Sea ice loss, melting glaciers, accelerated sea level rise, widespread wildfires, and longer, more intense heat waves. These are only a handful of consequences of climate change that are currently unfolding across the globe.
Although scientists unequivocally agree that climate change is real, there are still many myths and a lot of confusion around the subject. In this series of explainers, we will try to answer some of the most fundamental questions about climate change, the science behind it, and its impact. In the fourth instalment (you can scroll down to the end of this article for the first three parts), we try to answer the question: ‘What is the 1.5 degree Celsius threshold, and what happens when we breach it?’
What is the 1.5 degree Celsius limit?
In 2015, amid rising concerns over climate change impacts, the Paris Agreement was signed by 195 countries, which pledged to limit the rise in Earth’s temperature to “well below 2 degree Celsius above pre-industrial levels” by the end of the century, and going further, aim to “limit the temperature increase to 1.5 degree Celsius above pre-industrial levels.”
Simply put, by 2100, the world’s average surface temperature will have risen to not more than 1.5 degree Celsius warmer than pre-industrial levels.
The Paris Agreement didn’t specify what these pre-industrial levels were. Scientists, however, generally consider the years from 1850 to 1900 as a baseline. This is because it’s the earliest period with reliable, near-global measurements. Notably, some global warming had already taken place by that point due to human activity — the Industrial Revolution kicked off in the early 1700s. Nonetheless, it is crucial to have a reliable baseline with good historical data to measure the rising temperatures today.
Yearly surface temperature compared to the 20th-century average from 1880–2022. Blue bars indicate cooler-than-average years; red bars show warmer-than-average years. NOAA Climate.gov graph, based on data from the National Centers for Environmental Information.
Why 1.5 degree Celsius?
The 1.5 degree Celsius limit in the Paris Agreement was based “on a fact-finding report which concluded that, even global warming of 1.5 degree Celsius above the preindustrial average, over an extended, decades-long period, would lead to high risks for ‘some regions and vulnerable ecosystems,’” according to a report by the MIT News.
Therefore, it was decided to set the 1.5 degree Celsius limit as a “defence line”. It would ensure that the world avoids extreme and irreversible impacts of climate change that would unfold once global temperatures become 2 degree Celsius warmer than pre-industrial levels.
But it isn’t as if 1.5 degree Celsius is some magical number. Sergey Paltsev, deputy director of MIT’s Joint Program on the Science and Policy of Global Change, told MIT News: “There is nothing magical about the 1.5 number, other than that is an agreed aspirational target. Keeping at 1.4 is better than 1.5, and 1.3 is better than 1.4, and so on”.
He added, “The science does not tell us that if, for example, the temperature increase is 1.51 degrees Celsius, then it would definitely be the end of the world. Similarly, if the temperature would stay at 1.49 degrees increase, it does not mean that we will eliminate all impacts of climate change. What is known: The lower the target for an increase in temperature, the lower the risks of climate impacts.”
What happens when we breach the threshold?
The breach of the 1.5 degree Celsius limit, would lead to an increase in frequency, intensity and/or amount of heavy precipitation in many regions, and an increase in intensity or frequency of droughts in some regions, according to the Intergovernmental Panel on Climate Change (IPCC).
Moreover, higher global temperatures would make oceans warmer, resulting in a higher number of strong hurricanes with the possibility that they would quickly gain strength as they approach coastlines. Wildfires will also become more intense and last for a longer period. Sea ice melt will rapidly increase, which will contribute to sea level rise.
Most of these consequences have already started to emerge in recent years. Once the threshold is breached, they are only going to get worse.
How close are we to breaching the threshold?
In its 2023 State of Global Climate report, the World Meteorological Organisation (WMO) said there was a 66% chance that at least one of the years between 2023 and 2027 would cross the threshold. It means that one of these years would surpass the 1.5 degree Celsius warming limit across the entire calendar year for the first time.
2023 has already been declared the hottest year on record. It was 1.48 degree Celsius warmer than the average of the pre-industrial levels.
But haven’t we already breached the 1.5 degree Celsius limit?
According to the latest data by Europe’s Copernicus Climate Change Service (C3S), about 50% of days were more than 1.5 degree Celsius warmer than the pre-industrial levels — and two days in November were, for the first time, more than 2 degree Celsius warmer.
Daily global surface air temperature increase relative to the average for 1850–1900, the designated pre-industrial reference period, for 2023. The plot highlights temperature increases within three ranges: 1–1.5°C (orange), 1.5–2°C (red), and above 2°C (crimson). Source: ERA5. Credit: C3S/ECMWF
This, however, does not mean that the planet has breached the 1.5 degree and 2 degree Celsius thresholds. Those limits refer to long-term warming — which means global temperatures over a period of 20-30 years, on average, must not exceed 1.5 degree or 2 degree Celsius.
Here are the previous parts of the series: part 1, part 2, and part 3.
View of the snowless Tosa Maidan, in Budgam district, Sunday, Jan. 21, 2024. The forests of Jammu and Kashmir have been declared highly vulnerable to the effects of unpredictable weather patterns, early springs and above average temperatures which experts have attributed to climate change and global warming. (PTI Photo/S Irfan) 
Yearly surface temperature compared to the 20th-century average from 1880–2022. Blue bars indicate cooler-than-average years; red bars show warmer-than-average years. NOAA Climate.gov graph, based on data from the National Centers for Environmental Information.
Daily global surface air temperature increase relative to the average for 1850–1900, the designated pre-industrial reference period, for 2023. The plot highlights temperature increases within three ranges: 1–1.5°C (orange), 1.5–2°C (red), and above 2°C (crimson). Source: ERA5. Credit: C3S/ECMWF
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