A climate “tipping point” is a threshold where a small change causes a huge change in the Earth system—potentially irreversibly and abruptly. This makes tipping points a big concern as we face the threats presented by climate change.

In En-ROADS we have included several climate tipping points that are commonly brought up, but others are outside the scope of the model and we haven’t been able to include them. The overall insight of the model is true for tipping points as well: the lower we keep temperature change, the lower the likelihood of triggering large-scale, irreversible processes that would substantially alter the Earth’s climate.

Based on the Intergovernmental Panel on Climate Change’s (IPCC)
Assessment Report 6 (AR6) Working Group I The Physical Science Basis report (2021), we identified seven potential climate tipping points. For the purposes of En-ROADS, there are two impacts that we look at that tie back to tipping points: 1. Will it result in more greenhouse gases and thus more global temperature change? 2. Will it impact sea level rise? Other impacts like changes in forest cover or ocean circulation are significant impacts that could come from hitting a tipping point but are outside the scope of what En-ROADS covers.

Table 5.6 from the IPCC’s AR6 report provides an overview of the main tipping points they considered:

(IPCC AR6 WGI 5-739)

Because of the large range of uncertainty surrounding the timing and consequences of these kinds of events, we do not include tipping points in the Baseline scenario in En-ROADS, but you can still explore the implications of several. 

1. Emissions from Permafrost Thaw

How do I simulate emissions from permafrost in En-ROADS?

To simulate additional emissions of methane (CH4) from thawing permafrost, use the Assumptions pane (under the top “Simulation” menu). Under the section “Climate system sensitivities,” the final two sliders focus on permafrost. Increase the “Effect of temperature on methane emissions from permafrost and clathrates” slider to simulate the release of methane from permafrost. For further information about the slider, click on the arrow to the left of the slider. Use the “Temperature threshold for permafrost and clathrates” slider to set the temperature that will trigger the release of methane from permafrost and clathrates.

What is permafrost?
Permafrost refers to ground that is frozen year-round, located in areas such as the Arctic. When permafrost thaws, microbes break down the large amount of organic matter in the soil, releasing methane and CO2. There is approximately twice as much carbon stored in permafrost as there is carbon in the atmosphere—about 1460-1600 Gt

The processes that control permafrost thaw are challenging to model, and much remains unclear about the potential of permafrost to contribute to climate change. Most of the climate models included in the Coupled Model Intercomparison Project 6 (CMIP6) used by the IPCC in its 6th Assessment Report do not include permafrost dynamics, but those that do show a positive feedback process: emissions from permafrost will lead to more warming, which will accelerate the release of more emissions from permafrost (IPCC AR6 WGI 5-66). Further scientific research and modeling is needed. 

What about CO2 emissions from permafrost?

Currently, En-ROADS only includes an option for additional CH4 release from permafrost and does not include the option for additional release of CO2 from permafrost. It is unclear how much carbon from thawing permafrost will be released as CH4, or as CO2. According to the IPCC, “It is projected that CO2 released from permafrost will be 18 (3.1-41 5th-9th percentile range) PgC per 1°C by 2100 with the relative contribution of CO2 vs. CH4 remaining poorly constrained” (IPCC AR6 WGI 5-67). It is also uncertain whether permafrost carbon feedbacks increase linearly with temperature or at greater or smaller rate (IPCC AR6 WGI 5-67). The IPCC estimates the maximum CO2 or CH4 release in the 21st century as up to 240 PgC as CO2 and up to 5300 Tg of CH4 (low confidence), translating into a maximum rate of change over the 21st century of ≤1 ppm CO2 /year and ≤10 ppb CH4/year (IPCC AR6 WGI 5-79).

Methane release from clathrates

These En-ROADS Assumptions sliders also simulate the release of methane from clathrates (methane ice) which is found in the deep ocean. Large-scale release of methane from clathrates is unlikely in the next few centuries (IPCC AR6 WGI 5-81).

2. Dieback of the Amazon rainforest

Currently, En-ROADS does not include a way to simulate the effect of the dieback of the Amazon rainforest on emissions, removals, and global temperature. Since En-ROADS is a global model, it does not make distinctions between different forest types.

According to the IPCC AR6, “Continued Amazon deforestation, combined with a warming climate, raises the probability that this ecosystem will cross a tipping point into a dry state during the 21st century (low confidence).” (IPCC AR6 WGI TS-43)

3. Disappearance of Arctic sea ice, leading to lower albedo (the ability of the Earth’s surface to reflect light)

This is reversible, although the loss of albedo does increase warming. The impact of warming on Arctic sea ice can be viewed in En-ROADS under Graphs > Impacts > Probability of an Ice-Free Arctic Summer.

4. Melting of the Greenland and Antarctic ice sheets

While this process won’t directly cause more warming, it will have immense consequences for the millions of people who live along the coast.

To simulate greater or less ice sheet melt in En-ROADS than assumed in the Baseline scenario, go to Simulation > Assumptions > Sea level rise. 

View the effects in the “Sea Level Rise,” “Population Exposed to Sea Level Rise” graphs and the “Sea Level Rise—Flood Risk Map” under Graphs > Impacts. 

To see the long-term (multi-century) effects of warming on sea level rise, select the “Long-Term Equilibrium” map type at the bottom of the “Sea Level Rise—Flood Risk Map.” The Long-Term Equilibrium map is not affected by the Assumptions sliders, as long-term projections of sea level rise from ice sheet melt are already incorporated into the Long-Term Equilibrium map.

5. Coral reef dieoff

While this process won’t directly cause more warming, it will have major consequences for ocean life and the millions of people who rely on fishing for food and income.

En-ROADS does not model coral reef ecosystems, but you can use the “Ocean Acidification” graph under Graphs > Impacts to see the consequences of CO2 emissions on ocean acidity. CO2 dissolves into the ocean to form carbonic acid, decreasing the pH of the ocean (making it more acidic). This decreases the availability of carbonate, making it harder for corals to form their skeletons and shellfish to form their shells. In addition, warming leads to coral “bleaching” in which corals expel their symbiotic algae, which they rely on for food. Coral ecosystems are integral to ocean life, so their potential extinction has widespread repercussions.

6. Dieback of the boreal (taiga) forest

Expansion of temperate forests south and boreal forests to the north are expected to compensate for potential dieback of the current boreal forest (IPCC AR6 WGI 5-740).

7. Slowdown of the Atlantic Meridional Overturning Circulation (AMOC)

This is a regional effect that wouldn’t directly cause more warming, although it could accelerate the dieback of the Amazon rainforest. En-ROADS does not model ocean circulation with enough detail to simulate slowdown of the AMOC. The IPCC gives medium confidence to the projection that the AMOC will not abruptly collapse during the 21st century (IPCC AR6 WGI TS-38).