A pollen profile from varved lake sediments sampled at 10-year intervals and spanning the past 1000 years is analyzed to understand the effects of climate change and anthropogenic activity on forests in southwestern Quebec. The forests responded rapidly to changes in temperature and precipitation during the Medieval Warm Period and Little Ice Age as well as to land-use changes associated with the European Settlement of the area. The transition into the Little Ice Age was abrupt and had significant impact on the pollen production within a couple of decades. A synthesis of this record with other high-resolution and well-dated pollen data from the conifer-hardwood forest of eastern North America shows consistent results across the whole area, indicating that very-high resolution pollen data can provide insight into multi-decadal climate variability and its impact on forest vegetation. Tree-ring records from the region show inter-annual fluctuations not always consistent between sites, while high-resolution pollen data record multi-decadal to century changes which enable us to interpret climatic effects on plant communities.

A pollen profile from varved lake sediments sampled at 10-year intervals and spanning the past 1000 years is analyzed to understand the effects of climate change and anthropogenic activity on forests in southwestern Quebec. The forests responded rapidly to changes in temperature and precipitation during the Medieval Warm Period and Little Ice Age as well as to land-use changes associated with the European Settlement of the area. The transition into the Little Ice Age was abrupt and had significant impact on the pollen production within a couple of decades. A synthesis of this record with other high-resolution and well-dated pollen data from the conifer-hardwood forest of eastern North America shows consistent results across the whole area, indicating that very-high resolution pollen data can provide insight into multi-decadal climate variability and its impact on forest vegetation. Tree-ring records from the region show inter-annual fluctuations not always consistent between sites, while high-resolution pollen data record multi-decadal to century changes which enable us to interpret climatic effects on plant communities.

A high-resolution pollen record from Path Lake (43°87’00”N, 64°92’42”W, 10m asl) in Port Joli Harbour, Nova Scotia, Canada, was used to provide a paleo-ecological perspective on Holocene climate and vegetation variability within the context of local archaeological research. Pollen assemblages in the early Holocene reflect a post-glacial forest dominated by Pinus, Tsuga, Betula and Quercus. Shallow water aquatic and wetland taxa increased after 3400 cal. yr. BP in response to wetter climatic conditions. Increased settlement intensity of native inhabitants coincides with late-Holocene climate change at a regional scale, suggesting that environmental conditions may have influenced prehistoric human activities. European settlement, after 350 cal. yr. BP, was marked by a rise in Ambrosia, and peak charcoal accumulation rates after this time showed evidence of human disturbance on the landscape. This study suggests that environmental changes affected human exploitation of the landscape, and human activity altered forest composition in the late Holocene.

A pollen profile from varved lake sediments sampled at 10-year intervals and spanning the past 1000 years is analyzed to understand the effects of climate change and anthropogenic activity on forests in southwestern Quebec. The forests responded rapidly to changes in temperature and precipitation during the Medieval Warm Period and Little Ice Age as well as to land-use changes associated with the European Settlement of the area. The transition into the Little Ice Age was abrupt and had significant impact on the pollen production within a couple of decades. A synthesis of this record with other high-resolution and well-dated pollen data from the conifer-hardwood forest of eastern North America shows consistent results across the whole area, indicating that very-high resolution pollen data can provide insight into multi-decadal climate variability and its impact on forest vegetation. Tree-ring records from the region show inter-annual fluctuations not always consistent between sites, while high-resolution pollen data record multi-decadal to century changes which enable us to interpret climatic effects on plant communities.

A high-resolution pollen record from Path Lake (43°87’00”N, 64°92’42”W, 10m asl) in Port Joli Harbour, Nova Scotia, Canada, was used to provide a paleo-ecological perspective on Holocene climate and vegetation variability within the context of local archaeological research. Pollen assemblages in the early Holocene reflect a post-glacial forest dominated by Pinus, Tsuga, Betula and Quercus. Shallow water aquatic and wetland taxa increased after 3400 cal. yr. BP in response to wetter climatic conditions. Increased settlement intensity of native inhabitants coincides with late-Holocene climate change at a regional scale, suggesting that environmental conditions may have influenced prehistoric human activities. European settlement, after 350 cal. yr. BP, was marked by a rise in Ambrosia, and peak charcoal accumulation rates after this time showed evidence of human disturbance on the landscape. This study suggests that environmental changes affected human exploitation of the landscape, and human activity altered forest composition in the late Holocene.

Evidence for a human contribution to climatic changes during the past century is accumulating rapidly. Given the strength of the evidence, it seems natural to ask whether forcing projections can be used to forecast climate change. A Bayesian method for post-processing forced climate model simulations that produces probabilistic hindcasts of inter-decadal temperature changes on large spatial scales is proposed. Hindcasts produced for the last two decades of the 20th century are shown to be skillful. The suggestion that skillful decadal forecasts can be produced on large regional scales by exploiting the response to anthropogenic forcing provides additional evidence that anthropogenic change in the composition of the atmosphere has influenced our climate. In the absence of large negative volcanic forcing on the climate system (which cannot presently be forecast), the global mean temperature for the decade 2000-2009 is predicted to lie above the 1970-1999 normal with probability 0.94. The global mean temperature anomaly for this decade relative to 1970-1999 is predicted to be 0.35C (5-95% confidence range: 0.21C-0.48C). Reconstruction of temperature variability of the past centuries using climate proxy data can also provide important information on the role of anthropogenic forcing in the observed 20th century warming. A state-space model approach that allows incorporation of additional non-temperature information, such as the estimated response to external forcing, to reconstruct historical temperature is proposed. An advantage of this approach is that it permits simultaneous reconstruction and detection analysis as well as future projection. A difficulty in using this approach is that estimation of several unknown state-space model parameters is required. To take advantage of the data structure in the reconstruction problem, the existing parameter estimation approach is modified, resulting in two new estimation approaches. The competing estimation approaches are compared based on theoretical grounds and through simulation studies. The two new estimation approaches generally perform better than the existing approach. A number of studies have attempted to reconstruct hemispheric mean temperature for the past millennium from proxy climate indicators. Different statistical methods are used in these studies and it therefore seems natural to ask which method is more reliable. An empirical comparison between the different reconstruction methods is considered using both climate model data and real-world paleoclimate proxy data. The proposed state-space model approach and the RegEM method generally perform better than their competitors when reconstructing interannual variations in Northern Hemispheric mean surface air temperature. On the other hand, a variety of methods are seen to perform well when reconstructing decadal temperature variability. The similarity in performance provides evidence that the difference between many real-world reconstructions is more likely to be due to the choice of the proxy series, or the use of difference target seasons or latitudes, than to the choice of statistical method.

July temperature (TJUL) and total annual precipitation (ANNP) are reconstructed to better understand the spatial and temporal patterns of change in North America over the last 2,000 years using pollen databases. Using a customized application in R, the reconstructions use a composite averaging of multiple site reconstructions that show a distinct warmer Medieval Warm Period (MWP) compared to the colder Little Ice Age (LIA). Results show that, both multi-centennial scale periods are re- constructed as cooler than the last 50 years. Regional time series from several forested ecoregions show positive anomalies up to 0.6ºC during the MWP and anomalies up to -0.3ºC during the LIA. In order to test whether the TJUL reconstructions are biased to the modern calibration climate data, we show a distinct difference between the reconstructions when using station versus reanalysis-based modern TJUL fields. Reconstructions using station-based modern calibration data sets better reflect the centennial to multi-centennial scale climate variability as compared to the reanalysis-based modern calibration data sets that reveal a warm-bias. We justify the choice of the Whitmore et al. (2005) modern data set for large-scale pollen-based paleoclimate reconstructions. Finally we use Local Indicators of Spatial Association (LISA) to spatially filter the ANNP reconstructions in order to distinguish regional hydroclimate patterns from local site-specific conditions. Results show that a La Nina, positive North Atlantic Oscillation (+NAO) and positive Atlantic Multidecadal Oscillation (+AMO) state-like dominated both the MWP and Roman Warm Period (RWP), although the MWP was generally drier. In contrast, the Dark Ages Cold (DAC) period was likely dominated by El Nino, negative NAO and negative AMO state-like circulation. Minimum solar and high volcanic activity is likely to have contributed to more complex hydroclimate regional patterns during the LIA. The results presented in this dissertation can be used as benchmark data sets for future climate data-model comparisons in order to improve our understanding of natural climate variability during the past 2,000 years in the context of modern human-induced climate change.