The cascading effects of climate change and ocean acidification will be our undoing —positive feedback loops, ecological ignorance, runaway perturbations, and double whammies mean it’s uniquely existential

In summary, six of the nine proposed planetary boundaries (phosphorous, nitrogen, biodiversity, land use, atmospheric aerosol loading, and chemical pollution) are unlikely to be associated with existential risks. They all correspond to a degraded environment, but in our assessment do not represent existential risks. However, the three remaining boundaries (climate change, global freshwater cycle, and ocean acidification) do pose existential risks. This is because of intrinsic positive feedback loops, substantial lag times between system change and experiencing the consequences of that change, and the fact these different boundaries interact with one another in ways that yield surprises. In addition, climate, freshwater, and ocean acidification are all directly connected to the provision of food and water, and shortages of food and water can create conflict and social unrest.

Climate change has a long history of disrupting civilizations and sometimes precipitating the collapse of cultures or mass emigrations (McMichael, 2017). For example, the 12th century drought in the North American Southwest is held responsible for the collapse of the Anasazi pueblo culture. More recently, the infamous potato famine of 1846–1849 and the large migration of Irish to the U.S. can be traced to a combination of factors, one of which was climate. Specifically, 1846 was an unusually warm and moist year in Ireland, providing the climatic conditions favorable to the fungus that caused the potato blight. As is so often the case, poor government had a role as well—as the British government forbade the import of grains from outside Britain (imports that could have helped to redress the ravaged potato yields).

Climate change intersects with freshwater resources because it is expected to exacerbate drought and water scarcity, as well as flooding. Climate change can even impair water quality because it is associated with heavy rains that overwhelm sewage treatment facilities, or because it results in higher concentrations of pollutants in groundwater as a result of enhanced evaporation and reduced groundwater recharge. Ample clean water is not a luxury—it is essential for human survival. Consequently, cities, regions and nations that lack clean freshwater are vulnerable to social disruption and disease.

Finally, ocean acidification is linked to climate change because it is driven by CO2 emissions just as global warming is. With close to 20% of the world’s protein coming from oceans (FAO, 2016), the potential for severe impacts due to acidification is obvious. Less obvious, but perhaps more insidious, is the interaction between climate change and the loss of oyster and coral reefs due to acidification. Acidification is known to interfere with oyster reef building and coral reefs. Climate change also increases storm frequency and severity. Coral reefs and oyster reefs provide protection from storm surge because they reduce wave energy (Spalding et al., 2014). If these reefs are lost due to acidification at the same time as storms become more severe and sea level rises, coastal communities will be exposed to unprecedented storm surge—and may be ravaged by recurrent storms.

A key feature of the risk associated with climate change is that mean annual temperature and mean annual rainfall are not the variables of interest. Rather it is extreme episodic events that place nations and entire regions of the world at risk. These extreme events are by definition “rare” (once every hundred years), and changes in their likelihood are challenging to detect because of their rarity, but are exactly the manifestations of climate change that we must get better at anticipating (Diffenbaugh et al., 2017). Society will have a hard time responding to shorter intervals between rare extreme events because in the lifespan of an individual human, a person might experience as few as two or three extreme events. How likely is it that you would notice a change in the interval between events that are separated by decades, especially given that the interval is not regular but varies stochastically? A concrete example of this dilemma can be found in the past and expected future changes in storm-related flooding of New York City. The highly disruptive flooding of New York City associated with Hurricane Sandy represented a flood height that occurred once every 500 years in the 18th century, and that occurs now once every 25 years, but is expected to occur once every 5 years by 2050 (Garner et al., 2017). This change in frequency of extreme floods has profound implications for the measures New York City should take to protect its infrastructure and its population, yet because of the stochastic nature of such events, this shift in flood frequency is an elevated risk that will go unnoticed by most people.

4. The combination of positive feedback loops and societal inertia is fertile ground for global environmental catastrophes

Humans are remarkably ingenious, and have adapted to crises throughout their history. Our doom has been repeatedly predicted, only to be averted by innovation (Ridley, 2011). However, the many stories of human ingenuity successfully addressing existential risks such as global famine or extreme air pollution represent environmental challenges that are largely linear, have immediate consequences, and operate without positive feedbacks. For example, the fact that food is in short supply does not increase the rate at which humans consume food—thereby increasing the shortage. Similarly, massive air pollution episodes such as the London fog of 1952 that killed 12,000 people did not make future air pollution events more likely. In fact it was just the opposite—the London fog sent such a clear message that Britain quickly enacted pollution control measures (Stradling, 2016). Food shortages, air pollution, water pollution, etc. send immediate signals to society of harm, which then trigger a negative feedback of society seeking to reduce the harm.

In contrast, today’s great environmental crisis of climate change may cause some harm but there are generally long time delays between rising CO2 concentrations and damage to humans. The consequence of these delays are an absence of urgency; thus although 70% of Americans believe global warming is happening, only 40% think it will harm them (http://climatecommunication.yale.edu/visualizations-data/ycom-us-2016/). Secondly, unlike past environmental challenges, the Earth’s climate system is rife with positive feedback loops. In particular, as CO2 increases and the climate warms, that very warming can cause more CO2 release which further increases global warming, and then more CO2, and so on. Table 2 summarizes the best documented positive feedback loops for the Earth’s climate system. These feedbacks can be neatly categorized into carbon cycle, biogeochemical, biogeophysical, cloud, ice-albedo, and water vapor feedbacks. As important as it is to understand these feedbacks individually, it is even more essential to study the interactive nature of these feedbacks. Modeling studies show that when interactions among feedback loops are included, uncertainty increases dramatically and there is a heightened potential for perturbations to be magnified (e.g., Cox, Betts, Jones, Spall, & Totterdell, 2000; Hajima, Tachiiri, Ito, & Kawamiya, 2014; Knutti & Rugenstein, 2015; Rosenfeld, Sherwood, Wood, & Donner, 2014). This produces a wide range of future scenarios.

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Positive feedbacks in the carbon cycle involves the enhancement of future carbon contributions to the atmosphere due to some initial increase in atmospheric CO2. This happens because as CO2 accumulates, it reduces the efficiency in which oceans and terrestrial ecosystems sequester carbon, which in return feeds back to exacerbate climate change (Friedlingstein et al., 2001). Warming can also increase the rate at which organic matter decays and carbon is released into the atmosphere, thereby causing more warming (Melillo et al., 2017). Increases in food shortages and lack of water is also of major concern when biogeophysical feedback mechanisms perpetuate drought conditions. The underlying mechanism here is that losses in vegetation increases the surface albedo, which suppresses rainfall, and thus enhances future vegetation loss and more suppression of rainfall—thereby initiating or prolonging a drought (Chamey, Stone, & Quirk, 1975). To top it off, overgrazing depletes the soil, leading to augmented vegetation loss (Anderies, Janssen, & Walker, 2002).

Climate change often also increases the risk of forest fires, as a result of higher temperatures and persistent drought conditions. The expectation is that forest fires will become more frequent and severe with climate warming and drought (Scholze, Knorr, Arnell, & Prentice, 2006), a trend for which we have already seen evidence (Allen et al., 2010). Tragically, the increased severity and risk of Southern California wildfires recently predicted by climate scientists (Jin et al., 2015), was realized in December 2017, with the largest fire in the history of California (the “Thomas fire” that burned 282,000 acres, https://www.vox.com/2017/12/27/16822180/thomas-fire-california-largest-wildfire). This catastrophic fire embodies the sorts of positive feedbacks and interacting factors that could catch humanity off-guard and produce a true apocalyptic event. Record-breaking rains produced an extraordinary flush of new vegetation, that then dried out as record heat waves and dry conditions took hold, coupled with stronger than normal winds, and ignition. Of course the record-fire released CO2 into the atmosphere, thereby contributing to future warming.

Out of all types of feedbacks, water vapor and the ice-albedo feedbacks are the most clearly understood mechanisms. Losses in reflective snow and ice cover drive up surface temperatures, leading to even more melting of snow and ice cover—this is known as the ice-albedo feedback (Curry, Schramm, & Ebert, 1995). As snow and ice continue to melt at a more rapid pace, millions of people may be displaced by flooding risks as a consequence of sea level rise near coastal communities (Biermann & Boas, 2010; Myers, 2002; Nicholls et al., 2011). The water vapor feedback operates when warmer atmospheric conditions strengthen the saturation vapor pressure, which creates a warming effect given water vapor’s strong greenhouse gas properties (Manabe & Wetherald, 1967).

Global warming tends to increase cloud formation because warmer temperatures lead to more evaporation of water into the atmosphere, and warmer temperature also allows the atmosphere to hold more water. The key question is whether this increase in clouds associated with global warming will result in a positive feedback loop (more warming) or a negative feedback loop (less warming). For decades, scientists have sought to answer this question and understand the net role clouds play in future climate projections (Schneider et al., 2017). Clouds are complex because they both have a cooling (reflecting incoming solar radiation) and warming (absorbing incoming solar radiation) effect (Lashof, DeAngelo, Saleska, & Harte, 1997). The type of cloud, altitude, and optical properties combine to determine how these countervailing effects balance out. Although still under debate, it appears that in most circumstances the cloud feedback is likely positive (Boucher et al., 2013). For example, models and observations show that increasing greenhouse gas concentrations reduces the low-level cloud fraction in the Northeast Pacific at decadal time scales. This then has a positive feedback effect and enhances climate warming since less solar radiation is reflected by the atmosphere (Clement, Burgman, & Norris, 2009).

The key lesson from the long list of potentially positive feedbacks and their interactions is that runaway climate change, and runaway perturbations have to be taken as a serious possibility. Table 2 is just a snapshot of the type of feedbacks that have been identified (see Supplementary material for a more thorough explanation of positive feedback loops). However, this list is not exhaustive and the possibility of undiscovered positive feedbacks portends even greater existential risks. The many environmental crises humankind has previously averted (famine, ozone depletion, London fog, water pollution, etc.) were averted because of political will based on solid scientific understanding. We cannot count on complete scientific understanding when it comes to positive feedback loops and climate change.

5. It is multiplicative stresses (or “double whammies”) that should be our greatest concern

It is easy to see how positive feedback loops exacerbate existential risks. A second, but less obvious danger is the linkage of seemingly unrelated processes or phenomenon that increase risk. A good example is wildfires and tornadoes. Both of these represent natural disasters that can cause great damage. Until recently no one linked these two phenomena, and no one would have imagined that an increase in wildfires might cause an increase in tornados. However, researchers in 2016 documented a linkage between wildfires in Central America and the worst episode of tornadoes in North America’s recorded history (Saide et al., 2016)—more than 120 twisters in one day, which killed 316 people. The mechanism is that the aerosol particles produced by wildfires increase the vertical sheer in atmospheric wind speeds, which in turn makes tornadoes more likely and more severe.

While tornadoes and wildfires are both local there are other trends that are national or even global that entail interacting risks factors—or what the renowned ecologist Robert T. Paine called a “double whammy” (Paine, 1993). Paine makes the argument that whereas one perturbation or stress on its own might not be terribly worrisome, if an ecosystem is hit with two stresses or threats at the same time (or in quick succession) the result can be surprisingly catastrophic. For example, aging infrastructure in the United States (dams, bridges, levees, etc.) is often talked about as a disaster waiting to happen (Reid, 2008). Similarly, increased extreme rainfall is widely appreciated as a likely outcome of climate change. Putting the two together, we have a recipe for turning improbable events into something that should be expected. A specific example of what was once a highly unlikely tragedy, but is now perhaps a probable disaster is the failure of a large dam. If large aging dams fail due to the combination of decaying infrastructure and unprecedented rainfall, downstream communities could be destroyed. Existing dams were engineered for flood frequencies and rainfall regimes that have been replaced by much more extreme weather events. This should raise general concerns about flood-safety. Not only are the designs for major dams obsolete due to climate changes, the dams themselves are obsolete. In the United States alone, more than 85% of large dams will be more than 50 years old by 2020 (Hossain et al., 2009). Based on data from the National Performance of Dam Failures, the top ten causes of dam incidents in the United States are depicted in Fig. 2a. The most frequent type of incident was attributed to inflow floods—that is more than 1000 dam failures. The reason this is a global concern is that observations (Fig. 2b) in dry and wet regions all over the world show that extreme precipitation events have been increasing since the 1950s (Donat, Lowry, & Alexander, 2017). The combined effect of intensified rainfall and old dams pose a clear risk to communities worldwide.

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California, which has used dams and reservoirs to store water on a massive scale, recently suffered through several consecutive years of both low rainfall and high temperatures that produced a 5-year record-breaking drought (Diffenbaugh, Swain, & Touma, 2015). In early 2017, the state experienced massive amounts of precipitation leading to its wettest rainy season on record (Vahedifard, AghaKouchak, Ragno, Shahrokhabadi, & Mallakpour, 2017). The rainfall unleashed floods, landslides, and nearly collapsed the Oroville Dam, the tallest dam in North America. The tremendous water flows severely damaged the dam’s spillways, prompting the evacuation of about 190,000 people downriver of the dam (Park & Mclaughlin, 2017). This particular crisis is an example of how the intersection of climate change and infrastructure that is either aging or that was designed for different conditions can potentially lead to a catastrophe (Vahedifard et al., 2017). With the likelihood of more frequent extreme events in the future, situations like the one experienced at the Oroville Dam will become more common.

The intersection of climate change and human activity is also elevating the risk of severe wildfires in large portions of the world. Models suggest that precipitation was the primary driver behind global fire regimes during the preindustrial era, and then shifted towards an anthropogenic-driven regime during the industrial period (Pechony & Shindell, 2010). Now it appears that temperature will play a strong role in the 21st century in global wildfires (Pechony & Shindell, 2010). The combination of increasing temperatures at the global scale with increased propensity of wildfires due to human activity at the local level could lead to massive infernos (Bonan, 2008). Wildfire severity and frequency will be dramatically increased wherever the mean temperature in a region increases by 3 °C or more; unfortunately, in the Sahel, central Australia, central Asia, southern Africa, the western U.S., and in most of South America, warming is indeed expected to exceed 3 °C (Scholze et al., 2006). This is a global threat.

Sometimes there is irony in the way stresses combine to produce a catastrophe. Humans have adapted to heat waves by installing air conditioning. The combination of a heat wave, with increased demands for irrigation and air conditioning led to the largest ever power outage in India during 2012. Over 600 million were left without electricity and without air conditioning to mitigate the heat wave (Lundgren & Kjellstrom, 2013). Hospitals lost power and cities shut down. While it is possible to improve on the design of electric grids to reduce such massive outages (Fang, 2014), it is clear that the combination of extreme climate events and how humans respond to those heat waves has led to several massive power outages around the world (Klinger & Landeg, 2014). The irony is that air conditioning is an adaptation to heat—and the adoption of air conditioning routinely saves lives (Barreca, Clay, Deschenes, Greenstone, & Shapiro, 2016). But the adaptation that saves human lives can overburden an electric grid and make it much more susceptible to failure. Again it is the interconnections of stresses and the way we respond to environmental shocks that promulgates the greatest existential risk.

6. Can we be smart enough to avoid existential environmental catastrophes?

Two well-understood and much discussed causes of environmental collapse are: 1) over-exploitation of nature to such an extent that there is no longer sufficient natural capital to supply basic human needs (e.g., food and water), and 2) unintended consequences of technology. Over-exploitation of common natural resources has been called the “tragedy of the commons”, and a wide variety of public policies and government regulations have been devised to rectify this tragedy. The risks due to technological advances are also in a sense well-understood, or at least widely discussed. Often the “technology” that poses an existential risk is a technology that has been invented to forestall over-exploitation by increasing efficiencies. For example, industrialized agriculture may boost crop yields thereby forestalling famine, but at the same time industrial agriculture may leak so many nutrients into rivers that dead zones arise in coastal areas at the expense of fisheries.

History is filled with stories of human resourcefulness in response to environmental crises. What worries us most are not over-exploitation or unintended consequences of technology. Those are mistakes humans have made repeatedly and will continue to make, but our responses are often sufficient to correct the problem. Thus, in response to global over-fishing, systems of catch shares have been established all around the world, with great success at restoring fish resources wherever the policy is properly implemented and enforced, and where governments are effective (Costello, Gaines, & Lynham, 2008). What worries us is our ecological ignorance—especially our ignorance regarding positive feedbacks and interactions among stresses that amplify environmental perturbations in ways that surprise us. The numerous positive feedback loops described in Table 2, and the examples of “double whammies” outlined in Section 5 could well be our undoing.

Amplification of perturbations is characteristic of complex nonlinear systems with tight coupling. In 1973, the brilliant theoretical ecologist, Robert May, published a booked entitled, Stability and Complexity in Model Ecosystems (May, 1973). May used analyses of systems of nonlinear differential equations to show that complexity (measured by the number of species, and the number of interactions among those species), and strong coupling rendered ecosystems to be highly unstable. Only special configurations of ecosystems could mitigate this inherent instability. Although May’s initial models were criticized for being too unrealistic, his predictions about the challenge of maintaining stability in complex systems with many strong interactions have been verified using more realistic models (McCann, Hastings, & Huxel, 1998). Then in 1984, Yale sociologist Charles Perrow published a path breaking book entitled Normal Accidents (Perrow, 1984). In this book, Perrow argued that accidents such as the Three Mile Island meltdown should be expected because the nuclear energy system is complex, tightly coupled, and has an inherent catastrophic potential. Finally, in 2017, the MIT finance professor, Andrew Lo, argues that stock market crashes such as the 2008 collapse are to be expected because of the complexity of our financial systems, their tight coupling, and the speed of human responses to perturbations (Lo, 2017). All three of these treatises reveal a deep insight about environmental existential risk. Ecosystems are complex and tightly coupled. Add in humans with globalization, and the pressures on those ecosystems makes for a potentially fragile world.