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Global warming
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Global warming

Global warming is an increase over time of the average temperature of Earth's atmosphere and oceans. It is generally used to describe the temperature rise since the late 19th century and human causes of that rise.

Use of the term "global warming" in popular or scientific press usually implies a human influence.Climate change is a more neutral term which includes natural variability.

Table of contents
1 Terminology
2 Theories and their advocates
3 Temperature records
4 Scientific opinion
5 Theories to explain temperature change
6 Climate models
7 Potential effects
8 The Intergovernmental Panel on Climate Change
9 The Kyoto Protocol
10 External links
11 See also


The term global warming (GW) has meanings based on several usages:

In the 1970s it was unclear whether warming or cooling were more likely in the near future (next 100 years). By the late 1980s and early 1990s, the prospect that the earth's surface might become dangerously overheated captured public attention, and it has been a vigorously debated topic.

For attribution of change, see Anthropogenic global warming.

Theories and their advocates

Leaving the realm of scientific journals, the debate has spilled out into the public arena, with some politicians making the issue a component of their campaigns for high office, such as Al Gore, author of Earth in the Balance.

Nearly everything about global warming theories is controversial, not the least of which is whether there exists a scientific consensus sufficient to justify radical action to ameliorate its effects (see Kyoto Protocol).

Proponents of global warming theory (GWT) express a wide range of opinions. Some believe that the environmental damage will have such severe impact that immediate steps must be taken to reduce CO2 emissions, regardless of the economic costs to advanced nations such as the United States (the United States has the largest emissions of greenhouse gases of any country in absolute terms, and the second largest per capita emissions after Australia [1]). There are no known climatologists supporting this viewpoint. Others merely "believe in" the observed increase in temperature. Somewhere in between are those who support measures such as the Kyoto Protocol, intended to have minor climate effects.

GWT opponents similary cover a wide range. Some dismiss the theory as completely lacking any basis in fact; however, there are no known climate scientists adopting this position. Others, such as the well-known skeptic Patrick Michaels are milder, accepting that human influence has warmed the atmosphere while disputing the current and future warming given by the IPCC.

Controversial subjects are discussed further in the article Global warming controversy.

Temperature records

Over the past 20,000 years the dominant temperature signal has been the end of the last ice age, approximately 12,000 years ago [1]. Since then the temperature has been quite stable, though with various fluctuations, e.g. Medieval Warm Period or Little Ice Age. Over the past century or so the global temperature has increased by approximately 0.4 - 0.8 oC. For details see the article historical temperature record.

For attribution of change, see Anthropogenic global warming.

Northern Hemisphere temperature variations. See-also[1] [1]

All quantitative reconstructions depict temperatures as having been very roughly stable for the last 1,000 years but sharply rising in the last century (see Temperature record of the past 1000 years). These records, combined with attribution analysis, indicate that it is likely this is due to human activity (see anthropogenic climate change). Environmentalists have been quick to believe this and generally urge quick and radical action to save the environment (see Kyoto Protocol and UNFCCC).

Qualitative historical evidence show warming and cooling, and there is some evidence that these correlate with sunspots and other aspects of solar activity; for example, some historians argue that the Medieval Warm Period enabled the colonization of Greenland. Recent research suggests that episodes of warming and cooling such as the Medieval Warm Period or the Little Ice Age may have been regional, not global.

The difference between the interpretations of the historical record affects how the most recent warming trend is viewed: the quantitative records show the recent warming trend, and the current warmth, as unusual; from the qualitative record, many "skeptics" believe that the recent trend is not unusual, and reject calls for actions such as the Kyoto Protocol (see global warming controversy, historical temperature record, Temperature record of the past 1000 years).

The current conflict results in professional and personal disagreements as well as pressuring political forces. Presently this is particularly visible in various interpretations in topics such as the UNFCCC Kyoto Protocol (see global warming controversy).

The above paragraphs might give the impression that belief in the course of past climate change correlates strongly with advocacy for future actions: this is not necessarily so. It is possible, perhaps common, to study the past record and give no counsel on the future.

In the twentieth century, both marine and land-based thermometers have recorded such a warming from the 1880s to about 1940, followed by a lesser cooling from 1940 to 1975, and another period of warming from 1975 to present [1]. See Historical temperature record for more discussion; and anthropogenic climate change for attribution of the change.

Scientific opinion

A survey by Dennis Bray and Hans von Storch in 1996 showed a tendency of scientists in this field to agree that it is "certain that, without change in human behavior, global warming will definitely occur sometime in the future" - with the survey showing these scientists giving this statement an average score of 2.6 on a scale from 1 to 7, where 1 indicated complete agreement and 7 indicated complete disagreement.

See scientific opinion of global warming for further discussion of this and other opinion surveys of scientists.

Theories to explain temperature change

The climate system varies both through natural, "internal" processes as well as in response to variations in "external forcing" from both human and non-human causes, including changes in the Earth's orbit around the Sun (Milankovitch cycles), solar activity , and volcanic emissions as well as greenhouse gases. See Climate change for further discussion of these forcing processes.

Most climatologists accept that the earth has warmed recently. Somewhat more controversial is what may have caused this change. See Anthropogenic Climate Change for further discussion of "attribution" of change.

Greenhouse gas theory

The hypothesis that increases or decreases in greenhouse gas concentration would lead to higher or lower global mean temperature was first postulated in the late 19th century by Swedish chemist and 1903 Nobel Laureate Svante Arrhenius, largely as an attempt to explain ice ages. At the time his peers largely rejected his theory.

The theory that human greenhouse gas emissions have contributed to the warming of the Earth's atmosphere in the 20th century, has gained both adherents and opponents in the scientific community within the past 25 years. The IPCC was established to assess the risk of human-induced climate change; the United States National Academy of Sciences endorsed the theory. Atmospheric physicist Richard Lindzen and other skeptics oppose the theory.

The picture is actually more complicated than this. Atmospheric scientists know that adding carbon dioxide (CO2) to a dry atmosphere, with no other changes, will tend to make a planet's surface warmer. The issue is that we do not live on a planet with a dry atmosphere; there is an important amount of water vapor (humidity, clouds) in the Earth's atmosphere, and water vapor is a strong greenhouse gas. If adding CO2 to the atmosphere changes processes that regulate the amount of water vapor in the Earth's atmosphere, that could have a profound effect on the climate.

The effect of clouds is also critical. Clouds have competing effects on the climate; everyone has noticed that surface temperature drops when a cloud passes overhead on an otherwise hot, sunny summer day. So clouds cool the surface by reflecting sunlight back into space. But many people have also noticed that clear winter nights tend to be colder than cloudy winter nights. That is because clouds also radiate heat back to the surface of the Earth. Bottom line, clouds have competing effects on the climate. If CO2 changes the amount of distribution of clouds, it could have various complex effects on the climate.

Given this, it is not correct to imagine that there is a debate is between those who "believe in" and "oppose" the theory that adding CO2 to the Earth's atmosphere will result in warmer surface temperatures on Earth, on average. Rather, since it is known that adding CO2 to a dry atmosphere warms a planet, on one side of the debate are those who believe that adding CO2 to the Earth's atmosphere (which is not dry) still has a net warming effect. On the other side of the debate are those who theorize that changes in water vapor, clouds, and so on will act such as to exactly cancel out the warming effect that would be seen in a dry atmosphere. Unfortunately, the observed warming of the Earth over the past 50 years appears to be at odds with the skeptics' theory that climate feedbacks will exactly cancel out the CO2 warming.

Scientists have also studied this issue with computer models of the climate (see below). These models are accepted by the scientific community as being valid only after it has been shown that they do a good job of simulating known climate variations, such as the difference between summer and winter, the North Atlantic Oscillation, or El Nino. It is universally found that climate models that pass these tests always predict that the net effect of adding CO2 will be a warmer climate in the future, when all the water vapor and cloud changes are taken into account. The amount of predicted warming varies by model, however, which probably reflects the way different models depict clouds differently. Skeptics of "global warming" say that all the models are incorrect, but have been unable to produce a model of the climate that correctly simulates known natural variability but does not predict temperatures will increase in the future. Thus, the skeptics' theory that climate feedbacks will eliminate any CO2 warming effect is not supported by either the observations or any credible model.

CO2 at Mauna Loa since 1958
See-also [1] [1]

Coal-burning power plants, automobile exhausts, factory smokestacks, and other waste vents of the industrial age now pump about 22 billion tons of carbon dioxide (corresponding to 6 billion tons of pure carbon) and other greenhouse gases into the earth's atmosphere each year. The atmospheric concentration of CO2 has increased by 31% above pre-industrial levels since 1750. This is considerably higher than at any time during the last 420,000 years, the period for which reliable data exists, from ice cores. From less direct geological evidence it is believed that values this high were last attained 40 million years ago. About three-quarters of the anthropogenic emissions of CO2 to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation [1]. They are called greenhouse gases because they trap radiant energy from the sun that would otherwise be re-radiated back into space. (The fact that a natural greenhouse effect occurs is well-known and is not at issue in the debate over global warming. Without it, temperatures would drop by approximately 30°C, the oceans would freeze and life as we know it would be impossible.) What climatologists are concerned about, rather, is that increased levels of greenhouse gases in the atmosphere might cause more heat to be trapped.

Increases in CO2 measured since 1958 at Mauna Loa show a monotonically increasing atmospheric concentration of CO2. In fact, it is clear that the increase is faster than linear. On March 21, 2004, it was reported that the concentration in ppm reached 376ppm in 2003. South Pole records show similar growth [1].

Solar variation theory

Various hypotheses have been proposed to link terrestrial temperature variations to solar variations. The meteorological community has responded with skepticism, in part because theories of this nature have come and gone over the course of the 20th century.

Sami Solanki, the director of the Max Planck Institute for Solar System Research in Gottingen, Germany said:

The sun has been at its strongest over the past 60 years and may now be affecting global temperatures.... He continued: the brighter sun and higher levels of so-called "greenhouse gases" both contributed to the change in the Earth's temperature, but it was impossible to say which had the greater impact. [1]

The theories have usually been one of three types:

Various hypotheses have been proposed to link terrestrial temperature variations to solar variations. The theories have usually been one of three types:

The meteorological community has responded with skepticism, in part because theories of this nature have come and gone over the course of the 20th century.

Although correlations often can be found, the mechanism behind these correlations is a matter of speculation. Many of these speculative accounts have fared badly over time, and in a paper "Solar activity and terrestrial climate: an analysis of some purported correlations" (J. Atmos. and Solar-Terr. Phy., 2003 p801-812) Peter Laut demonstrates problems with some of the most popular, notably those by Svensmark and by Lassen (below).

In 1991, Knud Lassen of the Danish Meteorological Institute in Copenhagen and his colleague Eigil Friis-Christensen found a strong correlation between the length of the solar cycle and temperature changes throughout the northern hemisphere. Initially, they used sunspot and temperature measurements from 1861 to 1989, but later found that climate records dating back four centuries supported their findings. This relationship appeared to account for nearly 80 per cent of the measured temperature changes over this period (see graph). Sallie Baliunas, an astronomer at the Harvard-Smithsonian Center for Astrophysics, has been among the supporters of the theory that changes in the sun "can account for major climate changes on Earth for the past 300 years, including part of the recent surge of global warming." [1]

On May 6, 2000, however, New Scientist magazine reported that Lassen and astrophysicist Peter Thejll had updated Lassen's 1991 research and found that while the solar cycle still accounts for about half the temperature rise since 1900, it fails to explain a rise of 0.4 °C since 1980. "The curves diverge after 1980," Thejll said, "and it's a startlingly large deviation. Something else is acting on the climate. ... It has the fingerprints of the greenhouse effect."[1]

Later that same year, Peter Stott and other researchers at the Hadley Centre in the United Kingdom published a paper in which they reported on the most comprehensive model simulations to date of the climate of the 20th century. Their study looked at both natural forcing agents (solar variations and volcanic emissions) as well as anthropogenic forcing (greenhouse gases and sulphate aerosols). Like Lassen and Thejll, they found that the natural factors accounted for gradual warming to about 1960 followed by a return to late 19th-century temperatures, consistent with the gradual change in solar forcing throughout the 20th century and volcanic activity during the past few decades. These factors alone, however, could not account for the warming in recent decades. Similarly, anthropogenic forcing alone was insufficient to explain the 1910-1945 warming, but was necessary to simulate the warming since 1976. Stott's team found that combining all of these factors enabled them to closely simulate global temperature changes throughout the 20th century. They predicted that continued greenhouse gas emissions would cause additional future temperature increases "at a rate similar to that observed in recent decades."[1] A graphical representation of the relationship between natural and anthropogenic factors contributing to climate change appears in "Climate Change 2001: The Scientific Basis," a report by the Intergovernmental Panel on Climate Change (IPCC). [1]


Various other hypotheses have been proposed, including but not limited to:
  1. The current increase in temperature is predicted by the Milankovitch cycles theory, in which gradual changes in the Earth's orbit around the Sun and changes in the Earth's axial tilt affect the amount of solar energy reaching the Earth.
  2. The warming is within the range of natural variation and needs no particular explanation
  3. The warming is a consequence of coming out of a prior cool period - the Little ice age - and needs no other explanation.

Some skeptics would claim that the warming trend itself is not valid, and therefore does not need any explanation.

Jurassic global warming

It is thought by geologists that the Earth experienced global warming in the early Jurassic period, with average temperatures rising by 5° Celsius (9° Fahrenheit). Research by the Open University published in Geology (32, 157-160, 2004 [1]) indicates that this caused the weathering of rocks to be speeded up by 400%, a process that took around 150,000 years to return carbon dioxide levels to normal.

Climate models

Climate simulations do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or to anthropogenic forcing (see anthropogenic global warming). All models show that the warming occurring from approximately 1975 to 2000 is largely anthropogenic. These conclusions depend on the accuracy of the models used and on the correct estimation of the external factors.

The majority of scientists agree that important climate processes are incorrectly accounted for by the climate models but don't think that better models would change the conclusion. (Source: IPCC )

Critics point out that there are unspecified flaws in the models and unspecified external factors not taken into consideration that could change the conclusion above. Some unidentified critics say that the climate simulations are unable to fit the water vapor feedback, and handle clouds. Some indirect solar effects may be very important and are not accounted for by the models. Or then again, they might not be important at all. (Source: The Skeptical Environmentalist)

See also: climate models

Potential effects

Many public policy organizations and government officials are concerned that the current warming has the potential for harm to the environment and agriculture.

This is a matter of considerable controversy, with environmentalist groups typically emphasizing the possible dangers and groups close to industry questioning the climate models and consequences of global warming - and funding scientists to do so.

Due to potential effects on human health and economy due to the impact on the environment, global warming is a cause of great concern. Some important environmental changes have been observed and linked to global warming.

The examples of secondary evidence cited above (lessened snow cover, rising sea levels, weather changes) are examples of consequences of global warming that may influence not only human activities but also the ecosystems. Increasing global temperature means that ecosystems may change; some species may be forced out of their habitats (possibly to extinction) because of changing conditions, while others may spread. Few of the terrestrial ecoregions on Earth could expect to be unaffected.

Another cause of great concern is sea level rise. Sea levels are rising 1 to 2 centimetres (around half an inch) per decade, and some small countries in the Pacific Ocean are expressing concerns that if this rise in sea level continues, they soon will be entirely under water. Global warming causes the sea level to rise mainly because sea water expands as it warms, but some scientists are concerned that in the future, the polar ice caps and glaciers may melt. The IPCC TAR says: "Global mean sea level is projected to rise by 0.09 to 0.88 metres between 1990 and 2100, for the full range of SRES scenarios. This is due primarily to thermal expansion and loss of mass from glaciers and ice caps" [1]. Some researchers have found a negative correlation between sea level rise and average global temperature; water evaporates more quickly than it expands. (Source: Science and Environmental Policy Project)

As the climate gets hotter, evaporation will increase. This will cause heavier rainfall and more erosion. Many people think that it could result in more extreme weather as global warming progresses. The IPCC TAR says: "...global average water vapour concentration and precipitation are projected to increase during the 21st century. By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter. At low latitudes there are both regional increases and decreases over land areas. Larger year to year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected" [1].

Global warming can also have other, less obvious effects. The North Atlantic drift, for instance, is driven by temperature changes. It seems that it is diminishing as the climate grows warmer, and this means that areas like Scandinavia and Britain that are warmed by the drift might face a colder climate in spite of the general global warming. It is now feared that Global Warming may be able to trigger the type of abrupt massive temperature shifts which bracketed the Younger Dryas period. (See the discussion of chaos theory for related ideas.)

Global warming will probably extend the favourable zones for vectors conveying infectious disease, such as Malaria, Dengue fever, Yellow fever, ...

However, global warming can also have positive effects, since higher temperatures and higher CO2 concentrations improve the ecosystems' productivity. Satellite data shows that the productivity of the Northern Hemisphere has increased since 1982. On the other hand, an increase in the total amount of biomass produced is not necessarily all good, since biodiversity can still decrease even though a small number of species are flourishing. Similarly, from the human economic viewpoint, an increase in total biomass but a decrease in crop harvests would be a net disadvantage. In addition, IPCC models predict that higher CO2 concentrations would only spur growth of flora up to a point, because in many regions the limiting factors are water or nutrients, not temperature or CO2; after that, though greenhouse effects and warming would continue there would be no compensatory increase in growth.

The Intergovernmental Panel on Climate Change

Since it is such an important issue, governments need predictions of future trends in global change so they can take political decisions to avoid undesired impacts. Global warming is being studied by the Intergovernmental Panel on Climate Change (IPCC). The IPCC does not commission or carry out research itself, but rather disseminates the body of published research. The reports reflect the consensus of the published science.

The Kyoto Protocol

The Kyoto Protocol to the UNFCCC proposes binding greenhouse gas limits for developed countries.

External links

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See also


see Global warming/temp for a proposed change in the layout of this entry