Science of Climate Change

FREQUENTLY ASKED QUESTIONS ABOUT THE SCIENCE OF CLIMATE CHANGE

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E. Global Impacts of Climate Change

E.1 The global temperature has warmed by only 0.6°C in the last 100 years.  Such a change is much less than we get from one year to the next.  What’s the big deal?

Response: Natural variability in climate can cause large differences in conditions from one year to the next and one region to the next. However, the 0.6°C warming is a long term trend in the global average of all these variations in space and time. This average warming appears to exceed any changes detectable during at least the past 1000 years.  By comparison, it took only about 5°C of warming to cause the earth to slowly change from the last glacial period some 15,000 years ago, when large volumes of ice covered what is now Canada, to today’s conditions.

Explanation: Natural variability in climate can cause one region of the world to warm several degrees relative to the preceding year, while another cools a similar amount.   However, when such variability is averaged globally, the spatial variability is much less.  Likewise, averaging weather conditions over many years also reduces the year-to-year variability of climate. The trends in global temperature represent a long-term change is such average global conditions.  The 0.6°C warming over the past century makes the 20^th century the warmest of the past 1000 years, at least in the Northern Hemisphere (data is still inadequate for such comparisons in the Southern Hemisphere).  By comparison, the change in temperature between the last glacial maximum, which ended about 15,000 years ago, and today was about 4-6°C.  That change was associated with the transformation of the Canadian landscape from a large ice sheet several km thick to today’s mosaic of productive ecosystems (IPCC TASR Chapt 3).

E.2 What are the potential consequences of a few degrees of warming?

Response: A change in climate of that magnitude would significantly alter weather behaviour around the world from that which we are used to. Some of these changes are effectively irreversible. Since both ecosystems and human societies have adapted to the climates of today and the recent past, they will be ill-prepared to deal with the changes if these are too rapid to allow ecosystems and societies to adapt. For many developing countries, this may have very harmful effects on basic human values of where to live, what to eat and drink and how to live healthy lives. For all countries, increased frequency of severe weather events will enhance the risk of weather-related disasters.

Explanation: Ecosystems evolve slowly in response to changes in the average conditions and variability of past weather. Many species, like most trees, can respond only very slowly. Others have unique climate niches that may disappear, leaving them vulnerable to extinction. Likewise, the socio-economic infrastructure and culture of human societies are closely adapted to the climate within which these evolved, and a rapid climate change would make it difficult to adapt quickly, causing an increased risk of weather-related economic disasters. Experts also predict longer and more frequent extreme weather events such as heavy rains, droughts, floods, and severe storms whose impacts on humans and natural ecosystems could be significant (for example, longer and more frequent heat waves could increase heat ­stress related deaths). Regional changes in crop yields and productivity due to climate change are likely to increase the risk of famine, particularly in semi­arid and arid regions of the tropics and subtropics. Global warming is also expected to increase the potential transmission of infectious diseases such as malaria, dengue, and yellow fever through the expansion of the range in which disease­ carrying organisms can survive.

Reference: IPCC 2001, Synthesis Report, pg 91-92; IPCC 2001, WGII Chapters 4, 5, and 18.

E.3 What could be the consequences for global sea levels?

Response: Experts project that the average global sea level will rise 9 to 88 centimetres by 2100. This rise is primarily due to the combined effects of melting glaciers and the expansion of sea water as it warms.

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Explanation: As the oceans warm, the seawater expands. This alone could cause sea levels to rise between 10 and 40 cm by 2100, depending on how fast the heat penetrates into the ocean. Furthermore, mountain glaciers around the world are expected to continue to melt, adding another 5 to 15 cm of water to the ocean level as the water runs off to sea. Finally, slow changes in polar ice sheet thickness and extent could modify sea levels. Experts estimate that the combination of these factors could cause a rise by 2100 of between 9 and 88 centimeters, with levels continuing to rise centuries thereafter as all three factors continue to catch up with warmer global air temperatures and changing precipitation patterns.

Today, about 46 million people are at risk from flooding in low­ lying coastal areas, where 50 to 70% of the world’s population live. A 40 centimetre rise in the sea level would increase those whose land will be at risk from serious flooding or permanent inundation by up to 200 million, even after expected population growth is incorporated into the projection.  Adaptation measures, such as the installation or enhancement of protective sea walls and dikes, can help reduce the impacts on people. However, such measures will be costly. Estimates for protection of U.S. coastlines, for example, range from US$20 billion to US$150 billion. Furthermore, such protection will still leave an estimated 80 million additional people vulnerable to flooding during storm surges.

While Canadian coastlines are relatively rugged and hence less vulnerable to the impacts of sea level rise that those for many other countries, some coastlines are also low lying and or soft and hence vulnerable to erosion. Studies for PEI, for example, suggest that flooding levels in the Charlottetown area that now occur once every century could occur once every decade before end of century, and that up to 50% of current coastal property could be lost by 2100.

Reference: IPCC 2001, WGI, Chapter 11; IPCC 2001, WGII, page 363; McCulloch et al. 2002.

E.4  There appears to be a recent trend towards increased frequency and intensity of disasters related to extreme weather events. Is this linked to climate change?

Response: It is very difficult to establish trends in weather-related disasters or to attribute recent disasters to specific causes. Hence, the perceived increase in disasters in some parts of the world in recent years may not be real or may be entirely natural.  However, studies suggest that the frequency and severity of many types of extreme weather events that can cause disasters will change as the climate warms. Hence, many of the current weather-related disasters may be viewed as examples of what can be expected more often in the future as the global climate continues to warm.

Explanation: A weather-related disaster can occur when society and/or ecosystems are unable to effectively cope with an extreme weather event. That is, both the extremeness of the weather event and the sensitivity of ecosystems or society are factors. The dramatic rise in damages in recent years due to such disasters may therefore be at least partly attributed to demographic factors, such as increased human population in vulnerable regions and increased wealth. On the other hand, there are indications that there have also been increases in various types of extreme weather events, at least in some regions of the world. Since these events, by definition, occur infrequently and irregularly, they are difficult to link to global causes.  Furthermore, few events are without historical precedence, and, prior to the past few decades, most historical records of such events are not very accurate.  However, in many respects, the trends towards more intense and unusual extremes for some types of weather and climate events in some regions in recent years are broadly similar to those projected by climate models and related studies. Hence, while there is no hard proof to link recent disaster trends to climate change, many of these events can be considered as examples of what could happen more frequently in the future.

E.5 Why would global warming lead to more frequent and extreme weather events?

Response: Higher temperatures lead to higher rates of evaporation and precipitation, more frequent heat waves, less frequent cold extremes, and generally more energy for storms and other extremes. However, while models can provide useful clues as to the direction and significance of such changes, the processes involved are complex and the changes in extremes are difficult to predict accurately with current models.

Explanation: Most extreme events are complex responses to a number of factors, and hence their responses to warmer climates are difficult to assess. However, as the Earth warms, experts expect more frequent high temperature extremes and less frequent cold extremes, and that more precipitation will fall over shorter periods of time. This will likely increase the frequency of very heavy and extreme precipitation events, and of local flooding. Tornados and the intensity of thunderstorms and related extreme wind and hail events will also increase in some areas.  It is also expected that many regions of the world will experience more frequent, prolonged, or more severe droughts due to more rapid evaporation from plants, soils, lakes, and reservoirs. Increasing atmospheric moisture could also increase the intensity and frequency of blizzards and snow storms in some colder locations, while decreasing their frequency but increasing their intensity in more temperate latitudes. In effect, climate change will ‘load the dice’ with respect to the probability of occurrence of such events. There is as yet little consensus on how global warming will affect other extreme weather events such as tropical storms, cyclones and typhoons, although the potential maximum intensity of such storms is expected to increase.

E.6 Can scientists prove that recent extreme weather events are due to global warming?

Response: No.  Although by definition extreme events occur very rarely, most of the recent events have likely happened before. Furthermore, because of their complexity, it is still difficult to assess the natural probability of occurrence for many of these events. However, in many respects, many of the recent events are also consistent with what is expected more frequently in the future, and could therefore already have been influenced by warmer climates. At minimum, many of these provide a good reminder of what may happen more frequently in the future.

Explanation: These extreme events may simply be the result of natural variations in climate. While floods, heat waves, a severe El Niño, and other extreme events are expected to increase as the world warms, it is difficult to attribute any particular climate or weather event definitively to global warming or any other natural or human cause. Nor is it possible to rule out the role of climate change. This is partly because data on climate extremes in many regions of the world are inadequate to draw robust conclusions about possible changes in their frequency of severity that may have occurred on a global scale. Furthermore, the link between the frequency of extreme events and global warming can only be determined through statistical analyses of long-term data because the natural climate system can produce weather and climate events that appear to be uncharacteristic of the recent climate.

E.7 Will global warming take place gradually or rapidly?

Response: Climate model studies suggest that the response of the climate to human influences will be gradual. However, there is evidence that the Earth’s climate has occasionally made abrupt shifts in the distant past, primarily during periods of glacial climates or of climate change. Hence, similar abrupt changes, although unlikely, cannot be ruled out.

Explanation: There is clear evidence from paleoclimate data that the climate system underwent large-scale abrupt changes in climate during the past glacial maximum and the deglaciation process between 10,000 and 15,000 years ago. These appear to occur when the climate system is in an unstable mode, and to have caused regional changes in temperature over Greenland of up to 10°C within a few decades. Other regions around the world also seem to have experienced similar abrupt climate transitions. Such changes have not occurred during the past 10,000 years of stable Holocene climate. Some scientists, however, have expressed concern that a rapid, human induced climate change could return the climate to an unstable condition and once again trigger such events. Hence, while these are unlikely within at least the next century, they possibility cannot be ruled out. The risks appear to increase with increasing rates of change, and the consequences, should they occur, can be catastrophic, since they allow little chance of adaptation.

Reference: IPCC 2001, Synthesis Report, pp80-86; IPCC 2001, WGI, Chapters 4, 7, 9 and 11.

E.8 I hear that the 1997-98 El Niño may have been one of the most severe of this century, after another severe El Niño event only 15 years ago.  Is climate change causing more extreme El Niños?

Response:Several recent studies have indicated that the behaviour of El Niños during the past two decades has been very unusual when compared with historical records, and that warmer climates might indeed cause more intense El Niño type behaviour.  However, such linkages remain somewhat speculative, and more data and analysis will be required to prove they exist and to determine the mechanisms involved.

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Expanation: Various studies suggest that El Niño Southern Oscillation (ENSO) behaviour since 1976 has been very unusual, and perhaps without precedence in at least the past several centuries.  Some climate model studies of the effects of warmer climates on ENSO behaviour suggest that a warmer Pacific Ocean surface could induce stronger El Niño type behaviour until a new equilibrium in ocean climate occurs (centuries after changes in surface warming has stopped). However, other studies show little change. Furthermore, long-term model simulations of natural variability and paleo studies also indicate that the amplitude of El Niño events can vary substantially on century time scales.  Hence, research results to date are as yet inadequate to establish convincing linkages between recent El Niño behaviour and climate change.

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