9.2: Emissions Scenarios

The IPCC Scenarios

The primary conclusions of the previous section are that, approximately 55 million years ago, a sudden spike in atmospheric carbon (in the form of Carbon Dioxide and Methane) led to a rapid rise in atmospheric temperature and a sudden die-off of deep-ocean organisms (by up to 40%). The sudden spike in Carbon concentrations and Global Average temperatures is only “sudden” in geological terms- thousands to tens of thousands of years- but it does provide us with a “slow-motion” scenario of what could happen to our planet if continued runaway greenhouse warming takes hold. In short, we can use the PETM as a warning sign to get our acts together!

However, there are many unpredictable variables involved in getting our act together. It’s not as simple as assuming that current consumption trends will persist. For example, there was a noticeable decrease in fossil fuel consumption in both 2009 and 2020, driven by the 2008 Global Recession and the COVID-19 Pandemic, respectively. In both instances, there was a subsequent decrease in global Carbon emissions, though they quickly recovered. Neither the Recession nor the pandemic is an easily predictable event, but they had impacts on consumption. The same can be said of the onset of war, global natural catastrophes (such as asteroid collisions or major volcanic eruptions), or even the outcome of an election. All of these factors can impact global emissions by varying global growth and necessities. Thus, there is no convenient way to predict changes in Carbon emissions. Without the ability to predict emissions, we can’t make meaningful projections about future climate. The solution is to create a series of Emissions Scenarios that present “what-ifs” for use as input to climate models.

To address this issue, the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP) set up the Intergovernmental Panel on Climate Change (IPCC) in 1988. The IPCC is an intergovernmental body of the United Nations (UN). Its job is to "provide governments at all levels with scientific information that they can use to develop climate policies". The current set of future scenarios provided by the IPCC is in the form of Shared Socioeconomic Pathways (SSP’s), which are five scenarios that climate scientists can use to provide a likely range of temperature increases and subsequent environmental impacts. The five scenarios are presented in Table 8.2.1, and Figure 8.2.1 shows total global carbon dioxide emissions (in billions of tons) for these scenarios.

Table 8.2.1: Shared-Socioeconomic Pathway Scenarios from the IPCC Sixth Assessment Report.

SSP Scenario	Description
SSP1-1.9	Very low greenhouse gas emissions; CO2emissions cut to net-zero around 2050.
SSP1-2.6	Low greenhouse gas emissions; CO2emissions cut to net-zero around 2075.
SSP2-4.5	Intermediate greenhouse gas emissions; CO2around current levels until 2050, then falling but not reaching net-zero by 2100.
SSP3-7.0 (SSP3's Baseline)	High greenhouse gas emissions. CO2emissions will double by 2100.
SSP5-8.5 (SSP5's Baseline)	Very high greenhouse gas emissions. CO2emissions will triple by 2075.

Figure \(\PageIndex{1}\): Carbon dioxide emissions under the five primary SSP emissions scenarios. (CC-BY 4.0; Riahi et al via Our World In Data). Alternative Description of Image.

4. Which of the following scenarios would be the "worst-case scenario" in terms of emissions?
   1. SSP1-1.9
   2. SSP2-4.5
   3. SSP3-7.0
   4. SSP5-8.5
5. Based on the knowledge that additional greenhouse gases will lead to additional warming, which scenario would result in the most warming?
   1. SSP1-1.9
   2. SSP2-4.5
   3. SSP3-7.0
   4. SSP5-8.5

Figure 8.2.2 shows how carbon dioxide concentrations will change in each of the scenarios between now and 2100.

Figure \(\PageIndex{2}\): Shared Socioeconomic Pathways projected carbon dioxide concentrations. (CC-BY-SA 4.0; Sfdiversity via Wikimedia Commons). Alternative Description of Image.

6. Current Carbon Dioxide Concentrations are approximately 425 parts per million (ppm). According to Figure \(\PageIndex{2}\), which scenario is the only emissions scenario to show a decrease from 425 parts per million in carbon dioxide concentrations by 2100?
   1. SSP1-1.9
   2. SSP2-4.5
   3. SSP3-7.0
   4. SSP5-8.5
7. All other scenarios show at least some increase in Carbon Dioxide concentrations by 2100, with the SSP5-8.5 (Baseline) scenario showing:
   1. A decrease in carbon dioxide concentrations by 2100.
   2. A near-doubling in carbon dioxide concentrations by 2100.
   3. A near-tripling in carbon dioxide concentrations by 2100.
   4. A leveling-off in carbon dioxide by 2100.
8. Some of the "middle of the road" scenarios show Carbon Dioxide concentrations increasing, but then leveling off. Specifically, the SSP2-4.5 scenario shows a leveling off at ___________ parts-per-million by 2100
   1. 450
   2. 600
   3. 900
   4. 1200

These emissions scenarios are not limited to Carbon Dioxide emissions. Methane, which is another potent greenhouse gas, is also projected in the SSP scenarios. These projections are shown in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Methane Emissions under the five SSP scenarios. (CC-BY 4.0; Riahi et al via Our World In Data). Alternative Description of Image.

9. Interestingly, the SSP5 scenario doesn't show the greatest increase in Methane emissions. Instead, the ______________ scenario shows the highest, steadily increasing methane emissions levels by 2100.
   1. SSP1-1.9
   2. SSP1-2.6
   3. SSP2-4.5
   4. SSP3-7.0 (Baseline)
10. On the other hand, all of the other emissions scenarios show:
    1. A leveling-off sometime during the 21st century, followed by a decline in methane emissions.
    2. A continued, sustained increase in methane emissions until 2050, and then a decline in methane emissions.
11. However, the SSP5-8.5 baseline scenario shows.
    1. An immediate (by 2020) decline in methane emissions.
    2. An increase in methane emissions until approximately 2060, followed by a decline until 2100.
    3. A continued, sustained increase in methane emissions until 2090, with only a slight decline in emissions by 2100.

This occurs because while the SSP5-8.5 scenario is the most fossil-fuel-intensive, it also assumes the global population will decline to 7 billion by 2100. Comparatively, SSP3 shows the largest population increase, as the lack of a developed, globalized economy in many regions leads to higher birth rates in still-developing countries. Remember, the primary fossil fuel emissions by human activity are carbon dioxide. Since the population isn't increasing in SSP5-8.5, agricultural and land-use sources of methane are held constant into the next centuries. Because the atmospheric lifespan of methane is relatively short (about 12 years), actively cutting fossil/industrial sources while keeping agricultural rates flat drives a gradual decline in the global atmospheric concentration of methane. However, this is still offset by year-over-year increases in carbon dioxide emissions. All other emissions scenarios show some increase in population through the middle of the century, followed by an eventual decline by 2100. Thus, while it may look nice, the SSP5-8.5 scenario isn't a better emissions scenario. Figure \(\PageIndex{4}\) shows projected increases in global average temperature under each SSP scenario, relative to pre-industrial temperatures (year 1750 CE).

Figure \(\PageIndex{4}\): Global average temperature increase after 2005 for the five SSP scenarios. (CC-BY 4.0; Riahi et al via Our World In Data). Alternative Description of Image.

12. Regardless of the scenario, there has
    1. been almost 1°C of warming by 2005.
    2. been no warming by 2005.
13. Now examine the warming by 2100. Regardless of the scenario, there will be
    1. at least some additional warming (relative to 2005) by 2100.
    2. no warming at all by 2100.
    3. at least some cooling (relative to 2005) by 2100.
14. However, both SSP1 (1.9 and 2.6) scenarios project that temperatures will:
    1. Continue to sharply increase throughout the 21st century.
    2. Level off, and even begin decreasing by 2100.
15. Therefore, regardless of what we (as humans) do to reduce carbon emissions:
    1. We cannot limit warming by 2100.
    2. We can completely reverse warming to levels below 2005.
    3. There will be at least some additional warming, but we can limit it (meaning we can cause temperatures to stop increasing).
16. However, climate scientists argue (and the Paris Climate Agreement targets) that, if humans keep warming to below 1.5°C by 2100, we can avoid the most catastrophic impacts of climate change. Which scenario in Figure \(\PageIndex{4}\) has global temperature increases below 1.5°C by 2100?
    1. SSP3-7.0
    2. SSP2-4.5
    3. SSP1-2.6
    4. SSP1-1.9
17. Briefly return to Figure \(\PageIndex{1}\); the scenario you suggested in question 16 would require:
    1. No reductions in Carbon Dioxide Emissions
    2. Eventual (by 2060) reductions in Carbon Dioxide Emissions
    3. Immediate reduction in Carbon Dioxide Emissions
18. Therefore:
    1. We (humanity) must work to reduce Carbon Dioxide Emissions immediately.
    2. We (humanity) don't need to work on reducing Carbon Dioxide Emissions.
19. Which scenario shows the most warming by 2100?
    1. SSP1-1.9
    2. SSP2-4.5
    3. SSP3-7.0 (Baseline)
    4. SSP4-8.5 (Baseline)
20. This ____________ with the conclusion we made in question 5.
    1. Agrees
    2. Does not agree
21. Recall the PETM, when temperatures increased significantly (by upwards of 4-8°C) in a 1,000-10,000 year time period, such a significant increase:
    1. Is possible by 2100 in these SSP scenarios
    2. Is not possible by 2100 in these SSP scenarios

With an increase in temperature comes a number of other major consequences, such as (but certainly not limited to) sea-level rise. Figure 8.2.5 is a graph of global sea-level increases, relative to the 1995-2014 average sea level.

Figure \(\PageIndex{5}\): Global sea-level rise projections for each of the SSP scenarios. (Public Domain; NASA via Sea Level Projection Tool). Alternative Description of Image.

22. Regardless of scenario, there _______________________ by 2100.
    1. Will be at least some sea level rise.
    2. Will not necessarily be any sea-level rise.
23. Regardless of the scenario, all scenarios show a(n)
    1. Sustained increase in sea levels by 2100
    2. Leveling off of sea levels by 2100
    3. Sustained decrease in sea levels by 2100
24. The SSP5-8.5 scenario is the worst-case scenario, showing an approximately ________ increase in sea levels by 2100, with further increases beyond that.
    1. 0.2 meters
    2. 0.4 meters
    3. 0.8 meters
    4. 1.0 meter
25. Based on everything we have covered on this page:
    1. The warmer the temperatures get, the higher the sea levels will rise.
    2. There is no relationship between temperature increases and sea level rise.
    3. Warmer temperatures will cause sea levels to decrease.

Why does this matter? As sea levels rise, locations close to the ocean (including major airports such as San Francisco International Airport) will become more exposed to storm waves, increasing the risk of inundation during storms and even of complete inundation under all conditions. For regions such as the San Francisco Bay, this can permanently submerge homes, businesses, and airports (as mentioned above), expose them to storm waves, or cause beach erosion. This isn't even getting into the ecological impacts that sea-level rise will have on local wetlands.

Sea-level rise is just one impact of a warming world explored in the 2021 IPCC Report, nor is it the only impact it will experience. Other impacts not explored in this lab manual but of significance include, but are not limited to, global decreases in sea and land ice, changes in precipitation patterns (including droughts), and ecological stresses (such as those affecting coral reefs). As mentioned elsewhere in this lab manual, these impacts are being studied in entire degree programs and beyond, with new research being published regularly.