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Part 3


Earth’s CO2 concentration is the same today as it was 3 million years ago. Go back in time and see what sea levels were like in the past.

0.0 million years ago, sea levels were about:


feet higher
3M years ago
2M years ago
1M years ago
today’s carbon levels

Chapter 07

Understanding risk

Background gradient that looks like Earth
Podcast TIL About Uncertainty Episode #5 from MIT’s podcast, TILclimate Listen Now
TILclimate podcast thumbnail

How do we make choices in the face of uncertainty? In this episode of TILclimate (Today I Learned: Climate), MIT professor Kerry Emanuel joins host Laur Hesse Fisher to talk about climate risk. Together, they break down why the climate system is so hard to predict, what exactly scientists mean when they talk about “uncertainty”, and how scientists quantify and assess the risks associated with climate change.

When considering what to do about climate change, it’s helpful to think about it in terms of managing risk. Every one of us confronts various kinds of risks on a regular basis, from mundane risks like climbing a stepladder to replace a lightbulb, to highly consequential risks, like undergoing open-heart surgery. Whether or not we’re aware of it, each of these decisions involves two steps: estimating how likely something is to happen, and assessing the costs and benefits, both in human and monetary terms. What do we know about the probabilities and costs of climate change? And how should we consider tail risks: unlikely scenarios with potentially catastrophic outcomes?

In essence, risk is about probabilities and about costs, measured in human and monetary terms. For example, in deciding to ascend a stepladder to replace a lightbulb, we may estimate that the probability of falling off the ladder is small but of potentially great consequence, and weigh that against the large probability of successfully changing the bulb, with the attendant benefit of having light. This may be an easy one, but then there are the tough ones. A surgeon tells me that I have a 90% chance of surviving open-heart surgery. But if I do, I might have only a few years left to live. Given that the procedure will cost my family dearly whether it succeeds or not, should I go forward with it?

Catastrophic climate change is unlikely, but it is possible.

Figure 14 The extreme scenarios—massive or only little warming—are unlikely but possible. This is what we mean by “tail risk”.

As we explain in the next chapter, in the case of climate change, the most probable outcomes over the next century, barring any action to curtail the emission of greenhouse gases, incur serious costs to society. But if climate change is worse than what we currently think is the most likely outcome, we face the possibility of catastrophic outcomes, so catastrophic that it might be difficult to really attach any definite number to the likely costs. It becomes almost a philosophical question how much we might be willing to spend to avoid the unlikely, but not so comfortably improbable possibility of truly catastrophic outcomes.

To illustrate this a bit more concretely, take a look at Figure 14, which shows an estimate of the probability distribution of global mean temperature resulting from a doubling of CO2 relative to its pre-industrial value, made from 100,000 simulations with a particular climate model. We use this here as an illustration; it should not be regarded as the most up-to-date estimate of the probabilities of global temperature increases.

Figure 15 Sea level rose considerably after the last ice age but has been remarkably stable for the last 7,000–8,000 years.

08a Sea level rise

We begin by making a simple observation about past sea level rise and human civilization. Remember that as ice volume on Earth goes down, sea level goes up and vice versa. All that water locked in the ice came from the ocean, and so when there are extensive ice sheets there is less water in the ocean. Sea level must have been lower. How much lower? The answer is, roughly 130 meters (400 feet). We know this because we know the volume of land ice and also have direct geologic evidence of ancient shorelines.

Figure 15 illustrates sea level rise to modern values from its low point of about 130 meters (roughly 400 feet) below today’s level, about 22,000 years ago. Notice that sea level has been remarkably stable for the last 7,000–8,000 years—coincident with the time that human civilization developed.

Much damage would be done by a change in sea level of a few feet.

Civilization developed during a time of unusual climatic stability and is exquisitely tuned to the climate of the past 7,000-8,000 years.

Video 6 What most people get wrong about climate change Vox Video description: 195 countries just made a historic agreement to battle climate change. But it's easy to get overwhelmed by the politics and details. Here we zoom out from the present moment, taking a look at where we came from to get a new perspective on where we're headed.

And that is just the point. Because our prehistoric ancestors were nomadic, they did not build permanent cities. They probably didn’t even notice the 400 foot rise in sea level over 10,000 years (about 0.5 inch per year). Civilization developed during a time of unusual climatic stability and is exquisitely tuned to the climate of the past 7,000-8,000 years. But in our time, much damage would be done by a change in sea level of a few feet, let alone 400 feet. A modest climate shift in either direction will be highly problematic.

Runoff from melting ice in Greenland and West Antarctica is expected to further increase the rate of sea level rise over coming decades.

Projections range upward to an increase of around 1 meter (3 feet) by 2100, with a few estimates ranging as high as 2 meters (6 feet).

Video 7 Even as the Earth’s average temperature increases, it will take some time for that increase to have its full effect on sea levels. Vlogbrothers Video description: Graphs shouldn't be scary! They're just data! And even though I have a masters degree in environmental studies, this graph, once it was explained to me, changed how I felt about the impacts we were having on the planet completely. Of course, being scared by this graph depends on you caring about human life beyond your own lifespan which, like, seems like a fairly easy thing to me, but maybe not to a lot of other people.

Sea level rose through the 20th century and has continued to rise in the present one; its rate has increased to a little more than 0.1 inch per year, mostly owing to Thermal expansion is the tendency of matter to change its shape, area, and volume in response to a change in temperature.thermal expansion as ocean waters warm. Runoff from melting ice in Greenland and West Antarctica is expected to further increase the rate of sea level rise over coming decades, and projections range upward to an increase of around 1 meter (3 feet) by 2100, with a few estimates ranging as high as 2 meters (6 feet). Most of the thermal expansion effect and at least some of the glacial melting has been directly attributed to anthropogenic warming.

Elevated sea levels make coastal regions more susceptible to storm-induced flooding, as evidenced by the aftermath of Hurricane Sandy in 2012, for example. Rising seas also infiltrate aquifers, putting freshwater supplies at risk. Many cities, such as New York, are weighing the costs and benefits of adaptation strategies such as building massive storm barriers versus hardening individual buildings.

But owing to the slow heating of the oceans, sea level will not stop rising in 2100 even if by then we manage to eliminate emissions. The last time Earth’s atmosphere had a concentration of over 400 ppm of CO2 was during the Pliocene period, about 3 million years ago, during which time sea level was about 25 meters (80 feet) higher than it is today. It may take thousands of years, but that is where sea level is headed, and scientists are not confident about forecasting how fast land ice will melt. There is no way that coastal cities can adapt to that level of change; they would simply have to relocate.

Figure 16 The number of days each summer with extremely dangerous levels of heat and humidity is expected to go up.

08b Heat and humidity

Warming is also of direct concern. Human comfort is measured by a quantity called the wet-bulb temperature, which is the lowest temperature a damp surface can have in air of a given temperature and humidity. When the wet-bulb temperature exceeds about 35°C (95°F) the human body cannot transmit heat to the surrounding air fast enough to compensate for its internal production of heat, and body temperature rises to lethal values. This limiting wet-bulb temperature is very rarely exceeded in today’s climate, but such values are projected to become common in certain regions, such as the shores of the Persian Gulf, by late in this century. Mortality from heat waves is already of concern; for example, the 2003 heat wave in Europe is estimated to have killed at least 50,000 people. As mean temperatures climb, such heat waves become more common. However, deaths from hypothermia decline with increasing temperature, and as of this writing the data are ambiguous as to the net effect on mortality.

Figure 16 presents an estimate of the number of days each year, by the end of this century, in which the combination of heat and humidity will be extremely dangerous, under emissions scenario RCP 8.5 is a pessimistic projection that assumes no serious effort to curtail greenhouse gas emissions, and robust economic growth.RCP 8.5. (By comparison, such conditions today occur no more than once every 10 years, mostly in a small region of the Midwest.)

Figure 17 U.S. property losses due to sea level rise and stronger hurricanes are projected to increase.

08c Destructive storms

Violent storms are another risk to reckon with. Tropical cyclones cause on average more than 10,000 deaths and $700 billion (U.S.) in damages globally each year. There is now a strong consensus that the incidence of the strongest storms, which although small in number dominate mortality and damage statistics, will increase over time, even though there may be a decline of the far more numerous weaker events. The jury is still out on what might happen to the incidence and intensity of destructive winter storms and violent local storms such as tornadoes and hailstorms. Figure 17 shows projections of annual U.S. property losses as a result of the combination of higher sea levels and greater incidence of intense hurricanes.

Figure 18 In more acidic environments, mollusks, corals, and plankton have trouble building and maintaining their shells.

08d Ocean acidification

Increased atmospheric concentrations of CO2 lead to increases in the concentration of CO2 dissolved in ocean waters. This makes the oceans more acidic. Laboratory experiments show that as ocean acidity increases, organisms that build shells, including certain mollusks, corals, and plankton, begin to suffer declining ability to build and maintain their shells. Thus ocean acidification poses significant risks to marine ecosystems; but these risks are only now beginning to be quantified.

Video 8 Megadroughts in U.S. west projected to be the worst of the millennium. NASA Scientific Visualization Studio This video has no audio. Video description: This video shows a map of North America with increasingly dry soil moisture projections up until the year 2041. The south western parts of North America are most severely impacted.

08e Food and water

Perhaps the most consequential change, however will be the change in where and when rain falls. Physics tells us that as the climate warms, the frequency of storms will decline, but that when it rains it will rain substantially harder. Wet climates will generally become even wetter, while arid regions will become more so meaning that flash flooding and drought will be more frequent. These changes in the water cycle, which we are already starting to see, are especially worrying because of their impacts on our food and water resources.

These changes will become apparent first and be most severe in regions, such as the Middle East, that today have only marginal food and/or water supplies.

Figure 19 shows a projection of the effect of climate change on U.S. agricultural losses, relative to today’s 1-in-20 event. By the end of this century, today’s once in 20 years agricultural loss events could occur every other year.

Figure 19 By the end of this century, today’s once in 20 years agricultural loss events could occur every other year.

Political and social destabilization is perhaps the greatest and least predictable risk incurred by rapid climate change.

Historically, the disappearance of certain civilizations, such as that of the Anasazi in what is today the southwestern U.S., has been attributed to food and water shortages brought on by prolonged drought. Such shortages are also thought to cause or exacerbate mass migrations and armed conflict. The link between climate change and human conflict is well recognized in the defense community. For example, in its 2010 Quadrennial Defense Review, the U.S. Department of Defense states that: “climate change could have significant geopolitical impacts around the world, contributing to poverty, environmental degradation, and the further weakening of fragile governments. Climate change will contribute to food and water scarcity, will increase the spread of disease, and may spur or exacerbate mass migration.”

Political and social destabilization of a crowded, nuclear-armed world finely adapted to the highly stable climate of the last 7,000-8,000 years is perhaps the greatest and least predictable risk incurred by rapid climate change. Such existential risks are difficult to attach numbers to and represent extreme outcomes whose probability is not small under high-emissions scenarios.

Chapter 09

How long can we wait to act?

Figure 20 Even if emissions abruptly stopped, CO2 concentration will remain high for thousands of years.

Global climate change presents us with unprecedented challenges. Since climate science can do no more than estimate a broad set of possible outcomes ranging from the concerning to the catastrophic, society must treat the problem as one of risk assessment and management. At one extreme, we could elect to do nothing and gamble on an only moderately challenging outcome. But if we are wrong we will saddle children and their descendants with enormous problems. At the other extreme, we could make serious economic and other tangible sacrifices that might prove unnecessary. Unfortunately, waiting much longer to see which way things go is not a viable option since it takes thousands of years for CO2 levels in our atmosphere to decline once emissions stop. In fact, even if we were to magically cut all emissions today, we would still see CO2 levels of over 400 ppm until the year 3000. By the time the consequences of climate change become unequivocally clear, it will almost certainly be too late to do much about it. We must decide very soon.

Carbon dioxide is a greenhouse gas of special concern because of its long residence time in the atmosphere. Figure 20 shows estimates of the decline of CO2 levels assuming that emissions abruptly stop when concentrations reach various values. Over the first 100 years or so, concentrations fall fairly rapidly, but then the rate of decay drops off and it will take many thousands of years for concentrations to return to preindustrial values.

Figure 21 Even if emissions abruptly stopped, temperature will also remain high for thousands of years.

Temperature hardly drops over the first thousand years after emissions cease, mostly due to heat storage in the oceans.

Figure 21 shows projections of global mean temperature that correspond to the CO2 concentrations in Figure 20. Curiously, the temperature hardly drops at all over the first thousand or so years after emissions cease, reflecting mostly the effects of heat storage in the oceans. This is a crucial aspect of the challenge we face: absent technology for removing CO2 from the atmosphere, we will have to live with altered climate for many thousands of years. Thus we have a narrow time window within which to act.

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