cause increased extreme weather in Europe. The risks increase dramatically once we pass 1.5°C.
As with so many other aspects of the climate system, the more we learn about tipping elements, the more concerned we become. There may be more surprises ahead if we fail to limit global heating.
Scientists fear that crossing one tipping point could trigger others, resulting in a tipping cascade that might drive the planet irreversibly to a far hotter state. Such an outcome represents an existential threat to humanity. A recent report by the OECD notes that current climate policy does not adequately take these threats into account.
Fig 4. “Burning ember” diagrams indicating increasing risks at lower temperatures across scientific assessments.
Air pollution is the contamination of air caused by the presence of substances in the atmosphere that damage the health of humans and of other living beings. Air pollutants include gases like carbon monoxide (CO), carbon dioxide (CO2), chlorofluorocarbons, methane (CH4), ammonia (NH3), sulphur oxides (SOx), nitrous oxides (NOx), organic and inorganic particles (particulate matters, or PM), and biological molecules.
The concentration of air pollutants has increased massively since the industrial revolution as a result of human activities. Human activities that generate air pollutants include the combustion of fuels (coal, petroleum, natural gas), of wood and of other organic matter, forest and agriculture management strategies using controlled burns, cattle, and fumes emerging from paints, aerosols, sprays, and from other solvents.
Anthropogenic air pollution is one of the biggest public health hazards worldwide. While estimates vary, the latest study suggests that 8.7 million people die every year from air pollution due to the burning of fossil fuels. With a death toll of smoking and malaria combined, air pollution is one of the main contributors to global disease burden (the sum of mortality and morbidity), especially in low-income and middle-income countries. It is also associated with a myriad of negative outcomes short of death, such as heart disease, cancer, asthma, and reduced cognitive performance.
Fig 5. Air pollution across the globe using data from IQiAir.
Air pollution has continued to increase in recent decades. Between 1990 and 2015, mean concentrations of particle mass with diameter <2.5 µm (PM2.5) increased by 11.2% worldwide, and mean tropospheric (the lower layer of the atmosphere) ozone increased by 7.2% (PM2.5 and ozone are two indicators used to quantify exposure to air pollution). During this time period, ambient PM2.5 air pollution caused 17% of ischaemic heart disease, 14.2% of cerebrovascular disease, 16.5% of lung cancer, 24.7% of lower respiratory infections, and 27.1% of chronic obstructive pulmonary disease.
Heatwaves are events where temperatures are excessively higher than normal (above the 99% percentile for a specific area) for several (at least three) consecutive days. Climate change has unequivocally increased the frequency and duration of heatwaves in almost all regions of the world, but especially in South America, Africa, the Middle East, and Southwest Asia.
Heatwaves are dangerous because they directly harm human health, ultimately causing human death, and because they interfere with numerous systems, producing extreme events. For instance, when compounded with drought, heatwaves enhance fires and crop failure. Similarly, heatwaves induce thawing of permafrost and increase energy demand, further enhancing climate change.
Heatwaves are a significant health risk and have caused thousands of deaths, with scientists estimating that 37% of deaths related to heat exposure in the world between 1991 and 2018 were caused by climate change. Wet-bulb temperatures — a combined measure of heat and humidity — higher than 30 °C are especially dangerous for humans, have already been observed in some parts of the world, and are projected to occur more frequently under increased global heating.
Some regions have experienced higher mortality caused by heatwaves than others, with the highest number of climate change-induced deaths occurring in southern and western Asia, southeast Asia, and several countries in Central and South America. Similarly, heatwaves have caused stronger economic damages in poor tropical regions. Finally, extreme heat in combination with air pollution, another consequence of climate change, acts synergistically to cause human mortality.
An overall reduction in atmospheric greenhouse gases is the key solution to reduce mortality and damages brought by extreme heat, as adaptation to extreme heat is very difficult. Furthermore, technological solutions such as air conditioning (when available) puts pressure on electric grids, further inducing climate change, and increasing the chances of outages.
Fig 6. Projected warming across the globe at a 2°C average increase over pre-industrial levels.
Sea levels rise primarily due to two factors in a warming climate: thermal expansion (in warmer seas, water molecules have greater velocities which creates pressure and causes the body of water to expand), and ice-melt (from the polar ice caps and from glaciers). Global sea level already rose between 8 and 14cm in the 20th century. The amount by which sea levels will rise in the next decades and centuries is uncertain due to cascading effects. For example, a small rise in sea levels will inundate previously elevated parts of an ice sheet, which may cause it to melt faster, destabilise, and collapse, with knock-on effects in turn. The IPCC estimates that, depending on the amount of greenhouse gases that will be emitted, sea levels may rise up to 1.01m by 2100 and up to 1.88m by 2150 under very high greenhouse gas emission scenarios.
Sea level rise has and will have extraordinary impacts on human societies. Around one billion people live less than 10m above current high tide lines, including 230 million below 1m, and a third of the world lives in coastal communities. Even a 10cm sea level rise approximately doubles the odds of extreme flooding, especially in the tropics. Hundreds of millions of people will likely be on the move as sea levels rise, while low-lying island nations are likely to be entirely submerged – a reality tantamount to genocide. Bangladesh is a striking example: 20 million people are expected to lose their homes by 2050, as 17% of the country will be below water due to rising seas.
Fig 7. Global “hotspot” regions of changes in episodic coastal flooding by 2100. Circle size relates to change in the magnitude of inundation, and circle colour is related to the projected extreme sea level in 2100.
Ecosystems are being devastated at accelerating rates. This destruction is driven overwhelmingly by industrial activity, mainly by agriculture, and it is done in service of overconsumption by the world’s wealthiest (not due, despite much focus on it, to population growth). The destruction of ecosystems is stronger in biological hotspots (areas with high biodiversity) and endemisms (species whose entire distribution is confined to a small area). Many and varied elements cause ecosystem/ecological collapse (i.e., a transition beyond a bounded threshold in one or more variables that define the identity of the ecosystem), including global heating, habitat loss (caused by deforestation, for instance), over-farming (particularly for marine ecosystems), and modern farming practices such as the wide-spread use of pesticides.
It is hard to ascertain precisely how many species have already been lost, as scientists estimate that the majority of species have not even been described, particularly in the most biodiverse regions of the world (i.e., the tropics), and in marine ecosystems (where it is estimated that 91% of species await description). However, it seems that over the last 5 decades, vertebrate populations (fish, amphibians, reptiles, mammals and birds) have declined by around 70%. Only 4% of land vertebrates exist in the wild today, while the other 96% are made up of humans (36%) and their livestock (60%). Almost half of the world’s trees have been cut down since the start of human civilization.
Insect populations are relatively poorly studied, but there has been a decline in insect biomass over just the last few decades in multiple places, and it is estimated that 40% of all insect species are at risk of going extinct in the next few decades. Recent evidence suggests that even plankton (which generates around half of the world’s oxygen, and which are the basis of all marine ecosystems) may be at risk of catastrophic decline as the planet heats up. The natural world provides is of simply incalculable importance to humans. It provides the basis of our food, water, air, and resources for all humans. Without healthy ecosystems, human society cannot survive.
Fig 8. Expert estimates of changes in global biodiversity in terrestrial biomes (left column) and marine realms (right column) since 1500 (top row), until 2100, if current trends continue (middle row), or by 2100 if conservation efforts are intensified (bottom row).
Our food system is already broken. There are over 800 million people experiencing chronic hunger, more than 3 billion cannot afford a healthy diet, and a third of all food goes to waste. Nearly all farm subsidies — 90% out of 540 billion yearly — cause harm.
Crops, like other living beings, have ‘critical temperatures’ above which they die. As a result, heat waves kill huge proportions of the crops in the heated region. Food supply chains in the modern world are global, with 68.7% of national food supplies coming from other countries. Therefore, crop failures in one region have a global impact. As the world heats up, the number of extreme heatwaves has grown and will continue to grow rapidly (see above). Furthermore, around one third of all crops depend on animal pollinators, and insect populations are declining, which further stresses our crops. Droughts, too, are set to dramatically worsen. At 3°C of warming the global average drought duration is projected to last around 18 months. Therefore, without dramatic policy changes, food production this century will drop. This can cause a collapse of global societies, as an epidemic of starvation and food riots could sweep the world.
Since 1980, crop yields have consistently declined for most crops, and this decline is projected to accelerate – potentially dramatically – in coming years. While predictions vary, a number of studies predict declines in crop yields of around 50% within just a few decades (depending on the crop). For instance, some studies suggest that the average crop productivity— a measure of crop yield — by the end of the century could decline strongly for maize (predictions ranging from -6.4% to -24.1%). This is even more worrying when we consider that by 2050, we need a 70–100% increase in cereal food supply to feed the predicted world population of 9.8 billion people. The worst-case climate mitigation scenarios may also be an underestimation, because studies frequently only consider the effects of changing temperature and rainfall on crop yields, but do not include the effects of changing climate on human labour, the decline of pollinators, the interaction between all these factors, nor the effect on non-staple crops. Importantly, tropical and subtropical regions, where increasing temperatures have the largest impacts, will see large declines in maize. In contrast, in higher latitudes where wheat is generally grown, warming will increase gains.
Fig 9. Globally and in every region, the prevalence of food insecurity is higher among women than men.
Water is crucial for human life and for the functioning of ecosystems. Even today, there are two billion people who do not have access to safe drinking water, and four billion people, almost two thirds of humanity, experience severe water scarcity for at least one month a year. And the situation is becoming worse. Climate change is creating severe and unprecedented droughts. With each 0.50°C increase in warming, an additional 619 million people are projected to live in areas with a 25% likelihood of annual extreme drought (6-months of meteorological (lack of rainfall) and agricultural (lack of rain that affects soil moisture and crop growth) droughts). By 2050, some studies estimate that 0.50 to 3.1 billion people will be exposed to an increase in water scarcity due to climate change. Indeed, the IPCC estimates that between 800 million to 3 billion people are projected to experience different levels of water scarcity at 2°C, and nearly 4 billion at 4°C of warming.
Although all regions will be affected, droughts in the tropics are projected to be twice as frequent in comparison to the rest of the world. Moreover, with each increase in half a degree, the probability of drought is 1.5 times larger in forested than in non-forested areas, which can lead to increased tree mortality and reduced tree growth, further reducing a forest’s ability to capture carbon. Global hydroelectric producing capacity will also be exposed to at least a 50% likelihood of annual extreme drought. Cropland exposure to drought highly increases the likelihood of crop failure. The probability of crop yield failures is projected to be 25 times higher by 2050 across global breadbaskets, and already 4.5 times higher by 2030 under a high warming scenario. Crop failure can lead to famine, migration, and war.
Fig 10. Water scarcity index by 2050 according to the IPCC report.
Migration and Wars
Unless we force our world leaders into making both fast and dramatic economic and political changes in how we organise our economy and societies (see addressing root causes below), the net effect of the climate impacts we have described here will be increasingly catastrophic for human livelihoods.
Climate-induced displacement occurs when individuals and/or communities are forcibly displaced (either within or beyond borders), by short-term (hurricanes, wildfires, tsunamis, floodings, typhoons) and long-term (sea level rise, desertification, deforestation, rising temperatures) natural disasters which are triggered or intensified by the climate crisis. While it is difficult to establish a full picture, the United Nations High Commissioner for Refugees (UNHCR), calculates that weather-related disasters have induced around 21 million displacements annually since 2008. These numbers are getting worse: the World Bank estimates that ‘without urgent national and global climate action, South Asia, sub-Saharan Africa, and Latin America could witness more than 140 million people move within their countries’ borders by 2050’. And according to a report by the Institute for Economics & Peace (IEP), around 1.2 billion people could be displaced by 2050 due to climate change and its consequences. According to the IPCC report, about 3.3 billion people are living in countries with high human vulnerability to climate change. The number of refugees could well surge by billions over the next few decades: ‘unprecedented’ does not capture the depth of this crisis.
Fig 12. Forcibly displaced people worldwide.
Social dynamics are hard to predict, but we can seek guidance from historical analogues. One example which ties together many elements of the climate crisis is the Syrian civil war. In that case, the worst drought on instrumental record (made possible by the rapidly changing climate) preceded and precipitated the war. Large regions of the country saw three quarters of their animal and crop populations die. More than a million fled to cities to find work and food, with the additional pressure sparking social unrest, which escalated into civil war. This collapse led to half a million deaths and displaced ~5 million people from the country.
Addressing Root Causes
To avoid climate breakdown, we must first name its causes. The current capitalist economic model of endless growth on a finite planet is clearly unsustainable: by definition, that means that it must come to an end. The only question is whether we will choose to end it, or whether it ends via system collapse.
Importantly, there is a strong colonial dimension to climate breakdown. Nations already getting hit hardest are those in the Global South, which have historically contributed the least to the problem, are the ones suffering the most. These nations have historically and continue to be forced into carrying out labour-intensive extraction and production of raw materials for the consumption of the Global North, which is net appropriating trillions of dollars every year from the Global South. Climate breakdown is and will continue to accentuate existing inequality and injustices, both between and within nations. It is absolutely critical that any climate solutions centre justice.
Greenhouse gas emissions and environmental destruction are overwhelmingly driven by corporate expansion and industry. In addition to a North-South divide, there is grave inequality within countries. Most consumption – escalating massively during the neoliberal era – comes from the world’s richest, with the wealthiest 10% contributing around half of global personal emissions. Tackling gender, wealth, and social inequality are at the heart of combatting both climate breakdown and climate injustice. Though some would interpret these propositions as inherently ideological, they are conclusions that emerge out of robust scientific studies. Capitalism – particularly in its modern neoliberal, neocolonial incarnation – must be abandoned or transformed beyond recognition if human civilization is to survive.
The current economic model underpins virtually every aspect of society’s organisation, but is often invisibilized. Modern farms, in an effort to outcompete rivals, pump livestock full of antibiotics, use insect-destroying pesticides, and clear all available space of wildlife to sow profitable monocultures; technologies are often designed to break, so that replacements must be regularly purchased; relentless advertising creates discontent in order to push consumables upon the population. To escape the destruction of the natural world, the precepts underpinning this reality must be combatted.
On the immediate, pragmatic level, carbon emissions must be cut with urgency. Everything possible should be done to restore and sustain ecological systems: doing so could draw down around a third of the necessary carbon emissions and provide incalculable additional benefits (in terms of health, water and food security, resilience to environmental disasters, and more). The agricultural sector must be transformed to revolutionise our relationship to food and land. Housing must be reformed with efficiency front and centre. Energy systems must be overhauled so that power comes from sustainable technologies. These solutions require investment, but ultimately leave us wealthier, healthier and happier. At any rate, the cost of inaction is approximately the sum total of everything built over the past several thousand years, so inaction loses out in any conceivable cost-benefit analysis.
Nonviolent Civil Resistance
We have left it so late that traditional means of trying to steer the system onto a more sustainable pathway are not enough. Carbon emissions today are more than 60% higher than in 1990, when the first IPCC report came out. Continuing to only write academic papers, advise governments, and sign petitions is clearly insufficient.
This is because the problem is not an information deficit, but of power protecting the status quo. This includes not only fossil fuel companies that rake in record profits and unduly influence the policymaking process but also affluent, high-emitting individuals that — consciously or not — seek to protect their carbon-intense lifestyle.
We therefore need to build sufficient counter-power to prevent climate breakdown and secure a livable and sustainable future for all. Civil resistance, which is a method to change the status quo using nonviolent, noninstitutional means such as boycotts, occupations, and protests, is a key tool to build this power.
One core part of civil resistance is civil disobedience, which can be defined as a “constrained, communicative protest, contrary to law, that people engage in to support a change in governmental or nongovernmental practices.” It is a principled, nonviolent political act that challenges the status quo and that forces the state to engage with protesters.
Our use of nonviolent civil disobedience is principled in the sense that we remain committed to the democratic society of which we are a part. We respect our fellow citizens and want to engage with them to make them aware of the climate catastrophe we face together. Civil disobedience is both a communicative act — signalling to wider society that we are in an emergency — as well as a form of protest that cannot be ignored and requires a government response.
There are many examples of nonviolent civil disobedience being used to successfully challenge injustices. One of the most important examples of civil disobedience is the Civil Rights Movement led by Martin Luther King Jr. By using nonviolent civil disobedience, such as refusing to abide by segregation rules and holding large, disruptive marches, the Civil Rights Movement was able to end official racial segregation in the United States. Similarly, Gandhi's Satyagraha movement in India used nonviolent civil disobedience to successfully challenge British imperialism by holding marches and refusing to cooperate with government authorities. In both examples, mass movements successfully used nonviolent civil disobedience to challenge the status quo and advocate for positive change. Another important example is the Suffragette movement, where women (and their supporters) successfully used civil disobedience to demand the right to vote. Civil disobedience and civil resistance is practised all around the world and has had a very real impact in shifting the conversation and reducing carbon emissions.
To succeed in securing a livable and sustainable future for all, however, we need to massively increase the number of people determined to step up. Will you be one of them?