Chapter 3 – The Expected Consequences

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Introduction
1 – The Basic Science 2 – The Skeptic Debate 3 – The Expected Consequences 4 – The Solution 5 – A Letter to Your Grandchildren Acknowledgments To Learn More

“We served Republican presidents, but we have a message that transcends political affiliation: the United States must move now on substantive steps to curb climate change, at home and internationally. There is no longer any credible scientific debate about the basic facts: our world continues to warm, with the last decade the hottest in modern records, and the deep ocean warming faster than the earth’s atmosphere. Sea level is rising. Arctic Sea ice is melting years faster than projected.”
—William D. Ruckelshaus, Lee M. Thomas, William K. Reilly, and Christine Todd Whitman, former heads of the Environmental Protection Agency under Presidents Nixon, Reagan, George H. W. Bush, and George W. Bush, Aug. 1, 2013 (statement in the New York Times)

If you still have any doubts about the fact that global warming is not a partisan issue, the quote above should dispel them. It is from four Republican leaders of the Environmental Protection Agency who served in the past four Republican presidential administrations. You will not find any substantial difference between the urgency that these Republican administrators bring to the issue and what you would hear from recent Democratic administrators, or from President Obama or former Vice President Al Gore. Global warming transcends politics, and it transcends international borders.

To understand why the issue requires urgent action, we need to understand in more detail the types of consequences expected if we don’t act swiftly to curb the emissions that are causing global warming. By itself, the predicted rise in global average temperature of 2°C to 5°C (4°F to 9°F) by the end of this century might not sound so bad; it might even sound pleasant if you live in a cold climate today. But the expected consequences go far beyond a change in average temperature.

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Figure 3.1 This simple logical chain (flow chart) explains why we expect a variety of consequences from what we usually just call global warming.
You’ve probably heard great debate in the media about the consequences of global warming, such as whether it has caused recent weather events. But while there is room for legitimate debate over particular consequences, the basic science behind the consequences is easy to understand. We can see it with a simple chain of logic:

Starting point: As we’ve discussed, the underlying cause of recent global warming is human-caused emissions of carbon dioxide and other greenhouse gases.

Recall that: About half of this carbon dioxide is staying in the atmosphere, where it is responsible for the increasing carbon dioxide concentration (see figure 1.10), while much of the rest is being absorbed by the oceans.

In the atmosphere: The rising concentration of carbon dioxide and other greenhouse gases strengthens the greenhouse effect (see figure 1.4), and a stronger greenhouse effect means more total energy trapped in Earth’s atmosphere.
 

Therefore: Although we usually focus on warming, we should expect the consequences of the stronger greenhouse effect to include anything that can result from added energy in the atmosphere and oceans. Besides overall warming, these consequences can include changes in regional climates, more powerful storms and other extreme weather events, and melting of ice both on land and in the oceans.

In the oceans: The added carbon dioxide dissolves in the water, where chemical reactions make the water more acidic.
 

Therefore: Another effect of global warming is what we call ocean acidification, which can cause great damage to coral reefs and other ocean ecosystems.1 These changes, combined with pollution and overfishing, are likely to disrupt the food chain critical to sustaining the global fish stocks on which billions of the world’s people rely for food and livelihoods. The ecosystem changes in the ocean may also have feedback effects on other changes arising from global warming, potentially amplifying many other effects on human civilization.

Figure 3.1 summarizes this logical chain. In the rest of this chapter, I’ll discuss the major consequences identified above.

Consider heating a pot of water on a stove. The energy from the stove obviously heats the water, but that’s not all that it does. For example, some of the energy makes the gas flame or electric coil glow, some of it goes to the surrounding air, and some of it even drives “weather” (more technically, convection) in the pot by causing the water to circulate as heated water rises up from the bottom and cooler water sinks down from the top. In the same way, the added energy trapped by a stronger greenhouse effect in the atmosphere not only heats the air but also heats the oceans, melts ice, and drives more weather (and extreme weather).

It’s a surprisingly large amount. A recent analysis2 found that at the current rate at which human activities are emitting carbon dioxide and other greenhouse gases, we are causing the total energy of the atmosphere and oceans to increase by approximately 250 trillion joules each second. To put this in more concrete terms, that is the equivalent of, for example:

  • The energy that would be added by detonating four Hiroshima atomic bombs each second.
  • The energy that would be released by 500,000 lightning bolts each ­second.
  • Enough energy to have two places in the world being struck by a hurricane as powerful as Hurricane Sandy at all times.
  • Enough energy to power almost 3,000 tornadoes of the most destructive category (F5) each day.

With that much energy being added to the atmosphere and oceans,3 it should not be at all surprising that we would expect noticeable consequences. Indeed, you might wonder why the consequences aren’t even greater than they are, and the answer is that most of the energy is going into the gradual warming of the oceans and atmosphere.

Regional Climate Change

You’ve probably noticed that the terms “global warming” and “climate change” are often used interchangeably. There’s a reason for this: A warming of Earth’s average temperature is expected to mean regional changes in climate that can be much more or much less than average. They just have to all average out. In other words, while Earth as a whole may warm by “only” 2°C to 5°C during this century, different regions may experience more dramatic changes in climate, and these changes will also have numerous secondary impacts on our lives. This regional climate change is already under way, and we can expect greater change in the future.

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Figure 3.2 This map shows regional temperature changes, comparing the five-year period 2011–2015 to the average from 1951 to 1980. Notice that warming has affected almost all places on Earth, but some areas have warmed much more than others. (Gray areas represent a lack of data for the comparison.) NASA has produced an outstanding video of these changes, looking all the way back to 1880, which I’ve posted at https://youtu.be/4njRzbPdD34. Source: NASA, generated at data.giss.nasa.gov/gistemp/maps/
The most direct way to look at regional climate change is in terms of temperatures. Figure 3.2 compares average regional temperatures over the five-year period from 2011 to 2015 to the averages for the period 1951–1980. Notice that almost all regions of the world have been significantly warmer in recent years than they were a few decades earlier, but some regions — particularly in the far north — have warmed much more than others.

There are many other ways to see the climate change that is under way, but I probably don’t need to tell you that. By now, the changing climate is fairly obvious to most people, with frequent news reports of weather records being broken while reporters interview long-time ­residents saying they have never seen anything like the hurricane, blizzard, flood, or fire they’ve just experienced. Nevertheless, data like those shown in figure 3.2 prove it beyond any reasonable doubt: Climate change is already happening.

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Figure 3.3 The Colorado High Park wildfire may or may not have been tied to regional climate change, but we expect fires of this type to become more common in regions that become drier and hotter over time. Source: U.S. National Forest Service.
There are too many secondary effects to list fully, but here are a few examples:

  • Drought and floods: The changes in weather patterns cause some regions to become drier and others to become wetter. For example, the recent drought in California (California’s Sierra Nevada snowpack for 2015 was the lowest in at least 500 years) and the American Southwest is likely worse than it would have been without climate change, and the same is true for regions (such as in Pakistan) that have had increased flooding in recent years. You can understand these changes by recalling that warmer temperatures cause more evaporation of water. This extra evaporation tends to make dry places even drier and therefore more prone to drought, while at the same time making more total moisture available to fall as rain (or snow), thereby increasing the likelihood of floods.
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  • Wildfires: Drier and hotter conditions in turn lead to greater danger from wildfires (figure 3.3), and significant increases in wildfires already have been seen in many places around the world, including the American West, Alaska, Canada, Australia, and Russia.
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    Figure 3.4 This figure shows how the average timing of the first fall frost (orange) and last spring frost (yellow) has changed over the decades compared to the long-term average. Notice that the first fall frost is now coming more than four days later and the last spring frost more than four days earlier, for a net of eight additional frost-free days compared to the past average. The shorter frost season has been tied to the spread of pine beetles and other pests and diseases. Source: U.S. Environmental Protection Agency, based on a 2015 update to data from K. E. Kunkel et al., Geophys. Res. Lett. 31:L03201.

  • Dying forests: The pine forests of the Rocky Mountains are currently dying off due to an explosion in the population of pine beetles that kill the trees. This increase in the pine beetle population has been tied to the shorter and warmer winters that are a result of climate change (figure 3.4). Similar ecological effects are happening in many other regions around the world.
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  • Pests and diseases: Global warming is probably contributing to increases in the spread of insect-borne illnesses and crop-damaging pests. For example, diseases such as Zika, dengue fever, and chikungunya are transmitted by mosquitoes, which pick up these diseases by biting infected people and then, after some incubation period, spread them to other people through subsequent bites. Research has shown that warmer temperatures shorten the incubation period, which means the disease can spread more rapidly. Warmer ­temperatures also tend to increase the geographic ranges of insects, which is thought to be a major reason why insect-borne illnesses are spreading into regions that were once free of these diseases.
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  • Species extinctions: Some species cannot migrate fast enough as local climates change, causing them to go extinct as the ecosystems they rely upon are altered or disappear.

In brief, we expect that as the total amount of global warming continues to increase, future regional effects of climate change will essentially be magnified versions of what we are seeing already. The amount of magnification depends on how much the globe warms overall. For example, if you live in a region that has experienced increased drought or increased wildfires in recent years, you can expect these events to become even more common during the rest of this century. Similarly, if you are in a region that has experienced more flooding in recent years, massive floods that used to happen once in a generation may become your “new normal.”

As if the consequences described above (and below) aren’t bad enough, some scientists think there may be cause for even greater alarm: Vast amounts of both carbon dioxide and methane are currently stored in Arctic regions in permafrost, by which we mean ground or tundra in which temperatures generally remain below freezing all year round. The permafrost contains the remains of plants that have not decayed because of the freezing temperatures. If warming causes this permafrost to thaw, we might expect the material to decay and release its carbon dioxide and methane into the atmosphere. In that case, the concentrations of both of these greenhouse gases might rise even more dramatically and rapidly than they are already rising, effectively amplifying all the effects of global warming. I won’t say much more about this, but it’s worth keeping in mind that because of amplifying possibilities like this one, what we often think of as “worst-case” scenarios might not truly be the worst cases.

Storms and Extreme Weather

A second major consequence we can expect from global warming is more extreme weather events. As we’ve discussed, global warming really means an increase in energy in the atmosphere and oceans, and energy is what drives weather. With more energy, we expect hurricanes, thunderstorms, and other extreme weather events to become more numerous, more severe, or both. Notice that extreme events include severe winter weather, leading to the ironic result that global warming can lead to heavy winter snowfalls.

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Figure 3.5 Hurricane Patricia in October 2015 was one of the strongest hurricanes ever recorded and came during the second most active Pacific hurricane season on record. This photo was taken by astronaut Scott Kelly from the International Space Station. Source: Scott Kelly, NASA.
No, we cannot tie any particular storm to global warming. However, we can tie an overall trend to global warming. Many climate scientists use loaded dice as an analogy. Just as loading dice makes certain outcomes more likely than they would be by natural chance, global warming makes extreme weather events more likely than they would be otherwise. Consider 2015’s Hurricane Patricia (figure 3.5) as an example. This storm was one of the strongest hurricanes ever recorded, and it occurred during the second most active overall Pacific hurricane season on record. We cannot say that global warming caused this storm or this season. What we can say is that global warming makes storms like this and seasons like this more likely, and that we can therefore expect more years like this in the future. Cigarette smoking offers another analogy: We can’t be sure that smoking caused a particular person’s lung cancer, but we know that on average, the more you smoke, the more likely you are to get lung cancer. In the same way, as we add more greenhouse gases — and hence more energy — to the atmosphere, we should expect more extreme weather events.

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Figure 3.6 This graph shows the change in world natural catastrophes from 1980 through 2015. You can ignore the red portions of the bars at the bottom, which represent geological events (earthquakes, volcanoes) that are not affected by global warming. But all the other bar colors represent weather- or ­climate-related events. Although there is significant variability from one year to the next, the overall trend is clearly upward. Source: Munich Reinsurance Company, Topics Geo (2016 Issue).
It’s not easy to define “extreme,” and changes in human conditions can make storms that would once have been benign (for example, because they affected unpopulated areas) seem more severe simply because there are now more people in their path. This naturally introduces some uncertainty into statistics concerning extreme weather events. Nevertheless, the data strongly indicate an upward trend. Figure 3.6 shows a graph of recent natural catastrophes compiled by a leading insurance company. Aside from the red portions at the bottom, which represent nonclimate events such as earthquakes and volcanoes, the bars represent events that are linked to weather and climate. Notice that while there is quite a bit of variability from year to year, there’s been a marked overall increase from 1980 to the present, providing strong evidence for the “loaded dice” analogy for how global warming makes extreme weather more common.

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Figure 3.7 This graph shows the percentage change in heavy rain and snow events for different regions of the United States from 1958 to 2012. Notice that in all regions (except Hawaii), the trend has been toward heavier events; in other words, when it rains, it pours (and when it snows, it blizzards). Source: U.S. National Climate Assessment 2014 (nca2014.globalchange.gov).
Not at all! Remember that storms are driven by energy in the atmosphere and oceans, and global warming means more energy. So storms of all types — including winter storms — can become more severe. Indeed, figure 3.7 shows evidence of a trend toward “when it rains, it pours” (or “when it snows, it blizzards”) over recent decades, which is again consistent with what we might expect with global warming.

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Figure 3.8 Notice the increase in recent decades in the ratio of record high to record low temperatures for the 48 contiguous United States. This is a clear indication of a trend toward more hot-weather extremes. Similar data support the same conclusion globally. Source: National Center for Atmospheric Research, based on data from G. A. Meehl et al., U.S. Geophys. Res. Lett. 36:L23701 (2009).
Yes, and the evidence shows this to be the case, both nationally and globally. Figure 3.8 shows how the ratio of record high temperatures to record low temperatures has changed by decade since the 1950s. Notice that while record lows outpaced record highs in the 1960s and 1970s, in recent decades the record highs have significantly outpaced the record lows — a clear indication that hotter weather is becoming more common. Keep in mind that this is not just uncomfortable but dangerous: While they tend not to make as much news as major storms, heat waves kill more people globally than any other type of weather event.

Melting of Sea Ice

It’s fairly obvious that heat causes ice to melt, and therefore that we should expect global warming to contribute to a reduction in ice cover around the world. Broadly speaking, there are two categories of ice melt, each with a different set of consequences:

  1. The melting of sea ice, like that of the Arctic Ocean.
  2. The melting of glacial ice, by which we mean landlocked ice such as that in Greenland and Antarctica

We’ll focus on the melting of sea ice as our third major consequence of global warming, saving the melting of glacial ice for the discussion of sea level that follows.

The good news is that the melting of sea ice does not affect sea level, because this ice was already floating. The bad news is that it has many other detrimental effects. Most famously, it hurts polar bears, which depend on the ice in order to hunt seals, but it also affects weather patterns and has been implicated in changes in the jet stream and the so-called polar vortex that may have caused some of the strange weather in the U.S. in recent years. But these aren’t even the most serious consequences.

The two greatest threats posed by the melting of sea ice are these:

  1. Changes in ocean salinity (the amount of salt in the water): Melting ice adds fresh water to the oceans4 and therefore lowers the salinity of the ocean water in the regions where the ice melts. The lower salinity may in turn lead to changes in ocean currents and the productivity of fisheries. No one knows exactly how damaging these changes may prove to be, but at the extreme, they could be very dangerous. For example, changes in ocean currents could have dramatic effects on coastal climates around the world and on nutrient levels in surrounding waters, and any major changes in fishery production (as a result of nutrient level changes) could leave billions of people without a critical food source.
  2. Amplification of global warming: As I’ve already noted (see page 48), the melting of sea ice actually amplifies global warming, because ice reflects much more sunlight than water. Replacing ice with water therefore means that Earth absorbs more heat from the Sun. In other words, as sea ice melts, we get a reinforcing feedback that accelerates melting and makes all the other consequences of global warming even worse.

Because the weight of floating ice already contributes to sea level, and its weight does not change as it melts. You can easily prove this for yourself by grabbing a cup of ice water (in which the ice is all floating). Mark the water level when you start and when the ice has all melted, and you will see that it does not change.

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Figure 3.9 This map shows the declining extent of Arctic sea ice in September of three selected years. In 1980, the sea ice extended all the way to the edges of the red boundaries, including all the pink and white areas. In 1998, it extended only as far as the pink boundaries, and in 2012 only over the white region. Notice the huge decrease in the ice-covered area. Source: National Climate Assessment 2014, based on data from the National Snow and Ice Data Center.
Figure 3.9 shows the September ice coverage in the Arctic Ocean for 1980, 1998, and 2012. Note the clear and dramatic decline. The September change is shown because that is the month when sea ice is generally at its minimum after the summer melting, but similar results are found for other months. (We must compare the same month in each year, since obviously the ice coverage grows in winter and declines in summer.)

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Figure 3.10 This graph shows the change in the total area of the Arctic covered by sea ice in September of each year (when the sea ice is near its minimum after summer melting) since satellite records have been available. The black curve shows the actual data, and the blue line is a “best fit” that shows the declining trend. The average rate of decline for the more than three-decade period has been more than 13% per decade, and the nine lowest September ice extents (over the satellite record) have all occurred in the last nine years through 2015. Source: National Snow and Ice Data Center. (Latest monthly data available at nsidc.org/arcticseaicenews/)
Any time someone only shows selected data, as in figure 3.9, there’s always a risk that he or she has “cherry picked” the data to show a particular result, while other data might show something different. The way to tell whether the overall data are being fairly represented is to look at the larger data set. Figure 3.10 shows the year-by-year data for Septembers from 1979 through 2015. You can see that the extent of sea ice was indeed lower in 2012 than in the next three years, but the overall downward trend is still very clear. Moreover, notice that the nine lowest years on record for ice coverage have been the last nine years, a result that is statistically consistent only with a significant downward trend.

Sea Level Rise

A fourth major consequence of global warming is an expected rise in sea level. This rise actually comes from two distinct processes. First, although we don’t usually notice it, water expands very slightly as it gets warmer, and this thermal expansion has already been implicated in a measurable rise in sea level. The second, and potentially greater, contribution to rising sea level comes from melting of glacial ice, particularly in Greenland and Antarctica, which adds water to the oceans.

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Figure 3.12 This photograph shows damage in New Jersey from the Hurricane Sandy storm surge (2012). Source: Wikipedia/U.S. Air Force Master Sgt. Mark C. Olsen.
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Figure 3.11 This graph shows measurements of the overall rise in sea level since 1880, which is thought to be due primarily to thermal expansion of the water as ocean temperatures have risen. The shaded region shows the uncertainty range for the data. Note the overall rise of more than 8 inches, or 20 centimeters. The same effect is expected to increase sea level by up to another foot (30 centimeters) by 2100. Source: U.S. Environmental Protection Agency, based on data from NOAA and the Commonwealth Scientific and Industrial Research Organisation.
Figure 3.11 shows measurements of the change in sea level since 1880, indicating an overall rise of more than 8 inches (20 centimeters). Based on scientific understanding of how sea water expands with rising temperatures, it is thought that much of this increase has been due to thermal expansion (as opposed to ice melting). Assuming that global warming continues as predicted, ongoing thermal expansion is expected to cause a rise of another foot, or 30 centimeters, by 2100.
A sea level rise of a foot may not sound like much, but it is enough to cause flooding in many low-lying regions around the world. Moreover, its effects can be magnified by storms, causing storm surges to rise higher and go farther inland than they would otherwise. Indeed, many scientists suspect that the tremendous damage from 2012’s Hurricane Sandy (figure 3.12) was significantly magnified by the rise in sea level that has already occurred.

Toward the right side of figure 3.11, you can see that the satellite data show a smaller increase in sea level than the tidal data. This is not an error or any cause for concern, but a result of the fact that the “sea level” we observe along a coastline can be affected by at least two different processes: (1) an increase in the level of the ocean water, and (2) a decrease in the level of the land on the coastline relative to the ocean basin. Because tidal gauges measure sea level relative to the land level along coastlines, they account for both processes. In contrast, the satellite data measure only the actual height of the ocean surface, which explains the small difference in the data sets.

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Figure 3.13 While there are great uncertainties in estimating how rapidly sea level will rise, many experts expect the rise by 2100 to be at least about one meter (three feet), and a worst case might be as high as about 6 meters (20 feet). This map shows regions of the southeastern United States that would be flooded by a one-meter rise in sea level (red) and a six-meter rise in sea level (yellow). Other coastal regions around the world would experience similar inundation. Source: University of Arizona, Department of Geosciences, Environmental Studies Laboratory. If you want to see how sea level rise would affect other regions, there is a nice series of maps at www.globalwarmingart.com/wiki/Sea_Level_Rise_Maps_Gallery
The second contribution to rising sea level comes from melting of glacial ice, particularly in Greenland and Antarctica. Scientists cannot yet predict ice melting very well, but many experts expect it to increase sea level by at least one meter (three feet) — and possibly much more — by the end of this century. This would have severe consequences along coastlines. For example, the red in figure 3.13 shows coastal regions of the southeastern United States that would be flooded by a one-meter rise in sea level, and the yellow shows regions that would be flooded by a six-meter rise.5

Also keep in mind that while figure 3.13 shows only the southeastern United States, similar effects are expected globally. Some island nations (and other inhabited islands) may end up completely underwater, and many of the nations that will be affected by sea level rise are much less equipped to deal with the consequences than a wealthy country like the United States. Many political scientists and military analysts fear that sea level change alone could displace hundreds of millions of people from their homes, leading to political upheaval on top of climate upheaval.6

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Figure 3.14 These data from NASA’s GRACE satellites show changes in the mass of the Antarctic and Greenland ice sheets since 2002 (when the satellites were launched). Note that the rate of ice loss from Greenland is more than twice as large as the rate from Antarctica, which is presumably a result of the fact that temperatures have increased much more in the Arctic than in the Antarctic (see figure 3.2). Source: NASA (climate.nasa.gov/vital-signs/land-ice/)
What you heard is based on a study published in late 2015 (H. J. Zwally et al., J. Glaciol. 61, no. 230, pp. 1019–1036), and as I write in early 2016, scientists are actively debating the validity of the results. Nevertheless, even if the new study is correct, there’s still no reason to doubt that ice is melting overall on Earth; the study only questions ice loss in Antarctica. Here’s a brief summary of the issues.

There are two major methods used to measure changes in the Greenland and Antarctic ice sheets, both of which require satellite data. The first makes careful measurements of any changes in the strength of gravity over the ice sheets. These measurements should tell us how the total mass of the ice is changing with time (because the strength of gravity is determined by mass), and figure 3.14 shows the results from NASA’s GRACE satellites. The second method looks for changes in the elevation of the ice sheets. The new study used this method and found that even though there was elevation loss in many parts of Antarctica (as expected if mass has been lost), there was enough elevation gain in other parts of the continent to suggest a net gain in the overall average elevation of the ice sheet.

The question, then, is how to reconcile the net gain in ice elevation claimed by the new study with the loss of ice mass measured by the GRACE satellites. One possibility, of course, is that one or the other of the measurement sets is wrong. For example, perhaps either the mass measurements or the elevation measurements have some calibration problem that makes them inaccurate. Most scientists have great confidence in the GRACE results, because they have been checked and rechecked for more than a decade now, but somewhat less confidence in the newer claims that have not yet been checked so carefully. There’s also a way that both sets of measurements could be correct: Any elevation gain must be due to new snow that has fallen, which means it is at least possible that this snow is “fluffy” enough to have caused an overall elevation gain even while the total mass has declined. Climate scientists are working to determine which possibility is correct, which can make it difficult for nonscientists to know what to make of the debate. So for now, I’ll suggest that you focus on three key points:

  1. Even if the new study is correct, the amount of ice gain it claims for Antarctica is only about a third of the ice loss that GRACE has measured for Greenland. (Neither the new study nor any other studies have called the Greenland results into question.) In other words, any Antarctic ice gain is likely being more than made up for globally by Greenland’s rapid
    ice loss.
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  3. The new study also found that the trend in Antarctica is toward smaller ice gains and predicts that it will result in net ice loss within a few decades at most. In other words, even if it is correct, this study at best suggests that sea level rise will be less than otherwise estimated over the short term. That is, sea level rise will continue due to ice loss from Greenland and other sources, and it will accelerate once the Antarctic ice gain reverses.
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  5. This case is a great illustration of the way that science progresses through careful study in which results are checked and rechecked (and repeated or revised if necessary). This means it inevitably takes time for scientific debates to be resolved. It will take time for the details of this particular debate to be resolved, but this does not change the fact that 150 years of study have by now ensured that our basic 1-2-3 science is thoroughly understood and established. There is still much to learn about the precise consequences of global warming, but there is no denying that the basic problem is real.

Up to this point, we’ve talked only about the expected sea level rise by 2100, but we can’t expect the rise to stop there. If Earth warms enough, we might expect the polar caps to melt completely, returning our planet to the ice-free state of dinosaur times. So if you really want to understand the threat of sea level rise, we need to know how high it might go in this worst-case scenario. There are two general ways to answer this question. One is to look at geological data to see how much higher sea level has been in the past when Earth was ice-free. The other is simply to calculate the total volume of water in glacial ice and how much sea level would rise if that amount melted into the oceans. Both approaches yield the same dismaying answer: If the ice caps fully melt, sea level will rise by some 70 meters, or about 230 feet.

Although nearly all scientists suspect that such melting would take centuries or even millennia, it may already be inevitable7 unless we find a way to reverse the effects of global warming (such as by removing carbon dioxide from the atmosphere so that its concentration drops back down). If we try to put the best face on this possibility, a time scale of centuries would in principle allow our descendants to migrate inland as the coastline shifts. Still, it suggests the disconcerting possibility that future generations will have to send deep-sea divers to explore the underwater ruins of many of today’s major cities.

I’m not in the business of giving investment advice, and if you sell before sea level rises too much, you might make out quite well. But one thing is near certain: If your plan is to keep your beach-front property in your family for generations, then your only hope of success lies in our rapidly stopping and then reversing the effects of global warming.

Ocean Acidification

The fifth and last major consequence we’ll discuss is ocean acidification, which occurs as carbon dioxide dissolves in the oceans and, through a well-understood chain of chemical reactions, makes the oceans more acidic. As briefly noted in the introduction to this chapter, ocean acidification can have devastating consequences for ocean ecosystems, including killing coral reefs. It can therefore damage our civilization through both direct effects like reductions in fish stocks and indirect effects stemming from the way these changes in the oceans may affect other Earth systems, including the climate.

I won’t say much more about ocean acidification, partly because it has been less well studied than other consequences and partly because the details require discussions of ocean chemistry and ecosystems that are beyond the level of this book. Nevertheless, it’s important to keep in mind that because the oceans make up about three-fourths of Earth’s surface, anything that affects the oceans is likely to affect the rest of the world as well. It is possible that ocean acidification could be as devastating in its effects on our civilization as our other four consequences combined, and it is therefore very important to consider it as part of any overall discussion of global warming.

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Figure 3.15 This graph shows evidence for ocean acidification. The red curve is a repeat of the atmospheric carbon dioxide rise from figure 1.8. The blue curve shows measurements of the carbon dioxide concentration in the ocean, which is rising along with the atmospheric concentration. The green curve shows the change in the ocean pH, which is a measure of the water’s acidity; lower pH means greater acidity. The pH is going down as the carbon dioxide concentration goes up, demonstrating that the ocean is indeed becoming more acidic. Source: U.S. National Climate Assessment 2014 (nca2014.globalchange.gov)
Direct measurements show the acidification (figure 3.15), leaving no doubt that it is real. The effects on coral reefs and other ocean ecosystems have also been observed and measured.

We have discussed five major categories of consequences of global warming: regional climate change, storms and extreme weather, melting of sea ice, sea level rise, and ocean acidification. We’ve found that each of these major consequences is likely to lead to a number of secondary consequences, and feedbacks among the various consequences could potentially make the whole of the problem worse than the sum of its parts. While there is uncertainty about exactly how serious all the consequences of global warming will prove to be, there is little scientific doubt that these consequences are real.

We’ve also seen that these are not just consequences for the future. Evidence shows that we are already feeling the effects of many of them, though we expect them to worsen in the future. The bottom line is that the longer we continue adding carbon dioxide and other greenhouse gases to the atmosphere, the more detrimental to human life and civilization we should expect the consequences to be. With that in mind, it is time to turn our attention to what we can do about the problem before it gets completely out of hand.

Continue to Chapter 4 – The Solution