Science of Climate Change

FREQUENTLY ASKED QUESTIONS ABOUT THE SCIENCE OF CLIMATE CHANGE

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A. General Overview: What is Climate Change?

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

Response: Climate describes average day-to-day weather, including seasonal extremes and variations, for a specific location or region. In many respects, climate is what we can expect, and weather is what we get.

Explanation: Weather in any particular location or region can change quickly from hour to hour, day to day, season to season and year to year, even within an unchanging climate. Such changes include shifts in temperature, snow and rainfall, winds, and clouds. They are caused by an interplay of a number of factors, including rapid shifts in air circulation, slower variations in ocean conditions, or seasonal changes in the amount of sunshine.  The climate of a locality or region is calculated by averaging these weather conditions over an extended period of time, usually at least 30 years. The climate also describes how weather conditions can vary from these average values.  Such variations are described in statistical terms such as standard deviations or frequency of occurrence.

A.2 What is climate change?

Response: Climate change is a long-term shift or alteration in the climate of a specific location, a region or the entire planet. The shift is measured by changes in some or all the features associated with average weather, such as temperature, wind patterns and precipitation.  A change in the variability of climate is also considered climate change, even if average weather conditions remain unchanged.

Explanation: Climate change occurs when the climate of a specific location, region or the entire planet is altered between two different periods of time. This usually occurs when something alters the total amount of the sun's energy absorbed by the earth's atmosphere and surface, or changes the amount of heat energy from the earth's surface and atmosphere that escapes to space over an extended period of time. Such changes can involve both changes in average weather conditions and changes in how much the weather varies about these averages. They can be caused by natural processes like volcanic eruptions, changes in the sun's intensity, or very slow changes in ocean circulation or land surfaces which occur on time scales of decades, centuries or longer. Alternatively, humans can also cause climates to change by releasing greenhouse gases and aerosols into the atmosphere, by changing land surfaces, and by depleting the stratospheric ozone layer.  Both natural and human factors that can cause climate change are called ‘climate forcings', since they push, or ‘force' the climate to shift to new values.

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

Response: Climate change refers to general shifts in climate, including temperature, precipitation, winds, and other factors. This may vary from region to region. On the other hand, global warming (as well as global cooling) refers specifically to any change in the global average surface temperature. In other words, global warming or cooling is one type of planetary scale climate change. Global warming is often misunderstood to imply that the world will warm uniformly. In fact, an increase in average global temperature will also cause the circulation of the atmosphere to change, resulting in some areas of the world warming more, while other areas warming less than the average. Some areas can even cool.

Explanation: The initial response of the earth's atmosphere to a ‘climate forcing' is a change in flow of sun and heat energy through the atmosphere that 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 humidity 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 can 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' is the more accurate description of how climate system responds to a forcing. Unfortunately, although it can significantly misrepresent what really happens, the term ‘global warming' is still often used by media and others to describe climate change.

A.4 What is the “greenhouse effect” and how does it affect climate?

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 they were absent. This process is referred to as the ‘greenhouse effect because, in some respects, it resembles the role of glass in a greenhouse.

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Explanation: Earth is heated by sunlight. Although ozone in the stratosphere absorbs much of the harmful ultraviolet part of sunlight, most of the sun's energy passes through the atmosphere relatively unaffected by other gases in the atmosphere. About 31% of the sunlight is reflected back to space by clouds and the earth's surface, but the remainder warms the earth's surface, oceans and atmosphere. However, in order to keep the atmosphere's energy budget in balance, the warmed earth also emits heat energy back to space as infrared radiation. As this energy radiates upward, most of it is absorbed by clouds and molecules of greenhouse gases (including water vapour) 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, finally, the energy does escape from the atmosphere to space. However, because much of the energy has been recycled downward, surface temperatures become much warmer then if it the greenhouse gases were absent from the atmosphere. This natural process is known as the greenhouse effect. Without greenhouse gases, such as water vapour,­carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (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 amount of these greenhouse gases in our atmosphere has been relatively stable. Then a few centuries ago, their concentrations began to increase due to the increasing demand for energy caused by industrialization and rising populations, and due to changing land use and human settlement patterns.

Reference: IPCC, 2001 WGI, pp 89-90.

A.5 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 sunlight back to space. Both of these factors alter the amount of sunlight that is absorbed by the earth's climate system. Key human influences include changes in greenhouse gas concentrations, stratospheric ozone depletion, local air pollution and alterations in land use.  Most of these affect the amount of heat energy escaping to space, although some also change the amount of sunlight reflected to space.

Explanation: Changes in the intensity of sunlight reaching the earth can cause cycles of warming and cooling that have been a regular feature of the Earth's climatic history. Some of these solar cycles, like the four large glacial-interglacial swings during the past 400,000 years, extend over very long time scales and can have large amplitudes of 5 to 6°C. For the past 10,000 years, the earth has been in the warm interglacial phase of such a cycle. Other solar cycles are much shorter, with the shortest being the 11 year sunspot cycle.  However, the magnitudes of changes in climate for these shorter cycles are much smaller than those for the long cycles. Within the past 1000 years, for example, such changes have been within a range of about 1°C. Other natural causes of climate change include variations in ocean currents (which can alter the distribution of heat and precipitation) and large eruptions of volcanoes (which can sporadically increase the concentration of atmospheric particles, blocking out more sunlight).

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 carbon dioxide, which is released through the burning of fossil fuels (coal, oil and natural gas) and through deforestation and land degradation. An increase in greenhouse gases enhances the natural greenhouse effect and leads to an increase in the Earth's average surface temperature. At the regional scale, emissions of other polluting gases and particles into the atmosphere can also have large effects, although some of these can have opposing impacts. Sooty aerosols, for example, tend to warm regional climates, while sulphate aerosols will cool it by reflecting more sunlight. While their direct effects will be felt primarily within the industrialized regions, these aerosols can also indirectly alter average global temperatures and wind currents. Finally, human induced depletion of ozone in the stratosphere also tends to cool the earth's surface, while land use change can change the amount of sunlight reflected to space by the earth's surface and hence contribute to climate change.

A.6 Since greenhouse gases (e.g. carbon dioxide, methane, nitrous oxide, etc.) represent such a small fraction of the atmosphere, how can changes in their concentrations have a significant effect on the global climate?

Response: Most greenhouse gases are very effective in absorbing heat escaping from the earth and keeping it trapped, much like a blanket on a bed.  It takes only small amounts of these gases to significantly change the properties of the atmosphere.  This allows human emissions to have a significant effect on their influence on climate.

Explanation: 99% of the dry atmosphere consists of nitrogen and oxygen, which are relatively transparent to sunlight and infrared energy. Hence, they have little effect on the flow of sunlight and heat energy through the air.  By comparison, the atmospheric gases that cause the earth's natural greenhouse effect total less than 1% of the atmosphere.  However, these gases (including water vapour) collectively increase the earth's average surface temperature from -19°C to +14°C - a difference of about 33°C.  Furthermore, because the concentrations of these gases in the atmosphere are so low, it is possible for human emissions to have a significant effect on them.  For example, human emissions of carbon dioxide (CO₂) currently amount to roughly 28 billion tonnes per year and, over the next century, are expected to increase the concentration of carbon dioxide in the atmosphere from about 0.03% today to almost certainly 0.06% (a doubling), and possibly to 0.09% (a tripling).  Since the production of each molecule of carbon dioxide removes one molecule of oxygen from the atmosphere, a doubling of CO2 concentrations would only reduce the volume of oxygen in the atmosphere from 20.95% to about 20.92%.  That is, because the volume of oxygen is much larger, the same human activities have very little effect on its concentrations.

Reference: IPCC 2001 WG!, Chapter 1.

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