A global inventory of historical documentary evidence related to climate since the 15th century

Climatic variations have impacted societies since the very beginning of human history. In order to keep track of climatic changes over time, humans have thus often closely monitored the weather as well as natural phenomena influencing everyday life. Resulting documentary evidence from archives of societies enables invaluable insights into the past climate beyond the timescale of instrumental and early instrumental measurements. This information complements other proxies from archives of nature such as tree rings in climate reconstructions, as documentary evidence often covers seasons (e.g., winter) and regions (e.g., Africa, Western Russia, and Siberia, China) that are not well covered with natural proxies. While a mature body of research on detecting climate signals from historical documents exists, the large majority of studies is confined to a local or regional scale and thus lacks a global perspective. Moreover, many studies from before the 1980s have not made the transition into the digital age and, hence, are essentially forgotten. Here, I attempt to compile the first-ever systematic global inventory of documentary evidence related to climate extending back to the Late Medieval Period. It combines information on past climate from all around the world, retrieved from many studies on historical documentary sources. Historical evidence range from personal diaries, chronicles, administrative/ clerical documents to ship logbooks and newspaper articles. They include records of many sorts, e.g., tithes records, rogation ceremonies, extreme events like droughts and floods, as well as weather and phenological observations. The inventory, published as an electronic supplement, comprises detailed event chronologies, time series, proxy indices, and calibrated reconstructions, with the majority of the documentary records providing indications on past temperature and precipitation anomalies. The overall focus is on document-based time series with significant potential for climate reconstruction. For each included record series, extensive meta information and directions to the data (if available) are given. To highlight the potential of documentary data for climate science three case studies are presented and evaluated with different global reanalysis products. This comprehensive inventory promotes the first-ever global perspective on historical documentary climate records and, thus, lies the foundation for incorporating historical documentary evidence into climate reconstruction on a global scale, complementing early instrumental measurements as well as natural climate proxies.

Past climate conditions predict the influence of nitrogen enrichment on the temperature sensitivity of soil respiration

The response of soil carbon release to global warming is largely determined by the temperature sensitivity of soil respiration, yet how this relationship will be affected by increasing atmospheric nitrogen deposition is unclear. Here, we present a global synthesis of 686 observations from 168 field studies to investigate the relationship between nitrogen enrichment and the temperature sensitivity of soil respiration. We find that the temperature sensitivity of total and heterotrophic soil respiration increased with latitude. In addition, for total and autotrophic respiration, the temperature sensitivity responded more strongly to nitrogen enrichment with increasing latitude. Temperature and precipitation during the Last Glacial Maximum were better predictors of how the temperature sensitivity of soil respiration responds to nitrogen enrichment than contemporary climate variables. The tentative legacy effects of paleoclimate variables regulate the response through shaping soil organic carbon and nitrogen content. We suggest that careful consideration of past climate conditions is necessary when projecting soil carbon dynamics under future global change.

The Southern Ocean Radiolarian (SO-RAD) dataset: a new compilation of modern radiolarian census data

Radiolarians (holoplanktonic protozoa) preserved in marine sediments are commonly used as palaeoclimate proxies for reconstructing past Southern Ocean environments. Generating reconstructions of past climate based on microfossil abundances, such as radiolarians, requires a spatially and environmentally comprehensive reference dataset of modern census counts. The Southern Ocean Radiolarian (SO-RAD) dataset includes census counts for 238 radiolarian taxa from 228 surface sediment samples located in the Atlantic, Indian, and southwest Pacific sectors of the Southern Ocean. This compilation is the largest radiolarian census dataset derived from surface sediment samples in the Southern Ocean. The SO-RAD dataset may be used as a reference dataset for palaeoceanographic reconstructions, or for studying modern radiolarian biogeography and species diversity. As well as describing the data collection and collation, we include recommendations and guidelines for cleaning and subsetting the data for users unfamiliar with the procedures typically used by the radiolarian community. The SO-RAD dataset is available to download from https://doi.org/10.1594/PANGAEA.929903 (Lawler et al., 2021).

Inferring Past Climate from Moraine Evidence Using Glacier Modelling

<p>Glacier length fluctuations reflect changes in climate, most notably temperature and precipitation. By this reasoning, moraines, which represent former glacier extent, can be used to estimate past climate. However, estimating palaeoclimate from moraines is not a straight-forward process and involves several assumptions. For example, recent studies have suggested that interannual stochastic variability in temperature in a steady-state climate can cause a glacier to experience kilometre-scale fluctuations. Such studies cast doubt on the usefulness of moraines as climate proxy indicators. Detailed glacial geomorphological maps and moraine chronologies have improved our understanding of the spatial and temporal extent of past glacial events in New Zealand. Palaeoclimate estimates associated with these moraines have thus-far come from simple methods, such as the accumulation area ratio, with unquantifiable uncertainties. I used a numerical modelling approach to approximate the present-day glacier mass balance pattern, which includes the effects of snow avalanching on glacier mass balance. I then used the models to reconstruct palaeoclimate for Lateglacial and Holocene glacial events in New Zealand, and to better understand moraine-glacier-climate relationships. The climate reconstructions come from simulating past glacier expansions to specific terminal moraines, but I also simulated glacier fluctuations in response to a previously derived temperature reconstruction, and to interannual stochastic variability in temperature. The purpose behind each simulation was to identify the drivers of significant glacier fluctuations. The modelling results support the hypothesis that New Zealand moraine records reflect past climate, especially changes in temperature. Lateglacial climate was reconstructed to be 2-3 C lower than the present day. This temperature range agrees well with previous estimates from moraines and other climate proxy records in New Zealand. Modelled temperature estimates for the Holocene moraines are slightly colder than those derived from simpler methods, due to a non-linear relationship found between snowline lowering and glacier length. This relationship results from the specific valley shape and glacier geometry, and is likely to occur in other, similarly-shaped glacier valleys. The simulations forced by interannual stochastic variability in temperature do not show significant (>300 m) fluctuations in the glacier terminus. Such fluctuations can not explain the Holocene moraine sequence that I examined, which extends >2 km beyond the present-day glacier terminus. Stochastic temperature change could, however, in part, cause fluctuations in glacier extent during an overall glacier recession. Modelling shows that it is also unlikely that glaciers advanced to Holocene and Lateglacial moraine positions as a result of precipitation changes alone. For these reasons, temperature changes are a necessary part of explaining past glacier extents, especially during the Lateglacial, and the moraines examined here likely reflect changes in mean climate in New Zealand. The glacier modelling studies indicate that simpler methods, such as the accumulation area ratio, can be used to appropriately reconstruct past climate from glacial evidence, as long as the glacier catchment has a straight forward geometry, shallow bed slope and no tributary glaciers. Non-linear relationships between climate change and glacier length develop when valley shape is more complex, and glaciers within these systems are probably better simulated using a modelling approach. Using a numerical modelling approach, it is also possible to gain a greater understanding of glacier response time, length sensitivities, and estimates of ice extent in valleys within the model domain where geomorphic evidence is not available. In this manner, numerical models can be used as a tool for understanding past climate and glacier sensitivity, thus improving the confidence in the palaeoclimate interpretations.</p>

The Role of Climate and Bed Topography on the Evolution of the Tasman Glacier Since the Last Glacial Maximum

<p>Mountain glaciers respond to climatic changes by advancing or retreating, leaving behind a potentially powerful record of climate through moraine deposition. Estimates of past climate have been made based on the moraine record alone, using geometrical arguments; however, these methods necessarily ignore the effects of glacier dynamics and bed modification. Here, a one-dimensional coupled mass balance-flowline model is used to place constraints on the climate of the Late-glacial (13.5–11.6 kyr ago) and Last Glacial Maximum (LGM, 28 – 17.5 kyr ago) based on the well-mapped and -dated moraines at Tasman Glacier/Lake Pukaki, South Island, New Zealand. Due to the highly-dynamic nature of the system, distinct longitudinal bed profiles are considered for each of the glaciations modelled; the reconstructions show that terminal overdeepenings are likely present in all bed profiles, and hundreds of metres of sediment has been deposited in the glacier valley since the LGM. Using the coupled model and calculated bed topography, a 2.2°C temperature depression from the present is necessary to reproduce the Lateglacial ice extent, and 7.0°C is required for the early LGM, assuming presentday precipitation. The modelled Late-glacial ice extent is more sensitive to precipitation variability than that during the LGM, but the Tasman Glacier during both periods is primarily driven by temperature changes. While the Tasman Glacier shrank between the early and late LGM, modelling demonstrates that changes in bed topography due to erosion, transport and deposition of sediment are a major driver in reduction of glacier extent; a temperature increase of only 0.1°C is required to cause the transition between the two periods, which may be attributable to interannual, zero-trend climate variability. Thus, the consideration of the coupled glacier-sediment system is critical in accurately reconstructing past climate. Future work focusing on modelling this coupled system, such that the bed profile can evolve interactively with glacier flow, will be critical in better resolving transient events such as the early to late LGM transition.</p>

Inferring Past Climate from Moraine Evidence Using Glacier Modelling

<p>Glacier length fluctuations reflect changes in climate, most notably temperature and precipitation. By this reasoning, moraines, which represent former glacier extent, can be used to estimate past climate. However, estimating palaeoclimate from moraines is not a straight-forward process and involves several assumptions. For example, recent studies have suggested that interannual stochastic variability in temperature in a steady-state climate can cause a glacier to experience kilometre-scale fluctuations. Such studies cast doubt on the usefulness of moraines as climate proxy indicators. Detailed glacial geomorphological maps and moraine chronologies have improved our understanding of the spatial and temporal extent of past glacial events in New Zealand. Palaeoclimate estimates associated with these moraines have thus-far come from simple methods, such as the accumulation area ratio, with unquantifiable uncertainties. I used a numerical modelling approach to approximate the present-day glacier mass balance pattern, which includes the effects of snow avalanching on glacier mass balance. I then used the models to reconstruct palaeoclimate for Lateglacial and Holocene glacial events in New Zealand, and to better understand moraine-glacier-climate relationships. The climate reconstructions come from simulating past glacier expansions to specific terminal moraines, but I also simulated glacier fluctuations in response to a previously derived temperature reconstruction, and to interannual stochastic variability in temperature. The purpose behind each simulation was to identify the drivers of significant glacier fluctuations. The modelling results support the hypothesis that New Zealand moraine records reflect past climate, especially changes in temperature. Lateglacial climate was reconstructed to be 2-3 C lower than the present day. This temperature range agrees well with previous estimates from moraines and other climate proxy records in New Zealand. Modelled temperature estimates for the Holocene moraines are slightly colder than those derived from simpler methods, due to a non-linear relationship found between snowline lowering and glacier length. This relationship results from the specific valley shape and glacier geometry, and is likely to occur in other, similarly-shaped glacier valleys. The simulations forced by interannual stochastic variability in temperature do not show significant (>300 m) fluctuations in the glacier terminus. Such fluctuations can not explain the Holocene moraine sequence that I examined, which extends >2 km beyond the present-day glacier terminus. Stochastic temperature change could, however, in part, cause fluctuations in glacier extent during an overall glacier recession. Modelling shows that it is also unlikely that glaciers advanced to Holocene and Lateglacial moraine positions as a result of precipitation changes alone. For these reasons, temperature changes are a necessary part of explaining past glacier extents, especially during the Lateglacial, and the moraines examined here likely reflect changes in mean climate in New Zealand. The glacier modelling studies indicate that simpler methods, such as the accumulation area ratio, can be used to appropriately reconstruct past climate from glacial evidence, as long as the glacier catchment has a straight forward geometry, shallow bed slope and no tributary glaciers. Non-linear relationships between climate change and glacier length develop when valley shape is more complex, and glaciers within these systems are probably better simulated using a modelling approach. Using a numerical modelling approach, it is also possible to gain a greater understanding of glacier response time, length sensitivities, and estimates of ice extent in valleys within the model domain where geomorphic evidence is not available. In this manner, numerical models can be used as a tool for understanding past climate and glacier sensitivity, thus improving the confidence in the palaeoclimate interpretations.</p>

The Role of Climate and Bed Topography on the Evolution of the Tasman Glacier Since the Last Glacial Maximum

<p>Mountain glaciers respond to climatic changes by advancing or retreating, leaving behind a potentially powerful record of climate through moraine deposition. Estimates of past climate have been made based on the moraine record alone, using geometrical arguments; however, these methods necessarily ignore the effects of glacier dynamics and bed modification. Here, a one-dimensional coupled mass balance-flowline model is used to place constraints on the climate of the Late-glacial (13.5–11.6 kyr ago) and Last Glacial Maximum (LGM, 28 – 17.5 kyr ago) based on the well-mapped and -dated moraines at Tasman Glacier/Lake Pukaki, South Island, New Zealand. Due to the highly-dynamic nature of the system, distinct longitudinal bed profiles are considered for each of the glaciations modelled; the reconstructions show that terminal overdeepenings are likely present in all bed profiles, and hundreds of metres of sediment has been deposited in the glacier valley since the LGM. Using the coupled model and calculated bed topography, a 2.2°C temperature depression from the present is necessary to reproduce the Lateglacial ice extent, and 7.0°C is required for the early LGM, assuming presentday precipitation. The modelled Late-glacial ice extent is more sensitive to precipitation variability than that during the LGM, but the Tasman Glacier during both periods is primarily driven by temperature changes. While the Tasman Glacier shrank between the early and late LGM, modelling demonstrates that changes in bed topography due to erosion, transport and deposition of sediment are a major driver in reduction of glacier extent; a temperature increase of only 0.1°C is required to cause the transition between the two periods, which may be attributable to interannual, zero-trend climate variability. Thus, the consideration of the coupled glacier-sediment system is critical in accurately reconstructing past climate. Future work focusing on modelling this coupled system, such that the bed profile can evolve interactively with glacier flow, will be critical in better resolving transient events such as the early to late LGM transition.</p>

Supplemental Material: Differences between soil and air temperatures: Implications for geological reconstructions of past climate

The complete data used in text Figures 7–11.