Final Report Summary - ACCLIMATE (Elucidating the Causes and Effects of Atlantic Circulation Changes through Model-Data Integration)
Rapid changes in climate and ocean circulation, marked by shifts between cold (stadial) and warm (interstadial) phases have been observed in Greenland ice and North Atlantic marine sediment cores over the last glacial and deglaciation periods (i.e. the last 60 thousand years (ky)), highlighting the non-linear character of the climate system. The ACCLIMATE project aimed at combining a set of consistently dated Atlantic isotopic records with an isotope-enabled climate model through data assimilation in order to determine states of the climate system that best explain the observations over the last 40 ky, while being consistent with the model physics. Part of the project objectives have been reached but many obstacles unfortunately prevented us to complete the data-model integration work.
The project nevertheless led to the following major research outcomes:
- We built and published the first data set of marine records covering both low and high latitudes which is directly comparable to ice core data and climate model outputs over the last 40 ky. This data set is a unique source of knowledge of the leads and lags in the climate system, and hence of the mechanisms behind the climate system’s internal variability. It is available on the Pangaea (https://doi.org/10.1594/PANGAEA.900073) and Seanoe (https://doi.org/10.17882/59554) data repositories.
- New sedimentary Pa/Th data demonstrated that water mass transport decreased at mid-depth in the western equatorial Atlantic during all the stadials of the last glacial period. Moreover, we were able to show that the onset of these ocean circulation slowdowns preceded periods of increased rainfall over the adjacent continent by 120 to 1000 years, providing unprecedented evidence that shifts in the tropical rain belt may indeed be driven by changes in ocean circulation.
- The implementation of ocean circulation proxies in the iLOVECLIM climate model revealed that the response of the bottom water carbon isotopes to a slowdown of the overturning circulation lags the sedimentary Pa/Th response by a few hundred years.
- We showed that the decrease in sea surface temperature marking the onset of Heinrich Stadial 1 in the North Atlantic occurred within dating uncertainty of the sharp increase in dust flux recorded at ~17.5 cal. ky BP in Greenland ice cores, providing a new chronological marker for marine records subject to large uncertainties in radiocarbon dating.
- We demonstrated that non-linear interactions at the ice-sheets margin can explain the large-scale collapse of the northern hemisphere ice sheet during the last deglaciation, as well as a sea level rise of 1 to 2 m per century for the sole North American ice sheet.
Furthermore, ACCLIMATE led to the development of novel and unconventional methodologies:
- A new age-depth modeling routine for geological sequences with inherent depth uncertainty has been developed , which has raised a lot of interest in the scientific community. The software and user manual can be downloaded from the Zenodo public repository at https://doi.org/10.5281/zenodo.2527641.
- A portable proxy model, “Foraminifera as Modeled Entities” (FAME), has been developed, which computes the calcite oxygen isotopic ratio of the planktonic foraminifer species, accounting for the changes in growing season and depth habitat. The coupling of this innovative routine to isotope-enabled climate models permits the direct comparison of measured and simulated planktonic oxygen isotopes. FAME is available under the GNU General Public License https://www.gnu.org/licenses/gpl.html and is already being integrated as a standard in foraminifera modeling packages (e.g. https://doi.org/10.5194/cp-14-1851-2018).
- The new version of the ice sheet model GRISLI developed within ACCLIMATE has been used to better constrain the future evolution of the Greenland and Antarctic ice sheets within large international consortiums and in coupled simulations with a regional atmospheric model. It has also been used to infer basal temperature of the Antarctic ice sheet from satellite data.
- The downscaling tool developed over the course of ACCLIMATE is currently used in three spin-off projects.
Finally, the ACCLIMATE coring cruise has been the opportunity to develop outreach activities, including a website (http://www.sea.acclimateproject.eu/) targeted at schools (primary, middle and high schools) and the general public, which remains a resource for knowledge transfer towards schools, teachers and the general public.
The project nevertheless led to the following major research outcomes:
- We built and published the first data set of marine records covering both low and high latitudes which is directly comparable to ice core data and climate model outputs over the last 40 ky. This data set is a unique source of knowledge of the leads and lags in the climate system, and hence of the mechanisms behind the climate system’s internal variability. It is available on the Pangaea (https://doi.org/10.1594/PANGAEA.900073) and Seanoe (https://doi.org/10.17882/59554) data repositories.
- New sedimentary Pa/Th data demonstrated that water mass transport decreased at mid-depth in the western equatorial Atlantic during all the stadials of the last glacial period. Moreover, we were able to show that the onset of these ocean circulation slowdowns preceded periods of increased rainfall over the adjacent continent by 120 to 1000 years, providing unprecedented evidence that shifts in the tropical rain belt may indeed be driven by changes in ocean circulation.
- The implementation of ocean circulation proxies in the iLOVECLIM climate model revealed that the response of the bottom water carbon isotopes to a slowdown of the overturning circulation lags the sedimentary Pa/Th response by a few hundred years.
- We showed that the decrease in sea surface temperature marking the onset of Heinrich Stadial 1 in the North Atlantic occurred within dating uncertainty of the sharp increase in dust flux recorded at ~17.5 cal. ky BP in Greenland ice cores, providing a new chronological marker for marine records subject to large uncertainties in radiocarbon dating.
- We demonstrated that non-linear interactions at the ice-sheets margin can explain the large-scale collapse of the northern hemisphere ice sheet during the last deglaciation, as well as a sea level rise of 1 to 2 m per century for the sole North American ice sheet.
Furthermore, ACCLIMATE led to the development of novel and unconventional methodologies:
- A new age-depth modeling routine for geological sequences with inherent depth uncertainty has been developed , which has raised a lot of interest in the scientific community. The software and user manual can be downloaded from the Zenodo public repository at https://doi.org/10.5281/zenodo.2527641.
- A portable proxy model, “Foraminifera as Modeled Entities” (FAME), has been developed, which computes the calcite oxygen isotopic ratio of the planktonic foraminifer species, accounting for the changes in growing season and depth habitat. The coupling of this innovative routine to isotope-enabled climate models permits the direct comparison of measured and simulated planktonic oxygen isotopes. FAME is available under the GNU General Public License https://www.gnu.org/licenses/gpl.html and is already being integrated as a standard in foraminifera modeling packages (e.g. https://doi.org/10.5194/cp-14-1851-2018).
- The new version of the ice sheet model GRISLI developed within ACCLIMATE has been used to better constrain the future evolution of the Greenland and Antarctic ice sheets within large international consortiums and in coupled simulations with a regional atmospheric model. It has also been used to infer basal temperature of the Antarctic ice sheet from satellite data.
- The downscaling tool developed over the course of ACCLIMATE is currently used in three spin-off projects.
Finally, the ACCLIMATE coring cruise has been the opportunity to develop outreach activities, including a website (http://www.sea.acclimateproject.eu/) targeted at schools (primary, middle and high schools) and the general public, which remains a resource for knowledge transfer towards schools, teachers and the general public.