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Peer-reviewedCC BY 4.0Environmental Research Letters

An assessment of Northern Hemisphere warm spell trends across seasons: spring warm spells increasing faster than projected

  • Lualawi Mareshet Admasu1
  • Rachel H White1
  • Anaïs Orsi1
  1. 1University of British Columbia
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Abstract

Consecutive days of anomalously warm temperatures (warm spells) can severely impact human health and ecosystems, whatever time of year they occur. Previous research has typically focused only on warm-season warm spells—heatwaves; here, we assess trends in warm spells in all seasons across Northern Hemisphere midlatitude land using observational data (Berkeley Earth and ERA5) and CMIP6 climate models. Observational data show warm spell days increasing in all seasons, with mid-latitude land averages ranging from 0.8 d per decade in winter to 1.5 d per decade in summer over the last 70 years, with spring trends reaching 1.3 d per decade. CMIP6 models underestimate the observed increase in mid-latitude averaged spring warm spell days, with a multi-model mean value of 0.7 d per decade, and observational trends falling outside the CMIP6 model spread (model spread from 0.24 to 1.11 d per decade). CMIP6 models also overestimate fall warm spell trends, particularly with respect to the annual average trends, reporting trends as high as 1.17 times the annual average while observations report 0.85 times the annual average. The seasonality of the trends and associated model biases is strongly controlled by seasonal mean temperature trends. CMIP6 biases in seasonal temperature trends are partially linked to biases in surface albedo and soil moisture changes. These results have important implications for the accuracy of future projections of warm spells, particularly during the spring season.

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Cite

BibTeXRIS
BibTeX
@article{admasu2026assessment,
  title = {{An assessment of Northern Hemisphere warm spell trends across seasons: spring warm spells increasing faster than projected}},
  author = {Lualawi Mareshet Admasu and Rachel H White and Anaïs Orsi},
  journal = {Environmental Research Letters},
  year = {2026},
  month = oct,
  doi = {10.1088/1748-9326/aeb436},
  url = {https://iopscience.iop.org/article/10.1088/1748-9326/aeb436},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - An assessment of Northern Hemisphere warm spell trends across seasons: spring warm spells increasing faster than projected
AU  - Lualawi Mareshet Admasu
AU  - Rachel H White
AU  - Anaïs Orsi
PY  - 2026
DA  - 2026/10/22
JO  - Environmental Research Letters
DO  - 10.1088/1748-9326/aeb436
UR  - https://iopscience.iop.org/article/10.1088/1748-9326/aeb436
AB  - Consecutive days of anomalously warm temperatures (warm spells) can severely impact human health and ecosystems, whatever time of year they occur. Previous research has typically focused only on warm-season warm spells—heatwaves; here, we assess trends in warm spells in all seasons across Northern Hemisphere midlatitude land using observational data (Berkeley Earth and ERA5) and CMIP6 climate models. Observational data show warm spell days increasing in all seasons, with mid-latitude land averages ranging from 0.8 d per decade in winter to 1.5 d per decade in summer over the last 70 years, with spring trends reaching 1.3 d per decade. CMIP6 models underestimate the observed increase in mid-latitude averaged spring warm spell days, with a multi-model mean value of 0.7 d per decade, and observational trends falling outside the CMIP6 model spread (model spread from 0.24 to 1.11 d per decade). CMIP6 models also overestimate fall warm spell trends, particularly with respect to the annual average trends, reporting trends as high as 1.17 times the annual average while observations report 0.85 times the annual average. The seasonality of the trends and associated model biases is strongly controlled by seasonal mean temperature trends. CMIP6 biases in seasonal temperature trends are partially linked to biases in surface albedo and soil moisture changes. These results have important implications for the accuracy of future projections of warm spells, particularly during the spring season.
SN  - 1748-9326
ER  -