Article
Will Climate Change Become Unbearable When the World Reaches 1.5°C or 2°C?
In 2015, almost all countries agreed to try to limit warming to 1.5°C to 2°C above the pre-industrial average. However, the world is already 1.3°C warmer and current policies put it on track for more than 2°C of warming by the end of the century. What will warming beyond 1.5°C to 2°C mean for the Earth system, vulnerable communities, and ecosystems?
Key takeaways
- There is no sharp, global threshold below which climate risks are manageable and beyond which they are unbearable.
- Each incremental increase in temperature raises risks to ecosystems and human society while increasing the chances of fundamental changes to the Earth system.
- The point at which climate risks become unbearable is subjective, uncertain, and will vary substantially from region to region.
In January 2024, the global average temperature over a 12-month period was more than 1.5°C higher than the pre-industrial average for the first time. The record-shattering heat was accompanied by extreme weather around the world.
For example, temperatures exceeded 49°C in North Africa for the first time, and Autumn temperatures reached 39°C in France and 33°C in the United Kingdom.1 High temperatures fuelled wildfires that raged in Canada, burning an area more than double the previous record.2
Change in global annual average temperature relative to pre-industrial average,1850-1900
+1.50°C
1.25
1.00
0.75
0.50
0.25
0
1950
1975
2000
2023
Sources: JRA-3Q; ERA5
Storms killed thousands and displaced millions in 2023. Tropical Cyclone Freddy killed over 1,000 people in Malawi and displaced 1.4 million people across Southern Africa. Hurricane Otis, which killed dozens of people in Acapulco, Mexico, was the strongest Eastern Pacific hurricane to make landfall in decades.
This 12-month period of average temperatures exceeding 1.5°C comes just 8 years after the Paris Agreement set the goal of limiting long-term average global warming to well below 2°C, with an aim for 1.5°C. Global warming is typically measured as an average over decades, so the 1.5°C warming level has not been exceeded yet. However, surpassing 1.5°C now seems unavoidable,3 and temperatures are expected to reach about 2.5°C by the end of the century even if countries meet their 2030 emissions pledges.
What is the significance of the agreed-upon warming targets? What would crossing 1.5ºC or 2ºC mean in terms of climate change impacts?
Rising temperatures affect human health, agriculture, and ecosystems
Researchers have found that the increase in global temperature is roughly proportional to the cumulative amount of carbon dioxide emitted. So if twice as much carbon dioxide is emitted, the planet will warm about twice as much.4
As carbon dioxide emissions accumulate and temperatures rise, climate change impacts will worsen. Some physical climate hazards increase roughly linearly as temperatures rise. For example, if the world warms twice as much, the increase in the intensity of heatwaves, extreme rain, and drought are expected to be about twice as great.
While many physical aspects of climate change rise roughly linearly with rising temperatures, the impacts they have on human and ecological systems can rise much more dramatically. This is because there are thresholds in the response of many such systems beyond which changes become unbearable.
Humans have evolved to maintain an internal temperature of about 37°C, and sweating plays an important role in this process. However, when the air is very humid, this cooling mechanism breaks down because sweat does not evaporate as effectively.
As temperatures rise and humidity increases, people – especially those living in urban areas near the equator – will be increasingly exposed to dangerously hot and humid conditions.5 This will limit the ability to perform work safely outdoors, and if warming continues, heatwaves that become too extreme to survive unprotected will begin occurring in some places.6
Agriculture is another area affected by rising temperatures, weather extremes, and other aspects of climate change. Certain crop yields, such as maize, are projected to decrease significantly due to temperature increases and other changes in the climate.7 Limiting warming to 1.5°C could help to mitigate the impacts of rising temperatures on crops and bolster food security.8
The coral in the foreground has undergone bleaching while the one in the background has not.
Corals are animals that form reefs, providing food and habitat to a quarter of the world’s fish. When temperatures rise too high, the corals expel the algae living in their tissues, leading to bleaching. While bleaching is not always deadly to coral, it makes coral more vulnerable to other stresses, such as ocean acidification.
In 2024, the Great Barrier Reef suffered a record-breaking mass bleaching event, where almost half the reefs experienced record levels of heat stress and extreme bleaching was observed in all three of its regions for the first time. Coral reefs are expected to decline by 70 to 90% as warming exceeds 1.5°C, and become virtually non-existent above 2°C of warming.
Icebergs in the Lemaire Channel, West Antarctica. (Photo: Christopher Michel).
Climate tipping points
A climate “tipping point” is a threshold in part of the Earth system beyond which changes can be abrupt, significant, self-perpetuating, and potentially irreversible. As the global temperature rises, the number of Earth system elements at risk of crossing a tipping point increases, and some may even occur at around 1.5°C or less of warming.9
The Greenland and West Antarctic ice sheets are important parts of the Earth system that are vulnerable to global warming. They may both be close to thresholds of stability and could begin to collapse at warming levels between 1.5°C and 2°C.9 The collapse of these ice sheets would contribute metres to sea level rise over the course of the coming centuries, threatening coastal communities worldwide.10
There are large uncertainties in the temperature ranges where various tipping points may occur.9 For example, an ocean current called the Atlantic Meridional Overturning Circulation – or AMOC – transports heat away from the equatorial Atlantic and towards Northern Europe, regulating temperatures and playing an important role in global rainfall patterns. Models suggest AMOC may break down at 4°C of warming, though some projections suggest it could begin much sooner, even below 2°C of warming.9
Will climate change become unbearable past 1.5°C or 2°C?
There are no unambiguous thresholds in global temperature beyond which climate risks would become unbearable globally. Rather, the world faces a steepening slope of risk as temperatures rise. Every tenth of a degree of warming pushes ice sheets, rainforests, coral reefs, and more, farther from stable past conditions and increases the risk of them undergoing profound and potentially irreversible changes.
The impacts of these changes will not be felt evenly, however. For example, the loss of tropical coral reefs or the collapse of a local agricultural system could have devastating effects on the communities that depend on them but would have only indirect impacts on people in other parts of the world.
If at some point policymakers in a particular region judge the impacts of climate change to have become unbearable, they may turn to sunlight reflection methods, also known as solar radiation modification (SRM), as the only potential means of rapidly halting or reversing warming. However, these ideas come with risks and side effects of their own, and developing and deploying them will raise a host of sociopolitical challenges.
Open questions
- How effective could SRM be at preventing various climate tipping points? Are there some which might be worsened?
- How will the world react to certain regions experiencing increasingly unbearable climate risks?
- Could a powerful nation experiencing unbearable climate risks deploy SRM unilaterally?
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Endnotes
- Perkins-Kirkpatrick S, Barriopedro D, Jha R, et al. (2024). Extreme terrestrial heat in 2023. Nature Reviews Earth and Environment; 5(4):244-6. https://doi.org/10.1038/s43017-024-00536-y
- Jain P, Barber QE, Taylor SW, et al. (2024) Drivers and Impacts of the Record-Breaking 2023 Wildfire Season in Canada. Nature Communications. 15(1):6764 https://doi.org/10.1038/s41467-024-51154-7
- Peters GP. (2024). Is limiting the temperature increase to 1.5° C still possible? Dialogues on Climate Change. 29768659241293218. https://doi.org/10.1177/29768659241293218
- Leduc M, Matthews HD, de Elía R. (2016). Regional estimates of the transient climate response to cumulative CO2 emissions. Nature Climate Change. 6(5):474-8. https://doi.org/10.1038/nclimate2913
- Yang J, Zhao L, Oleson K. (2023). Large humidity effects on urban heat exposure and cooling challenges under climate change. Environmental Research Letters; 18(4):044024. https://doi.org/10.1088/1748-9326/acc475
- Vanos J, Guzman-Echavarria G, Baldwin JW, et al. (2023). A physiological approach for assessing human survivability and liveability to heat in a changing climate. Nature communications. 14(1):7653. https://doi.org/10.1038/s41467-023-43121-5
- Hasegawa T, Wakatsuki H, Ju H, et al. (2022). A global dataset for the projected impacts of climate change on four major crops. Scientific data. 9(1):58 https://doi.org/10.1038/s41597-022-01150-7
- IPCC. (2022). Impacts of 1.5°C Global Warming on Natural and Human Systems. In: Global Warming of 1.5°C. Cambridge University Press, pp. 175–312. https://doi.org/10.1017/9781009157940.005
- McKay DIA, Staal A, Abrams JF, et al. (2022). Exceeding 1.5°C global warming could trigger multiple climate tipping points. Science. 377(6611):eabn7950. https://doi.org/10.1126/science.abn7950
- Golledge NR. (2020). Long‐term projections of sea‐level rise from ice sheets. Wiley Interdisciplinary Reviews: Climate Change. 11(2):e634. https://doi.org/10.1002/wcc.634
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