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Climate can be described as the sum of weather. While the weather is quite variable, the trend over a longer period, the climate, is more stable. However, the climate still changes over time scales of decades to millennia. Ice ages are the prototypical example of a long time scale change. Natural climate changes are due to both the internal dynamics of the climate system and changes in external climate forcings.
Historical temperature records and proxy records of climate variables show fluctuations on all time scales. Some of these changes can be plausibly attributed to external forcing factors such as the cool temperatures of the Maunder minimum of the 1800ís, which may have been caused by a decrease in solar irradiation.1 The eruption of Mt. Pinatubo in 1991 caused a cooling of the Earthís surface due to the injection of light reflecting aerosol particles into the stratosphere.2
Natural and human systems have adapted to the prevailing amount of sunshine, wind, and rain. While these systems can adapt to small changes in climate, adaptation is more difficult or even impossible if the change in climate is too rapid or too large. This is the driving concern over anthropogenic, or human induced, climate change. If climate changes are too rapid then many natural systems will not be able to adapt and will be damaged and societies will need to incur the costs of adapting to a changed climate.
Weather and climate are driven by the absorption of solar radiation and the subsequent re-distribution of that energy through radiative, advective, and hydrological processes. The Earthís surface temperature is primarily determined by the balance between the absorption and emission of radiation. A change in this radiative balance is termed a radiative forcing, which is measured in Watts per square meter.
Naturally occurring greenhouse gases, primarily water vapor and carbon dioxide, trap thermal radiation from the Earthís surface and this effect keeps the surface warmer than it would be otherwise. Human activities are causing an enhancement of the natural greenhouse effect by substantially increasing the atmospheric concentrations of greenhouse gases. For example, the atmospheric concentration of carbon dioxide has already risen by about 30% from its pre-industrial level and methane concentrations are more than double their pre-industrial value. Further substantial increases in carbon dioxide concentrations are inevitable, at least in the near term, as world-wide use of fossil fuels continues to increase.
Figure IV.1.1 Global Mean Surface Temperature Change: Departure from the 1951-1980 Mean.
The relationships between the atmospheric concentration of greenhouse gases and their radiative effects are well quantified. Forcing from the long-lived greenhouse gases: carbon dioxide, methane, and nitrous oxide, is presently about 2.5 Watts per meter squared (W/m2). Of this total, 1.6 W/m2 is from carbon dioxide alone. The total anthropogenic forcing is uncertain, particularly because the magnitude of the negative forcing associated with sulfate aerosols is unclear. While changes in solar irradiance may have affected global climate in the last century, a 0.15% change irradiance, the order of estimated changes, results in only a 0.36 W/m2 forcing.
There are still significant uncertainties in moving from greenhouse gas emissions, particularly those of carbon dioxide, to atmospheric concentrations. However the largest difficulty is moving from changes in the concentration of greenhouse gases to changes in climate. The largest source of uncertainty lies in determining the magnitude of climate feedbacks. For example, an increase in trapped radiation and the associated warming is expected to increase the level of water vapor in the atmosphere, which would tend to further enhance the greenhouse effect ó a positive feedback. An example of a negative feedback would be an increase in clouds that reflected more sunlight back into space. The actual feedback from changes in clouds is uncertain since they also act to trap outgoing infrared radiation.
It is the balance between positive and negative feedbacks which will determine the net effect of increased greenhouse gases. While climate models agree that the net effect will be warming, the amount of warming (and other changes) given by various models is different. The current central warming estimate, developed by the Intergovernmental Panel on Climate Change (IPCC), is a global average temperature rise of two degrees centigrade by the year 2100.3
The primary tools for study of the climate system, particularly in the context of the anthropogenic greenhouse effect, are complex computer models known as General Circulation Models, or GCMs. Since these global models must operate on a relatively large spatial scale, small scale phenomena such as the formation and properties of clouds, rainfall, and turbulent processes cannot be explicitly represented and must be parameterized. Improving parameterizations smaller scale phenomena is one of the primary goals of climate modelers.
The accuracy of GCMs in simulating present climatic conditions has steady improved, although there are still significant errors for some features, such as cloud cover. While this lends increasing confidence in the results, models can only be rigorously tested against recent climatic conditions. Their accuracy in simulating future climate can never be fully tested.
An impression of the range in estimates is given by combining the IPCC low emissions scenario with a low climate sensitivity and combining the high emissions scenario with a high climate sensitivity. The resulting range is one to three and a half degrees centigrade global average temperature increase by 2100.3 Note that the temperature change in a specific region will often be quite different than the global average.
Estimating the impacts of climate change requires information on a regional level. The spatial resolution of general circulation models is too coarse to provide such information. While techniques have been developed to produce higher resolution climate change data, reliable regional projections of future climate change are still not possible. In addition, the spatial pattern of climate changes is different for different GCMs, although a number of broad similarities are present.
CO2 Emissions Factors
|20 Tg C/EJ
|15 Tg C/EJ
|26 Tg C/EJ
|21 Tg C/EJ
|1 Gt Carbon
|3.664 Gt CO2
Table 1: Carbon content of various fossil fuels in terms of
The principal anthropogenic greenhouse gas is carbon dioxide, with a substantial contribution from methane. While chlorofluorocarbons (CFCs) are potent greenhouse gases, the stratospheric ozone depletion that they cause partially cancels out their direct radiative effect. Sulfate aerosols, formed in the atmosphere from sulfur dioxide produced primarily by the use of coal, are a crucial contributor to climate change, although in the opposite direction since they act to cool the Earthís surface.
Carbon dioxide represents about 60% of the positive anthropogenic radiative forcing. The largest source of carbon dioxide is from the use of fossil fuels. The carbon content of different fossil fuels varies, with natural gas having the lowest carbon content and coal the highest (Table 1). Typical generation efficiencies also vary greatly. Combined-cycle natural gas generating plants are the preferred generating mode today due to their high efficiency, while traditional uses of non-commercial fuels such as wood are generally quite inefficient.
Land-use changes, primarily tropical deforestation, contribute about a fifth of current carbon dioxide emissions; however, the importance of deforestation, relative to fossil fuel emissions, is expected to continue to diminish in the future.
Unlike other greenhouse gases, carbon dioxide is not destroyed in the atmosphere but instead cycles between the atmosphere, terrestrial biosphere, and oceans. Because of this complicated cycle carbon dioxide does not have a single atmospheric lifetime. Only about half the carbon dioxide emitted today remains in the atmosphere, some portion of which will remain there for centuries. The rest is absorbed in either the ocean or the terrestrial biosphere. It is carbon dioxide sequestered by ancient forests that we are burning today as fossil fuels.
There are numerous sources of anthropogenic methane emissions, including fossil fuel use (natural gas) and production, ruminant animals, waste disposal, and rice agriculture. While methane is a potent greenhouse gas, with twenty one times the radiative effect of carbon dioxide per molecule, it has a much shorter atmospheric lifetime. Methane is oxidized in the atmosphere in roughly a decade while carbon dioxide is essentially indestructible and stays in the atmosphere until absorbed by the oceans or terrestrial biosphere. Therefore carbon dioxide is the greenhouse gas of primary concern, due to its long atmospheric lifetime and the large quantity that is released into the atmosphere.
Sulfate aerosols are light colored particles, part of the haze seen in industrialized areas. They act in the opposite sense to greenhouse gases, reflecting light and tending to cool the Earthís surface. The best estimate of the cooling effect of sulfate aerosols is that they have offset a bit more than a third of the global-average warming due to anthropogenic greenhouse gases released to date. However, the radiative effects of aerosols are quite uncertain, and their cooling effect could be significantly different than the current ìbest guessî value. Sulfate aerosols are expected to caused an indirect effect by acting as condensation nuclei and thus causing clouds to be denser and more reflective. The magnitude of the indirect effect is very uncertain.
Even though these aerosols, along with those caused by biomass burning, tend to cool the atmosphere they can not exactly cancel the warming caused by greenhouse gasses even if the magnitude of the two effects were equal. While greenhouse gases such as carbon dioxide and methane are fairly evenly distributed in the atmosphere, aerosols are concentrated near their sources. Thus sulfate aerosol cooling effects are concentrated near heavily industrialized regions, particularly the eastern United States and western Europe. While the climate effect of these compounds might be considered beneficial, when sulfur dioxide and sulfate aerosols are eventually removed from the atmosphere they acidify the soil, which damages natural and agricultural systems.
Energy use is the primary source of greenhouse gases. The main factors that drive energy use are economic growth and population growth. Contrary to most popular conceptions, it is economic growth not population growth that is the primary driver, both historically and in model projections, of greenhouse gas emissions. Population growth is, however, still a significant contributor to increased future greenhouse gas emissions.
Emissions of most greenhouse gasses are expected to continue to increase in the future. Greenhouse gas emissions from developing and developed countries are currently comparable in magnitude. However, most of the growth in greenhouse gas emissions will occur in developing countries, where economic growth rates are much larger than those in industrialized regions. If developing countries follow the energy-intensive development path followed by the presently industrialized countries then atmospheric concentrations of greenhouse gases will increase dramatically.
One of the largest uncertainties in future greenhouse emissions is the effect of technological change. If renewable energy sources become cost-effective, if there are major gains in the efficiency of energy utilization, or if there is a large increase in the use of nuclear energy (fission or fusion), then emissions of greenhouse gases may be substantially restrained.
Central projections of greenhouse gas emissions result in a doubling of anthropogenic concentrations of carbon dioxide before the end of the next century. This is likely to result in a an ìaverage rate of warming [which] would probably be greater than any seen in the last 10,000 years.î 3 However, if favorable technological developments are assumed to occur then carbon dioxide emissions could stabilize or even fall by the end of the next century. An ongoing debate has been over the rate at which such developments would occur either with or without policy intervention.
The importance of the climate change issue stems from the impact of changes in climate on human and natural systems. The two most well known consequences of climate change are an increase in global-mean temperature and a rise in sea level. The primary components of sea-level rise are thermal expansion and the melting of small (continental) glaciers. However there are other changes in climate could be as important, or even so, than changes in the mean climate state. These include changes in precipitation and climate variability, particularly changes in the intensity and/or severity of extreme events such as droughts, floods, or tropical storms. The extent to which any of these latter changes might occur is still quite uncertain.
The level of damages from climate change is also uncertain. Although changes in climate will be beneficial in some areas, net costs are expected from a change in climate due to increases in the concentrations of greenhouse gases. Coastal regions are heavily populated and are particularly sensitive to climate changes, particularly sea-level rise. Agricultural activities are very sensitive to climate. However, damage estimates for this sector are uncertain since the extent to which rising levels of atmospheric carbon dioxide will enhance crop growth is not clear. Other sectors that will be affected by climate change include forestry, air quality, water resources, human health, and energy use.
The anticipated rate of anthropogenic climate change is greater than the natural rate at which climate has changed in the past. This has led to considerable concern that the rate of anthropogenic climate change will be greater than the rate at which some natural systems are able to adapt. If the climate changes to a state that is outside the range of tolerance of an individual species then that species must migrate to a suitable area. Plant species migrate very slowly, and the migration of many animal and plant species is severely limited by human development. Many ecosystems, such as wetlands, are particularly vulnerable to a change in climate or a rise in sea-level.
There are two principal responses to climate change, mitigation and adaptation. The rate at which carbon dioxide, methane, and other greenhouse gasses are released into the atmosphere can be decreased. This is termed mitigation and would reduce the magnitude of future climate change. Emission reductions can occur through either reduced energy demand, use of more efficient energy production technologies, and/or use of energy sources that produce no net greenhouse gas emissions. Carbon-free energy sources include renewable energy, geothermal energy, and nuclear energy.
Reductions in energy use can be obtained by direct policy measures, such as a carbon tax, and by improvements in the efficiency of energy using and producing equipment. Modern energy production technologies, such as combined-cycle power plants, are significantly more efficient than older power plants. However, in the long term, stabilization of carbon dioxide concentrations will require the development of non-fossil energy supplies, that is, renewable and/or nuclear energy.
The second choice is adaptation, that is adjusting to the effects of future climate change. While richer countries can build sea walls or shift agricultural production, these actions will take away resources from other activities. Poorer countries are more vulnerable to climate change since they are generally more dependent on natural resources and they lack the economic resources with which to cope with damages.
Efforts to reduce anthropogenic effects on climate are strongly affected by the inertia present in climate and human systems. The effect of increasing concentrations of greenhouse gases is strongly moderated by the thermal inertia of the oceans. On the human side, the systems by which we generate and use energy, along with society in general, also change slowly.
Responding to climate change also requires an informed public. Studies of public ìenvironmental valuesî have found widespread support for environmental protection and even a general willingness to forgo economic gains in favor of the environment.4 Public understanding of the climate change issue is, however, flawed. The connection between energy use and climate change is practically nonexistent in the public mind. In addition, a majority of people confuse climate change with pollution and ozone depletion ó often expressing the view that climate change can be abated through pollution controls.
Much of the public debate over climate change has confused the issue of detection of climate change with the inevitability of climate change. The consensus of the scientific community is clear: increasing emissions of greenhouse gases will inevitably cause the levels of greenhouse gases in the Earthís atmosphere to rise, which will change the Earthís climate. While the inevitability of climate change is generally accepted, the magnitude and nature of these changes are still uncertain.
While anthropogenic climate change has not been unambiguously detected, the evidence for a human effect on climate is mounting. The surface temperature of the earth has risen by about half a degree centigrade over the last century. This rate of change is similar in magnitude to natural climate changes but also well within the range of the possible effects of the historical rise in greenhouse gas concentrations. 5
Unambiguously detecting climate change through the record of global mean temperature is not possible at this point since, while we may detect warming we cannot uniquely attribute a general warming to anthropogenic influence. Fingerprint detection is a more promising technique. This scheme involves using GCMs to identify distinctive spatial patterns caused by anthropogenic influence. A number of studies using this technique have recently found evidence of human influence on climate. These studies, plus other changes in weather and temperature patterns, lead working group I of the IPCC to conclude that, while there still many uncertainties, ìthe balance of evidence suggests that there is a discernible human influence on global climate.î3
The degree to which the climate will change in the future is still uncertain. However climate change may lead to significant damage to both human and natural systems. Estimates of the cost of reducing greenhouse gas emissions are also uncertain and a definitive cost-benefit calculation which compares climate change damages to mitigation costs is not possible at this time.
Stripped of the baggage associated with political and economic interests, much of the debate over climate change boils down to differences in values. Technological change and a general increase in wealth through economic growth will leave the world better able to deal with this issue in the future. However, some, perhaps small, amount of damage will accrue in the interim. A risk-averse viewpoint argues for mitigation of greenhouse gas emissions as soon as possible to avoid the possibility of harm. An opposite view advocates waiting until we are more certain about climate change effects (and more able to effect changes). This part of the debate will be better informed, but not solved, by improved science.
Further information, references, and much quantitative information, can be found in the IPCC reports. The most recent report is in three volumes. The first volume6 reports on climate science; the second on impacts, adaptations, and mitigation; and the third on economic and social dimensions. The policy-maker summaries are available on the internet.3 The 1990 IPCC report also contains much useful information and discussion, some not repeated in later reports.7
Carbon Dioxide concentrations:
Neftel, A., H. Friedli, E. Moor, H. Lotscher, H. Oeschger, U. Siegenthaler, B. Stauffer. 1994. ìHistorical CO2 record from the Siple Station ice core.î pp. 11-14. In Trends ë93: A Compendium of Data on Global Change, edited by T. A. Boden, D.P. Kaiser, R. J. Sepanski and F. W. Stoss. Oak Ridge, Tennessee: Carbon Dioxide Information Analysis Center.
Keeling, C.D., and T.P. Whorf. 1991. ìAtmospheric CO2 records from sites in the SIO air sampling network.î pp. 16-26. In Trends ë93: A Compendium of Data on Global Change, edited by T. A. Boden, D.P. Kaiser, R. J. Sepanski and F. W. Stoss. Oak Ridge, Tennessee: Carbon Dioxide Information Analysis Center. Updated 1995 at CDIAC: http://cdiac.esd.ornl.gov/ftp/
Historical temperature change:
Jones, P.D., T.M.L. Wigley, and K.RE. Briffa. 1994, ìGlobal and hemispheric temperature anomaliesóland and sea instrumental records.î pp. 603-608. In Trends ë93: A Compendium of Data on Global Change, edited by T. A. Boden, D.P. Kaiser, R. J. Sepanski and F. W. Stoss. Oak Ridge, Tennessee: Carbon Dioxide Information Analysis Center. (updated)