| Title: |
How well can we quantify when 1.5 °C of global warming has been exceeded? |
| Authors: |
Thorne, Peter W.; Nicklas, John M.; Kennedy, John J.; Calvert, Bruce; Fox-Kemper, Baylor; Richardson, Mark T.; Simmons, Adrian; Hawkins, Ed; Rhode, Robert; Cowtan, Kathryn; Abram, Nerilie J.; Andersson, Axel; Noone, Simon; Marbaix, Phillipe; Lenssen, Nathan; Olonscheck, Dirk; Walsh, Tristram; Outten, Stephen; Bethke, Ingo; Samset, Bjorn H.; Smith, Chris; Pirani, Anna; Fuglestvedt, Jan; Rajamani, Lavanya; Betts, Richard A.; Kent, Elizabeth C.; Trewin, Blair; Morice, Colin; Osborn, Tim; Burgess, Samantha N.; Geden, Oliver; Parnell, Andrew; Forster, Piers M.; Hewitt, Chris; Hausfather, Zeke; Masson-Delmotte, Valerie; Marotzke, Jochem; Gillett, Nathan; Seneviratne, Sonia I.; Schmidt, Gavin A.; Chan, Duo; Brönnimann, Stefan; Reisinger, Andy; Menne, Matthew; Rojas Corradi, Maisa; Kadow, Christopher; Huybers, Peter; Stephenson, David B.; Wallis, Emily; Rogelj, Joeri; Schurer, Andrew; McKinnon, Karen; Zhai, Panmao; Driouech, Fatima; Moufouma Okia, Wilfran; Vazifehkhah, Saeed; Szopa, Sophie; Merchant, Christopher J.; Hirahara, Shoji; Ishii, Masayoshi; Engelbrecht, Francois A.; Li, Qingxiang; Lee, June-Yi; Cannon, Alex J.; Cassou, Christophe; Schuckmann, Karina; Delju, Amir H.; Murtagh, Ellie |
| Source: |
eISSN: 1866-3516 |
| Publication Year: |
2026 |
| Collection: |
Copernicus Publications: E-Journals |
| Description: |
Parties to the 2015 Paris Agreement agreed to limit the long-term increase in global average temperature to well below 2 °C and pursue efforts to keep temperatures below 1.5 °C relative to pre-industrial levels. As the world is fast approaching the 1.5 °C warming level on a sustained basis, and with 2024 likely the first year that was over 1.5 °C warmer than 1850-1900, there is ever increasing interest in how we will know whether and when 1.5 °C warming since pre-industrial has been reached or exceeded with respect to a long-term average. This paper represents a comprehensive community methodological overview, building on the IPCC 6th assessment. It explains why there is no straightforward answer and proposes clear and reasoned ways forward. Existing challenges are as follows. Firstly, the Paris Agreement text contains definitional ambiguities around 'pre-industrial', 'global average temperature', whether the assessment should be on realised or long-term human-induced warming, and over what time frame the long-term temperature goal applies. Then, there are intrinsic limitations of observational records which get more uncertain further back in time due to data sparsity and measurement heterogeneity. Finally, in a non-stationary climate, multidecadal mean indicators of global temperature change will either lag behind the change or must rely on expected future temperature changes (based on extrapolation, initialized predictions, or scenario-based and constrained projections). Our analysis shows that knowing 'whether we are there yet' is a multifaceted and inherently probabilistic problem that includes information on the definition of a specific level of global warming, temperature changes over multiple timescales, and also potentially includes unpacking the attribution of human-caused changes from observed variations. Given the policy relevance of understanding where the world stands relative to 1.5 °C, or any other level of global warming since pre-industrial, there ... |
| Document Type: |
text |
| File Description: |
application/pdf |
| Language: |
English |
| DOI: |
10.5194/essd-2025-825 |
| Availability: |
https://doi.org/10.5194/essd-2025-825; https://essd.copernicus.org/preprints/essd-2025-825/ |
| Accession Number: |
edsbas.A7F77D18 |
| Database: |
BASE |