Holocene palaeoclimate and palaeovegetation on the islands of Flores and Pico

Palaeoclimatic stability is regarded as an important factor in explaining patterns of endemism in the Azorean flora. However, modelling simulations and quantitative reconstructions for the last 6000 years suggest considerable palaeoclimatic variability. Here we explore the link between Holocene palaeoclimate and palaeovegetation on the islands of Flores and Pico. Modern pollen assemblages indicate that most major plant communities can be detected using pollen analysis and that, in some cases, the pre-colonisation vegetation was quite similar to present-day relict vegetation. A 200–500-year pollen record from Alagoinha, a low-elevation mire in western Flores, shows that Juniperus brevifolia-dominated communities were widespread at lower elevations prior to large-scale deforestation. Today these communities are generally restricted to higher elevations. While our results are preliminary, there appears to be a weak link between palaeovegetation (which was primarily influenced by volcanism, soil formation and human impact) and palaeoclimatic changes detected through geochemical proxies. Even if the Azorean palaeoclimate varied substantially, its impact on the pristine vegetation, at least in terms of pollen production, was relatively small.

Stacking of discontinuous regional palaeoclimate records: Chironomid-based summer temperatures from the Alpine region

Since multi-site reconstructions are less affected by site-specific climatic effects and artefacts, regional palaeotemperature reconstructions based on a number of sites can provide more robust estimates of centennial- to millennial-scale temperature trends than individual, site-specific records. Furthermore, reconstructions based on multiple records are necessary for developing continuous climate records over time scales longer than covered by individual sequences. Here, we present a procedure for developing such reconstructions based on relatively short (centuries to millennia), discontinuously sampled records as are typically developed when using biotic proxies in lake sediments for temperature reconstruction. The approach includes an altitudinal correction of temperatures, an interpolation of individual records to equal time intervals, a stacking procedure for sections of the interval of interest that have the same records available, as well as a splicing procedure to link the individual stacked records into a continuous reconstruction. Variations in the final, stacked and spliced reconstruction are driven by variations in the individual records, whereas the absolute temperature values are determined by the stacked segment based on the largest number of records. With numerical simulations based on the NGRIP δ18O record, we demonstrate that the interpolation and stacking procedure provides an approximation of a smoothed palaeoclimate record if based on a sufficient number of discontinuously sampled records. Finally, we provide an example of a stacked and spliced palaeotemperature reconstruction 15000–90 calibrated 14C yr BP based on six chironomid records from the northern and central Swiss Alps and eastern France to discuss the potential and limitations of this approach.

Southern Hemisphere high-resolution palaeoclimate records of the last 2000 years

This study presents a comprehensive assessment of high-resolution Southern Hemisphere (SH) paleoarchives covering the last 2000 years. We identified 174 monthly to annually resolved climate proxy (tree ring, coral, ice core, documentary, speleothem and sedimentary) records from the Hemisphere. We assess the interannual and decadal sensitivity of each proxy record to large-scale circulation indices from the Pacific, Indian and Southern Ocean regions over the twentieth century. We then analyse the potential of this newly expanded palaeoclimate network to collectively represent predictands (sea surface temperature, sea level pressure, surface air temperature and precipitation) commonly used in climate reconstructions. The key dynamical centres-of-action of the equatorial Indo-Pacific are well captured by the palaeoclimate network, indicating that there is considerable reconstruction potential in this region, particularly in the post AD 1600 period when a number of long coral records are available. Current spatiotemporal gaps in data coverage and regions where significant potential for future proxy collection exists are discussed. We then highlight the need for new and extended records from key dynamical regions of the Southern Hemisphere. Although large-scale climate field reconstructions for the SH are in their infancy, we report that excellent progress in the development of regional proxies now makes plausible estimates of continental- to hemispheric-scale climate variations possible.

Holocene land-cover reconstructions for studies on land cover-climate feedbacks

The major objectives of this paper are: (1) to review the pros and cons of the scenarios of past anthropogenic land cover change (ALCC) developed during the last ten years, (2) to discuss issues related to pollen-based reconstruction of the past land-cover and introduce a new method, REVEALS (Regional Estimates of VEgetation Abundance from Large Sites), to infer long-term records of past land-cover from pollen data, (3) to present a new project (LANDCLIM: LAND cover – CLIMate interactions in NW Europe during the Holocene) currently underway, and show preliminary results of REVEALS reconstructions of the regional land-cover in the Czech Republic for five selected time windows of the Holocene, and (4) to discuss the implications and future directions in climate and vegetation/land-cover modeling, and in the assessment of the effects of human-induced changes in land-cover on the regional climate through altered feedbacks. The existing ALCC scenarios show large discrepancies between them, and few cover time periods older than AD 800. When these scenarios are used to assess the impact of human land-use on climate, contrasting results are obtained. It emphasizes the need for methods such as the REVEALS model-based land-cover reconstructions. They might help to fine-tune descriptions of past land-cover and lead to a better understanding of how long-term changes in ALCC might have influenced climate. The REVEALS model is demonstrated to provide better estimates of the regional vegetation/land-cover changes than the traditional use of pollen percentages. This will achieve a robust assessment of land cover at regional- to continental-spatial scale throughout the Holocene. We present maps of REVEALS estimates for the percentage cover of 10 plant functional types (PFTs) at 200 BP and 6000 BP, and of the two open-land PFTs "grassland" and "agricultural land" at five time-windows from 6000 BP to recent time. The LANDCLIM results are expected to provide crucial data to reassess ALCC estimates for a better understanding of the land suface-atmosphere interactions.