Global Warming

Global warming

Global Warming: A remarkable global warming trend has become evident since 1900. It is thought to be driven by the rapid increase in atmospheric CO2 and other greenhouse gases due to modern human activities.

Atmospheric CO2 measured at Mauna Loa, Hawaii. The yearly minima occur towards the end of growing season in the Northern Hemisphere.

Global Warming and the Greenhouse Effect

Global Warming

During the last century the climate has shown an unusual warming trend. The data indicate that the 20th Century was the warmer than any century since 1400. The temperature data before 1400 are inadequate to make a firm comparison. This trend is commonly thought to be driven by the rapid build-up of atmospheric greenhouse gases (CO2, CH4, etc.) produced by the burning of fossil fuels, and other human activities. The first graph shows the recent concurrent increases in atmospheric CO2 and the mean surface temperature. The second graph shows the annual cycle in the CO2 measurements at Mauna Loa. During the Northern summer the growth of plants reduces the amount of CO2 in the atmosphere. The potential problems which could arise from continued global warming caused the international community in 1988 to form an International Panel on Climate Change (IPCC) which issues periodic reports. Some 2,500 climate scientists serve on the panel. In 1995 it made its first definite determination that the present warming was basically driven by the anthropogenic generation of greenhouse gases. There were three major points.

(1) The anthropogenically generated greenhouse gases in the atmosphere are increasing rapidly and will almost certainly continue to do so for at least several decades. The United States is the largest producer of greenhouse gases. In 1993 the President spoke of trying to reduce the emission of greenhouse gases to the 1990 level by the year 2000. However in the fall of 1997 the Energy department announced that in the United States, energy source emissions had actually increased by 8% from 1991 through 1996. The rapid development of the poorer nations is also contributing an ever increasing share of the production of these gases.

(2) The IPCC panel agreed that the increase of greenhouse gases over the last 100 years has caused a noticeable global warming. There is considerable variability in the 'natural' climate system and this is not yet entirely understood. Thus the human driven warming is just beginning to emerge from the noise. However the warming signal is expected to become very clear in a few decades due to the rapid increase in the greenhouse gases.

(3) Present climate prediction models can give only a rough idea of what the results of global warming will be. Some agreed-upon effects include a rise in sea level and a warmer, moister climate at high latitudes. Changes in regional surface temperatures and precipitation patterns are expected to cause considerable perturbations in present global agricultural patterns. However the prediction models are not accurate enough to tell us exactly what these future patterns will be. A strong effort to slow the growth of atmospheric greenhouse gases would help some, but the Earth's population must learn to adapt to the changing conditions. Both the slowing of the greenhouse gas emissions and the adaptation to change present scientific and political challenges.

The Greenhouse Effect

The mean surface temperature of the Earth is close to 288 K, while satellite measurements show that the Earth-atmosphere system radiates to space with an effective temperature close to 255 K (Ramanathan et al., 1989 and Raval & Ramanathan, 1989). The atmosphere acts as a blanket which warms the surface of the Earth; this is known as the greenhouse effect of the atmosphere. Most of the mass of the atmosphere resides in the lower portion (0 to 11 kilometers above sea level) known as the troposphere. In this region the temperature decreases steadily down to about 216 K at the top of the troposphere. It is the troposphere that is chiefly responsible for the warming. Several minor gases in the atmosphere absorb the infrared radiation emitted by the warm surface. These gases then reradiate, at their own cooler temperature, both upward and also back towards the surface. This absorption and reradiation is repeated until the radiation eventually escapes to cold space. It is this infrared trapping by the atmosphere that produces the greenhouse effect. The primary contributors are water vapor, clouds, CO2, O3, CH4, N2O, and the halocarbons. Water vapor is the most important greenhouse gas. While CO2 is fairly evenly distributed throughout the atmosphere, water vapor has a high spatial and temporal variability. An increase in the greenhouse gas, increases the trapping effect and is expected to cause an increase in the surface temperature (Hansen et al., 1997; IPCC, 1996; Mackay and Khalil, 1995).

The Earth's climate and biosphere are driven by the absorption of shortwave, high energy, photons from the Sun. The exhaust heat is ultimately radiated to cold space in the form of longwave, low energy, photons. How much energy is absorbed, where geographically and at what time of the year, are important climate parameters. The fractions that are absorbed at the surface and in the atmosphere are also important. The same is true for the exhaust radiation. Clouds, and to a lesser extent aerosols, can effect both the Earth's absorption and emission of radiation. An increase in cloud cover decreases the solar radiation reaching the surface while at the same time increasing the downward longwave radiation at the surface. The net warming or cooling effect of the cloud depends on many factors including the cloud's altitude, optical thickness, and the clear sky insolation. Both measurements and theory indicate that the Earth-atmosphere system is close to a steady state condition in which the absorbed and emitted radiant energy balance over the course of a year (Barkstrom et al., 1989). When the absorbing and radiating patterns are changed, the Earth's climate has to adjust. This is termed 'radiative forcing of climate'. Increasing the amount of greenhouse gases in the atmosphere produces such a radiative forcing of climate.

Natural Climate Variability

The difficulty in predicting the future comes from our uncertainties concerning both the complex climate system and the future rate of anthropogenic generation of greenhouse gases. We know from historical records that the Earth has gone through several periodic glacial (ice age) and interglacial cycles with both the mean surface temperature and the atmospheric CO2 being higher in the interglacial (about 280 ppmv of CO2) than in the glacial periods (about 200 ppmv of CO2). Global plant growth increases in interglacial periods and this should draw increased amounts of CO2 out of the atmosphere. Indeed there is evidence that the total amount of CO2 that went into terrestrial plants at the start of our present interglacial period was greater than that which went into the atmosphere. According to Sundquist (1993) the deep ocean is the logical source of the CO2 that went into the atmosphere and vegetation during this transition period. The exact procedure for drawing the CO2 out of the deep ocean is uncertain but it seems related to the general climate change at the end of an ice age (Sundquist 1993). The glacial cycles are thought to be driven by the long term cyclic variations in the Earth's orbit about the Sun and the tilt of the Earth's axis of rotation. In this reckoning we are somewhat past the middle of the present interglacial period. Another important factor is thought to be changes in the thermohaline driven deep ocean circulation, particularly in the North Atlantic. During the present warm interglacial period there have been alternating warmer and colder periods, but the causes of these smaller cycles are still being discussed. Suggested causes include some combination of actual small changes in the solar intensity, variations in volcanic activity, and changes in ocean circulation patterns (see IPCC 1995 and 1996). In the 17th and 18th centuries a cold period, known as the little ice age, occurred. We are presently in a natural warming period following this cold swing. However the warming in the 20th century stands out as unusual in comparison with known climate history.

Human Activity and Climate Prediction

The recent increase of atmospheric greenhouse gases has been closely linked to the great expansion of the burning of fossil fuels and changes in land use since 1800. In pre- industrial times, the CO2 concentration in the atmosphere was about 280 parts per million by volume (ppmv) and for over 1000 years fluctuated by only about +/- 10 ppmv. By 1994 the concentration had increased to 358 ppmv. As the graph shows, the rate of increase has risen noticeably since 1950. About half of the modern increase has occurred since 1950. The present growth rate is about 1.5 ppmv/yr. Human activity also produces large amounts of sulphate aerosols. These tend to have a radiative cooling effect and this partially offsets the warming caused by the greenhouse gases (Mitchell et al., 1995). Prediction models which consider both the greenhouse gases and the sulphate aerosols show an even higher correlation between human activity and the global temperature increase than do models which treat only the greenhouse gases. Once in the atmosphere the CO2 tends to stay there for from 50 to 200 years before it is eventually absorbed by vegetation, the ocean , and other sinks. The length of time that the other greenhouses gases remain in the atmosphere varies with the gas from a few years to many hundreds of years. Global primary plant production and respiration adds fresh CO2 to the atmosphere at about the same rate as it absorbs old CO2 from the atmosphere. In a global steady state situation the intake and production balance over the course of some time period such as a year or a decade.. The same is true of the ocean sink. In nature of course there is never an exact balance, and significant swings can occur, However the large human driven increase in the production of CO2 and other greenhouse gases has produced a significant unbalancing of the system.

General circulation models (GCM) and particularly coupled atmosphere-ocean models are the preferred tools for studying the complex interactions associated with global warming and radiative forcing of climate. Although not perfect, these models enhance our ability to understand the climate system and possible future climate changes. The IPCC (1995) study (p. 5) states: "Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability, and because there are uncertainties in key factors. These include the magnitude and patterns of long-term natural variability and the time-evolving pattern of forcing by, and response to, changes in concentrations of greenhouse gases and aerosols, and land surface changes. Nevertheless, the balance of evidence suggest that there is a discernible human influence on global climate." Climate changes of note during the last century include a global sea level rise of between 10 and 25 cm and a global mean surface air temperature increase of between about 0.3 and 0.6 degrees centigrade. Night-time temperatures over land have generally increased more than daytime temperatures. (IPCC 1995, p.4) " ... the recent warming has been greatest over the mid-latitude continents in winter and spring". In addition "precipitation has increased over land in high latitudes of the Northern Hemisphere, especially during the cold season."

It is expected that the human influence on climate will become very visible in the 21st century. Modelers have tried to see what the climate may be like in 2100 (IPPC 1995, p. 5). "The IPCC has developed a range of scenarios, IS92a-f, of future greenhouse gas and aerosol precursor emissions based on assumptions concerning population and economic growth, land -use, technological changes, energy availability and fuel mix during the period 1990 to 2100. Through understanding of the global carbon cycle and of atmospheric chemistry, these emissions can be used to project atmospheric concentrations of greenhouse gases and aerosols and the perturbation of natural radiative forcing. Climate models can then be used to develop projections of future climate." The scenarios include a probability range from best case (extreme corrective action) to worst case (little corrective action) with the 'most probable' lying in-between. In 2100 it is predicted, that relative to 1990 the most probable global mean temperature will be 2 degrees C higher (+1 to +3.5 degrees C). (IPCC 1995, p.6) "Because of the thermal inertia of the oceans, only 50-90% of the eventual equilibrium temperature change would have been realized by 2100 and temperature would continue to increase beyond 2100, even if concentrations of greenhouse gases were stabilized by that time." The mean sea level should have risen another 50 cm (15 cm to 90 cm) by 2100 and should continue to rise even after the global mean temperature becomes stabilized.

Parameters Involved and Questions:

The global mean temperature and the greenhouse gases are essential parameters involved in the global warming studies. However most climate parameters seem to be involved to a greater or lesser extent in the complicated Global Warming process. Water vapor and clouds are very important, but their effect can be variable. As the surface temperature increases, at least the lower atmosphere will warm and atmospheric water vapor will increase. This will increase the greenhouse effect but the exact increase depends somewhat on how the water vapor is distributed vertically in the atmosphere. The total radiative forcing on the surface is also dependent on any accompanying change in cloud amount and type. For instance optically thick clouds sharply increase the albedo. If in addition the cloud tops are low, such clouds contribute little to the greenhouse effect. In a region dominated by such clouds, the warming caused by an increase in greenhouse gases could be canceled by the reduction in solar heating. On the other hand, high thin cirrus clouds contribute to the greenhouse warming without greatly reducing the insolation.

Other important questions involve how the polar ice sheets will melt, and just how the Earth's carbon cycle works and how it might change as the climate warms. In the most probable scenario, most of the rise in sea level by 2100 will be due to thermal expansion of the water as the mean temperature rises. However if high ice melting should occur, the sea level would rise by 90 cm instead of the expected 50 cm. The human input of CO2 into the atmosphere constitutes only a small fraction of the natural carbon cycle. The normal cycling of CO2 between the atmosphere and the land and ocean constitutes the vast majority of the total. The problem is that the natural cycle is close to equilibrium, In the natural cycle about as much CO2 is absorbed from the atmosphere as is released to it. However, the anthropogenic CO2 has unbalanced the system. It tends to go into the atmosphere and stay there for many years. At present the new excess of atmospheric CO2 is being slowly absorbed back into the land and ocean, but we need to know more abo ut the processes involved. As mentioned above, there is some evidence that in past global warming periods the oceans actually released more CO2 than they absorbed (Sundquist, 1993). If this should start to occur in the near future we would certainly have a worst case scenario. These are only some of the global warming questions that require additional study.

Additional Discussions of Global Warming on the Web

A list of some references to Global Warming on the Web:
http://www.msnbc.com/news/106541.asp

IPCC Web site:
http://www.ipcc.ch/

NOAA K-12 Climate information:
http://www.fsl.noaa.gov/~osborn/CLIMGRAPH2.html

References:

Barkstrom, B.R., E. Harrison, G. Smith, R. Green, J. Kibler, R. Cess, and the ERBE Science Team, 1989, Earth Radiation Budget Experiment (ERBE) archival and April 1985 results, Bull. Amer. Meteor. Soc., 70, 1254-1262.

Hansen, J., M. Sato, and R. Ruedy, 1997: Radiative forcing and climate response, J. Geophys. Res., 102, 6831 - 6864.

IPCC, 1995: Climate Change 1994: radiative forcing of climate change and an evaluation of the IPCC IS92 emission scenarios, edited by: Houghton, J.T., L.G. Meira Filho, J. Bruce, H. Lee, B.A. Callander, E. Haites, N. Harris and K. Maskell, Cambridge University Press, 339 pp.

IPCC, 1996: Climate Change 1995: The Science of Climate Change, contribution of Working Group I to the Second Assessment Report of IPCC, edited by: Houghton, J.T., L.G. Meira Filho, B.A. Callander, N. Harris and K. Maskell, Eds., with J. A. Lakeman, Production Ed., Cambridge University Press, 572 pp.

MacKay, R. M., and M. A. K. Khalil, 1995: Doubled CO2 experiments with the Global Change Research Center two-dimensional statistical dynamical climate model, J. Geophys. Res., 100, 21127 - 21135.

Mitchell, J. F. B., T. C. Johns, J. M. Gregory, and S. F. B. Tett, 1995: Climate response to increasing levels of greenhouse gases and sulphate aerosols, Nature, 376, 501-504.

Ramanathan, V., B. R. Barkstrom and E. F. Harrison, 1989: Climate and the Earth's Radiation Budget, Physics Today, 42, no. 5 (May), 22-33.

Revel, A. and V. Ramanathan, 1989: Observational determination of the greenhouse effect, Nature, 342, 758-761.

Sundquist, E. T., 1993: The global carbon dioxide budget, Science, 259, 934-941.


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