About This Report

The CCCR draws primarily from existing sources of information that have been peer-reviewed and are publicly available. Exceptions include updates to published results, which were deemed acceptable as long as the method used to update data was citable. In addition, projections of future climate change for Canada, for some variables, are directly available as output from climate models. In this case, the projections specific to Canada have not always been published, although the models used to produce these projections have been comprehensively evaluated and documented in the peer-reviewed literature. Where relevant, quantitative information that has not been through external peer review was considered for inclusion; authors were required to judge the quality and reliability of the information and to maintain a copy of it. All chapters of this report underwent external peer review (see Section 1.3.2).

It is recognized that Indigenous observations and knowledge systems contribute significantly to our understanding of changing climate. This knowledge is incorporated in other reports of this assessment, where the holistic perspective of Indigenous knowledge systems contributes to a fuller understanding of climate change impacts and of the ability of human and natural systems to adapt.

Environment and Climate Change Canada, as the focal point for climate science expertise in the federal government, led the development of the CCCR. Authors with recognized expertise were invited to lead individual chapters of the CCCR. The lead author team, assessment coordinators, and representative stakeholders came together for an initial scoping meeting in February 2017 to discuss the overall aims and scope of the CCCR and to develop initial chapter outlines. Lead authors then formed chapter author teams with the required expertise to comprehensively assess chapter topics. Discussions at the scoping meeting with a panel of stakeholders provided key input to developing the CCCR to be effective and relevant.

External review of the report included both a targeted review by invited reviewers with specific subject-matter expertise and an open review involving other experts who registered through the National Assessment portal (https://www.nrcan.gc.ca/environment/impacts-adaptation/19924). In addition, members of the National Assessment Advisory Committee, National Adaptation Platform, and the Atmosphere-Related Research in Canadian Universities (ARRCU) working group were invited to participate in the external review.

Each chapter of the CCCR begins with a set of key messages: those findings the authors felt are most relevant to the target audiences. The key messages include quantitative information about how various components of the climate system — such as the atmosphere and cryosphere — have changed over specified periods of time and projections of additional change this century. These quantitative results emerge from the body of evidence assessed within each chapter of the CCCR. As a science assessment involves critically analyzing the knowledge base and deciding on the level of confidence in results (see Section 1.1), conveying the degree of certainty and uncertainty about results is important. In the CCCR, authors have done this for key messages by adopting the calibrated uncertainty language of the IPCC (Mastrandea et al., 2010⁶). This allows authors and audiences to distinguish between what is well known and widely accepted and what is not well known or not agreed upon.

Two metrics are used to communicate the degree of certainty about key messages (see Figure 1.2):

• Confidence in the validity of a result based on the type, amount, quality, and consistency of evidence (e.g., mechanistic understanding, theory, data, models, and expert judgment) and the degree of agreement. Confidence is expressed qualitatively.
• Likelihood of a result occurring based on quantified measures of uncertainty expressed probabilistically (based on statistical analysis of observations or model results, or expert judgment). Likelihood is expressed quantitatively.

When a quantified measure of uncertainty (i.e., likelihood) is provided but no confidence level is given, a high or very high confidence level is implied. Calibrated uncertainty language is italicized where used in the CCCR (e.g., likely or very likely), and definitions are provided in Figure 1.2 and at the first use of one of these terms in each chapter. Confidence statements are used more frequently in this report than are likelihood statements because the requirements for an assessment of likelihood – quantified estimates of uncertainty – were not available in the supporting literature in many cases.

Some Key Messages include statements that are expressed without a confidence level. These are not to be construed as “no confidence” statements. In fact, the opposite is true; these are factual statements. For example, a Key Message from Chapter 7 includes the statement “Relative sea level in different parts of Canada is projected to rise or fall, depending on local vertical land motion.” By definition, relative sea level is expressed as the mean sea level relative to a local reference land level; therefore, this statement is a fact. In another example in Chapter 4, the following statement is part of a Key Message: “Annual and seasonal mean temperature is projected to increase everywhere, with much larger changes in northern Canada in winter.” This is a conditional statement of fact. Based on the assumptions driving the climate model, the output is as described. In other cases, factual statements represent assessed findings for which evidence and understanding are overwhelming. There is an example of this in Chapter 2, where the first key message includes the statement “Warming of the climate system during the Industrial Era is unequivocal, based on robust evidence from a suite of indicators.”

The application of calibrated uncertainty language is an important feature of the scientific assessments undertaken by the IPCC, and the same approach is adopted here. In essence, the author teams identify relevant scientific literature, data, or model outputs; evaluate the amount of evidence and the level of agreement (or disagreement); and then apply their collective expert judgment to make an assessment of confidence, or where possible, likelihood. The wording of each key message is a collective effort of the chapter author team, aimed at providing clear and concise messages that are underpinned by the literature cited in the body of the chapters. Summary statements are included in each chapter to provide a traceable account of the authors’ assessment of the supporting evidence for key messages.

This report assesses observed and projected changes in climate for Canada. Therefore, it refers to both past and future time periods. Past changes are assessed over periods of time for which there are sufficient data records, either based on direct measurements (observations) and/or satellite data. This means that observed changes are described almost exclusively for time periods beginning sometime in the 20th century. The CCCR does not assess changes in current climate relative to conditions that existed in the distant past (paleoclimate) but does provide information on levels of greenhouse gases in the atmosphere going back to the mid-18th century using values directly measured from ice cores (see Chapter 2, Section 2.3).

To assess past changes in the climate, sufficiently long climate data records are needed for a detectable change to emerge from the natural fluctuations in the climate system. In general, estimates of change have smaller uncertainty with longer records. In the CCCR, different time periods for describing observed changes in climate are reported, reflecting the length of the available data record as well as the time when the work was completed. Some of these records extend back to the early 20th century, many begin after the mid-20th century, and satellite-based records start in the late 1970s at the earliest.

There are also no standard time periods for assessing and discussing future changes in climate. Time periods of interest depend on the needs and concerns of the user of the information; for some, near-term changes and impacts are of most concern, whereas, for others, their interest demands a longer-term view. In the CCCR, assessment of future climate changes is based primarily on results from coordinated experiments to model global climate (see Chapter 3, Box 3.1). These experiments have designated time periods describing near-term, mid-21st century, and late 21st century conditions. The near-term time period has, naturally, shifted forward in time while the mid-21st century (2046–2065) and late 21st century (2081–2100) time periods remained the same. In the CCCR, we consider 2031–2050, about 10–30 years from now, as the near-term period. Where changes in climate are discussed relative to the end-of-the-century, this should be understood to mean by the year 2100. The near-term and late-century time periods are used in Chapter 4 to provide projections of future changes for Canada for various indicators and indices related to temperature and precipitation. Elsewhere in the report, results from the fifth phase of the Coupled Model Intercomparison Project (CMIP5, see Chapter 3, Box 3.1) may be discussed with respect to the CMIP5 defined near-term time period (2016–2035). Climate models project future changes relative to a reference period. In the CMIP5 experiments, the reference period was 1986–2005, which was representative of “current climate” at the time the experimental set-up was established.

The core content of the CCCR is contained within Chapters 2 to 7. Chapters 2 and 3 focus on global-scale climate changes, and Chapters 4 to 7 focus on changes in climate across Canada. The global context presented in Chapters 2 and 3 is useful for understanding changes in Canada, recognizing the interconnectedness between changes occurring within a country or region and those occurring throughout the world. Chapter 2 provides a historical perspective on global-scale changes, summarizing observations of change throughout the Earth’s climate system and current understanding of the causes of these observed changes. Chapter 3 looks at future global-scale changes, describing the Earth system models used to project future climate change as well as the various emission scenarios — alternative plausible futures — used to drive these models. In addition to background on the methods of projecting future climate change, a synopsis of future changes at the global scale is presented, focusing on changes in temperature and precipitation, along with a discussion of how future global temperature change is related to the total amount of human emissions of carbon dioxide over time.

Chapters 4 to 7 each cover observed changes, understanding of causes of change (specifically, the contributions of human influences and natural climate variability), and future changes, for different components of the climate system in Canada. Chapter 4 assesses past and future changes in temperature and precipitation for Canada, including changes in temperature and precipitation extremes, and presents analyses of some recent individual extreme events and their causes. Chapter 5 covers the cryosphere — those parts of Canada with frozen water, including snow, sea ice, land ice (glaciers and ice caps), freshwater ice (lake and river ice), and permafrost. Chapter 6 assesses past and future climate-related changes to Canada’s freshwater availability through key components of the water cycle, including streamflow, surface water levels (lakes and wetlands), soil moisture, drought, and groundwater. Chapter 7 looks at changes occurring in the three oceans surrounding Canada, including physical and chemical changes.

The final chapter of this report (Chapter 8) provides a short synopsis of changes for Canada as a whole and then a synthesis of changes assessed in Chapters 4 to 7 for northern Canada and for the five regions of southern Canada. This chapter may be useful for readers who prefer an overview of changes by geographical region before reading in-depth about changes in different components of the climate system in the core chapters. This chapter also helps transition to subsequent reports being prepared as part of the National Assessment (see Section 1.1).