Global warming


Global warming is an increase over time of the average temperature of
Earth's atmosphere and oceans. The term may be used to describe theories
explaining such an increase (see global warming hypothesis) or the crisis
that many advocates say will arise if no one does anything about to prevent
such an increase from occurring.

Since 1990, the prospect that the earth's atmosphere might heat up too much
-- because of carbon dioxide and other "greenhouse gases" -- has been a
hotly debate topic.

Temperature records

Depending on what data one chooses to emphasize, different conclusions are
possible (see historical temperature record).

All quantitative reconstructions show temperatures as having been 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.

Qualitative historical evidence show patterns of warming and cooling, which
correlate with sunspots and other aspects of solar activity; thus, some
historians argue that the Medieval Warm Period enabled the colonization of
Greenland.

The difference between the qualitative and the quantitative matters, because
it affects how the most recent warming 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.

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 consel 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.

Scientific opinion

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 solar activity and volcanic emissions
as well as greenhouse gases.

See scientific opinion of global warming for discussion of an opinion survey
of scientists done on this topic.

The greenhouse gas theory

The hypothesis that a increases or decreases in greenhouse gas concentration
would lead to a 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. Climate researcher Stephen Schneider, who thought in the 1970s that
aerosols might lead to global cooling, has become a strong proponent of the
global warming hypothesis. Atmospheric physicist Richard Lindzen and Harvard
astronomer Sallie Baliunas oppose the theory. The IPCC endorsed the theory,
as did the United States National Academy of Sciences.

Automobile exhausts, coal-burning power plants, factory smokestacks, and
other waste vents of the industrial age now pump six billion tons of carbon
dioxide and other greenhouse gases into the earth's atmosphere each year.
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. Concentrations of greenhouse gases in the atmosphere are
currently at approximately 25% above pre-industrial values. 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.

The solar variation theory

Various theories 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. The theories have usually been one of
three types:

   * Solar irradiance changes directly affecting the climate. This is
     generally considered unlikely, as the variations seem to be small
   * Variations in the ultraviolet component having an effect. The UV
     component varies by more than the total.
   * Effects mediated by changes in cosmic rays (which are affected by the
     solar wind, which is affected by the solar output) such as changes in
     cloud cover.

Although often correlations are presented and the mechanism is speculative.
Many of these effects 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."

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."

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." 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).

Global warming controversy

A separate article on socio-political issues surrounding the theory of
global warming

Historical temperature record

Over the past 20,000 years the dominant temperature signal has been the end
of the last ice age, approximately 12,000 years ago. Since then the
temperature has been quite stable, though with various fluctuations, e.g.
Medieval Warm Period or Little Ice Age.

Evidence for a current warming period

According to the IPCC, Earth has seen a significant increase in average
global surface temperature over the last 150 years.

The main evidence for global warming comes from thermometer measurements
from land stations and ships, which independently show much the same
warming since 1860. The data from these stations, particularly those located
outside of the United States, show an average surface temperature increase
of 0.4 to 0.8 Celsius degrees during the 20th century. Most of the warming
occurred during two periods: 1910 to 1945 and 1976 to 2000. (Source:
Intergovernmental Panel on Climate Change, the IPCC).

Secondary evidence comes from observed variations in the snow cover and ice
extent, global average sea level, precipitation, cloud cover, El Ni–o and
extreme weather events during the 20th century.

For example, satellite data shows a 10% decrease of snow cover since the
late 1960s, and the Northern Hemisphere spring and summer sea-ice extent has
decreased by about 10% to 15% since the 1950s and there has been a
widespread retreat of mountain glaciers in non-polar regions throughout the
20th century.

Evidence of global warming is uneven with certain places, especially the
north shore of Alaska, showing dramatic rises temperature far above the
average for the globe as a whole.

Evidence against a current warming period

Weather balloons and satellites measuring the temperature of the atmosphere
above the earth's surface since 1979 show a warming trend in the troposhere
of 0.074 ¡C / decade up to August 2003 [9]; see satellite temperature
measurements for details. A National Academy of Sciences panel that reviewed
the apparent discrepancy concluded that "the warming trend in global-mean
surface temperature observations during the past 20 years is undoubtedly
real and is substantially greater than the average rate of warming during
the twentieth century. The disparity between surface and upper air trends in
no way invalidates the conclusion that surface temperature has been rising."

The satellite data are controversial and incorporate a number of corrections
for, amongst other effects, orbital drift of the satellites. Other
analyses of the same data produce different (higher) trends.

Climate models

Climate simulations (vague) do not unambiguously attribute the warming that
occurred from 1910 to 1945 to either variation in solar radiation (internal
and natural forcing) or to anthropogenic forcing. The models are used to
estimate the relative importance of the various factors mentioned above.
Most models show that warming occurring from 1975 to 2000 needs
anthropogenic greenhouse gas emissions to be explained.

The IPCC (see below), a United Nations science and public policy
organization, published a report saying that scientists believe that
anthropogenic greenhouse gases "play an important role in global warming."

This conclusion depends on the accuracy of the models used and on the
correct estimation of the external factors. The majority of scientists agree
that important climate features are incorrectly accounted for by the climate
models but don't think that better models would change the conclusion.

Critics point out that there are flaws in the models and external factors
not taken into consideration that could change the conclusion above.
Some critics say that the climate simulations are unable
to fit the water vapor feedback, and handle clouds. Critics also point out
that the sun may have a share of responsibility for the observed global
warming greater than now thought by the majority of the scientific
community. Some indirect solar effects may be very important and are not
accounted for by the models. So, they argue, the share of global warming
caused by anthropogenic greenhouse gases may be lower than thought.

The IPCC says Complex physically-based climate models are required to
provide detailed estimates of feedbacks and of regional features. Such
models cannot yet simulate all aspects of climate (e.g., they still cannot
account fully for the observed trend in the surface-troposphere temperature
difference since 1979) and there are particular uncertainties associated
with clouds and their interaction with radiation and aerosols. Nevertheless,
confidence in the ability of these models to provide useful projections of
future climate has improved due to their demonstrated performance on a range
of space and time-scales.

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. As a consequence, the sea level could rise several metres. At the
moment, scientists are not expecting any major ice melting in the next 100
years. (Sources: IPCC for the data and the mass media for the general
perception that climate change is important.) Some researchers have found a
negative correlation between sea level rise and average global temperature;
water evaporates more quickly than it expands.

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.

Global warming can also have other, less obvious effects. The North Atlantic
drift, for instance, is driven by temperature changes. It seems as though 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.

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). In its last report, the IPCC made some
predictions about future climate change. These predictions are the basis for
current political and scientific discussion.

IPCC predictions are based on the same models used to establish the
importance of the different factors in global warming. These models need
data about anthropogenic emissions of greenhouse gases and aerosols. These
data are predicted from economic models based on 35 different scenarios.
Scenarios go from pessimistic to optimistic, and predictions of global
warming depend on the kind of scenario considered.

None of these scenarios consider any kind of measures to avoid global warming.

Nevertheless, proponents of the IPCC assessment say that the current climate
models are good in predicting surface temperatures and that this is
significant. They furthermore argue that it is surface temperatures that
will have the greatest and most direct effect on the environment,
agriculture and the stability of polar ice.

The IPCC says that it has corrected the land station data to account for the
urban heat island effect. Most temperature measuring stations are in rural
areas, and the urban measurements are simply corrected against the rural
ones. The change resulted in a 0.05 degree Celsius lowering of temperature
estimates according to the National Academy of Science's book, Reconciling
Observations of Global Temperature Change (2000). This is far too small to
account for the nearly 0.4 degree rise in temperatures observed during the
late 20th century. The difference is small because most urban measuring
stations have had a heat island effect ever since measurements were first
taken. In some cases, the effect has decreased due to decreasing population
density and increasing energy efficiency.

In its last report, IPCC stated that average surface temperature is
projected to increase by 1.4 to 5.8 Celsius degrees over the period 1990 to
2100, and the sea level is projected to rise by 0.1 to 0.9 metres over the
same period. The wide range in predictions is based upon several different
scenarios that assume different levels of future CO2 emissions. Each
scenario then has a range of possible outcomes associated with it. The most
optimistic outcome assumes an agressive campaign to reduce CO2 emssions,
while the most pessimistic is a "business as usual" scenario. The more
realistic scenarios fall in between.

IPCC uses the best available predictions and their reports are under strong
scientific scrutiny. The IPCC concedes that there is a need for better
models and better scientific understanding of some climate phenomena, as
well as the uncertainties involved. Critics point out that the available
data is not sufficient to determine the real importance of greenhouse gases
in climate change. Sensitivity of climate to greenhouse gases may be
over-estimated or under-estimated estimated because of some flaws in the
models and because the importance of some external factors may be
misestimated.

On the other hand, predictions are based on scenarios, and the IPCC did not
assign any probability to the 35 scenarios used.

The Kyoto Protocol

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