Many regions of the world have experienced a net warming of the atmosphere over the past century. Evidence suggests that this warming may be due to more than just average natural variability in climate. It coincides with measured increases in concentrations of certain atmospheric gases, such as carbon dioxide, methane, and nitrous oxide, which can potentially enhance the natural "greenhouse effect" of the atmosphere. Although human activities are thought to be responsible for most of this rise in concentrations of greenhouse gases, it is not yet possible to offer a rigorous proof of this association. A number of approaches may, in time, be able to provide such proof, and trend analysis of climate elements and their derivatives is one of the most promising. In 1992, a State of the Environment Report (SOE Report No. 92-2) on temperature change in Canada attempted to answer the questions "What is happening?" and "Why is it happening?" It reported long-term trends towards higher annual temperatures in most regions of Canada. The report showed that Canadian mean daily air temperatures had risen by approximately 1.1°C over the 1895-1991 period compared with global temperature increases of about half that amount. The greatest increases were in the western and northwestern regions of Canada in the extreme northeastern regions of the country, the trend over the past half-century was slightly downward. The present report, another contribution of the Atmospheric Environment Service to the State of the Environment Report Series, introduces an additional question: "Why is it significant?" The answer is complex and incomplete, as research continues, but some provisional suggestions are made. The report's theme is climate variability and change. It expands on past discussions of temperature by considering seasonal and daily patterns of long-term temperature change. It indicates that the greatest rises have occurred in winter and spring and during the nighttime hours. Lake ice data are also examined, providing corroboration of temperature trend patterns. This analysis shows that there has been an average shortening of the winter ice season (by about a week), mostly attributable to earlier spring breakups. The report also examines Canadian trends in precipitation and cloudiness, two other climate elements that are significant to the understanding of global climate change. On a national scale, precipitation has been increasing since 1948. Considered regionally, the increases have been primarily over eastern and northern Canada. Elsewhere there is no trend or even a slight downward trend, as in the Prairie climate region. Since the 1950s, cloudiness, too, has been on the increase, with the Great Lakes/St. Lawrence climate region experiencing the greatest increases, in spring, in summer, and particularly in the fall. By contrast, the Pacific coast region has experienced a significant decrease in the fall. The areas that have had the greatest temperature increases (in western Canada) have not experienced proportional. increases in precipitation but are receiving more cloudiness. This may be a result of increased evaporation, as also seems to be the case in eastern regions, where there has been increasing precipitation with much lower temperature increases. These studies have not progressed far enough to furnish more certain evidence of cause and effect, but it can be said that none of the regional trends in climate is unexplainable in physical terms. Understanding long-term changes in climate depends not only upon a variety of physical evidence, but also on natural indicators such as plant and animal behaviour. This State of the Environment Report establishes a base for further multi-element analyses intended to broaden the factual foundation and increase our understanding of how global change can affect Canada.