Causes of global warming
Main articles: attribution of recent climate change, scientific opinion on climate 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. Climatologists accept that the earth has warmed recently. Somewhat more controversial is what may have caused this change. See attribution of recent climate change for further discussion.
Carbon dioxide during the last 400,000 years and the rapid rise since the Industrial RevolutionAtmospheric scientists know that adding carbon dioxide (CO2) or methane (CH4) to an atmosphere, with no other changes, will tend to make a planet's surface warmer. Indeed, greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be 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 CO2 or CH4 to the Earth's atmosphere will result in warmer surface temperatures on Earth, on average. Rather, the debate is about what the net effect of the addition of CO2 and CH4 will be, and whether changes in water vapor, clouds, the biosphere and various other climate factors will cancel out its warming effect. The observed warming of the Earth over the past 50 years appears to be at odds with the skeptics' theory that climate feedbacks will cancel out the warming.
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Greenhouse gas emissions
Greenhouse gas trendsCoal-burning power plants, automobile exhausts, factory smokestacks, and other waste vents of the human environment contribute about 22 billion tons of carbon dioxide and other greenhouse gases into the earth's atmosphere each year. Animal agriculture, manure, natural gas, rice paddies, landfills, coal, and other sources contribute about 250 million tons of methane each year. About half of human emissions have remained in the atmosphere. The atmospheric concentrations of CO2 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 420,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 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 [8].
The longest continuous instrumental measurement of CO2 mixing ratios began in 1958 at Mauna Loa. Since then, the annually averaged value has increased monotonically from 315 ppmv (see the Keeling Curve). The concentration reached 376 ppmv in 2003. South Pole records show similar growth [9]. The monthly measurements display small seasonal oscillations.
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Alternative theories
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Solar variation theory
Incoming solar radiation and greenhouse gases (green and blue) during 420,000 years.Main article: Solar variation theory
Various hypotheses have been proposed to attribute terrestrial temperature variations to variations in solar output.
In the IPCC TAR, it was reported that 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 [10]. 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 [11].
Work published since the IPCC TAR, has revised the assessment of solar contribution to the post 1950 warming. "The best estimate of the warming from solar forcing is estimated to be 16% or 36% of greenhouse warming depending on the solar reconstruction."."Do Models Underestimate the Solar Contribution to Recent Climate Change" Peter A. Stott, et al, Journal of Climate, 15 DECEMBER 2003
However, a number of studies have suggested that additional solar variation feedbacks may exist which have not been incorporated in the present models or that the relative importance of solar variation may be underestimated [12] [13]. Such claims are disputed (e.g. [14] [15]) but form an active area of current research. The outcome of this debate may play a key role in determining how much climate change is attributed to human vs. natural factors.
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Other theories
Various other hypotheses have been proposed, including but not limited to:
The warming is within the range of natural variation and needs no particular explanation.
The warming is a consequence of coming out of a prior cool period — the Little Ice Age — and needs no other explanation.
The warming trend itself has not been clearly established, and therefore does not need any explanation.
At present, none of these has more than a small number of supporters within the climate science community.
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Climate models
Main article: General circulation model
Scientists have 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 Niño. All climate models that pass these tests also predict that the net effect of adding greenhouse gases will be a warmer climate in the future. The amount of predicted warming varies by model, however, which probably reflects the way different models depict clouds differently.
As noted above, climate models have been used by the IPCC to anticipate a warming of 1.4°C to 5.8°C between 1990 and 2100 [16]. They have also been used to help determine the causes of recent climate change by comparing the observed changes to those that the models predict from various natural and human derived forcing factors.
The most recent climate models can produce a good match to observations of global temperature changes over the last century. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions. Adding simulation of the ability of the environment to sink carbon dioxide suggested that rising fossil fuel emissions would decrease absorption from the atmosphere, amplifying climate warming beyond previous predictions, although "Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses" [17].
Another suggested mechanism whereby a warming trend may be amplified involves the thawing of tundra, which can release the potent greenhouse gas, methane, that is trapped in large quantities in permafrost and ice clathrates [18].
Uncertainties in the representation of clouds are a dominant source of uncertainty in existing models, despite clear progress in modeling of clouds [19]. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability. Further, all such models are limited by available computational power, so that they may overlook changes related to small scale processes and weather (e.g. storm systems, hurricanes). However, despite these and other limitations, the IPCC considers climate models "to be suitable tools to provide useful projections of future climates" [20].
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Issues
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The relation between global warming and ozone depletion
Main article: Ozone depletion
Although they are often interlinked in the popular press, the connection between global warming and ozone depletion is not strong. There are four areas of linkage:
Global warming from CO2 radiative forcing is expected (perhaps somewhat surprisingly) to cool the stratosphere. This, in turn, would lead to a relative increase in ozone depletion and the frequency of ozone holes.
Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposed effects: reduced ozone allows more solar radiation to penetrate, thus warming the troposphere. But a colder stratosphere emits less long-wave radiation, tending to cool the troposphere. Overall, the cooling dominates: the IPCC concludes that observed stratospheric O3 losses over the past two decades have caused a negative forcing of the surface-troposphere system [21] of about –0.15 ± 0.10 W m–2 [22].
One of the strongest predictions of the GW theory is that the stratosphere should cool. However, although this is observed, it is difficult to use it for attribution (for example, warming induced by increased solar radiation would not have this upper cooling effect) because similar cooling is caused by ozone depletion.
Ozone depleting chemicals are also greenhouse gases, representing 0.34 ± 0.03 W/m2, or about 14% of the total radiative forcing from well-mixed GHG's [23].
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The relation between global warming and global dimming
Main article: Global dimming
Some scientists now consider that the effects of the recently recognized phenomenon of global dimming (the reduction in sunlight reaching the surface of the planet, possibly due to aerosols) may have masked some of the effect of global warming. If this is so, the indirect aerosol effect is stronger than previously believed, which would imply that the climate sensitivity to greenhouse gases is also stronger. Concerns about the effect of aerosol on the global climate were first researched as part of concerns over global cooling in the 1970s.
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Pre-human global warming
It is thought by some geologists that the Earth experienced global warming in the early Jurassic period, with average temperatures rising by 5 °C (9 °F). Research by the Open University published in Geology (32: 157–160, 2004 [24]) indicates that this caused the rate of rock weathering to increase by 400%. As a result of this, carbon dioxide levels dropped back to normal over roughly the next 150,000 years.
Sudden release of methane clathrate (a greenhouse gas) has been hypothesized as a cause of past global warming. Two events possibly linked in this way are the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. However, warming at the end of the last ice age is thought to not be due to clathrate release [25].
The greenhouse effect has also been invoked to explain how the Earth made it out of the Snowball Earth period. During this period all silicate rocks were covered by ice, thereby preventing them to combine with atmospheric carbon dioxide. The atmospheric carbon dioxide level gradually increased until it reached about 350 times current levels. At this point temperatures were raised to an average of 50 °C, hot enough to melt the ice. Increased amounts of rainfall would quickly wash the carbon dioxide out of the atmosphere. Thick layers of abiotic carbonate sediment which can be found on top the glacial rocks from this period are believed to be formed by this rapid carbon dioxide removal process.
Using paleoclimate data for the last 500 million years (Veizer et al. 2000, Nature 408, pp. 698-701) concluded that long-term temperature variations are only weakly coupled to CO2 variations. Shaviv and Veizer (2003, [26]) extended this by arguing that the biggest long-term influence on temperature is actually the solar system's motion around the galaxy. Afterwards, they argued that over geologic time a change in CO2 concentrations comparable to doubling preindustrial levels, only results in about 0.75 °C warming rather than the usual 1.5-4.5 °C reported by climate models [27]. In turn Veizer's recent work has been discussed and criticised on RealClimate.org [28].
Leading palaeoclimatologist William Ruddiman has argued (eg Scientific American, March 2005) that human influence on the global climate began around 8000 years ago with the development of agriculture. This prevented CO2 (and later methane) levels falling as rapidly as they would have done otherwise. Ruddiman argues that without this effect, the Earth would be entering, or already have entered, a new ice age. However other work in this area (Nature 2004) argues that the present interglacial is most analogous to the interglacial 400,000 years ago that lasted approximately 28,000 years, in which case there is no need to invoke the spread of agriculture for having delayed the next ice age.
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Public controversy
Main article: Global warming controversy
Leaving the realm of scientific journals, the debate has spilled out into the public arena. In the United States, some politicians have made the issue a component of their campaigns for high office. Global warming is a more central and sustained issue, however, for the European Union.
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Effects
Main article: Effects of global warming
The predicted effects of global warming are many and various, both for the environment and for human life. The primary effect of global warming is increasing carbon dioxide and increasing global average temperature. From this flow a variety of secondary effects, including sea level rise, impacts on agriculture, reductions in the ozone layer (see below), increased extreme weather, and the spread of disease. In some cases, the effects may already be being experienced, although it is generally difficult to attribute specific natural phenomena to long-term global warming.
The extent and likelihood of these consequences is a matter of considerable controversy. A summary of possible effects and our current understanding can be found in the report of the IPCC Working Group II [29].
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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.
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Destabilisation of ocean currents
Main article: Destabilisation of ocean currents
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Environmental refugees
The termini of the glaciers in the Bhutan-Himalaya. Glacial lakes have been rapidly forming on the surface of the debris-covered glaciers in this region during the last few decades. According to USGS researchers, glaciers in the Himalaya are wasting at alarming and accelerating rates, as indicated by comparisons of satellite and historic data, and as shown by the widespread, rapid growth of lakes on the glacier surfaces. The researchers have found a strong correlation between increasing temperatures and glacier retreat.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 millenia. 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. [30] [31] [32]
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Spread of disease
It has been claimed that global warming will probably extend the favourable zones for vectors conveying infectious diseases such as malaria. An example of this may be the recent extension to the north Mediterranean region of bluetongue disease in domesticated ruminants associated with mite bites. Another is the increase of hantavirus infection, Crimean-Congo hemorrhagic fever, tularemia and rabies in wide areas of Russia during 2004–2005 associated with a population explosion of rodents and their predators. Some of this, however is blamed on breakdowns in governmental vaccination and rodent control programs.[33] 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 [34].
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Financial effects
Financial institutions, including the world's two largest insurance companies, Munich Re and Swiss Re, warn in a joint study (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.
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Possible positive effects
The NOAA projects that by the 2050s, there will only be 54% of the volume of sea ice there was in the 1950s.Global warming may also have positive effects. Plants form the basis of the biosphere. They utilize the sun's energy to convert water, nutrients, and CO2 into usable biomass. Plant growth can be limited by a number of factors, including soil fertility, water, temperature, and CO2 concentration. Thus, an increase in temperature and atmospheric CO2 can stimulate plant growth in places where these are the limiting factors. Satellite data shows that the productivity of the northern hemisphere has indeed 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 smaller 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. Moreover, 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.
Melting Arctic ice may open the Northwest Passage in summer, which would cut 5,000 nautical miles from shipping routes between Europe and Asia. This would be of particular relevance for supertankers which are too big to fit through the Panama Canal and currently have to go around the tip of South America. According the Canadian Ice Service, the amount of ice in Canada's eastern Arctic Archipelago decreased by 15 percent between 1969 and 2004 [35].
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Mitigating and adapting to global warming
Main article: Mitigation of global warming
"Mitigation of global warming" covers all actions aimed at reducing the extent or likelihood of global warming. The world's primary international agreement on combating climate change is the Kyoto Protocol. Various other strategies include development of new technologies, nuclear power, renewable energy, biodiesel, electric cars (and hybrids), and fuel cells, Energy conservation, carbon taxes and carbon sequestration schemes.
Adaptation stategies accept some warming as a given and focus on preventing or reducing undesirable consequences: for example defending against rising sea levels or ensuring food security.
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