The earth's climate has suffered through many changes over time, with events ranging from ice ages to warm, interglacial periods (such as the present era) documented. However, the most rapid warming period observed has occurred in recent decades and scientists from the Intergovernmental Panel on Climate Change (IPCC) [18, 19] have determined that there is a greater than 90 percent chance that most of the current warming is due to the increases in greenhouse gas (GHG) emissions caused by human activities, with the increase in global average temperatures resulting in severe effects on the Earth's climate.
Data reported by the IPCC show that the concentration of atmospheric CO2 has increased from a pre-industrial age value of approximately 280 to 379 ppm in 2005 (see Table 1.1), with emissions from fossil fuel combustion being the main source of this greenhouse gas. The energy sector is central to this, accounting for over 60%  of the world's greenhouse gas emissions. As indicated in Section 3.3.5, the energy sector generates emissions of all the principal GHGs and, in particular, carbon dioxide, methane and nitrous oxide, throughout the whole fuel life-cycle, from extraction to combustion. Emissions of GHGs also depend on external factors such as weather conditions, economic growth, and fuel prices, among others.
According to data from the International Energy Agency , in 2005 the United States ranked first in CO2 emissions from fuel combustion, while Canada and Mexico were seventh and twelfth, respectively. North America was therefore one of the major contributors of greenhouse gases in the world. Figure 1.3 shows that the contribution of North America to global GHG emissions was 19.4% that year. Canada and Mexico each contributed less than 2% and the United States contributed 16%. GHG emissions from electricity generation using fossil fuels in North America accounted for 6% of the global GHG emissions in 2005. Data in this figure were compiled based on the six major IPCC categories (energy, industrial processes, solvent use, agriculture, land-use change and forestry, and waste); and the global GHG emissions estimate from the World Resources Institute .
Figure 1.4 shows CO2 emissions per capita for 1990–2007 from fossil fuel-based electricity generation in North America. The emissions data used to create this graph were taken from the GHG emissions inventory reports of each country [22, 23, 24]. A slight decreasing trend is observed after year 2000 in all three countries but the data also suggest that per capita emissions increased from 1990 to 2000. The per capita CO2 emission values estimated on the basis of the 2005 data used for the present report were 3.75, 1.12 and 8.07 t/capita for Canada, Mexico and the United States, respectively; these are in close agreement with the values shown in this figure.
In the period from 1990 to 2005, total national GHG emissions increased by 23.5% . In 2005, Canada contributed 2% of the total worldwide emissions of GHGs, but it was one of the highest per capita emitters, with 22.7 t CO2-eq of GHGs per capita. However, a slight reduction of 1.3% in GHG emissions was observed between 2004 and 2005. Among all the greenhouse gases, CO2 comprised the largest portion of Canada’s emissions from all GHG sources, with 78% .
As in Mexico and the United States, energy-related activities, such as stationary combustion sources, transport, mining, and oil and gas exploration activities generated by far the largest portion of GHG emissions in Canada, with 81% of the total. Electricity generation (including utilities and industry) contributed 17% of that total, with CO2 accounting for over 99% of the GHG emissions from this sector . Per capita CO2 emissions from electricity generation in 2005 were 3.85 t CO2 (see Figure 1.4), whereas the per capita share of CO2 emissions per Gross Domestic Product (GDP) was 110 g/US$.1
2Data for Mexico are for 2006