4.1: Setting the Stage for Mitigation

In this section, we will look at two key questions:

1. Why should we mitigate climate change?
2. How much time do we have to begin mitigation efforts?

Why should we mitigate climate change?

The scientific findings presented in Chapter 1 make a compelling case for mitigation of climate pollutants. Unmitigated warming along a business-as-usual pathway presents serious and possibly existential threats to human society and natural ecosystems. Human societies have already experienced significant impacts from the 1°C of warming that has occurred since the Industrial Revolution, including increases in extreme weather events such as heat waves, droughts, and flooding; a 40% loss of summer sea ice in the Arctic; and major episodes of coral reef bleaching. Future warming could cause major population displacements due to sea level rise and extreme weather, as well as massive disruption and extinction of natural species. These impacts could become catastrophic and pose existential threats if warming were to exceed 4°C. The long lifetime of carbon dioxide in the atmosphere means that the effects would linger for centuries to millennia, affecting our children, grandchildren, and generations still unborn.

The impacts of climate change will be felt in almost every aspect of human society and social systems and in natural ecosystems as well. Sea level rise, floods, and forest fires will threaten residential and commercial buildings, as well as the insurance companies that could face rising liability costs as damage to insured properties increases. Employment in the energy sector will be affected by major shifts as the industry transitions from fossil fuels to renewables and other low-carbon energy sources. Agriculture will be heavily affected by shifts in growing zones; in particular, millions of agricultural workers in the subtropics could be displaced by drought and heat waves. Even recreation will be affected. For example, increasing snowmelt is already beginning to affect the ski industry, migration of species can affect recreational fishing, and increasing temperatures and more frequent heat waves will affect your opportunities and ability to enjoy outdoor sports.

As you will read in the coming chapters, mitigation of climate change will require a shift away from fossil fuels (coal, oil, and natural gas) as our primary energy source. Fossil fuels currently supply about 80% of the energy used worldwide, and they are by far the largest source of carbon dioxide emissions.

There are many co-benefits to moving away from fossil fuels. Beyond their warming effects, emissions associated with the use of fossil fuels are also a major health hazard. Aside from the future warming avoided, significant health co-benefits will result from phasing out fossil fuels. Fossil fuel combustion generates black carbon, which can cause heart disease and lung cancer, and ozone, which aggravates respiratory conditions and inhibits growth of agricultural crops. Air pollution (outdoors and indoors) is estimated to cause 7 million premature deaths each year, with about half of those deaths attributed to pollutants associated with fossil fuel burning. Full implementation of the short-lived climate pollutant (SLCP) mitigation measures discussed in this chapter and in Chapter 15 could save 2.4 million lives that would have been lost to outdoor pollution and 3 million lives otherwise lost to indoor pollution each year, and it could save up to 140 million tons of staple crops (maize, rice, soybean, and wheat) that would have been destroyed by ozone exposure.

A shift from fossil fuels to low-carbon energy sources would have other co-benefits as well. While there would be job losses in traditional fossil fuel industries, there would also be significant new employment opportunities in sectors such as renewables and energy storage. In light of the rapid advances in energy storage technology and dramatic decreases in the price of wind and solar energy over the past decade, renewables have the potential to provide abundant, affordable energy for all people and dramatically improve the lives of the 3 billion global poor.

How much time do we have to begin mitigation?

The short answer is: not much. Humanity has reached a crossroads; the consequences of our actions over the next decade or so will affect our descendants and the planet for centuries and millennia to come. Given the scope and scale of transformations that will be required, it’s clear that we must begin mitigation efforts now and bring them up to full speed by the middle of this century. We can see this more clearly by focusing on two approximate time periods: between now and 2030, and between 2030 and 2050.

Since the beginning of the Industrial Revolution, humans have emitted approximately 2 trillion metric tons (actually 2.2 trillion tons as of 2017) of CO₂ into the Earth’s atmosphere. About 44% of these 2 trillion tons still remain in the atmosphere (the rest has been taken up by the oceans, land plants, and soil organisms). By 2030, under a business-as-usual scenario we will have added another 1 trillion tons, bringing cumulative emissions to 3 trillion tons, and by 2050 they will reach 4 trillion tons. In short, unchecked emissions would lead to a warming of 1.5°C by 2030 and more than 2˚C by 2050.

In Table 4.1.1 we show the actual or projected warming that would be realized in a given year, as well as a quantity called “committed warming,” a term that has different meanings depending on the context. Here, we define the term committed warming as follows: it is the warming that will ultimately happen even if CO₂ concentrations stay at current levels. The warming continues to increase even after the concentrations have stopped increasing because Earth takes roughly a decade or two to adjust to increased CO₂ in the atmosphere. Currently, the Earth’s surface temperature is constantly playing catch-up as we continue to increase concentrations of CO₂ and other super pollutant greenhouse gases.

Figure 4.1.1 shows possible future temperature trajectories. The purple line represents measured global temperatures from 1950 to about 2010, and the labeled lines represent future temperature projections under different scenarios. The business-as-usual scenario is represented by the gray line that borders the colored zones. The other labeled lines represent mitigation pathways that we’ll discuss later in this chapter and in Chapter 15 (a stylized version of this curve can be seen on the title page of the book).

Figure 4.1.1 Projections of future warming, showing business-as-usual and pathways for CO₂ mitigation only, SLCP mitigation only, and mitigation of both CO₂ and SLCPs. The purple line represents the historical temperature record. From Ramanathan et al. 2017.

If unmitigated emissions continue, we will emit another trillion tons between 2030 and 2050, making the total emissions 4 trillion tons. At that point we will be committed to 3°C warming, well into the “danger zone” of severe impacts on climate, not all of which can be foreseen at the present. In that case, we would actually reach 3°C warming around 2070.

Table 4.1.1 Emissions, actual or projected warming, and committed warming under a business-as-usual scenario

Year	Cumulative CO2	Actual or Projected Warming	Committed Warming
2017	2.2 trillion tons	1°C	1.5°C
2030	3 trillion tons	1.5°C	2°C
2050	4 trillion tons	2.2°C	3°C

If we do not mitigate emissions during this century, the temperature of the Earth will increase by at least 4°C by 2100. More specifically, climate models show a 1 in 2 chance (50% probability) that temperatures by 2100 will be at least 4°C warmer than the preindustrial era, with a 1 in 20 chance (5% probability) that warming will be 6°C or greater. As we saw in Section 1.4, warming exceeding 4°C could represent an existential threat to human society and natural systems. Although the risk of this level of warming is “only” 1 in 20 based on current projections, most people would find this an unacceptable level of risk for a possibility with such serious consequences. As pointed out in Chapter 1, few people would choose to board a plane if there was a 1 in 20 chance that it would crash.