Global Temperatures Surpass 1.5°C Above Pre-Industrial Levels
The 2024 State of the Climate Report by the World Meteorological Organization (WMO) confirmed that the global average annual temperature in 2024 was 1.55°C higher than the reference period of 1850-1900. This marks a significant milestone, as the global temperature briefly exceeded 1.5°C for at least one month in 2016, 2017, 2019, 2020, and 2023. However, 2024 was the warmest year on record in the 175 years of global temperature monitoring.
Ambiguities in the Paris Agreement
The Paris Agreement, adopted during the 2015 UN Climate Change Conference (COP21), more than a decade ago, sets the target to “keep the global temperature increase well below 2°C and to pursue efforts to limit it even further to 1.5°C, recognizing that this would significantly reduce the risks and impacts of climate change.”
This agreement is a political-diplomatic commitment between countries that have ratified the United Nations Framework Convention on Climate Change (UNFCCC). However, its wording contains ambiguities and raises two fundamental questions:
- What does “relative to pre-industrial levels” mean? This period defines the baseline temperature reference for measuring global warming without ambiguity.
- When will the temperature increase be considered to have surpassed 1.5°C? In other words, for how long must the temperature exceed this threshold?
Understanding “Pre-Industrial Levels”
The pre-industrial level refers to the global average temperature before the Industrial Revolution, which serves as a benchmark for assessing the effects of global warming. The UNFCCC tasked the Intergovernmental Panel on Climate Change (IPCC) with defining what constitutes pre-industrial levels, resulting in the Special Report on Global Warming of 1.5°C.
The chosen period was from 1850 to 1900, a span of fifty years. This selection is due to the availability of high-quality temperature observations for both land and ocean surfaces, allowing for precise comparisons. During this time, global air temperatures were relatively stable, with an approximate average of 13.84°C.
With human influence on climate still relatively small before the widespread and intensive use of fossil fuels, this period was deemed a suitable reference to evaluate human activities’ impact on climate.
This period extends the classic 30-year period defined by the World Meteorological Organization (WMO), which is based on standard climatological norms established by the WMO in 1872.
Sustained Temperature Increase Over Two Decades
Despite the global temperature exceeding 1.5°C, we cannot definitively say that we have surpassed the threshold set by the Paris Agreement.
Global temperatures do not increase gradually. We must differentiate between natural climate variability—caused by phenomena like El Niño and volcanic activity—and the variability that defines its long-term trend, which is the ongoing warming process. The climate system fluctuates across different time scales with short- and long-term temporal frequencies.
The 1.5°C figure refers to an average global warming trend, not a single-year anomaly that could be exceptionally warmer or cooler than the long-term average. But for how long must this temperature increase persist to be considered a trend?
The Second Periodic Review of the UNFCCC’s long-term global goal clarified that “the target is assessed over decades” (COP22 in Sharm el-Sheikh, Egypt, in 2022). The IPCC, in its report on the physical basis of climate change, defines the moment of surpassing as “the midpoint of the first 20-year period during which the global average surface air temperature consistently exceeds the threshold temperature.” Using this average helps ensure that warming trends are attributed to human intervention rather than natural variations.
Therefore, it will take 20 years to confirm if maintaining the 1.5°C threshold as a trend is indeed the case. Humanity will only know with certainty if we have reached the Paris Agreement’s limit in hindsight, which poses the risk of delaying recognition and the corresponding reaction.
Consensus is Crucial
Global warming is accelerating. The 2021 IPCC report indicated, in almost all emission scenarios, that the 1.5°C threshold “could be reached as early as the 2030s.” The recent WMO Annual Global Climate Update Decadal 2025-2029 suggests that it is likely (with a 70% probability) that the average quinquennial temperature from 2025-2029 will surpass 1.5°C.
Determining when we enter a 20-year period with an average warming of 1.5°C is not merely a temperature tracking exercise; it’s crucial for climate risk management and adaptation planning.
Evaluating the global average temperature increase using the average warming of recent decades will postpone formal recognition of when Earth surpasses the 1.5°C limit. This could lead to distractions and delays when urgent climate action is most needed.
A single, agreed-upon metric describing the surpassing of the 1.5°C threshold—there are already alternatives—should be defined and agreed upon, anticipating the events leading up to it. The consequences will be severe. While this increase may seem distant, available observations suggest we might reach it sooner than anticipated.
The occurrence of the first year with a 1.5°C warming would imply that the period reaching the lower bound of the Paris Agreement’s target has already begun, and the expected impacts at this warming level are already manifesting.
