A limited role for unforced internal variability in twentieth-century warming
The early twentieth-century warming (EW; 1910–45) and the mid-twentieth-century cooling (MC; 1950–80) have been linked to both internal variability of the climate system and changes in external radiative forcing. The degree to which either of the two factors contributed to EW and MC, or both, is sti...
| Main Authors: | , , , , , , , , , |
|---|---|
| Format: | Journal article |
| Published: |
American Meteorological Society
2019
|
| _version_ | 1797091873552072704 |
|---|---|
| author | Haustein, K Otto, F Venema, V Jacobs, P Cowtan, K Hausfather, Z Way, R White, B Subramanian, A Schurer, A |
| author_facet | Haustein, K Otto, F Venema, V Jacobs, P Cowtan, K Hausfather, Z Way, R White, B Subramanian, A Schurer, A |
| author_sort | Haustein, K |
| collection | OXFORD |
| description | The early twentieth-century warming (EW; 1910–45) and the mid-twentieth-century cooling (MC; 1950–80) have been linked to both internal variability of the climate system and changes in external radiative forcing. The degree to which either of the two factors contributed to EW and MC, or both, is still debated. Using a two-box impulse response model, we demonstrate that multidecadal ocean variability was unlikely to be the driver of observed changes in global mean surface temperature (GMST) after AD 1850. Instead, virtually all (97%–98%) of the global low-frequency variability (.30 years) can be explained by external forcing. We find similarly high percentages of explained variance for interhemispheric and land–ocean temperature evolution. Three key aspects are identified that underpin the conclusion of this new study: inhomogeneous anthropogenic aerosol forcing (AER), biases in the instrumental sea surface temperature (SST) datasets, and inadequate representation of the response to varying forcing factors. Once the spatially heterogeneous nature of AER is accounted for, the MC period is reconcilable with external drivers. SST biases and imprecise forcing responses explain the putative disagreement between models and observations during the EW period. As a consequence, Atlantic multidecadal variability (AMV) is found to be primarily controlled by external forcing too. Future attribution studies should account for these important factors when discriminating between externally forced and internally generated influences on climate. We argue that AMV must not be used as a regressor and suggest a revised AMV index instead [the North Atlantic Variability Index (NAVI)]. Our associated best estimate for the transient climate response (TCR) is 1.57 K (60.70 at the 5%–95% confidence level). |
| first_indexed | 2024-03-07T03:38:34Z |
| format | Journal article |
| id | oxford-uuid:bd20efc1-f18e-41d9-ac10-9f8bc78057ba |
| institution | University of Oxford |
| last_indexed | 2024-03-07T03:38:34Z |
| publishDate | 2019 |
| publisher | American Meteorological Society |
| record_format | dspace |
| spelling | oxford-uuid:bd20efc1-f18e-41d9-ac10-9f8bc78057ba2022-03-27T05:29:23ZA limited role for unforced internal variability in twentieth-century warmingJournal articlehttp://purl.org/coar/resource_type/c_dcae04bcuuid:bd20efc1-f18e-41d9-ac10-9f8bc78057baSymplectic Elements at OxfordAmerican Meteorological Society2019Haustein, KOtto, FVenema, VJacobs, PCowtan, KHausfather, ZWay, RWhite, BSubramanian, ASchurer, AThe early twentieth-century warming (EW; 1910–45) and the mid-twentieth-century cooling (MC; 1950–80) have been linked to both internal variability of the climate system and changes in external radiative forcing. The degree to which either of the two factors contributed to EW and MC, or both, is still debated. Using a two-box impulse response model, we demonstrate that multidecadal ocean variability was unlikely to be the driver of observed changes in global mean surface temperature (GMST) after AD 1850. Instead, virtually all (97%–98%) of the global low-frequency variability (.30 years) can be explained by external forcing. We find similarly high percentages of explained variance for interhemispheric and land–ocean temperature evolution. Three key aspects are identified that underpin the conclusion of this new study: inhomogeneous anthropogenic aerosol forcing (AER), biases in the instrumental sea surface temperature (SST) datasets, and inadequate representation of the response to varying forcing factors. Once the spatially heterogeneous nature of AER is accounted for, the MC period is reconcilable with external drivers. SST biases and imprecise forcing responses explain the putative disagreement between models and observations during the EW period. As a consequence, Atlantic multidecadal variability (AMV) is found to be primarily controlled by external forcing too. Future attribution studies should account for these important factors when discriminating between externally forced and internally generated influences on climate. We argue that AMV must not be used as a regressor and suggest a revised AMV index instead [the North Atlantic Variability Index (NAVI)]. Our associated best estimate for the transient climate response (TCR) is 1.57 K (60.70 at the 5%–95% confidence level). |
| spellingShingle | Haustein, K Otto, F Venema, V Jacobs, P Cowtan, K Hausfather, Z Way, R White, B Subramanian, A Schurer, A A limited role for unforced internal variability in twentieth-century warming |
| title | A limited role for unforced internal variability in twentieth-century warming |
| title_full | A limited role for unforced internal variability in twentieth-century warming |
| title_fullStr | A limited role for unforced internal variability in twentieth-century warming |
| title_full_unstemmed | A limited role for unforced internal variability in twentieth-century warming |
| title_short | A limited role for unforced internal variability in twentieth-century warming |
| title_sort | limited role for unforced internal variability in twentieth century warming |
| work_keys_str_mv | AT hausteink alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT ottof alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT venemav alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT jacobsp alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT cowtank alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT hausfatherz alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT wayr alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT whiteb alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT subramaniana alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT schurera alimitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT hausteink limitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT ottof limitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT venemav limitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT jacobsp limitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT cowtank limitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT hausfatherz limitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT wayr limitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT whiteb limitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT subramaniana limitedroleforunforcedinternalvariabilityintwentiethcenturywarming AT schurera limitedroleforunforcedinternalvariabilityintwentiethcenturywarming |