Putting Us on Track to a Low-Carbon Future

As outlined in the Introduction, significant reductions in greenhouse gas emissions are needed to avoid the worst impacts of climate change. But how much effort is needed? What types of reductions and emissions pathways are needed in order to avoid the worst parts of climate change? And how we do know where to focus our efforts? This chapter tackles these questions and highlights the sectors where our efforts will have the greatest impact.

Avoiding the Worst Impacts of Climate Change

The level of greenhouse gases in the atmosphere is measured in parts per million, or the number of greenhouse gas particles per million particles in the atmosphere. The impact of gases other than carbon dioxide is measured by equating those gases to an equivalent amount of carbon dioxide, called carbon dioxide equivalent (COe). The equivalence of gases ranges widely. For example, 1 molecule of methane equals about 30 molecules of carbon dioxide, whereas other chemicals such as fluorinated gases, used primarily as refrigerants, are thousands of times more potent than carbon dioxide per molecule. Notably, the equivalence value varies based on the timeframe over which the gas is evaluated (methane has a higher equivalence over 20 years than over 100 years, for example) and as the science of climate change advances. The total amount of COe in the atmosphere includes CO as well as all the other gases that contribute to climate change.

There is broad consensus that preventing the worst impacts of climate change requires keeping global warming below two degrees Celsius through the end of the 21st century. To have at least a 50/50 chance of limiting warming to two degrees, we must limit concentrations of COe to 500 parts per million by 2100, although some overshoot of this target in previous years is okay. Yet in 2015, COe concentrations measured 485 parts per million, and they have been increasing at a rate of 2–4 parts per million per year. To achieve the 500 parts per million target by 2100, immediate on-the- ground action is needed. But what does this mean in terms of emissions?

Climate change and the warming that drives it are a function of the total amount of carbon in the atmosphere. In other words, it is a stock problem, not a flow problem, as discussed in the Introduction. Therefore, it is useful to think of emissions, and necessary emission reductions, in terms of cumulative totals rather than annual amounts. Significant action to reduce emissions will be needed throughout the 21st century, but for simplicity and given the growing uncertainty in years further out, we focus on the necessary reductions between now and 2050.

Without additional action to reduce greenhouse gas emissions, just over 2 trillion tons of COe will be emitted between 2016 and 2050. Although climate models vary, they show that in order to meet the 500 parts per million target, cumulative total emission reductions of 25 to 55 percent relative to a business-as-usual scenario are necessary between 2016 and 2050.

For this book, we rely on modeling completed in 2013 as part of the Low Climate Impact Scenarios and the Implications of Required Tight Emissions Control Strategies (LIMITS) exercise. In particular we rely on the modeling done by Pacific Northwest National Laboratory and the Joint Global Change Research Institute using the Global Change Assessment Model, evaluating emissions between 2010 and 2050. More information on the Global Change Assessment Model, the LIMITS study, and emission scenarios from the Intergovernmental Panel on Climate Change is provided in Appendix II.

The results of the LIMITS study suggest that to have a 50/50 shot at staying under two degrees of warming we need to reduce cumulative greenhouse gas emissions by at least 41 percent between 2010 and 2050 (Figure 1-1).

This value is global; emission reductions needed from individual countries will vary, depending on their development status. For example, the most industrialized countries will need to achieve significantly deeper reductions than the 41 percent global number to compensate for other emerging economies with high rates of economic development. It's also worth noting that a 41 percent reduction in cumulative emissions entails much greater annual emission reductions in later years as emission reductions are phased in. In 2050, global annual emission reductions of 65 percent relative to business-as-usual will be necessary, with the more economically developed regions needing to achieve reductions of 70 percent or more.

This book evaluates potential reductions at a global scale. According to the Global Change Assessment Model results discussed earlier, we need cumulative greenhouse gas emission reductions of just over 40 percent between 2020 and 2050 relative to business as usual to give ourselves a 50/50 shot at staying under two degrees of warming. This is the target we aim for in this book.

The Paris Agreement: A Good First Step

In December 2015, 189 countries responsible for nearly 99 percent of the world's greenhouse gas emissions signed the Paris Agreement, in which they agreed to make an effort to limit emissions over the next 10 to 30 years. The centerpiece of the Paris Agreement is each country's specific emission reductions targets.

If the targets are all met, they would collectively move emissions a good share of the way to the two-degree pathway. As shown in Figure 1-2, the Paris Agreement commitments, on their own, move the emission curve about a third of the way to the two-degree pathway relative to business-as-usual. If existing policies and the Paris pledges are extended to 2100 with the same degree of effort, the emission curve moves about 80 percent of the way to the two-degree pathway. Despite the United States' decision to withdraw from the Paris Agreement, commitments from remaining countries still cover more than 80...