Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change?
Many previous studies have shown that a solar forcing must be greater than a CO _2 forcing to cause the same global mean surface temperature change but a process-based mechanistic explanation is lacking in the literature. In this study, we investigate the physical mechanisms responsible for the lowe...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
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IOP Publishing
2016-01-01
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| Series: | Environmental Research Letters |
| Subjects: | |
| Online Access: | https://doi.org/10.1088/1748-9326/11/4/044013 |
| _version_ | 1797748676324491264 |
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| author | Angshuman Modak Govindasamy Bala Long Cao Ken Caldeira |
| author_facet | Angshuman Modak Govindasamy Bala Long Cao Ken Caldeira |
| author_sort | Angshuman Modak |
| collection | DOAJ |
| description | Many previous studies have shown that a solar forcing must be greater than a CO _2 forcing to cause the same global mean surface temperature change but a process-based mechanistic explanation is lacking in the literature. In this study, we investigate the physical mechanisms responsible for the lower efficacy of solar forcing compared to an equivalent CO _2 forcing. Radiative forcing is estimated using the Gregory method that regresses top-of-atmosphere (TOA) radiative flux against the change in global mean surface temperature. For a 2.25% increase in solar irradiance that produces the same long term global mean warming as a doubling of CO _2 concentration, we estimate that the efficacy of solar forcing is ∼80% relative to CO _2 forcing in the NCAR CAM5 climate model. We find that the fast tropospheric cloud adjustments especially over land and stratospheric warming in the first four months cause the slope of the regression between the TOA net radiative fluxes and surface temperature to be steeper in the solar forcing case. This steeper slope indicates a stronger net negative feedback and hence correspondingly a larger solar forcing than CO _2 forcing for the same equilibrium surface warming. Evidence is provided that rapid land surface warming in the first four months sets up a land-sea contrast that markedly affects radiative forcing and the climate feedback parameter over this period. We also confirm the robustness of our results using simulations from the Hadley Centre climate model. Our study has important implications for estimating the magnitude of climate change caused by volcanic eruptions, solar geoengineering and past climate changes caused by change in solar irradiance such as Maunder minimum. |
| first_indexed | 2024-03-12T16:08:12Z |
| format | Article |
| id | doaj.art-1aa829b468444e73a56c258de363dbf8 |
| institution | Directory Open Access Journal |
| issn | 1748-9326 |
| language | English |
| last_indexed | 2024-03-12T16:08:12Z |
| publishDate | 2016-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | Environmental Research Letters |
| spelling | doaj.art-1aa829b468444e73a56c258de363dbf82023-08-09T14:14:47ZengIOP PublishingEnvironmental Research Letters1748-93262016-01-0111404401310.1088/1748-9326/11/4/044013Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change?Angshuman Modak0Govindasamy Bala1Long Cao2Ken Caldeira3Divecha Center for Climate Change and Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560012, IndiaDivecha Center for Climate Change and Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560012, India; Interdisciplinary center for water research, Indian Institute of Science, Bangalore-12, IndiaSchool of Earth Sciences, Zhejiang University , Hangzhou, Zhejiang 310027, People’s Republic of ChinaDepartment of Global Ecology, Carnegie Institution for Science, Stanford, CA 94305, USAMany previous studies have shown that a solar forcing must be greater than a CO _2 forcing to cause the same global mean surface temperature change but a process-based mechanistic explanation is lacking in the literature. In this study, we investigate the physical mechanisms responsible for the lower efficacy of solar forcing compared to an equivalent CO _2 forcing. Radiative forcing is estimated using the Gregory method that regresses top-of-atmosphere (TOA) radiative flux against the change in global mean surface temperature. For a 2.25% increase in solar irradiance that produces the same long term global mean warming as a doubling of CO _2 concentration, we estimate that the efficacy of solar forcing is ∼80% relative to CO _2 forcing in the NCAR CAM5 climate model. We find that the fast tropospheric cloud adjustments especially over land and stratospheric warming in the first four months cause the slope of the regression between the TOA net radiative fluxes and surface temperature to be steeper in the solar forcing case. This steeper slope indicates a stronger net negative feedback and hence correspondingly a larger solar forcing than CO _2 forcing for the same equilibrium surface warming. Evidence is provided that rapid land surface warming in the first four months sets up a land-sea contrast that markedly affects radiative forcing and the climate feedback parameter over this period. We also confirm the robustness of our results using simulations from the Hadley Centre climate model. Our study has important implications for estimating the magnitude of climate change caused by volcanic eruptions, solar geoengineering and past climate changes caused by change in solar irradiance such as Maunder minimum.https://doi.org/10.1088/1748-9326/11/4/044013radiative forcingefficacyclimate feedbackfast cloud adjustmentsrapid land surface warming |
| spellingShingle | Angshuman Modak Govindasamy Bala Long Cao Ken Caldeira Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change? Environmental Research Letters radiative forcing efficacy climate feedback fast cloud adjustments rapid land surface warming |
| title | Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change? |
| title_full | Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change? |
| title_fullStr | Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change? |
| title_full_unstemmed | Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change? |
| title_short | Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change? |
| title_sort | why must a solar forcing be larger than a co2 forcing to cause the same global mean surface temperature change |
| topic | radiative forcing efficacy climate feedback fast cloud adjustments rapid land surface warming |
| url | https://doi.org/10.1088/1748-9326/11/4/044013 |
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