Global Warming

Global warming refers to the observed increases in the average temperature of the Earth's atmosphere and oceans in recent decades.

The scientific opinion on climate change is that the average global temperature has risen 0.6 ą 0.2 °C over the 20th century, and that it is very likely that "Most of the warming observed over the past 50 years is attributable to human activities". The increased volumes of carbon dioxide and other greenhouse gases (GHGs) released by the burning of fossil fuels, land clearing and agriculture, and other human activities, are the primary sources of the human-induced component of warming.

Observational sensitivity studies and climate models referenced by the Intergovernmental Panel on Climate Change (IPCC) predict that global temperatures may increase by between 1.4 and 5.8 °C between 1990 and 2100. The range of uncertainty is primarily the result of the difficulty of predicting the volume of future carbon dioxide emissions, but there is also some uncertainty about the climate sensitivity.

The increase in global temperatures is expected to result in other changes, including rises in sea level and changes in the amount and pattern of precipitation. These changes may increase the frequency and intensity of extreme weather events, such as floods, droughts, heat waves, and hurricanes. It may cause higher or lower agricultural yields, glacier retreat, reduced summer streamflows, and contribute to biological extinctions. Although warming is expected to affect the number and magnitude of these events, it is difficult to connect particular events to global warming. Although most studies focus on the period up to 2100, warming (and sea level rise due to thermal expansion) is expected to continue past then, since CO2 has a long average atmospheric lifetime.

There are a few scientists who contest the view that humanity's actions have played a significant role in increasing recent temperatures. However, more significant uncertainties exist regarding how much climate change should be expected in the future, and there is a hotly contested political and public debate over what, if anything, should be done to reduce or reverse future warming, and how to cope with the consequences.

The term 'global warming' is a specific case of the more general term 'climate change' (which can also refer to cooling, such as occurs during Ice ages). In principle, 'global warming' is neutral as to the causes, but in common usage, 'global warming' generally implies a human influence. However, the UNFCCC uses 'climate change' for human-caused change, and 'climate variability' for other changes. Some organizations use the term 'anthropogenic climate change' for human-induced changes.

Relative to the period 18601900, global temperatures on both land and sea have increased by 0.75 °C. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C/decade against 0.13 °C/decade (Smith, 2005)). Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C per decade since 1979. Over the one or two thousand years before 1850, world temperature is believed to have been relatively stable, with possibly local fluctuations, such as the Medieval Warm Period or the Little Ice Age.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree Celsius. Similar estimates prepared by the World Meteorological Organization and the UK Climatic Research Unit concluded that 2005 was still only the second warmest year, behind 1998.

Depending on the timeframe, a number of temperature records are available. These are based on different data sets, with different degrees of precision and reliability. An approximately global instrumental temperature record begins in about 1860; contamination from the urban heat island effect is believed to be small.

A longer-term perspective is available from various proxy records for recent millennia; see temperature record of the past 1000 years for a discussion of these records and their differences. The attribution of recent climate change is clearest for the most recent period of the last 50 years, for which the most detailed data are available. Satellite temperature measurements of the tropospheric temperature date from 1979.

Causes

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 solar activity, volcanic emissions, and greenhouse gases. Climatologists accept that the earth has warmed recently, but the cause or causes of this change is more controversial, especially outside the scientific community.

Adding carbon dioxide (CO2) or methane (CH4) to an atmosphere, with no other changes, will tend to make a planet's surface warmer; greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be an estimated 30 °C lower, and the Earth uninhabitable. It is therefore not correct to say that there is a debate between those who "believe in" and "oppose" the theory that adding carbon dioxide or CH4 to the Earth's atmosphere will result in warmer surface temperatures on Earth, absent indirect mitigating effects. Rather, the debate is about what the net effect of the addition of carbon dioxide and CH4 will be.

Due to the thermal inertia of the earth's oceans and slow responses of other indirect effects, the Earth's current climate is not in equilibrium with the forcing imposed by increased greenhouse gases. Climate commitment studies indicate that, even if greenhouse gases were stabilised at present day levels, a further warming of perhaps 0.5 °C to 1.0 °C would still occur.

Greenhouse gases in the atmosphere

The atmospheric concentrations of carbon dioxide and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. This is considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that carbon dioxide values this high were last attained 40 million years ago. About three-quarters of the anthropogenic emissions of carbon dioxide 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.

The longest continuous instrumental measurement of carbon dioxide mixing ratios began in 1958 at Mauna Loa. Since then, the annually averaged value has increased monotonically from 315 ppmv as shown by the Keeling Curve. The concentration reached 376 ppmv in 2003. South Pole records show similar growth. The monthly measurements display small seasonal oscillations.

Methane is produced biologically and released from gas pipelines. Some biological sources are "natural" such as termites and others are attributable to human activity such as agriculture, e.g., rice paddies. Recent evidence suggests that forests may also be a source (RC; BBC). Note that this is a contribution to the natural greenhouse effect, and not to the anthropogenic greenhouse effect (Ealert).

Future carbon dioxide levels are expected to continue rising due to ongoing fossil fuel usage, though the actual trajectory will depend on uncertain economic, sociological, technological, and natural developments. The IPCC Special report on emissions scenarios gives a wide range of future carbon dioxide scenarios, ranging from 541 to 970 parts per million by 2100.

Globally, the majority of anthropogenic greenhouse gas emissions arise from fuel combustion. The remainder is accounted for largely by "fugitive fuel" (fuel consumed in the production and transport of fuel), emissions from industrial processes (excluding fuel combustion), and agriculture: these contributed 5.8%, 5.2% and 3.3% respectively in 1990. Current figures are broadly comparable.

Around 17% of emissions are accounted for by the combustion of fuel for the generation of electricity. A small percentage of emissions come from natural and anthropogenic biological sources, with approximately 6.3% derived from agriculturally produced methane and nitrous oxide.

Positive feedback effects, such as the expected release of possibly as much as 70,000 million tonnes of methane from permafrost peat bogs in Siberia, which have started melting due to the rising temperatures, may lead to significant additional sources of greenhouse gas emissions. [9]. Note that anthropogenic emissions of other pollutants - notably sulphate aerosol - exert a cooling effect; this can account for the plateau/cooling seen in the temperature record in the middle of the 20th century, though this may also be due to intervening natural cycles.

Various alternative hypotheses have been proposed to explain the observed increase in global temperatures, including but not limited to:

The warming is within the range of natural variation.

The warming is a consequence of coming out of a prior cool period - the Little Ice Age.

The warming is a result of variances in solar irradiance.

At present, none of these have much support within the climate science community as an explanation for recent warming.

Solar Variation Theory

Studies reported in the IPCC Third Assessment Report (TAR) in general failed to detect solar forcing in the climate record. Volcanic and solar forcings might account for half of the temperature variations prior to 1950, but that the net effect of such natural forcings was roughly neutral since then.

In particular, the change in climate forcing from greenhouse gases since 1750 was estimated to be 8 times larger than the change in forcing due to increasing solar activity over the same period.

Since the TAR, various studies (Lean et al., 2002, Wang et al., 2005) have suggested that changes in irradiance since pre-industrial times are less by a factor of 3-4 than in the reconstructions used in the TAR (e.g. Hoyt and Schatten, 1993, Lean, 2000.).

Stott et al. estimated solar forcing to be 16% or 36% of greenhouse warming. In general the level of scientific understanding of the variance in direct solar irradiance is very low.

However, some researchers (e.g. ) have proposed that feedbacks from clouds or other processes enhance the direct effect of solar variation. Solanki, et al (2004) found that solar activity for the last 60 to 70 years is at its highest level in 8000 years; Muscheler et al. disagree, finding other comparably high levels of activity in the past.

Solanki reports that past solar behavior implies that there is only an 8% probability that this current period of high activity can last another 50 years.

Effects of Global Warming

The predicted effects of global warming are many and various, both for the environment and for human life. These effects include sea level rise, impacts on agriculture, reductions in the ozone layer, increased intensity and frequency of extreme weather events, and the spread of disease. In some cases, the effects may already be being experienced, although it is impossible to attribute specific natural phenomena to long-term global warming. In particular the relationship between global warming and hurricanes is still being debated.

Four new papers correlating climate change with increased hurricane intensity seem to be making the case that the two phenomena are linked; a draft WMO statement acknowledges the different viewpoints.

The extent and likelihood of these consequences is a matter of considerable controversy. A summary of possible effects and recent understanding can be found in the report of the IPCC Working Group II. Global warming is already causing death and disease across the world through flooding, environmental destruction, heatwaves and other extreme weather events, according to some scientists.

Effects on Ecosystems

Secondary evidence of global warming - lessened snow cover, rising sea levels, weather changes - provides 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 flourish. Few of the terrestrial ecoregions on Earth could expect to be unaffected.

Many of the species at risk are arctic fauna such as polar bears, emperor penguins, many salt wetland flora and fauna species, and any species that inhabit the low land areas near the sea. Species that rely on cold weather conditions such as gyrfalcons, and snowy owls that prey on lemmings, which use the cold winter to their advantage, will be hit hard. Global warming can also leave vegetation more suseptible to insect infestation. Droughts weaken a tree's immune and defense systems and allow for insects such as the Spruce Budworm to colonize forests and decrease the productivity and value of the trees and forest as a whole.

Impact on Glaciers

Global warming has led to negative glacier mass balance, causing glacier retreat around the world. Oerlemans (2005) showed a net decline in 142 of the 144 mountain glaciers with records from 1900 to 1980. Since 1980 global glacier retreat has increased significantly.

Similarly, Dyurgerov and Meier (2005) averaged glacier data across large scale regions (e.g. Europe) and found that every region had a net decline from 1960 to 2002, though a few local regions (e.g. Scandinavia) have shown increases. Some glaciers that are in disequilibrium with present climate have already disappeared and increasing temperatures are expected to cause continued retreat in the majority of alpine glaciers around the world. Upwards of 90% of glaciers reported to the World Glacier Monitoring Service have retreated since 1995.

Of particular concern is the potential for failure of the Hindu Kush and Himalayan glacial melts. The melt of these glaciers is a large and reliable source of water for China, India, and much of Asia, and these waters form a principal dry-season water source. Increased melting would cause greater flow for several decades, after which "some areas of the most populated region on Earth are likely to 'run out of water'.

Destabilization of Ocean Currents

There is also some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localised cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift.

Environmental Refugees

Even a relatively small rise in sea level would make some densely settled coastal plains uninhabitable and create a significant refugee problem. If the sea level were to rise in excess of 4 metres almost every coastal city in the world would be severely affected, with the potential for major impacts on world-wide trade and economy.

Presently, the IPCC predicts sea level rise of less than 1 meter through 2100, but they also warn that global warming during that time may lead to irreversible changes in the Earth's glacial system and ultimately melt enough ice to raise sea level many meters over the next millennia. It is estimated that around 200 million people could be affected by sea level rise, especially in Vietnam, Bangladesh, China, India, Thailand, Philippines, Indonesia and Egypt.

An example of the ambiguous nature of environmental refugees is the emigration from the island nation of Tuvalu, which has an average elevation of approximately one meter above sea level. Tuvalu already has an ad hoc agreement with New Zealand to allow phased relocation and many residents have been leaving the islands. However, it is far from clear that rising sea levels from global warming are a substantial factor - best estimates are that sea level has been rising there at approximately 1-2 mm/yr, but that shorter timescale factors - ENSO, or tides - have far larger temporary effects.

Spread of Disease

Global warming may extend the range of vectors conveying infectious diseases such as malaria. Bluetongue disease in domesticated ruminants associated with mite bites has recently spread to the north Mediterranean region. Hantavirus infection, Crimean-Congo hemorrhagic fever, tularemia and rabies increased in wide areas of Russia during 2004-2005. This was associated with a population explosion of rodents and their predators but may be partially blamed on breakdowns in governmental vaccination and rodent control programs.

Similarly, despite the disappearance of malaria in most temperate regions, the indigenous mosquitoes that transmitted it were never eliminated and remain common in some areas. Thus, although temperature is important in the transmission dynamics of malaria, many other factors are influential.

Financial Effects

Financial institutions, including the world's two largest insurance companies, Munich Re and Swiss Re, warned in a 2002 study (UNEP summary) that "the increasing frequency of severe climatic events, coupled with social trends" could cost almost 150 billion US dollars each year in the next decade. These costs would, through increased costs related to insurance and disaster relief, burden customers, tax payers, and industry alike.

According to the Association of British Insurers, limiting carbon emissions could avoid 80% of the projected additional annual cost of tropical cyclones by the 2080s. According to Choi and Fisher (2003) each 1% increase in annual precipitation could enlarge catastrophe loss by as much as 2.8%.

The United Nations' Environmental Program recently announced that severe weather around the world has made 2005 the most costly year on record, although there is no way to prove that [a given hurricane] either was, or was not, affected by global warming. Preliminary estimates presented by the German insurance foundation Munich Re put the economic losses at more than 200 billion U.S. dollars, with insured losses running at more than 70 billion U.S. dollars.

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