A
new study concludes that the 1.5°C threshold was already passed years ago and that climate scientist got the global warming timelines seriously wrong.
Here is the summary of the paper.
Key Findings
Earlier Onset of Warming: Using 300 years of temperature data preserved in Caribbean sclerosponge skeletons, the study finds that industrial-era global warming began in the mid-1860s—over 80 years earlier than indicated by instrumental sea surface temperature (SST) records.
Pre-Industrial Baseline Redefined: The pre-industrial period is defined by stable ocean mixed-layer (OML) temperatures (<±0.1 °C) from 1700 to the early 1860s, with a notable cooling in the early 1800s due to major volcanic eruptions.
Magnitude of Warming: By 2020, global mean surface temperatures (GMST) were already 1.7 ± 0.1 °C above the pre-industrial baseline, which is 0.5 °C higher than the most recent IPCC estimates.
Land vs. Ocean Warming: Land temperatures have risen faster than ocean temperatures, especially since the late 20th century.
By 2020, land areas had warmed to nearly 2 °C above pre-industrial levels, while the oceans lagged behind due to their higher heat capacity.
Implications for Climate Targets:
The study projects that the 2 °C global warming threshold will be reached by the late 2020s—almost two decades earlier than current mainstream projections. This suggests that the Paris Agreement’s targets are now much more challenging to achieve.
Methodology
Sclerosponge Thermometry: The researchers analyzed the ratio of strontium to calcium (Sr/Ca) in the skeletons of long-lived Caribbean sclerosponges (Ceratoporella nicholsoni), which reliably records past OML temperatures.
Calibration: The Sr/Ca thermometer was calibrated against modern (post-1963) instrumental SST records, achieving a high correlation (R² = 0.91).
Temporal Coverage: The sclerosponge record extends back to 1700, providing a continuous and well-dated archive of mixed-layer ocean temperatures, including periods not covered by instrumental records.
Significance and Context
Reconciliation of Records: The study helps resolve inconsistencies between land and ocean temperature records and questions the adequacy of the commonly used 1850–1900 reference period for defining "pre-industrial" climate.
Policy Implications: The findings imply that the world has already surpassed the 1.5 °C warming threshold targeted by the Paris Agreement, and that the window for limiting warming to 2 °C is closing rapidly.
Call for Urgency: The authors emphasize the urgent need to halve emissions by 2030, as current rates of warming—especially on land—make climate targets increasingly difficult to meet.
Expert Commentary
The study’s novel use of sclerosponge proxies provides a more sensitive and longer-term perspective on global warming, but experts note the need for further validation of the temperature sensitivity of this proxy across a broader range of conditions.
