Periodic Reporting for period 3 - PALAEO-RA (A Palaeoreanalysis To Understand Decadal Climate Variability)
Reporting period: 2021-10-01 to 2023-03-31
Climatic variations at decadal scales, such as phases of accelerated warming, weak monsoons, or widespread subtropical drought, have profound effects on society and the economy. Understanding such variations re-quires insights from the past. However, no data sets of past climate are available to study decadal variability of large-scale climate with state-of-the-art diagnostic methods. Currently available data sets are limited to statistical reconstructions of local or regional surface climate. The PALAEO-RA project will produce the first ever comprehensive, 3-dimensional, physically consistent reconstruction of the global climate system at a monthly scale for the past six centuries. This palaeoreanalysis is based on combining information from early instrumental measurements, historical documents, and proxies (e.g. tree rings) with a large ensemble of climate model simulations. To achieve this novel combination, a completely new data assimilation system for palaeoclimatological data will be developed. The unique data sets produced in this project will become reference data sets for studying past climatic variations (i) for diagnostic studies of interannual-to-decadal variability, (ii) as a benchmark for model simulations and (iii) for climate impact studies. Using the data produced, the project will analyse episodes of slowed or accelerated global warming, decadal subtropical drought periods, episodes of expanding or contracting tropics, slowed or strengthened monsoons, changes in storm tracks, blocking and associated weather extremes, and links between Arctic and midlatitude climate. The analyses will provide new insights into the processes governing decadal variability of weather and climate.
In the first project phase we have compiled early instrumental climate data; work which included a global inventory of such data, the compilation of already electronically available data, and data rescue focusing on data from the 18th and early 19th century.
Similarly, an inventory of global documentary climate data was produced and most of the data series were compiled. In order to use these data in an assimilation approach, forward models need to be developed, and work towards this end was undertaken in the first project phase.
The climate model simulations requires monthly sea-surface temperatures and sea ice as boundary conditions. Such a data set was constructed for the past millennium and is ready for being used in the simulations.
The atmospheric model ECHAM6 was set up and simulations started. Within the first project phase, the period 1850 to present could be simulated (31 members). At the same time, work was directed towards improve the assimilation scheme.
Within the project and in collaboration with other projects, several analyses of climate variability could be performed. Climate variability in the early 19th century was analysed and specifically the effects of the Tambora eruption in 1815. The project also analysed decadal changes in flood frequency in the 19th century as well as past internannual-to-decadal droughts in the USA and their relation to atmospheric circulation. Furthermore, we analysed the causes of climatic changes since the Little Ice Age, finding that several volcanic eruptions in a short period can have particularly long-lasting effects on the climate system.
Similarly, an inventory of global documentary climate data was produced and most of the data series were compiled. In order to use these data in an assimilation approach, forward models need to be developed, and work towards this end was undertaken in the first project phase.
The climate model simulations requires monthly sea-surface temperatures and sea ice as boundary conditions. Such a data set was constructed for the past millennium and is ready for being used in the simulations.
The atmospheric model ECHAM6 was set up and simulations started. Within the first project phase, the period 1850 to present could be simulated (31 members). At the same time, work was directed towards improve the assimilation scheme.
Within the project and in collaboration with other projects, several analyses of climate variability could be performed. Climate variability in the early 19th century was analysed and specifically the effects of the Tambora eruption in 1815. The project also analysed decadal changes in flood frequency in the 19th century as well as past internannual-to-decadal droughts in the USA and their relation to atmospheric circulation. Furthermore, we analysed the causes of climatic changes since the Little Ice Age, finding that several volcanic eruptions in a short period can have particularly long-lasting effects on the climate system.
The project went far beyond the state of the art in several respects. The inventor of early instrumental data is the first of its kind and will be useful for future efforts in climate reconstruction, but also a resource for historians. The same is true for the inventory of documentary data. The data digitising effort was substantial. In this project alone, 678 records were digitised, comprising 7155 station years. The project also went beyond the state of the art in terms of analyses. A particular achievement was the publication of an analysis of the causes of climatic changes since the Little Ice Age, which caught media attention.
During the next project phase we will finalise the simulations, the compilation of input data, and the improvement the assimilation scheme. Then these elements will be put together and the production of the reanalysis will be started. With the new product together with existing reconstructions, the compiled data and our model simulations, further climate analyses will be possible. They will contribute to better understand the mechanisms underlying multidecadal climatic changes.
During the next project phase we will finalise the simulations, the compilation of input data, and the improvement the assimilation scheme. Then these elements will be put together and the production of the reanalysis will be started. With the new product together with existing reconstructions, the compiled data and our model simulations, further climate analyses will be possible. They will contribute to better understand the mechanisms underlying multidecadal climatic changes.