There is significant evidence that northern Canada's climate has already undergone substantial change. These changes have meant significant impacts for physical, natural and human systems in Canada's North. Climate models suggest that such trends will continue into the future, and therefore shifts in Arctic systems are expected for some time to come. This introductory paper is the first in a series published in two issues of Ambio presenting work conducted for northern chapters of two recent Canadian national climate science assessment initiatives, From Impacts to Adaptation: Canada in a Changing Climate 2007 and Human Health in a Changing Climate: A Canadian Assessment of Vulnerabilities and Adaptive Capacity. Collectively, these assessments involved the work of 24 scientists with expertise in a variety of disciplines and regions in the Canadian North. These assessment processes adopted aspects of a vulnerability approach to climate assessment, primarily through a review of existing and projected exposures and elements of adaptive capacity based on existing literature. In so doing, they have contributed towards a more comprehensive understanding of climate impacts and adaptations across the northern regions of the country. This paper provides an overview and introduction to the series of papers contained in the two issues of Ambio.

Understanding the implications of climate change on northern Canada requires a background about the size and diversity of its human and biogeophysical systems. Occupying an area of almost 40% of Canada, with one-third of this contained in Arctic islands, Canada's northern territories consist of a diversity of physical environments unrivaled around the circumpolar north. Major ecozones composed of a range of landforms, climate, vegetation, and wildlife include: Arctic, boreal and taiga cordillera; boreal and taiga plains; taiga shield; and northern and southern Arctic. Although generally characterized by a cold climate, there is an enormous range in air temperature with mean annual values being as high as −5°C in the south to as low as −20°C in the high Arctic islands. A similar contrast characterizes precipitation, which can be >700 mm y⁻¹ in some southern alpine regions to as low as 50 mm y⁻¹ over islands of the high Arctic. Major freshwater resources are found within most northern ecozones, varying from large glaciers or ice caps and lakes to extensive wetlands and peat lands. Most of the North's renewable water, however, is found within its major river networks and originates in more southerly headwaters. Ice covers characterize the freshwater systems for multiple months of the year while permafrost prevails in various forms, dominating the terrestrial landscape. The marine environment, which envelops the Canadian Arctic Archipelago, is dominated by seasonal to multiyear sea ice often several meters thick that plays a key role in the regional climate. Almost two-thirds of northern Canadian communities are located along coastlines with the entire population being just over 100 000. Most recent population growth has been dominated by an expansion of nonaboriginals, primarily the result of resource development and the growth of public administration. The economies of northern communities, however, remain quite mixed with traditional land-based renewable resource-subsistence activities still being a major part of many local economies.

This article reviews the historical, instrumental, and future changes in climate for the northern latitudes of Canada. Discussion of historical climate over the last 10 000 years focuses on major climatic shifts including the Medieval Warm Period and the Little Ice Age, and how these changes compare with those most recently experienced during the period of instrumental records. In reference to the latter, details are noted about observed trends in temperature and precipitation that have been recorded over the last half century, which exhibit strong west to east and north to south spatial contrasts. A comprehensive review of future changes is also provided based on outputs from seven atmosphere–ocean global climate models and six emission scenarios. Discussion focuses on annual, seasonal, and related spatial changes for three 30-year periods centered on the 2020s, 2050s, and 2080s. In summary, substantial changes to temperature and precipitation are projected for the Canadian North during the twenty-first century. Although there is considerable variability within the various projections, all scenarios show higher temperature and, for the most part, increasing precipitation over the entire region.

The physical environment of the Canadian North is particularly sensitive to changes in climate because of a large concentration of cryospheric elements including both seasonal and multiyear forms of freshwater and sea ice, permafrost, snow, glaciers, and small ice caps. Because the cryosphere responds directly to changes in air temperature and precipitation, it is a primary indicator of the effects of climate variability and change. This article reviews the major changes that have occurred in the recent historical record of these cryospheric components at high latitudes in Canada. Some changes have been less pronounced in the Canadian North than elsewhere, such as changes in sea-ice coverage, whereas others have been potentially more significant, such as ablation of the extensive alpine and high-Arctic small glaciers and ice caps. Projections of future changes are also reviewed for each cryospheric component. Discussion about two other physical components of the North intrinsically linked to the cryosphere is also included, specifically: i) freshwater discharge to the Arctic Ocean via major river networks that are fed primarily by various forms of snow and ice, and ii) the related rise in sea level, which is strongly influenced by ablation of the cryosphere, and coastal stability, which also depends on the thermal integrity of coastal permafrost.

Northern Canada is projected to experience major changes to its climate, which will have major implications for northern economic development. Some of these, such as mining and oil and gas development, have experienced rapid expansion in recent years and are likely to expand further, partly as the result of indirect effects of changing climate. This article reviews how a changing climate will affect several economic sectors including the hydroelectric, oil and gas, and mining industries as well as infrastructure and transportation, both marine and freshwater. Of particular importance to all sectors are projected changes in the cryosphere, which will create both problems and opportunities. Potential adaptation strategies that could be used to minimize the negative impacts created by a climate change are also reviewed.

Climate variability and change is projected to have significant effects on the physical, chemical, and biological components of northern Canadian marine, terrestrial, and freshwater systems. As the climate continues to change, there will be consequences for biodiversity shifts and for the ranges and distribution of many species with resulting effects on availability, accessibility, and quality of resources upon which human populations rely. This will have implications for the protection and management of wildlife, fish, and fisheries resources; protected areas; and forests. The northward migration of species and the disruption and competition from invading species are already occurring and will continue to affect marine, terrestrial, and freshwater communities. Shifting environmental conditions will likely introduce new animal-transmitted diseases and redistribute some existing diseases, affecting key economic resources and some human populations. Stress on populations of iconic wildlife species, such as the polar bear, ringed seals, and whales, will continue as a result of changes in critical sea-ice habitat interactions. Where these stresses affect economically and culturally important species, they will have significant effects on people and regional economies. Further integrated, field-based monitoring and research programs, and the development of predictive models are required to allow for more detailed and comprehensive projections of change to be made, and to inform the development and implementation of appropriate adaptation, wildlife, and habitat conservation and protection strategies.