SSP5-8.5 is often said to be a "worst-case" scenario, but as illustrated by the image below, current temperatures are on track or even exceeding SSP5-8.5 projections, given that the rise in
and 2025 resembled a La Niña year. So, has the IPCC been downplaying the danger? It sure makes one wonder whether SSP5-8.5 is the worst-case scenario. Indeed, Business As Usual (BAU) may turn out to be even worse, so let's have a closer look at what the outlook for some of the worst-case scenarios could be.
How much could temperatures rise?
The image below, adapted from ClimateReanalyzer, shows the Coupled Model Intercomparison Project Phase 6 (CMIP6) for the SSP5-8.5 scenario pointing at a temperature rise of 1.661°C in February 2025, of 4.388°C in February 2083 and of 5.163°C in February 2100, when using a 1901-2000 base (temperatures will be higher when a genuinely
pre-industrial base is used).
The map below shows the CMIP6 SSP5-8.5 rise versus 1881-1920 in February 2100. The map shows that the temperature rise in areas on land (where most people live) could be as much as 8°C higher in Feb 2100 in the SSP585 model.
The map warns that temperatures over large parts of the Arctic may be as much as 20°C higher than 1881-1920 in February 2100. This would suggest that by 2100 the snow and ice cover in the Arctic will have declined dramatically and that huge amounts of greenhouse gases will likely have been released from the seafloor of the Arctic Ocean and from thawing terrestrial permafrost, with huge albedo changes as well as loss of the latent heat buffer, further accelerating the temperature rise over the years. There are further contributors to a rapid and potentially huge temperature rise, so the SSP5-8.5 model may severely underestimate the temperature rise. Indeed, the SSP5-8.5 model may not be the worst-case scenario.
CMIP6 SSP5-8.5 can be used to project temperature rises beyond 2100, as illustrated by the image below, from an
earlier post and from a
2016 paper by Brian O'Neill et al.
In the above analysis, CO₂ emissions keep rising in CMIP6 SSP5-8.5 to about 35 GtC in 2100, to fall gradually after 2100 (a), while atmospheric CO₂ concentrations keep rising and remain at levels higher than 2000 ppm beyond 2250 (b), while radiative forcing (RF) rises to and remains at 12.5 W/m⁻² (c) and global mean temperature change rises to 8°C in 2300 (d).
Equilibrium climate sensitivity (ECS) can be used to convert RF into °C temperature change. ECS in
IPCC AR6 is 3, i.e. lower than in CMIP models. A
study led by James Hansen concludes the IPCC ECS is too low and suggests to use 1.2°C per W/m⁻², which in a 8.5 W/m⁻² scenario would correspond with a 10.2°C temperature anomaly in 2100. Hansen et al. add that equilibrium global warming for today’s GHG amount is 10°C, which is reduced to 8°C by today’s human-made aerosols.
A 2020
analysis by Jorgen Randers et al. (image on the right) points out that, even if all greenhouse gas emissions by people could stop immediately and even if the temperature anomaly could fall to 0.5°C above pre-industrial, greenhouse gas levels would start rising again after 2150 and keep rising for centuries to come.
The image on the right is created with content from a study by
Christina Schädel et al. The study concludes that permafrost fires and thaw will release 231 Gt CO₂e (63 Gt C) for each degree Celsius rise in temperature from a 389-691 Gt pool of carbon. By comparison, the total emissions by people is now about 55 Gt CO₂e.
In other words, the danger is even more menacing when including the full wrath of rising temperatures in the Arctic, i.e. including the impact of permafrost fires and permafrost thaw, and the impact of destabilizing sediments at the seafloor of the Arctic Ocean resulting in eruptions of huge amounts of methane, as well as the impact of albedo loss and loss of the latent heat buffer of declining sea ice and permafrost, all of which comes on top of existing global emissions.
How fast could temperatures rise?
Next to the size of the temperature change, the rate of change is also important. Large changes did take place in prehistoric times, but they typically did take a long time to evolve. The current temperature rise looks set to be huge and also looks set to be rising at accelerating speed, dwarfing anything seen in previous extinction events. The image below, from an
earlier post, shows extinction rates and temperature changes for the five major extinction events (grey vertical lines
).
“The paleoclimate record shouts to us that, far from being self-stabilizing, the Earth's climate
system is an ornery beast which overreacts even to small nudges” (Wally Broecker)
The image below further illustrates that temperatures may currently be rising much faster than they ever did in history.
Contributors to a potentially huge temperature rise
The image below uses NASA Land-only temperature anomalies versus 1880-1920, illustrating that temperature could rise more rapidly than SSP5-8.5 suggests. The image illustrates that the 1.5°C threshold was crossed for temperatures on land since 2015, when politicians pledged (at the
Paris Agreement) to take efforts to prevent a rise of more than 1.5°C from pre-industrial from occurring.
Note that this 1880-1920 base is not pre-industrial; temperature anomalies can be higher when using a genuinely
pre-industrial base.
The image below has a polynomial trend added that points at 3°C getting crossed on land in early 2027.
As discussed, crossing 3°C on land is important, since most people live on land and there are indications that such a rise will cause many species (including humans) to
go extinct.
Meanwhile, the 3-year running average for the mean rate of atmospheric CO₂ growth through January 2026 broke 8.00 ppm per 3 years, reaching a new record high growth rate of 8.06 ppm per 3 years, as illustrated by the
Eliot Jacobson image below.
Below are 14 contributors to a potentially huge temperature rise:
1. High and rising greenhouse gas concentrations
2. Earth Energy Imbalance rapid and accelerating rise
3. Rapid and accelerating decline in Earth Albedo
- Sea ice decline
- Snow and ice cover on land decline
- The aerosol masking effect getting reduced
- Lower clouds reflectivity decline
4. Further feedback kicking in with accelerating ferocity
-
Water vapor feedback
- Ocean stratification, acidification and hypoxia (dead zones)
- Polar amplification of the temperature rise
-
Jet Stream distortion and more extreme weather events
- Sea currents such as AMOC and SMOC slowing down
- Decline in the capacity of oceans and land to take up CO₂ and heat
5. Thinning of sea ice resulting in loss of the ability to consume incoming ocean heat
6. More fuel getting burned worldwide
7. Worldwide rise in agricultural emissions
8. Depletion of soil moisture and water from lakes, rivers and aquifers
9. Deforestation, loss of soil carbon and associated emissions
- Forest fires increase
- Trees becoming more vulnerable to droughts, fires, pests and diseases
- More trees cut and burned to create pasture, for construction and energy use
- Increase in ozone due to storms and lightning
10. Loss of wildlife and biodiversity
11. Pollution of water and soil
- Oil spills on sea, infrastructure collapse on land
- Pesticides, agricultural chemicals, etc.
- Plastic and PFAS contamination
- Flooding and fires in urban areas (waste, toxic substances from warehouses, etc.)
- Pollution from military activities
12. Politicians hardly take environmental and
climate action
13. Many media focus on selling consumables instead of on climate action
14. Meanwhile, a new El Niño may emerge in the course of 2026
While each of these points is alarming in itself, they can also amplify each other and together they can cause a dramatic and rapid temperature rise, as discussed in a
recent post and as illustrated and supported by the rapidly rising polynomial trend in the image below, which is based on many years of historic data and which adds further weight to important warnings.
When could humans go extinct?
As the likeliness of a huge and accelerating temperature rise, the severity of its impact, and the ubiquity and the imminence with which it will strike all become more apparent and manifest—the more sobering it is to realize that a mere
3°C rise will likely suffice to cause human extinction.
A 2018 study (by
Strona & Bradshaw) indicates that most life on Earth will disappear with a 5°C rise. What does this mean for humans?
Terrestrial vertebrates are more in danger than many other species, since they depend on numerous other species for food. Humans are terrestrial vertebrates and humans are also large warm-blooded mammals with high metabolic rates, thus requiring more habitat. It also takes a long time for humans to reach maturity. Additionally, humans have become addicted to processed food, fossil fuels, plastic, etc. Furthermore, humans require large amounts of fresh water, including for sweating when temperatures rise. A 3°C rise may therefore suffice to cause humans to disappear, as illustrated by the image below.
• Greenhouse gas rising
• Permafrost and wildfire carbon emissions indicate need for additional action to keep Paris Agreement temperature goals within reach - by Christina Schädel et al.
• The climate change runaway chain reaction-like process - by Andrew Glikson
Amplifying feedbacks leading to accelerated planetary temperatures also discussed on facebook at:
https://www.facebook.com/SamCarana/posts/10165533175460161• Pre-industrial
https://arctic-news.blogspot.com/p/pre-industrial.html
• When Will We Die?
https://arctic-news.blogspot.com/2019/06/when-will-we-die.html
• Extinction
https://arctic-news.blogspot.com/p/extinction.html
• The threat of seafloor methane eruptions
https://arctic-news.blogspot.com/2025/11/the-threat-of-seafloor-methane-eruptions.html
• Feedbacks in the Arctic
https://arctic-news.blogspot.com/p/feedbacks.html
• Water Vapor Feedback
https://arctic-news.blogspot.com/2024/09/water-vapor-feedback.html
• Jet Stream
https://arctic-news.blogspot.com/p/jet-stream.html• Endangerment Finding in danger?
https://arctic-news.blogspot.com/2025/04/endangerment-finding-in-danger.html
• Transforming Society
https://arctic-news.blogspot.com/2022/10/transforming-society.html
• Climate Plan
https://arctic-news.blogspot.com/p/climateplan.html
• Climate Emergency Declaration
https://arctic-news.blogspot.com/p/climate-emergency-declaration.html
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