"Today, we face the reality that human activities have altered the Earth's atmosphere and changed the balance of our natural climate."⁽¹⁾

Climate change has often been described as "one of the most pressing environmental challenges."⁽²⁾ Our lifestyles, our economies, our health and our social well-being are all affected by climate. Changes in climate have the potential to impact all regions of the world and virtually every economic sector. Although impacts will not be evenly distributed around the globe, all countries will need to deal, in one way or another, with climate change.

Our Changing Climate

"An increasing body of observations gives a collective picture of a warming world and other changes in the climate system"⁽³⁾

Climate is naturally variable, and has changed greatly over the history of the Earth. Over the past two million years, the Earth's climate has alternated between ice ages and warm, interglacial periods. On shorter time scales, too, climate changes continuously. For example, over the last 10 000 years, most parts of Canada have experienced climate conditions that, at different times, were warmer, cooler, wetter and drier than experienced at present. Indeed, with respect to climate, the only constant is that of continuous change.

There are a number of factors that drive climate variability. These include changes in the Earth's orbit, changes in solar output, sunspot cycles, volcanic eruptions, and fluctuations in greenhouse gases and aerosols. These factors operate over a range of time scales but, when considered together, effectively explain most of the climate variability over the past several thousand years. These natural drivers alone, however, are unable to account for the increase in temperature and accompanying suite of climatic changes observed over the 20th century (Figure 1).

Figure 1: global instrumental temperature record and modelled reconstructions: a) using only natural drivers, and b) including natural drivers, greenhouse gases and aerosols (from reference 4).

Over the last century, global mean surface temperature has risen by about 0.6ºC (Figure 1; reference 5). Although not unprecedented, this rate of warming is likely to have been the greatest of any century in the last thousand years.⁽⁵⁾ All regions of the world have not warmed by the same amount; certain areas have warmed much more than others, and some comparatively small areas have even experienced cooling. The timing of warming has also been variable. Most of the warming occurred over two distinct time periods of the 20th century (Figure 1a; reference 5); there have been seasonal differences in the amount of warming observed (see reference 6 for Canadian data); and night-time minimum temperatures have increased by about twice as much as daytime maximum temperatures.⁽⁵⁾

This warming observed over the 20th century has been accompanied by a number of other changes in the climate system.⁽⁵⁾ For example, there has very likely been an increase in the frequency of days with extremely high temperatures, and a decrease in the number of days of extreme cold.⁽⁵⁾ Global sea level has risen, while sea-ice thickness and extent has decreased. The extent of snow and ice cover has very likely declined, and permafrost thickness has decreased in many northern areas. In the northern hemisphere, annual precipitation has very likely increased and heavy precipitation events have likely become more common.⁽⁵⁾

Why have these changes in climate been occurring? Much research has addressed this question, and the answer has become increasingly confident over time: "most of the warming observed over the last 50 years is attributable to human activities."⁽³⁾ That is to say that recent changes in climate can only be explained when the effects of increasing atmospheric concentrations of greenhouse gases are taken into account (Figure 1).

The Greenhouse Effect

Greenhouse gases, such as water vapour, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), are emitted through natural processes, including plant decomposition and respiration, volcanic eruptions, and ocean fluxes (e.g., evaporation). Once in the atmosphere, these gases trap and reflect heat back toward the Earth's surface through a process known as the greenhouse effect. Although this process is necessary for maintaining temperatures capable of supporting life on Earth, human activities, such as the burning of fossil fuels and land-use changes, have significantly increased the concentrations of greenhouse gases in the atmosphere over the past century. For example, the atmospheric concentration of CO2 has increased by about 30% since the industrial revolution, from 280 parts per million (ppm) in the late 1700s to about 372 ppm in 2002 (Figure 2; reference 7). Humans have also introduced other, more potent greenhouse gases, such as halocarbons (e.g., chlorofluorocarbons) to the atmosphere. This buildup of greenhouse gases due to human activity enhances the Earth's natural greenhouse effect.

Figure 2: Trends in atmospheric CO2, CH4 and N2O during the last 1 000 years (from reference 3).