Science of Climate Change

FREQUENTLY ASKED QUESTIONS ABOUT THE SCIENCE OF CLIMATE CHANGE

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B. Human Influences on the Atmosphere

B.1 How much have atmospheric greenhouse gas concentrations increased in recent years?

Response: Since the Industrial Revolution began, concentrations of CO₂ have increased by about 31 percent, methane has more than doubled, and nitrous oxide has risen by 17 percent. There is clear evidence that these increases are mostly due to the burning of fossil fuels for transportation, heating and electricity and other human activities. Carbon dioxide accounts for about two thirds of the predicted increases in the greenhouse effect that these changes have caused to date.

Explanation: Data from cores extracted from polar ice sheets, which contain fossilized air bubbles that provide samples of the chemical composition of the atmosphere in the distant past, show that the atmospheric concentration of carbon dioxide was very stable between 10,000 and 250 years ago, remaining between 260 and 280 parts per million by volume (ppmv). During the past 250 years, this has increased to about 370 ppmv, with most of the increase occurring in recent decades. Meanwhile, the concentrations of methane and nitrous oxide, which were also both quite stable throughout the past 10,000 years, have increased by 151% and 17%, respectively.  Concentrations of ozone in the troposphere have also increased. Finally, there is also evidence of significant concentrations of a number of other trace gases, particularly halocarbons, that were largely absent in the pre-industrial atmosphere.

Reference: IPCC, 2001 WGI, pp 39-42.

B.2 How do scientists know that the atmospheric buildup of greenhouse gases is due to human activity?

Response: A number of factors clearly point to the role of human activities as the primary source of these increases in greenhouse gas concentrations. For example, the current rate of rise in concentrations agrees well with changes in rate of human emissions, and is unprecedented in many millennia of atmospheric history.  Furthermore, trends in ratios of carbon isotopes in atmospheric carbon dioxide and in the distribution of CO₂ in the atmosphere are consistent with emissions from human sources. Similar evidence demonstrates the role of humans in increases in the other greenhouse gases.

Explanation: The rapid rise in greenhouse gas concentrations during the past century is consistent with trends in human emissions, and unprecedented in at least the last 420,000 years and likely in the past 20 million years. Furthermore, the concentration of CO₂ molecules in the atmosphere containing the radioactive carbon 14 atom (after adjustment for atomic explosion testing activities in the 1950s) is declining. This is consistent with increased concentrations of burning of coal, oil and natural gas, all of which contain ‘old' carbon that has no carbon 14. Changes with time in ratios of carbon 13 and carbon 12 in oceans are also consistent with human emissions, as is the north-south gradient in atmospheric concentrations of CO₂. Finally, carbon budget models, which can now reproduce the global carbon cycle quite accurately, point to human emissions. Similar studies have been undertaken for methane and nitrous oxide, which also indicate a major human contribution. However, the exact magnitude of the human role for these gases is less well understood because of the uncertainty surrounding the many biological processes involved in both their natural and human emissions. Finally, trace gases such as the halocarbons and sulphur hexafluoride have no significant natural sources. There is strong evidence that changes in their concentrations are entirely caused by human emissions.

B.3  The amount of carbon dioxide (CO₂) added to the atmosphere by human activities each year is only a small fraction of that released from natural sources. How can our actions significantly change the concentration of atmospheric CO₂?

Response: Over thousands of years, the large natural emissions of CO₂ into the atmosphere by oceans and land ecosystems have been almost perfectly offset by the large amounts of CO₂ removed from the atmosphere through natural processes such as photosynthesis and ocean absorption.  Human emissions have upset this balance.  Just as an accumulating deficit in a financial budget can cause a large debt, this imbalance has, over time, caused a large accumulation of additional carbon dioxide in the atmosphere.

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Explanation: Human emissions of carbon dioxide into the atmosphere, currently estimated at about 28 billion tonnes annually, represent approximately 5% of the average natural flow of carbon dioxide into the atmosphere through plant and soil respiration and venting from the surface waters of the oceans (a total of about 550 billion tonnes each year).  However, natural emissions are offset by the natural absorption processes such as the uptake of CO₂ by plant photosynthesis, as well as absorption by the oceans. Like a bank account, changes in the amount of carbon dioxide in the atmosphere (the "balance" of the global carbon budget) are determined by the net average difference between inflow (emissions, or ‘sources') and outflow (uptake, or ‘sinks'), not by the magnitude of the flows themselves.  Air samples from the distant past, trapped as bubbles within ice buried deep within the Greenland and Antarctic polar ice sheets, can provide good indicators of how this ‘balance' has changed over the past 420,000 years. These provide clear evidence that, during the pre-industrial period of the current interglacial (the past 10,000 years), the atmospheric concentration of carbon dioxide varied by only a few percent from an average value of 280 parts per million by volume (ppmv).  This implies that the natural carbon budget was, on average, well balanced (i.e. on average, inflow equaled outflow) during this time period.  This, together with other sources of evidence, indicates that the cumulative effect of a small but increasing imbalance introduced into the carbon budget by humans is the principle cause for the 31% increase in CO₂ concentrations noted over the past several centuries. It is, in effect, the accumulated human ‘debt' within the global carbon budget.

Reference: IPCC, 2001 WGI, Chapter 3.

B.4 Don't volcanoes naturally release far more CO₂ into the atmosphere each year than humans?

Response: No. On a global scale, volcanoes release less than 1% of human emissions of carbon dioxide and hence are a minor contributor to changes in its atmospheric concentrations. Furthermore, emissions from volcanoes have always been part of the natural cycle, which has been in approximate balance for many millennia, until the industrial revolution.

Explanation: Most recent estimates by volcanic experts with the U.S. Geological Survey suggest  that, globally, volcanoes release about 150 million tonnes (Mt) of CO₂ into the atmosphere each year. By comparison, humans annually emit more than 22 billion tonnes (Gt) of CO₂ from fossil fuel combustion alone, and another 6 or so Gt of CO₂ from deforestation activities.  That is more than 100 times as great as volcanic emissions.

Mount Etna, in Sicily, is the largest single volcanic emitter of CO₂, estimated at 25 Mt of CO₂ per year. By comparison, emissions from Mount St. Helens following its eruption several decades ago were less than 2 Mt of CO₂/year.

Reference: Gerlach 1991.

B.5  Which human activities contribute the most greenhouse gases to the atmosphere

Response: The use of fossil fuel currently accounts for between 70 and 90% of all human emissions of carbon dioxide. Fossil fuels are used for transportation, manufacturing, heating, cooling, electricity generation, and other applications. The remainder of the carbon dioxide emissions comes from human land use activities -- ranching, agriculture and the clearing and degradation of forests.  For other greenhouse gases, primary sources include the production and transport of fossil fuels, agricultural activities, waste management and industrial processes.

Explanation: Each year, humans release more than 22 billion tonnes of carbon dioxide into the atmosphere through the burning of fossil fuels for energy. However, deforestation activities, forest degradation and agricultural land mismanagement also add large amounts of carbon dioxide emissions - between 2 and 9 billion tonnes - each year. Some, but not all of these land use emissions are being offset by growth of new forests and improved soil management in some regions of the world.

Methane emissions occur both naturally and as a result of human activities. It is the second most significant greenhouse gas, next to carbon dioxide. Rice cultivation, cattle and sheep ranching, and decaying material in landfills all release methane, as do coal mining, oil drilling operations, and leaky gas pipes. Nitrous oxide come from both natural sources and human activities. Fossil fuel combustion, industrial practices, and agricultural practices including the use of chemical fertilizers all increase atmospheric nitrous oxide. The industrial production of chlorofluorocarbons (CFCs)þand other halocarbons - used in refrigeration, air conditioning, and as solvents - have added other greenhouse gas, but many of these sources are gradually being eliminated under existing international agreements because they deplete the stratospheric ozone layer. Ozone in the troposphere (the lower part of the atmosphere) is another important greenhouse gas resulting from industrial activities. It is created naturally, but is also produced by atmospheric reactions caused by smog precursors such as nitrogen oxide from motor vehicles and power plants.

In Canada, about one-third of all greenhouse emissions are caused by the production of energy for Canadians and for exports, almost equally divided between the production of electricity from the combustion of fossil fuels and the exploration and production of coal, oil and natural gas for the energy market. Another 27% is produced by the transportation of goods and people across Canada - whether by truck, car, airplane, train, boats or other means. Manufacturing and construction add another 17%, while non-electrical use of energy in the residential, commercial and institutional sector emit about 13% and the agricultural sector almost 9%.

Similar sources of emissions occur in other countries, although the ratios differ with the type of economy, culture and climate.

Reference: Environment Canada Report on Greenhouse Gas Inventory (2002).

B.6 Humans also release a lot of carbon dioxide into the atmosphere by breathing. Are we supposed to stop breathing to stop climate change?

Explanation: Each human being takes in large volumes of carbon each year through the plant food, meat and fish he or she eats. That food carbon was amassed through photosynthetic and other processes that removed carbon dioxide from the atmosphere, either directly (for plants) or through the food chain (for animals, birds and fish). Most of the carbon taken in by a human is removed again through respiration (as carbon dioxide) and body wastes. During the years of growth, a small amount is retained each year to accumulate human body mass. This represents a small net sink for carbon, and stops accumulating for a mature human of stable mass. Upon death, many human bodies are buried underground, providing a small long-term sink for carbon.

Unlike ruminant animals, which have different digestive systems, gases erupting from humans through flatulence or belching contain very little methane and hence do not contribute significantly to increased methane concentrations.

B.7 I understand water vapour dominates the natural greenhouse effect. Doesn't this make changes in the concentrations of other greenhouse gases insignificant?

Response: No! While water vapour represents about two-thirds of the natural greenhouse gases, changes in its concentrations are determined primarily by changes in atmospheric temperature and related effects on the hydrological cycle. As increases in other greenhouse gases warm the atmosphere and surface, the amount of water vapour also increases, amplifying the initial warming effect of the other greenhouse gases.

Background: Water vapour is indeed one of the most potent and abundant greenhouse gases in the atmosphere. If the effects of all greenhouse gases other than water vapour were ignored, the natural greenhouse effect would be about 60-70% of observed values, compared to about 25% if only CO₂ were present. However, humans have little direct effect on water vapour concentrations. Rather, its concentrations respond to changes in temperature and other natural atmospheric processes. Warmer atmospheric temperatures, whether caused by increased greenhouse gas concentrations or other causes, increase the amount of water vapour that the atmosphere can hold. Likewise, warmer surface temperatures increase the rate of global evaporation of water from land ecosystems and ocean surfaces. Much of the increased evaporation comes down again as increased precipitation, but some remains in the atmosphere as water vapour. During recent decades, for example, a rise in global temperatures has been accompanied by an increase in global precipitation and observations of rising moisture content of the atmosphere over many parts of the world. The increase in water vapour also affects other aspects of the climate system, particularly clouds. Most scientists agree that the overall effect of the direct and indirect feedbacks caused by increased water vapour content of the atmosphere significantly enhances the initial warming that caused the increase - that is, it is a strong positive feedback. However, the magnitude of this effect depends on where the increases take place within the atmosphere. If these occur in atmospheric regions where air is already near saturation levels, the additional effect is small. If, on the other hand, it occurs in dry air like that over deserts or in the upper troposphere, the effect can be very large. Most models suggest that the enhancement effect will be quite large (on the order of 60%). However, this feedback is very complex, and its magnitude remains one of the key uncertainties in climate models.

Reference: IPPC, 1990 WGI, pp 47-48.

B.8  Don't human emissions of aerosols cool the climate and therefore offset emissions of greenhouse gases?

Response: Many human activities also result in the emission of sulphate and sooty aerosols, biomass burning particles and soil dust.  These aerosols, in addition to directly reflecting or absorbing sunlight, can alter cloud processes and have significant impacts on regional climates. However, some aerosols cause warming while others cause cooling. While their effects are not well understood, most studies indicate that their complex role during the past century has been significant but are secondary to that of the greenhouse gases. Because emissions of these aerosols are now being controlled in many countries to reduce local air pollution, the relative effect of greenhouse gases on the climate is expected to be much more important than that of aerosols in the future.

Explanation: Many of the same human activities that release greenhouse gases also release aerosols (small solid particles and liquid droplets) into the atmosphere. These include sulphate aerosols and soot from the burning of fossil fuels, biomass aerosols from the burning of vegetation, and mineral dust from agricultural activities.  Some, like soot, are dark and thus absorb sunlight and warm the atmosphere. Others, like sulphate aerosols, reflect sunlight and cause cooling. These aerosols can also make clouds brighter and last longer.  Since, unlike long-lived greenhouse gases, aerosols only remain in the lower atmosphere for days to weeks, they do not spread around the world but remain concentrated in and downwind of industrial or agricultural regions. Because they are not evenly distributed, their effect is much greater in some parts of the world than others, and hence they have a complex effect on climate that includes changes in circulation and in cloud characteristics as well as local areas of warming and cooling.

Globally, some of their effects cancel out. However, while very uncertain, it is likely that these aerosols have ‘masked' some of the impacts of the greenhouse gases. If their emissions were to stop overnight, they would quickly disappear, unmasking again that part of the effect of greenhouse gases they were offsetting on a regional scale.

Many countries have already undertaken programs to reduce the emissions of these gases to improve local air quality, and hence their emissions are decreasing in most industrialized regions. However, they continue to increase in other industrializing regions. It is likely that these regions will also need to curtail emissions in the future to protect local air quality. Experts estimate that the role of aerosols will be far less than that for greenhouse gases in the decades to come.

B.9 What other human activities affect the climate?

Response: Humans also affect the climate through ozone depletion in the stratosphere, which slightly cools the surface, and by changing the reflectivity of the earth's surface through land use change (primarily a warming effect). These effects are believed to be relatively small compared to those for greenhouse gases.

Explanation: Ozone depletion allows more ultraviolet radiation to reach the lower atmosphere but also reduces the greenhouse effect of the ozone involved. Since the latter dominates, this has resulted in a slight surface cooling in recent decades. This depletion is expected to stabilize and subsequently be reversed in the future as the effect of measures under the Montreal Protocol reduce the concentrations of ozone depleting substances in the stratosphere.

Deforestation, reforestation, desertification, soil cultivation and urbanization are all processes that can affect the surface albedo (the amount of sunlight reflected by the earth's surface back to space). These effects are complex, and depend on the time of year. For example, replacing forests in mid-latitudes with agricultural fields can decrease albedo in the spring and fall (when the bare soils are exposed to the sun) but increase albedo in winter (when the fields are covered with snow rather than a tree canopy). Some studies suggest these impacts can be significant. However, other studies indicate that, while these effects can have important local impacts on climate, the net global effect is secondary to that of past changes in greenhouse gas concentrations, particularly since the land area involved is a relatively small area of the total earth's surface.

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