We can think of greenhouse gases—carbon dioxide (CO2) and a number of other gases such as methane and nitrous oxide—as acting like a blanket. They cover the Earth and keep the warmth in, heating up the surface where we live. The greater the concentration of greenhouse gases in the atmosphere, the thicker the blanket, and the warmer we are.

Until the Industrial Revolution, humans had little impact on the climate. Average surface temperatures varied somewhat over the centuries due to natural variation, but within the last ten thousand years—since the beginning of agricultural civilization—they have been a relatively constant 15°C.

The Industrial Revolution threatens to change that. The Industrial Revolution was based on fossil fuels.

Technologies such as the steam engine were able to convert the energy stored in fossil fuels into motion, enabling the mechanization of many tasks. The beginning of the industrial revolution was powered by coal. Over time, economies have diversified their fuel mix and now rely heavily on three fossil fuels: coal, oil, and natural gas. All three, when burned to produce energy, release CO2 into the atmosphere.

As a result of industrialization and related changes such as population increases, global emissions went from near zero in 1850 to around 34 billion tons of greenhouse gases in 1992 and 45 billion tons in 2010. Globally, about 65% of this total is from fossil fuels, 12% from deforestation, and the rest from emissions of other greenhouse gases such as methane and nitrous oxide, largely from agriculture. In developed countries such as the United States about 80% of emissions come from fossil fuels because these countries have largely eliminated deforestation.

The resulting increase in the thickness of the greenhouse gas blanket will increase global temperatures. So far, temperatures have increased by about 1°C from their preindustrial level. If we continue to emit, temperatures may go up anywhere from around 2°C to as much as 6°C or even more, depending on how much we emit and how sensitive the atmosphere turns out to be to greenhouse gases. These are global averages. Many places, such as northern land masses, may heat up far more, and other places, such as the area over the ocean, may heat up less.

There are three key features of the greenhouse gas blanket that lead to the conclusion that we must eventually reduce emissions to zero. The first is that CO2 in the atmosphere is effectively permanent. The CO2 we emit today will continue to influence the climate for tens or even hundreds of thousands of years.1 There is, moreover, no way to remove it, at least using any technology that we have now or that is foreseeable.

The second feature is that temperatures continue to go up when we emit more. As the greenhouse gas blanket gets thicker, we get warmer.

The amount of warming we will get is uncertain, and the science enormously complex. Notwithstanding years of work, we cannot pin it down. There is, however, a simple way to understand the core relationship between emissions and temperatures: temperatures go up linearly with the total amount of CO2 emitted in the past.2 All we need to know is the cumulative emissions of carbon to know what the likely temperature increase will be.

Figure 6.1 is a simplified representation of this relationship. The x-axis shows cumulative emissions of carbon, the sum of all emissions in the past, regardless of when they occurred.3 The y-axis shows the expected increase in global average temperatures. The central black line reflects the current best estimate of how sensitive temperatures are to emissions, a value known as climate sensitivity. Using this estimate, there is a 50/50 chance of the specified temperature increase for a given level of cumulative emissions. For example, for cumulative emissions of 1 trillion tons of carbon, we have a 50/50 chance of a 2°C temperature increase. For a cumulative emission of 1.5 trillion tons, there is a 50/ 50 chance of a 3°C temperature increase.

We do not know how much temperatures will increase for a given level of emissions, a value known as the climate sensitivity. The light gray lines reflect uncertainty regarding climate sensitivity. If we get lucky and the climate is relatively insensitive (the bottom gray line), we can emit

Temperature as a Function of Cumulative Emissions

figure 6.1 Temperature as a Function of Cumulative Emissions.

1.5 trillion tons of carbon before we have a 50/50 chance of a 2°C temperature increase. If we are unlucky and the upper-gray line represents the true climate sensitivity, we can emit only about 800 billion tons before temperatures increase by 2°C.

Figure 6.1 points to a key conclusion. Whatever limit we set on temperature increases, emissions have to stop once we meet this target. For example, if the target is 2°C, we can only emit 1 trillion tons of carbon. Any more than that would lead to a likely temperature increase greater than 2°C. The same holds for any other limit we set. A 4°C limit means we can emit at most 2 trillion tons.

Said another way, on human timescales, the atmosphere is a nonrenewable resource. For a given temperature increase, it can hold a fixed amount of carbon and no more. The atmosphere is not like agricultural land, which can be replenished, or fisheries or forests, which if left alone, regrow. It is a strictly limited resource. As a result, unless we decide to let temperatures increase indefinitely, emissions have to go zero. This is true even given uncertainty about the climate sensitivity. Regardless of which line we are on— the upper or lower gray line or somewhere in the middle— emissions have to go to zero to stop temperature increases.4

The third central feature of climate change is that there is a limit to tolerable temperature increases. The harms from climate change are even more uncertain than the extent of temperature increases, but they will go up rapidly as temperatures increase. We have already experienced about a 1°C temperature increase. There have been some harms but they have not been extreme. Economic growth has continued. Problems other than climate change dominate our attention. At the other end of the spectrum, temperature increases of 5°C or 6°C would almost certainly be catastrophic. The last time temperatures were 6°C warmer was around 40 million years ago. There was no ice anywhere on Earth and sea levels were an astounding 60 meters higher than they are today.5 Many ecosystems that depend on the current climate would collapse. To get a sense of the magnitude, the last ice age, when much of North America and Europe were covered with mile high glaciers, was only 6°C colder than today. There is no possibility that policies that would lead to this sort of warming are desirable. There is a limit to tolerable temperature increases.

These facts—that if we continue to emit greenhouse gases, temperatures keep on going up, that the harms will likely go up rapidly as temperatures increase, and that there is a limit to tolerable temperature increases—mean that we eventually have to reduce emissions to zero. This is true even if the temperature turns out to be relatively insensitive to greenhouse gases and even if the harms turn out to be on the lower end of the possibilities. The only thing that varies with these factors is the total allowable emissions before we must stop. When we think about the ethics of climate change we have to think of it as the ethics of using a vital, nonrenewable resource.6

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