Climate is the average state, and our expectation, of the weather. It may be thought of as a local condition, or we can apply it to larger regions or to the whole planet. In recent years the concept has been applied not only to atmospheric behaviour, but also to the oceans, freshwater, glaciers and the living cover and soils of Earth. Taken together, these form the climate system. The usual measures of climate are air and sea temperatures, precipitation (rain and snow), wind velocity, humidity and cloudiness. We have fairly accurate tabulations of these quantities for the past century and can express climate in terms of their average values and variability, usually over 30-year periods.

Climate change is defined by the United Nations Framework Convention on Climate Change (UNFCCC) as "a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods."

The Greenhouse Effect

Earth's temperature is determined partly by the greenhouse effect, a natural process that retains heat in the atmosphere. Without this natural greenhouse effect, Earth's average temperature would be -18ºC instead of +15ºC. The greenhouse effect depends on the presence of various greenhouse gases (GHGs) in the atmosphere to trap the sun's heat near Earth's surface, thereby raising Earth's temperature and making life possible.

The thermodynamics of climate warming were understood in the 19th century, even though measurements in the upper atmosphere were not yet possible. The greenhouse effect was hypothesized by the French mathematician Joseph Fourier (1827), and linked to past changes in climate by the Swedish chemist Svante Arrhenius (1896). British scientist Guy S. Callendar (1938) was the first to propose that humans could actually emit greenhouse gases in sufficient quantities to induce a significant global warming. This was confirmed in the late 1950s by Charles Keeling, who had established a CO2 monitoring station on top of the Mauna Loa volcano, Hawaii, which is still operational today.

Today, measurements show that the concentration of the primary greenhouse gas, carbon dioxide, has already increased by 35% over preindustrial levels, largely as a result of emissions from the combustion of coal, oil and natural gas for energy, and from deforestation. Concentrations of other important greenhouse gases (eg, methane, nitrous oxide, tropospheric ozone and synthetic gases such as fluorocarbons) have also increased significantly as a result of anthropogenic (attributed to humans) emissions.

Some of these emissions remain in the atmosphere for decades or centuries, and may mix through both hemispheres and up into the stratosphere. This implies that climate change must be dealt with as a global issue. A concentration of greenhouse gases over Canada may have originated anywhere in the world, and at any time since the Industrial Revolution.

Climates Vary

Recent changes show in weather records. Longer-term changes can be identified by geological and chemical studies of Earth. These studies show that liquid water has been present throughout the history of Earth; hence, the world average surface-air temperature has always been above 0°C. In fact, this temperature was usually higher than the present 15°C. During the past 30 million years a cooling of the air and the oceans has taken place, culminating in repeated glacial episodes during the past 2 million years. The last cold episode peaked about 18 000 years ago, when ice sheets covered North America as far south as the Ohio and Missouri valleys and the plateaus of Washington. Parts of the Yukon, the Northwest Territories and Alaska, however, escaped glaciation.

About 10 000 years ago, Canada's climate appears to have rebounded to conditions much like those of today. The ice sheets melted slowly, vanishing over Labrador-Ungava and Keewatin about 4000 years ago. Forests and prairies recolonized the country and with them came animals and human populations (see Biogeography). Conditions were warmer than at present until about 2000 years ago, then cooled slightly.

Warming Effect

It has become clear that the world's surface has been getting warmer since the 19th century. Mean global annual surface temperature is believed to have risen by 0.6ºC to 0.9ºC in the past century. The 10 warmest years in the recorded global climate history of the past century were 1997, 1998, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 and 2009. During that time, 2005 had the largest departure from the average, with temperatures averaging 0.62ºC above the average for the 20th century. In Canada, the records of 132 carefully run stations across the country have shown a larger increase of 1.4ºC since 1948, when comprehensive nation-wide record keeping began. Precipitation is harder to measure, chiefly because of snowfall and its water content. Nationally, precipitation appears to have increased by about 10% between 1955 and 1980, and has remained high most years since then. Warming occurred until the 1940s, followed by moderate cooling into the mid-1970s and pronounced warming since the 1980s.

In the past 2 decades much new satellite data has been added to global climatic data. We also have measurements of the various trace gases that are thought to be influencing surface climate - notably carbon dioxide, methane, nitrous oxide, ozone and various pollutants, all greenhouse gases (see Air Pollution). All are increasing in concentration globally.

Does this observed global warming, stronger in Canada than elsewhere, signify a real change in climate? The atmosphere is endlessly variable in properties and behaviour, but these variations are normally seen as fluctuations about an unchanging average background; the fluctuations, in this view, are as much part of the climate as the averages. We have to ask whether the warming of the past century is something more lasting, ie, a genuine change in the climate itself, because the atmospheric changes have been accompanied by a rise in concentrations of the so-called greenhouse gases listed earlier. These gases retard the flow of solar energy back to space and hence tend to warm Earth's surface.

Predictive Models

One way to answer questions about climate change is to construct mathematical models of the global climate system and see how they behave when the greenhouse gases increase. This has been done in many centres around the world, including at the Meteorological Service of Canada (formerly Canada's Atmospheric Environment Service). The models predict that if the build-up continues at the predicted rates, temperatures will likely rise by at least another 1.1ºC by 2100, relative to today (for low future greenhouse gas emission scenarios and low climate sensitivity), and possibly by as much as 6.4ºC (for high emission scenarios and high climate sensitivity). Sea levels could rise by between 20 and 60 cm due to the thermal expansion of ocean waters only. Melting of glacial ice will add to those levels.

Canadian winters are likely to warm by 2 to 3 times the global average and become wetter. Summers in southern Canada are also expected to warm more than global averages and become drier; northern regions of Canada would warm much less in summer because of the influence of sea ice and the cold Arctic Ocean waters. The Intergovernmental Panel on Climate Change (IPCC) assesses the results of climate change studies. Most countries (including Canada) are signatories of a Framework Convention on Climate Change that commits them to measures aimed to bring the warming under control.

The IPCC has 3 working groups. Working Group I assesses the scientific aspects of the climate system and climate change. Working Group II addresses the vulnerability of socio-economic and natural systems to climate change, negative and positive consequences of climate change, and options for adapting to it. Working Group III assesses options for limiting greenhouse gas emissions and otherwise mitigating climate change. A task force carries out work on National Greenhouse Gas Inventories.

Study of Ancient Climates

Further evidence against which the present warming can be assessed comes from the study of ancient climates. There are rich repositories of information in such materials as annual tree rings, the pollen content of bogs and lake sediments (see Palynology), the composition of sediments in the deep ocean, the isotopic composition of cave calcite and aragonite, and above all the geochemistry of the major ice caps and their exchanges of water with the deep oceans. From these sources of information we have constructed a dated sequence of climatic changes back through the Holocene and Pleistocene epochs (with the alternation of ice ages and intervening warmer intervals in which we live ourselves) (see Geological History).

The geochemistry and lithology of the older rocks, even back to the Proterozoic Era in the planet's earlier days, have yielded a picture of how the atmosphere, oceans and continents have evolved, and alongside the dated physical record we have learned how life has evolved in partial harmony with the fluctuating climate. Much of this history of ancient climate has been gathered in the past 3 decades.

Against this ancient record the changes now in progress still seem impressive. For the Northern Hemisphere, it is likely that the warming of the past half-century is unprecedented in at least the past 1300 years. Unfortunately, data for the Southern Hemisphere remains as yet too sparse to draw conclusions for that region. Furthermore, a large variety of studies indicate that the recent warming cannot not be adequately explained by natural causes of climate change, but is very likely due to the effect of human emissions of greenhouse gases. It is therefore timely to conclude that the study of climate change is crucial to the welfare of our species, and of all other life; and that Canada is well placed to make a powerful contribution to our understanding of what is going on.

Our Natural Endowment

The predicted changes seem small. But the forests, grasslands, wetlands and tundra of our natural endowment are sensitive even to small changes in temperature and precipitation. So also are our energy consumption and the well-being of global society, particularly in tropical regions. These changes would dramatically alter ecozones across Canada and affect both the natural environment and the socio-economic and cultural activities of Canadians in many complex ways.

Some impacts would be beneficial, others costly and potentially disastrous. The most problematic consequences would likely be due to changed frequencies and intensities of extreme weather-related events, since these are difficult to predict and often exceed the ability of ecosystems and societies to cope.

Recent increases in extreme weather and climate events in Canada and globally have included severe loss of forests to fire, extreme floods, breakup of ice shelves on the Antarctic Peninsula and increased frequency of intense winter storms and damaging hurricanes. There is no proof that such unusual weather is already occurring as a result of global warming, but these events serve as useful examples of what may be in store for Canadians in future decades.

However, in our attempts to minimize our impact on the planet, we must take care to avoid the human tendency to rush headlong into an endeavour without fully realizing its potential for harm. An example is the effort to lessen the impact of the oil industry by replacing oil-based fuel with "clean" biofuel energy. If such fuels are derived from crops grown on lands that have been deforested to generate these crops, the carbon dioxide emissions from the deforestation process can far exceed any benefits in offsetting fossil fuel emissions.

On grounds of self-interest, therefore, we have no choice but to understand what is in progress, and proceed with caution to take any countermeasures we can devise.