Interaction between Climate Change and Greenhouse Gas Emissions 5 1.2 PAST AND FUTURE CLIMATE CHANGE Changes in climate are not new: Earth has long been subjected to sequential glacials, interglacials, and warm periods, and all parts of Canada have been warmer, cooler, wetter, and drier at various times in the past. A number of natural factors control climatic variability, including Earth’s orbit, changes in solar output, sunspot cycles, and volcanic eruptions ( Chapter 2 ). However, the present climatic change is unprecedented in character: it cannot be explained by these factors alone. The recently observed increase in global temperature is strongly related to increases in the concentration of GHGs in the recent past, 2 increases that are directly attributable to human activities. Over the course of the 20th century global mean temperature has risen by about 0.6°C, and is projected to continue to rise at an average rate of 0.1 to 0.2°C per decade for the next few decades then increase to a rate of warming of between 1.4 and 5.8°C per decade by 2100. 2 Average temperatures across Canada are expected to rise at twice the global rate. In general, Canadian temperatures have been increasing steadily over the last 58 years, with winter temperatures above normal between 1985 and 2005 ( Figure 1.2 ). At the same time, in general, over the last 58 years, winter precipitation has been decreas- ing ( Figure 1.3 ) across Canada. In southern Canada, surface temperatures have increased by 0.5 to 1.5°C during the 20th century. The greatest warming has occurred in western Canada, with up to 6°C increase in the minimum temperature. In addition, the frequency of days with extreme temperature, both high and low, is expected to increase, snow and ice cover to decrease, and heavy precipitation events to increase. 3 During the second half of the 21st century, heat sums, measured in growing degree days, across southern Canada are expected to increase by between 40 and 100%.