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State and trends


2.8 Climate change


2.8.1 Introduction In North America, climate change traditionally occupied a theoretical dimension; something that will happen in the future. Evidence to the contrary was relegated to the Arctic where extreme transformations due to high temperatures have been underway for at least two decades. In the past decade, however, the effects of climate change are manifesting at lower latitudes: in hurricanes that generate human and infrastructural disasters in important cities, intense and enduring regional droughts, ecosystem and species migration that can disrupt livelihoods and lifestyles, and even in the temperature and precipitation extremes associated with shifting weather patterns experienced across the continent during all seasons. These extremes damage the built environment, challenge regional and global food security, and threaten the livelihoods and opportunities of citizens throughout both countries (Horton et al. 2015; Williams 2014; Scott et al. 2015; IPCC 2014a; AAFC 2015).


2.8.2 Newly erratic atmospheric systems


The West and South of North America have been experiencing drought and all parts of the continent have suffered from extreme summer heat waves since at least 2000. However, unexpected impacts are dominating the continent’s Atlantic coastal region. Patterns such as the North Atlantic oscillation and the Atlantic multi-decadal oscillation affect weather and climate from Hudson Bay to the Arabian Peninsula. Temperature trends over the past few decades show a conspicuous region of cooling in the northern Atlantic. This cooling trend tracks a slowing in the south to north Atlantic current, the Atlantic meridional overturning circulation (AMOC) that includes the Gulf Stream, over the 20th century and particularly since 1970. Some of this slowing is likely due to contributions of cold fresh water from the melting of the Greenland ice sheet. Close inspection of data suggests that the AMOC’s weakness after 1975 is unprecedented in the past millennium. Further melting of the Greenland ice sheet in the coming decades could contribute to further weakening


Figure 2.8.1: An analysis of pressure over North America


Shown above is an analysis of pressure over North America for February 20th, 2015 illustrating a strong ridge of high pressure over the western USA and Alaska and a deep trough in the east, which is allowing polar air to drive south. Source: Adapted from Weather Bell 2015


of this south to north circulation in the North Atlantic. An accelerated rate of sea level rise along the continent’s east coast is already apparent and attributed to this slowing of the Gulf Stream (Rahmstorf et al. 2015).


These circulation changes may bring anomalously persistent and cold oceanic and atmospheric temperatures to the North Atlantic that also enhance sea-level rise along the coastline of North America. At the same time, the ice-free status of the Arctic coast in summer may be another complex factor contributing to the very apparent changing climate on the North American continent (Francis and Skific 2015; Rahmstorf et al. 2015).


New research suggests that disproportionate Arctic warming and the consequent weakening of expected equator-to- pole temperature gradients cause the northern hemisphere


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