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The observed late twentieth century warming must be assessed in relation to natural long-term variations of the climatic system. Here, we present a Northern Hemisphere (NH) temperature reconstruction for the mid-to-late Holocene of the past 6000 years, based on a synthesis of existing paleo-temperature proxies that are capable of revealing centennial-scale variability. This includes 56 published temperature records across the NH land areas, with a sampling resolution ranging from 1 to 100 years and a time span of at least 1000 years. The composite plus scale (CPS) method is adopted with spatial weighting to develop the NH temperature reconstruction. Our reconstruction reveals abrupt cold epochs that match well the Bond events during the past 6000 years. The study further reveals two prominent cycles in NH temperature: 1700-2000-year cycle during the mid-to-late Holocene and 1200-1500-year cycle during the past 3500 years. Our reconstruction indicates that the late twentieth century NH temperature and its rate of warming are both unprecedentedly high over the past 5000 years. By comparing our reconstruction with the projected temperature increase scenarios, we find that temperature by the end of the twenty-first century would likely exceed any peaks during the mid-to-late Holocene. [ABSTRACT FROM AUTHOR]

It has long been assumed that deteriorating climate (cooling and warming above the norm) could shrink the carrying capacity of agrarian lands, depriving the human population of sufficient food. Population collapses (i.e. negative population growth) follow. However, this human-ecological relationship has rarely been verified scientifically, and evidence of warming-caused disaster has never been found. This research sought to explore quantitatively the temporal pattern, spatial pattern and triggers of population collapses in relation to climate change at the global scale over 1100 years. Various countries/regions in the Northern Hemisphere (NH) during the pre-industrial era. We performed time-series analysis to examine the association between temperature change and country-wide/region-wide population collapses in different climatic zones. All of the known population collapse incidents in the NH in the period 800-1900 were included in our data analysis. Nearly 90% of population collapses in various NH countries/regions occurred during periods of climate deterioration characterized by shrinking carrying capacity of the land. In addition, we found that cooling dampened the human ecosystem and brought about 80% of the collapses in warmer humid, cooler humid and dry zones, while warming adversely affected the ecosystems in dry and tropical humid zones. All of the population collapses and growth declines in periods of warm climate occurred in dry and tropical humid zones. Malthusian checks (famines, wars and epidemics) were the dominant triggers of population collapses, which peaked dramatically when climate deteriorated. Global demographic catastrophes and most population collapse incidents occurred in periods with great climate change, owing to overpopulation caused by diminished carrying capacity of the land and the resultant outbreak of Malthusian checks. Impacts of cooling or warming on land carrying capacity varied geographically, as a result of the diversified ecosystems in different parts of the Earth. The observed climate-population synchrony challenges Malthusian theory and demonstrates that it is not population growth alone but climate-induced subsistence shortage and population growth working synergistically, that cause large-scale human population collapses on the long-term scale. [ABSTRACT FROM AUTHOR]