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The future of global catastrophic risk events from climate change » Yale Climate Connections

Four times since 1900, human civilization has suffered global catastrophes with extreme impacts: World War I (40 million killed), the 1918-19 influenza pandemic (40-50 million killed), World War II (40-50 million killed), and the COVID-19 pandemic (an economic impact in the trillions, and a 2020-21 death toll of 14.9 million, according to the World Health Organization).

These are the only events since the beginning of the 20th century that meet the United Nations’s definition of global catastrophic risk (GCR): a catastrophe global in impact that kills over 10 million people or causes over $10 trillion (2022 USD) in damage.

But human activity is “creating greater and more dangerous risk” and increasing the odds of global catastrophic risk events, by increasingly pushing humans beyond nine “planetary boundaries” of environmental limits within which humanity can safely operate, warns a recent United Nations report, “Global Assessment Report on Disaster Risk Reduction – Our World at Risk: Transforming Governance for a Resilient Future” (GAR2022) and its companion paper, “Global catastrophic risk and planetary boundaries: The relationship to global targets and disaster risk reduction” (see July post, “Recklessness defined: breaking 6 of 9 planetary boundaries of safety“).

These reports, endorsed by United Nations Secretary-General António Guterres, make the case that the combined effects of disasters, economic vulnerabilities, and overtaxing of ecosystems are creating “a dangerous tendency for the world to tend toward the Global Collapse scenario. This scenario presents a world where planetary boundaries have been extensively crossed, and if GCR events have not already occurred or are in the process of occurring, then their likelihood of doing so in the future is extreme … and total societal collapse is a possibility.”

Figure 1. The nine planetary boundaries beyond which there is a risk of destabilization of the Earth system, which would threaten human societal development, April 2022 version. (Image credit: Stockholm Resilience Institute; plot annotated for clarification)
Figure 2. Types of global catastrophic risk (GCR) events. (Image credit: Thomas Cernev, 2022, Global catastrophic risk and planetary boundaries: The relationship to global targets and disaster risk reduction, United Nations Office for Disaster Risk Reduction)

Global catastrophic risk (GCR) events

Human civilization has evolved during the Holocene Era, the stability of which is now threatened by human-caused climate change. As a result, global catastrophic risk events from climate change are growing increasingly likely, the U.N. May 2022 reports conclude. There are many other potential global catastrophic risk events, both natural and human-caused (Figure 2), posing serious risks and warranting humanity’s careful consideration. But the report cautions of “large uncertainty both for the likelihood of such events occurring and for their wider impact.” (Note that there is at least one other type of Global Catastrophic Risk event the report omits: an intense geomagnetic storm. A repeat of the massive 1859 Carrington Event geomagentic storm, which might crash the electrical grid for 130 million people in the U.S. for multiple years, could well be a global catastrophic risk event.)

Five types of GCR events with increasing likelihood in a warmer climate

1) Drought
The most serious immediate global catastrophic risk event associated with climate change might well be a food-system shock caused by extreme droughts and floods hitting multiple major global grain-producing “breadbaskets” simultaneously. Such an event could lead to significant food prices spikes and result in mass starvation, war, and a severe global economic recession. This prospect exists in 2022-23, exacerbated by war and the COVID-19 pandemic.

The odds of such a food crisis will steadily increase as the climate warms. The author of this post presented one such scenario in an op-ed published in The Hill last year, and insurance giant Lloyds of London detailed another such scenario in a “food system shock” report issued in 2015. Lloyds gave uncomfortably high odds of such an event’s occurring—well over 0.5% per year, or more than a 14% chance over a 30-year period.

2) War
In his frightening book Food or War, published in October 2019, science writer Julian Cribb documents 25 food conflicts that have led to famine, war, and the deaths of more than a million people – mostly caused by drought. For example, China’s drought and famine of 1630-31 led to a revolt that resulted in the collapse of the Ming Dynasty. Another drought in China in the mid-nineteenth century led to the Taiping rebellion, which claimed 20-30 million lives.

Since 1960, Cribb says, 40-60% of armed conflicts have been linked to resource scarcity, and 80% of major armed conflicts occurred in vulnerable dry ecosystems. Hungry people are not peaceful people, Cribb argues, and ranks South Asia – India, Pakistan, Bangladesh, and Sri Lanka – as being at the most risk of future food/water availability conflicts. In particular, nuclear powers India and Pakistan have a long history of conflict, so climate change can be expected to increase the risk of nuclear war between them. A “limited” nuclear war between India and Pakistan, 100 bombs dropped on cities. would be capable of triggering a global “nuclear winter” with a death toll up to two billion, Helfand (2013) estimated. 

3) Sea-level rise, combined with land subsidence
During the coming decades, it will be very difficult to avoid a global catastrophic risk event from sea-level rise, when combined with coastal subsidence from groundwater pumping, loss of river sedimentation from flood-control structures, and other human-caused effects: A moderate global warming scenario (RCP 4.5) will put $7.9-12.7 trillion dollars of global coastal assets at risk of flooding by 2100, according to a 2020 study by Kirezci et al., “Projections of global-scale extreme sea levels and resulting episodic coastal flooding over the 21st Century.” While this study did not take into account assets that inevitably will be protected by new coastal defenses to be erected, neither did it consider the indirect costs of sea-level rise from increased storm surge damage, mass migration away from the coast, salinification of fresh water supplies, and many other factors. A 2019 report by the Global Commission on Adaptation estimated that sea level rise will lead to damages of more than $1 trillion per year by 2050.

Furthermore, sea-level rise, combined with other stressors, might bring about megacity collapse – a frightening possibility with infrastructure destruction, salinification of fresh water resources, and a real estate collapse potentially combining to create a mass exodus of people, reducing the tax base of the city to the point that it can no longer provide basic services. The collapse of even one megacity might have severe impacts on the global economy, creating increased chances of a cascade of global catastrophic risk events. One megacity potentially at risk of this fate is the capital of Indonesia, Jakarta, with a population of 10 million). Land subsidence (up to two inches per year) and sea-level rise (about 1/8 inch per year) are so high in Jakarta that Indonesia currently is constructing a new capital city in Borneo. Plans call for moving 8,000 civil servants there in 2024, and eventually move 1.5 million workers from Jakarta to the new capital by 2045.

4) Pandemics
As Earth’s climate warms, wild animals will be forced to relocate their habitats and increasingly enter regions with large human populations. This development will dramatically increase the risk of a jump of viruses from animals to humans that could lead to a pandemic, according to a 2022 paper by Carlson et al. in Nature, “Climate change increases cross-species viral transmission risk.” Bats are the type of animal of most concern.

Note that in the case of the 1918-19 influenza GCR event, a separate GCR event helped trigger it: WWI, because of the mass movement of troops that spread the disease. The U.N. reports emphasize that one GCR event can trigger other GCR events, with climate change acting as a threat multiplier.

Figure 3. Predicted change in surface temperature 51-100 years after a failure of the Atlantic Meridional Overturning Circulation. Catastrophic cooling is predicted to affect Northern Europe, the edge of arctic sea ice  reach northern France, and temperatures in the U.S. fall 1-2 degrees Celsius (1.8-3.6°F). Sea ice edges are shown in bright blue; the sea ice edge would remain virtually unchanged in the Southern Hemisphere, but advance significantly equatorward in the Northern Hemisphere, reaching northern France. (Image credit: modified from Orihuela-Pinto et al., 2022, Interbasin and interhemispheric impacts of a collapsed Atlantic Overturning Circulation, Nature Climate Change,

5) Ocean current changes
Increased precipitation and glacial meltwater from global warming could flood the North Atlantic with enough fresh water to slow down or even halt the Atlantic Meridional Overturning Circulation (AMOC), the ocean current system that transports warm, salty water from the tropics to the North Atlantic and sends cold water to the south along the ocean floor. If the AMOC were to shut down, the Gulf Stream would no longer pump warm, tropical water to the North Atlantic. Average temperatures would cool in Europe by three degrees Celsius (5.4°F) or more in just a few years – not enough to trigger a full-fledged ice age, but enough cooling to bring snows in June and killing frosts in July and August, as occurred in the famed 1816 “year without a summer” caused by the eruption of Mt. Tambora. In addition, shifts in the jet stream pattern might bring about a more La Niña-like climate, causing an increase in drought to much of the Northern Hemisphere, greatly straining global food and water supplies.

study published in August 2021 looked at eight independent measures of the AMOC, and found that all eight showed early warning signs that the ocean current system may be nearing collapse. “The AMOC may have evolved from relatively stable conditions to a point close to a critical transition,” the authors wrote.

Figure 4. A pteropod shell is shown dissolving over time in seawater with a lower pH. When carbon dioxide is absorbed by the ocean from the atmosphere, the chemistry of the seawater is changed. (image credit: NOAA)

6) Ocean acidification
The increased carbon dioxide in the atmosphere is partially absorbed by the oceans, making them more acidic. Since pre-industrial times, the pH of surface ocean waters has fallen by 0.1 pH units, to 8.1 – approximately a 30 percent increase in acidity. Increased acidity is harmful to a wide variety of marine life, and acidic oceans have been linked to several of Earth’s five major extinction events through geologic time.

Under a business-as-usual emission scenario, continued emissions of carbon dioxide could make ocean pH around 7.8 by 2100. The last time the ocean pH was this low was during the middle Miocene, 14-17 million years ago. The Earth was several degrees warmer and a major extinction event was occurring.

7) A punishing surprise
In 2004, Harvard climate scientists Paul Epstein and James McCarthy conclude in a paper titled “Assessing Climate Stability” that: “We are already observing signs of instability within the climate system. There is no assurance that the rate of greenhouse gas buildup will not force the system to oscillate erratically and yield significant and punishing surprises.” Hurricane Sandy of 2012 was an example of such a punishing surprise, and climate change will increasingly bring low-probability, high impact weather events – “black swan” events – that no one anticipated. As the late climate scientist Wally Broecker once said, “Climate is an angry beast, and we are poking at it with sticks.”

Figure 5. An 18 km-high volcanic plume from one of a series of explosive eruptions of Mount Pinatubo on June 12, 1991, viewed from Clark Air Base. Three days later, the main eruption produced a plume that rose nearly 40 km, penetrating well into the stratosphere. Pinatubo’s sulfur emissions cooled the Earth by about 0.5 degree Celsius (0.9°F) for 1-2 years. (Photograph by David H. Harlow, USGS.)

Volcanic eruptions: A decreasing likelihood in a warming climate

Climate change can also be expected to reduce the likelihood of one type of global catastrophic risk event: the impacts of a massive volcanic eruption. A magnitude-seven “super-colossal” eruption with a Volcanic Explosivity Index of seven (VEI 7) occurred in 1815, when the Indonesian volcano Tambora erupted. (The Volcanic Explosivity Index is a logarithmic scale like the Richter scale used to rate earthquakes, so a magnitude 7 eruption would eject ten times more material than a magnitude 6 eruptions like that of Mt. Pinatubo in the Philippines in 1991.)

The sulfur pumped by Tambora’s eruption into the stratosphere dimmed sunlight so extensively that Northern Hemisphere temperatures fell by about 0.4-0.7 degree Celsius (0.7-1.0°F) for 1-2 years afterward. The result: the famed Year Without a Summer in 1816. Killing frosts and snow storms in May and June 1816 in Eastern Canada and New England caused widespread crop failures, and lake and river ice were observed as far south as Pennsylvania in July and August. Famine and food shortages rocked the world.

Verosub (2011) estimated that future eruptions capable of causing “volcanic winter” effects severe enough to depress global temperatures and trigger widespread crop failures for one to two years afterwards should occur about once every 200-300 years, which translates to a 10-14% chance over a 30-year period. An eruption today like the Tambora event of 1815 would challenge global food supplies already stretched thin by rising population, decreased water availability, and conversion of cropland to grow biofuels.

However, society’s vulnerability to major volcanic eruptions is less than it was, since the globe has warmed significantly in the past 200 years. The famines from the eruption of 1815 occurred during the Little Ice Age, when global temperatures were about 0.9 degree Celsius (1.6°F) cooler than today, so crop failures from a Tambora-scale eruption would be less widespread than is the case with current global temperatures. Fifty years from now, when global temperatures may be another 0.5 degree Celsius warmer, a magnitude seven eruption should be able to cool the climate only to 1980s levels. However, severe impacts to food supplies still would result, since major volcanic eruptions cause significant drought. (To illustrate, in the wake of the 1991 climate-cooling VEI 6 eruption of the Philippines’ Mt. Pinatubo, land areas of the globe in 1992 experienced their highest levels of drought for any year of the 1950-2000 period.)

Unfortunately, the future risk of a volcanic global catastrophic risk event may be increasing from causes unrelated to climate change, because of the increasing amount of critical infrastructure being located next to seven known volcanic hot spots, argued Mani et al. in a 2021 paper, “Global catastrophic risk from lower magnitude volcanic eruptions.” For example, a future VEI 6 eruption of Washington’s Mount Rainier could cost more than $7 trillion over a 5-year period because of air traffic disruptions; similarly, a VEI 6 eruption of Indonesia’s Mount Merapi could cost more than $2.5 trillion.


Complex systems like human cultures are resilient, but are also chaotic and unstable, and vulnerable to sudden collapse when multiple shocks occur. Jared Diamond’s provocative 2005 book, Collapse: How Societies Choose to Fail or Succeed, described flourishing civilizations or cultures that eventually collapsed, like the Greenland Norse, Maya, Anasazi, and Easter Islanders. Environmental problems like deforestation, soil problems, and water availability were shown to be a key factor in many of these collapses.

“One of the main lessons to be learned from the collapses of the Maya, Anasazi, Easter Islanders, and those other past societies,” Diamond wrote, “is that a society’s steep decline may begin only a decade or two after the society reaches its peak numbers, wealth, and power. … The reason is simple: maximum population, wealth, resource consumption, and waste production mean maximum environmental impact, approaching the limit where impact outstrips resources.”

Some of Diamond’s conclusions, however, have been challenged by anthropologists. For example, the 2010 book, Questioning Collapse: Human Resilience, Ecological Vulnerability, and the Aftermath of Empire, argued that societies are resilient and have a long history of adapting to, and recovering from, climate change-induced collapses. But a 2021 paper by Beard et al., “Assessing Climate Change’s Contribution to Global Catastrophic Risk,” argued, pointed to “reasons to be skeptical that such resilience can be easily extrapolated into the future. First, the relatively stable context of the Holocene, with well-functioning, resilient ecosystems, has greatly assisted recovery, while anthropogenic climate change is more rapid, pervasive, global, and severe.”

To paraphrase, one can think of the nine planetary boundaries as credit cards, six of those nine credit cards charged to the hilt to develop civilization as it now exists. But Mother Nature is an unforgiving lender, and there is precious little credit available to help avoid a cascade of interconnected global catastrophic risk events that might send human society into total collapse, if society unwisely continues its business-as-usual approach.

Avoiding climate change-induced global catastrophic risk events is of urgent importance, and the UN report is filled with promising approaches that can help. For example, it explains how systemic risk in food systems from rainfall variability in the Middle East can be reduced using traditional and indigenous dryland management practices involving rotational grazing and access to reserves in the dry season. More generally, the encouraging clean energy revolution now under way globally needs to be accelerated. And humanity must do its utmost to pay back the loans taken from the Bank of Gaia, stop burning fossil fuels and polluting the environment, and restoring degraded ecosystems. If we do not, the planet that sustains us will no longer be able to.

Bob Henson contributed to this post.

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