Millennial-scale climate variability in North America during the past 14,000 years

• Main Author: Viau, Andre Ernest J
• Format: Thesis
• Language: English
• Published: Université d'Ottawa / University of Ottawa 2003
• Subjects: Physical Geography.; Geophysics.; North Atlantic
• Online Access: https://dx.doi.org/10.20381/ruor-19531; http://www.ruor.uottawa.ca/handle/10393/28971

Variations in the Earth's climate occur on many time and space scales. A recent focus of paleoclimate research is the so-called 1500-year North Atlantic quasi-periodic cycle, and has revolved around three main themes. First, what are the underlying causes and physical mechanisms governing these millennial-scale variations? Next, are they global or restricted to certain sensitive regions of the planet? Last, what is the magnitude of the temperature changes of these variations, and do they vary in time and space? This dissertation explores millennial-scale climate variability in North America during the past 14,000 years using a dense network of fossil pollen data, which is used as proxy for climate variations. Three independent approaches are used to quantify these changes. A mixture modelling analysis of radiocarbon dates on pollen transitions, a principal component analysis of pollen diagrams from all of North America, and a mean July temperature reconstruction based on the method of modern analogue (MAT) all reveal millennial-scale climate variability throughout North America during the past 14,000 years. The identified transitions generally correlate well with other proxy-climate records from the North Atlantic region. However, certain mismatches occurred particularly at 9, 6 and 4 ka BP. If we assume the dominant millennial-scale period is 1150-years, the records become more consistent. North American temperature variability was not unidirectional nor uniformly distributed in space, suggesting large-scale ocean-atmospheric reorganizations at the transitions. Correlation between the proxy-climate and cosmogenic nuclide records supports a variable solar output hypothesis as the fundamental cause for century to millennial-scale climate variability. The mean July temperature of North America varied on the order of 0.2 to 0.4°C during the Holocene and 0.4° and 0.6°C during the deglaciation. Temperature was more variable during the late glacial, possibly due to the impact on the climate of massive meltwater pulses into the North Atlantic, further amplified through ocean dynamic processes. Recent global warming estimated as an increase in temperature of 0.4--0.6°C, is therefore greater than the estimated natural variation of the past 10,000 years, providing further evidence that recent increases in atmospheric CO2 have played a major role in modern warming.