A.1 What is climate and how does it differ from weather?

Response: Climate describes average day-to-day weather for a specific location or region experienced over an extended period of time. In many respects, climate is what we can expect, and weather is what we get. For example, the climate of Edmonton indicates that we should expect maximum temperatures on an average day in January to reach -8°C. However, in January 2007, actual daily maximum temperatures varied from a low of -20.3°C on January 13 to a high of +4.3°C on January 18.

Explanation: Weather in any particular location or region can change from hour to hour, day to day, season to season, and year to year. Such changes include shifts in temperature, snow and rainfall, winds, and clouds. They are caused by a complex interplay of various factors, including rapid shifts in global wind patterns, slower variations in ocean conditions, and seasonal changes in the amount of sunshine. Averaging over an extended period of time - usually at least 30 years - allows the characterization of weather we might expect at that location or region. Such climate statistics can also be used as a reference for assessing the probability of getting weather that significantly differs from these averages, including the risk of extreme weather events.

A.2 What is climate change?

Response: Climate change is a long-term shift or alteration in the climate of a specific location, region or the entire planet. The shift is measured by changes in some or all of the features associated with average weather, such as temperature, wind patterns and precipitation. It can involve both changes in average weather conditions and changes in how much the weather varies about these averages. "Climate change" is distinguished from "climate variability" by the persistence of the change over time so that a measurable difference is observed between two periods of time.

Explanation: At the global scale, climate change occurs in response to a change in the amount of energy flowing into or out of the Earth's climate system. This occurs when something alters either the amount of the sun's radiation absorbed by the Earth's atmosphere and surface, or the amount of heat radiation emitted from the Earth's surface and atmosphere to space (see Figure A.4). The climate system responds to this imbalance in energy input versus output by warming or cooling until a radiation energy balance is restored. Since the factors that cause the initial change in the energy balance push or 'force' the climate to change, these factors are generally referred to as 'climate forcings'. Colloquially, positive forcings are often referred to as 'warming factors' while negative forcings are called 'cooling factors'. Climate forcings can be natural phenomena or can arise from human activities.

The factors that affect regional climate change are much more complex. That is because, in addition to being affected by global climate change, regional climates are also affected by a myriad of other factors operating on smaller time and space scales, and by changes in wind and ocean patterns due to internal fluctuations of the climate system.

A.3 What is the difference between climate change and global warming?

Response: Global warming (as well as global cooling) refers specifically to a sustained warming (cooling) of the global average surface temperature. Global warming is often misunderstood to imply that the world will warm uniformly. In fact, it will affect the climate of one region very differently from another. As a result, some areas of the world will warm more, while others will warm less than the average. Some areas may even cool. Furthermore, an increase in average global temperature will also cause changes in other aspects of the climate system, such as precipitation and winds, affecting weather patterns around the world. In other words, global warming is only one aspect of climate change. Hence, the term 'climate change' more clearly describes the situation that the world is facing today.

Explanation: The initial response of the Earth's atmosphere to a 'climate forcing'¹ is a change in flow of solar and heat energy through the atmosphere that in turn causes temperatures at the surface, in the atmosphere and within the oceans to change. However, these changes in temperature are more rapid over land than water, and can cause changes in many other aspects of the climate. For example, warmer temperatures would cause more evaporation, higher concentrations of water vapour in the atmosphere, changes in cloud cover and in rain or snowfall, more snow and ice melt, and changes in winds and ocean currents, and so forth. Many of these secondary changes also affect temperature, resulting in a complex interplay of different processes that may amplify the increase in temperature in some regions and moderate changes, or even cause cooling, in others. In other words, a climate forcing that causes global warming also causes many other aspects of the climate to change in complex ways. Therefore, the term 'climate change'² is the more accurate description of how the climate system responds to a forcing.

¹ See A.2 for a description of climate forcing.
² The term 'climate change' is used preferentially throughout this document. The term 'global warming', when it is used, refers specifically to the increase in global average surface temperature.

A.4 What is the "greenhouse effect" and how does it influence the climate system?

Response: The greenhouse effect describes the role of the atmosphere in insulating the planet from heat loss, much like a blanket on our bed insulates our bodies from heat loss. The small concentrations of "greenhouse gases" within the atmosphere that cause this effect allow most of the sunlight to pass through the atmosphere to heat the planet. However, these gases absorb much of the outgoing heat energy radiated by the Earth itself, and return much of this energy back towards the surface. This keeps the surface much warmer than if these gases were absent. This process is referred to as the 'greenhouse effect' because, in some respects, it resembles the role of glass in a greenhouse. The greenhouse effect makes the Earth livable. Without it, the Earth would be too cold to support life as we know it.

Explanation: The Earth is heated by sunlight. In turn, the Earth radiates heat energy out to space. It is this balance between incoming solar (shortwave) radiation and outgoing infrared (longwave) radiation that determines the temperature of the Earth. However, gases and solid and liquid particles within the atmosphere, as well as properties of the Earth's surface, affect the flow of solar and heat energy by reflecting, scattering or absorbing and re-radiating some of it. About 31% of the incoming sunlight is reflected back to space by clouds and the Earth's surface. The remainder of the solar energy warms the Earth's surface, oceans and atmosphere. Much of the harmful ultraviolet part of sunlight is absorbed in the stratosphere by ozone (O₃). Thus, the ozone layer not only protects the Earth's ecosystems from harm, it also retains a portion of the sun's energy in the upper atmosphere. However, while some atmospheric particles can absorb significant amounts of solar energy, most gases within the atmosphere absorb very little, allowing most of the Sun's energy to pass through to warm the surface. The warmed Earth then emits heat energy (infrared radiation) back towards space. Most of this outgoing radiation is absorbed by clouds and molecules of greenhouse gases in the lower atmosphere. These re-radiate the energy in all directions, some back towards the surface and some upward, where other molecules higher up can absorb the energy again. This process of absorption and re-emission is repeated until the energy escapes from the atmosphere to space. Since much of this heat energy has been recycled downward, surface temperatures become much warmer than if the greenhouse gases were absent from the atmosphere. This natural process is known as the greenhouse effect. Without naturally occurring greenhouse gases, such as water vapour, CO₂, CH₄ and N₂O, the Earth's average temperature would be -19°C instead of +14°C, or 33°C colder. Over the past 10,000 years (the period since the end of the most recent glaciation), the amount of these greenhouse gases in our atmosphere has been relatively stable. Then a few centuries ago, their concentrations began to increase due to human activities. This has enhanced the natural greenhouse gas effect, and caused the Earth's climate to change.

Reference: Le Treut et al., 2007

Figure A.4. A simple diagram of the natural greenhouse effect. Naturally occurring greenhouse gases in the atmosphere insulate the Earth from heat loss. The Earth's average temperature is much warmer than it would otherwise be because of the natural greenhouse effect. In a stable climate, the net solar energy absorbed by the Earth's atmosphere, surface and oceans is, on average, equal to the net heat energy returned back to space by the Earth's surface and atmosphere. Enhancing the greenhouse effect will alter the Earth's energy balance and will have a warming effect on Earth's climate (Environment Canada, 2007a).

A.5 What are the primary gases that produce the natural greenhouse effect and what are their relative roles?

Response: Important naturally occurring greenhouse gases include water vapour, CO₂, CH₄, O₃ and N₂O. Without the natural greenhouse effect, Earth's average temperature would be -19°C instead of +14°C, or 33°C colder. About two thirds of the natural greenhouse effect is from water vapour. Another one quarter is due to CO₂.

Explanation: Water vapour is the single most important absorber of the outgoing long-wave infrared radiation. If the radiative effects of other greenhouse gases are ignored, water vapour is responsible for roughly 60 to 70% of the natural greenhouse effect, while CO₂ alone would make up only about 25%.

However, the atmospheric concentration of water vapour varies in response to changes in temperature and other factors such as changes in soil moisture and vegetation. The total amount of water vapour in the atmosphere will increase under a warmer climate as a result of the atmosphere's increased ability to hold more water vapour before it becomes saturated and condenses the water vapour into raindrops or snowflakes. Furthermore, warmer surface temperatures will result in more evaporation of surface moisture into water vapour. The tight coupling between atmospheric temperature and atmospheric water vapour is the reason why changes in water vapour are considered a climate feedback rather than a climate forcing. The water vapour feedback is positive, meaning the increase in water vapour will cause additional absorption of infrared radiation, further enhancing the greenhouse effect.

References: Le Treut et al., 2007; Shine et al., 1990.

A.6 What causes climate change?

Response: Changes in climate can be caused both by natural events and processes and by human influences. Key natural factors include changes in the intensity of sunlight reaching the Earth and in the concentration of volcanic dust (which reflects and scatters sunlight) in the stratosphere. Both of these factors alter the amount of sunlight that is absorbed by the Earth's climate system. Changes in atmospheric concentrations of greenhouse gases due to natural processes have also contributed to past changes in climate. Key human influences include emissions of gases and particles that affect the atmospheric concentrations of greenhouse gases, cause O3 depletion in the stratosphere and create regional air pollution. Land use change due to expansion of agriculture, urbanization and other factors are also contributors. Most of these human influences affect the amount of heat energy escaping to space, although some also change the amount of sunlight reflected to space.

Explanation: Any factor that affects the balance between the amount of radiative solar energy absorbed by the Earth's climate system and the radiative heat energy released back to space pushes the climate towards a new state, and hence is a climate forcing. One example of a climate forcing that has been a regular feature of the Earth's climatic history is the changing annual and/or seasonal intensity of sunlight reaching the Earth. Some changes, like the large 100,000-year glacial-interglacial swings detected in polar ice core data and ocean sediments, appear to be caused by cyclic variations in the Earth's orbit around the Sun. These orbital changes affect both the distance between the Sun and the Earth and the angle of Earth's exposure to sunlight. Such long-term cycles can cause changes in average global surface temperatures on the order of 4 to 7°C between glacial and interglacial periods. For the past 10,000 years, the Earth has been in the warm interglacial phase of such a cycle. Other cycles in solar intensity are caused by changes in the amount of energy released from the Sun itself. These solar activity cycles can be on much shorter time scales, with the shortest being the well-known 11-year sunspot cycle. Finally, other natural changes in climate forcings include large eruptions of volcanoes, which can sporadically increase the concentration of atmospheric particles for short periods of time, temporarily blocking out more sunlight.

However, the magnitudes of naturally induced changes in climate during the current interglacial period have been much smaller than those caused by the long orbital cycles. Within the past several thousand years, for example, net changes in average global surface temperatures due to natural climate forcings appear to have been within a range of about 1°C.

Most scientists are now convinced that human activities are also changing the climate. The main cause of such change is the increasing atmospheric concentration of greenhouse gases. Particularly important is the increase in CO₂, which is released by humans primarily through the burning of fossil fuels (coal, oil and natural gas) and through deforestation. An increase in greenhouse gases enhances the natural greenhouse effect and leads to an increase in the Earth's average surface temperature. Emissions of other polluting gases and particles into the atmosphere can also be significant. However, many of these do not stay in the atmosphere long, and hence their roles in influencing climate change may be large at a regional scale but relatively modest when averaged globally. Some of these can also have opposing climate forcing effects. Dark, sooty aerosols, for example, tend to absorb both solar and heat radiation energy, and cause a warming influence. On the other hand, sulphate aerosols reflect and scatter incoming sunlight, both directly and by altering the amount and brightness of clouds, and tend to cool the climate. While the immediate effects of such aerosols will be felt primarily within the industrialized regions from where most emissions originate, aerosols can also indirectly alter average global temperatures and wind currents. Human-induced depletion of O₃ in the stratosphere also tends to cool the Earth's surface (see B.11). Finally, land use change can alter the albedo of the Earth's surface, making it either more or less reflective. In this way, changing land use can contribute to climate change.

A.7 Is there any evidence that past changes in greenhouse gas concentrations have been linked to climate change?

Response: Yes. For example, the relationship between greenhouse gases and climate change is strongly supported by the analyses of ice samples taken from deep within ice sheets in Antarctica and Greenland. These samples provide excellent archives of fossilized air bubbles trapped within the ice, and thereby provide a record of the variations in concentrations of atmospheric greenhouse gases over hundreds of thousands of years. The relative concentrations of different oxygen and hydrogen isotopes in the ice itself can also indicate how regional air temperatures have changed over time. These analyses indicate that the atmospheric concentrations of CO₂, CH₄ and N₂O have remarkably strong correlations with the air temperature over Antarctica and Greenland.

Explanation: Studies of polar ice cores have demonstrated that atmospheric greenhouse gas concentrations are linked to changes in past climate. The latest scientific analyses of ice core samples from Antarctica, for example, provide records of climate and greenhouse gas variations over the last 650,000 years. As shown in the accompanying figure, there is good agreement between these records. During glacial periods, average local temperatures in Antarctica were some 10°C colder than today, while CO₂, CH₄ and N₂O concentrations dropped to their lowest values of 200 parts per million (ppm), 400 parts per billion (ppb) and 220 ppb, respectively. During warm interglacials, when temperatures were similar to or slightly warmer than today, gas concentrations rose to 300 ppm, 700 ppb and N₂O ->280 ppb, respectively. These records also indicate that the current concentration of CO₂, CH₄ and N₂O are unprecedented for the last 650,000 years. There are also indications that, over million-year time scales, the Earth's climate was warm during periods of high CO₂ concentrations and much cooler during low CO₂ concentration periods, providing additional evidence of the tight coupling between greenhouse gas concentrations and climate. Current scientific understanding is that while changes in solar forcing were likely to have initiated climate warming or cooling, changes in greenhouse gas concentrations strongly amplified the initial change in climate.

Figure A.7. The greenhouse gas (CO₂, CH₄, and N₂O) and deuterium (δD) records for the past 650,000 years from the European Project for Ice Coring in Antarctica (EPICA) ice core sample and other ice cores, with marine isotope stage correlations (labelled at lower right) for stages 11 to 16. δD, a proxy for air temperature, is the deuterium/hydrogen ratio of the ice, expressed as a per mil deviation from the value of an isotope standard. More positive values indicate warmer conditions. Data for the past 200 years from other ice core records and direct atmospheric measurements at the South Pole are also included (Brook, 2005).

A.8 Ice core data indicate that, during glacial-interglacial cycles, changes in CO₂ concentrations lag those in polar temperatures. Doesn't this indicate that climate change causes CO₂ concentrations to change, not the other way around?

Response: Studies of ice core records indeed indicate that in the past, changes in CO₂ concentrations appear to have been a response to changes in climate, not the initial cause. However, there is clear evidence that the initial changes in climate, believed to be triggered by variations in the Earth's orbit around the Sun, resulted in a rapid release of greenhouse gases, particularly CO₂, that substantially enhanced the initial warming. In fact, model studies suggest that approximately half of the magnitude of the 4 to 7°C global temperature swing between glacial and interglacial periods can be attributed to greenhouse gas feedbacks.

Explanation: Analyses of polar ice core samples have demonstrated that, during the 650,000 years of that record, changes in CO₂ were never the initial cause of the slow swings in climate from glacial to interglacial conditions. Such swings appear to have been triggered by changes in the Earth's orbit around the Sun. However, the initial changes in climate appear to have caused a rapid increase in the release of CO₂, CH₄ and N₂O from various natural sources. The prime source of CO₂ emissions was likely the deep oceans. This response appears to have been so rapid that any lag between temperature change and CO₂ response was on the order of several centuries to one millennium - rapid on geological time
scales. Climate model simulations demonstrate that the full magnitude of temperature changes cannot be explained solely by the climate forcing due to orbital changes. Rather, the responsive changes in greenhouse gas concentrations appear to have contributed about 50% of the change in climate. This, in turn, provides evidence of the important role of greenhouse gases in the climate system, and that increases in their concentrations due to direct emissions by humans would also cause the climate to warm.

Reference: Jansen et al., 2007.