On the climate sensitivity and historical warming evolution in recent coupled model ensembles
<p>The Earth's equilibrium climate sensitivity (ECS) to a doubling of atmospheric <span class="inline-formula">CO<sub>2</sub></span>, along with the transient climate response (TCR) and greenhouse gas emissions pathways, determines the amount of future wa...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/20/7829/2020/acp-20-7829-2020.pdf |
| _version_ | 1819227050672652288 |
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| author | C. M. Flynn T. Mauritsen |
| author_facet | C. M. Flynn T. Mauritsen |
| author_sort | C. M. Flynn |
| collection | DOAJ |
| description | <p>The Earth's equilibrium climate sensitivity (ECS) to a doubling of atmospheric <span class="inline-formula">CO<sub>2</sub></span>, along with the transient climate response (TCR) and
greenhouse gas emissions pathways, determines the amount of future warming. Coupled climate models have in the past been important tools to estimate
and understand ECS. ECS estimated from Coupled Model Intercomparison Project Phase 5 (CMIP5) models lies between 2.0 and 4.7 <span class="inline-formula">K</span> (mean of
3.2 <span class="inline-formula">K</span>), whereas in the latest CMIP6 the spread has increased to 1.8–5.5 <span class="inline-formula">K</span> (mean of 3.7 <span class="inline-formula">K</span>), with 5 out of 25 models exceeding
5 <span class="inline-formula">K</span>. It is thus pertinent to understand the causes underlying this shift. Here we compare the CMIP5 and CMIP6 model ensembles and find
a systematic shift between CMIP eras to be unexplained as a process of random sampling from modeled forcing and feedback distributions. Instead,
shortwave feedbacks shift towards more positive values, in particular over the Southern Ocean, driving the shift towards larger ECS values in many
of the models. These results suggest that changes in model treatment of mixed-phase cloud processes and changes to Antarctic sea ice representation
are likely causes of the shift towards larger ECS. Somewhat surprisingly, CMIP6 models exhibit less historical warming than CMIP5 models, despite an
increase in TCR between CMIP eras (mean TCR increased from 1.7 to 1.9 <span class="inline-formula">K</span>). The evolution of the warming suggests, however, that several of
the CMIP6 models apply too strong aerosol cooling, resulting in too weak mid-20th century warming compared to the instrumental
record.</p> |
| first_indexed | 2024-12-23T10:35:13Z |
| format | Article |
| id | doaj.art-f8df15dddb74434a83137b7abd878971 |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-12-23T10:35:13Z |
| publishDate | 2020-07-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-f8df15dddb74434a83137b7abd8789712022-12-21T17:50:19ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242020-07-01207829784210.5194/acp-20-7829-2020On the climate sensitivity and historical warming evolution in recent coupled model ensemblesC. M. FlynnT. Mauritsen<p>The Earth's equilibrium climate sensitivity (ECS) to a doubling of atmospheric <span class="inline-formula">CO<sub>2</sub></span>, along with the transient climate response (TCR) and greenhouse gas emissions pathways, determines the amount of future warming. Coupled climate models have in the past been important tools to estimate and understand ECS. ECS estimated from Coupled Model Intercomparison Project Phase 5 (CMIP5) models lies between 2.0 and 4.7 <span class="inline-formula">K</span> (mean of 3.2 <span class="inline-formula">K</span>), whereas in the latest CMIP6 the spread has increased to 1.8–5.5 <span class="inline-formula">K</span> (mean of 3.7 <span class="inline-formula">K</span>), with 5 out of 25 models exceeding 5 <span class="inline-formula">K</span>. It is thus pertinent to understand the causes underlying this shift. Here we compare the CMIP5 and CMIP6 model ensembles and find a systematic shift between CMIP eras to be unexplained as a process of random sampling from modeled forcing and feedback distributions. Instead, shortwave feedbacks shift towards more positive values, in particular over the Southern Ocean, driving the shift towards larger ECS values in many of the models. These results suggest that changes in model treatment of mixed-phase cloud processes and changes to Antarctic sea ice representation are likely causes of the shift towards larger ECS. Somewhat surprisingly, CMIP6 models exhibit less historical warming than CMIP5 models, despite an increase in TCR between CMIP eras (mean TCR increased from 1.7 to 1.9 <span class="inline-formula">K</span>). The evolution of the warming suggests, however, that several of the CMIP6 models apply too strong aerosol cooling, resulting in too weak mid-20th century warming compared to the instrumental record.</p>https://www.atmos-chem-phys.net/20/7829/2020/acp-20-7829-2020.pdf |
| spellingShingle | C. M. Flynn T. Mauritsen On the climate sensitivity and historical warming evolution in recent coupled model ensembles Atmospheric Chemistry and Physics |
| title | On the climate sensitivity and historical warming evolution in recent coupled model ensembles |
| title_full | On the climate sensitivity and historical warming evolution in recent coupled model ensembles |
| title_fullStr | On the climate sensitivity and historical warming evolution in recent coupled model ensembles |
| title_full_unstemmed | On the climate sensitivity and historical warming evolution in recent coupled model ensembles |
| title_short | On the climate sensitivity and historical warming evolution in recent coupled model ensembles |
| title_sort | on the climate sensitivity and historical warming evolution in recent coupled model ensembles |
| url | https://www.atmos-chem-phys.net/20/7829/2020/acp-20-7829-2020.pdf |
| work_keys_str_mv | AT cmflynn ontheclimatesensitivityandhistoricalwarmingevolutioninrecentcoupledmodelensembles AT tmauritsen ontheclimatesensitivityandhistoricalwarmingevolutioninrecentcoupledmodelensembles |