The 2015 Paris Agreement sets out basic science-based safeguards for global climate action with the aim of “holding the increase in the global average temperature to well below 2°C above pre-industrial levels.” levels and to pursue efforts to limit the temperature increase to 1.5°C”. These goals were reaffirmed in the Glasgow Climate Pact in 2021, which recognised that the impacts of climate change would be significantly lower at 1.5°C compared to 2°C.

The quantification of global warming is internationally accepted as the change in global surface temperature (GST) compared to the mean GST for the period 1850–1900, which serves as a pre-industrial reference period given the availability of historical data. The Intergovernmental Panel on Climate Change (IPCC) uses physical science reports as the authoritative source of information on the state of GST change. The IPCC uses two primary metrics to quantify warming: global mean surface temperature (GMST) and global surface air temperature (GSAT). GSAT is the metric chosen by the IPCC to quantify the years of threshold exceedances, such as the 1.5°C level.

New benchmark time series 1850–2024 and projection to 2034, called the “ClimTrace GST record”, presents updated data and projections for this key metric. This record shows a clear exceedance of 1.5°C by the annual average in 2024, with a value of 1.62 [1.55–1.69] °CThe 20-year average used by the IPCC to monitor threshold exceedances remained below 1.5°C in 2024 (1.39 [1.29–1.49]°C). However, this threshold is projected to be exceeded in 2028 [2025–2032]This immediacy underscores the importance of better quantifying the Paris goals and taking effective action in the near future.

To improve tracking and quantification of the Paris goals, the study proposes a simple four-class definition for assessing compliance or exceedance of targets based on global warming levels (GWL), defined as 20-year averages of GSAT. The classes are:

1. T1.5C (Target-1.5 °C): GWL ≤ 1.5 °C (compliance with the Paris effort target).
2. WB2C (Well-below-2 °C): 1.5 °C < GWL ≤ 1.7 °C (compliance with the Paris limit target of "well below 2°C").
3. RB2C (Risk-below-2 °C): 1.7 °C < GWL ≤ 2.0 °C (exceeding the Paris target, risky below 2°C).
4. EX2C (Exceedance-2 °C): GWL > 2.0 °C (exceeding 2 °C).

The WB2C threshold of 1.7°C was chosen to reflect IPCC projections and to be consistent with the threshold used in AR6 for reporting remaining carbon budgets.

Based on GSAT results from ClimTrace, this quantification approach shows adherence to the T1.5C target from the current year 2024 (CGWL 1.39 ± 0.10 °C) to 2027. Subsequently Compliance with the WB2C target applies from 2028 to 2035 (CYGWL 2028 1.50 ± 0.10 °C). Exceeding the WB2C class (and thus the first full violation of the T1.5C and WB2C targets) occurs in 2036 if warming over the next 10 years follows the SSP2-4.5 scenario (SSP2-4.5 CYGWL 2036 1.71 ± 0.10 °C). The SSP2-4.5 scenario reaches the EX2C class around 2047 (SSP2-4.5 CYGWL 2047 2.00 ± 0.10 °C).

Standardization of this approach, including consolidation of the GST record and class definitions, is proposed in the context of the IPCC to provide a standardized assessment of compliance with the Paris Agreement worldwide, including for the UN global stocktake process. The ClimTrace GST record and associated time series are publicly available to assist in this standardization and to incentivize climate action across domains. Spring

The article is published in the journal Communications Earth & Environment.

Glossary of key terms

• Global Surface Temperature (GST): A collective term for global mean surface temperature (GMST) and global surface air temperature (GSAT), which is used to quantify global warming.
• Global Mean Surface Temperature (GMST): A synthetic combination of near-surface air temperature over land and sea surface temperature over oceans.
• Global Surface Air Temperature (GSAT): The global average of near-surface air temperature over both land and oceans.
• Pre-industrial level: A reference period, usually 1850–1900, used to measure global temperature change.
• Paris Agreement: An international treaty that aims to hold the increase in global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the increase to 1.5°C.
• 1.5°C target: The more ambitious goal of the Paris Agreement to limit global warming to 1.5°C above pre-industrial levels.
• Well below 2°C target: The main limit on global warming set by the Paris Agreement.
• Intergovernmental Panel on Climate Change (IPCC): The leading international body for assessing climate change. Its reports provide scientific information for policy decisions.
• ClimTrace GST record: New reference time series of global surface temperature (GMST and GSAT) from 1850 to the present with predictions and projections.
• 20-year moving average: An average temperature calculated over a 20-year period, which helps smooth out short-term fluctuations and identify long-term warming trends.
• Scale factor GMST to GSAT (fGMST2GSAT): Factor used to relate the change in GMST and GSAT.
• Shared Socioeconomic Pathways (SSP): Scenarios of future socio-economic development that are used to model potential future greenhouse gas emissions and associated climate change.
• Current Global Warming Level (CGWL): The estimated level of global warming for the current year, often quantified using a 20-year average.
• Projected Global Warming Level (PGWL): Projected level of global warming for future years.
• Class T1.5C (Paris effort goal Target-1.5 °C): Global warming level ≤ 1.5°C.
• Class WB2C (Paris limit goal Well-below-2 °C): Global warming level between 1.5°C and 1.7°C.
• Class RB2C (Paris excess Risky-below-2 °C): Global warming level between 1.7°C and 2.0°C.
• Class EX2C (Paris excess Exceedance-2 °C): Global warming level > 2.0°C.
• Uncertainty: The range of values within which the true value is expected to lie, quantified, for example, as the 90% confidence interval (90% CI).