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Global Warming
Answers to Frequently Asked Questions
| Is there a natural greenhouse effect? | |
| Yes. The average temperature at the surface of
the earth is 15ºC. Without the natural greenhouse effect, it would be
around 33ºC cooler (-18ºC). This natural greenhouse effect is caused by
naturally-occurring greenhouse gases, such as carbon dioxide, methane, and
water vapour. Most climate scientists think that, over the next century,
mankind's emissions of greenhouse gases will increase the greenhouse
effect by 5-15%, causing a warming
of 1.8-5.4ºC.
A good introduction to natural greenhouse effect, as well as the enhanced greenhouse effect that will occur from mankind's emissions of greenhouse gases, is provided by the University of California's Virtual Museum.
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| Is water vapour the most important greenhouse gas? | |
| It depends what is meant by 'important'.
Greenhouse gases act by absorbing radiation that is emitted from the
earth's surface, and water vapour, because it is so abundant, absorbs more
radiation than any other. However, the amount of water vapour is governed
by the air temperature, among other things (warmer air can hold more water
vapour). So it is the amount of these other greenhouse gases that
determines, to a degree, the amount of water vapour. Water vapour,
therefore, is important because it acts to amplify the warming due
to other greenhouse gases. For a fairly simple explanation of the role of
water vapour in the greenhouse effect, see this
page. Working out the proportion of the natural greenhouse effect that is due to water vapour is not easy. Greenhouse gases act by trapping a range of frequencies of infrared radiation. Different greenhouse gases absorb some unique frequencies, but some shared frequencies. This means that if you were to remove one greenhouse gas, much of the effect would remain. For a more detailed explanation, see The Earth's Annual Global Mean Radiation Budget. As shown in Table 3 of that document, in cloudless skies, water vapour contributes around 60% of the greenhouse effect, and carbon dioxide around 26%. Most of the rest is caused by ozone and methane. The degree to which water vapour will amplify the effects of increases in other greenhouse gases is a crucial issue, but one that is currently far from resolved. Most climate models show that doubling the concentration of CO2 in the atmosphere will increase temperatures by 1.1ºC, and that water vapour feedback will increase this to 1.8ºC (further amplification comes as snow and ice is melted, thereby reducing albedo). However, it is not currently known with certainty how much water vapour increases for a given increase in temperature, and the changes in cloud cover are also uncertain (see also Are climate models accurate?). What evidence there is suggests that water vapour in the stratosphere has increased by more than predicted, but water vapour in the lower troposphere by less than predicted (Minschwaner et al., 2004). For a selection of scientific papers on this topic, see The Climate Sensitivity Publication Exchange. See The Earth Observatory for a discussion of recent results in context.
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| Are greenhouse gases increasing? | |
| At the end of the last ice age, the concentration of CO2
increased by around 100 ppm (parts per million) over around 8,000 years, or around 1.25
ppm per century. Since the start of the industrial revolution, the rate of increase has
accelerated markedly: since 1860, the concentration of CO2 has increased by
around 80 ppm, just over 50 ppm per century. The rate of CO2 accumulation
has continued to increase, and it currently stands at around 150
ppm/century – over 200 times faster than the background rate for the past 15,000
years. The result can be clearly seen in the Law Dome Ice Core, which shows the increase
in CO2 that has occurred over the last
1000 years. Other greenhouse gases have also increased markedly, according to the US Govt.
Carbon Dioxide Information Analysis Center.
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| What is causing the increase in CO2 and other greenhouse gases? | |
| Some greenhouse gases, such as CFCs, are synthetic, and so we
know that their source is anthropogenic (man made). However, other greenhouse gases, such
as CO2 and methane, have natural sources as well as anthropogenic ones. In
fact, natural sources of these greenhouse gases are far greater than anthopogenic ones. We
know, also, that levels of these natural greenhouse gases have fluctuated in the past
entirely naturally. How, then, do we know that anthropogenic sources are the cause of the
current increase? There are several reasons for attributing the rise in greenhouse gases
to anthropogenic, rather than natural, emissions. The first clue comes from comparing the current increase with changes that have occurred in the past (see Paleoclimate FAQ). Records from ice cores show that the current increase in CO2 and other greenhouse gases is much more rapid than has occurred naturally in the past. In addition, the current concentration of GHG is far greater than at any time over the past 400,000 years, and is still rising rapidly. This strongly suggests that the cause is different to that which has caused greenhouse gases to increase in the past. The second line of evidence comes from changes in the isotopic concentration of atmospheric carbon. Carbon consists of three main isotopes, C12 (the most common), C13 and C14. Fossil fuel is depleted in both C13 and C14. The main natural sources of carbon in the atmosphere are the ocean and the biosphere (plants and animals). However, carbon from the ocean is depleted in C14 but not C13, whereas carbon from plants and animals is depleted in C13 but not C14. By tracking changes in these isotopes, it can be shown that fossil fuels are the major source of the modern increase. For more information on this, see Robert Grumbine's carbon dioxide FAQ. The third reason for thinking that anthropogenic sources are the main cause of the increase is to look at the full picture – absorption as well as emission. Greenhouse gases are cycled naturally - they are emitted, and then they are either absorbed or broken down (here are illustrations showing the global carbon cycle and the methane cycle). If the amount of gas emitted is equal to the amount absorbed, then the cycle is balanced, and there will no accumulation in the atmosphere. Taking the carbon cycle as an example, it's possible to add up all the sources of carbon emissions to the, and add up all the sources of absorption from the atmosphere, as follows: Emission to the atmosphere: 191 GtC/yr natural + 8 GtC/yr manmade It can be seen that, if there were no manmade sources, absorption would be greater than emission, and so atmospheric levels of carbon would actually decrease. This is a surprising result, and it is caused by the fact that increasing temperatures have resulted in increased plant growth (see A Greener Greenhouse). The increase absorption, however, is not great enough to offset the increased emissions from human activity, and so CO2 continues to accumulate (see also Will increased plant growth absorb the excess CO2?). Fossil fuel, however, is not the only source of the increase in CO2 and methane. Changes in land use also have made a significant proportion of the increase in atmospheric CO2 - around a quarter of the total in 1990, for example. Major sources of methane include rice paddies, livestock, and landfill sites (see methane cycle and Robert Grumbine's methane FAQ).
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| Is the Earth absorbing more radiation than it emits? | |
| Greenhouse gases cause the greenhouse effect because
radiation from the sun can penetrate the atmosphere and be absorbed by the earth, but
radiation from the earth cannot escape as easily. This trapped radiation causes the earth
to heat. A recent analysis of satellite data (detailed in this press release,
this news report, and this technical
summary) and this have shown that this is, indeed, occurring. The analysis of
the spectrum of radiation emitted from the earth shows that those frequencies which are
predicted to be absorbed by greenhouse gases really are being absorbed.
An increased greenhouse effect at the earth's surface has also been demonstrated experimentally (Philipona et al, 2004)
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| Is the recent warming caused by changes in solar radiation? | |
| Much solar radiation is trapped high in the atmosphere, and
so it is not possible to directly measure solar activity without the use of satellites.
These satellites have only been available since 1979 (a graph of solar
activity, as measured by a series of satellites, is provided in this article
from NASA's Earth Observatory). It is clear that there has been no recent rise in solar
activity that could explain the recent rise in temperature. As with
satellite measurements of tropospheric
temperatures, it has proved to be very difficult to remove the errors
caused when one satellite is replaced by the next. As a result, there is some
controversy over whether these satellites show that the sun has
remained constant over the past 20 years, or increased by 0.1% over that
time. An increase of 0.1% would not, in itself, account for the warming
that has been observed in the 20th century over that time. There is no doubt that solar variability plays an important role in global climate change. In the 1400s to 1700, decreased solar activity (the 'Maunder Minimum) led to a period of global cooling, known as the 'Little Ice Age' that was particularly acute in Europe (discussed here). Looking further back, it is possible that 1,500-year cycles in solar activity have caused periodic changes in the earth's climate (Bond et al 2000. Abstract, full paper). A cold snap that occurred 10,300 years ago also appears to be linked to a decrease in solar radiation. The amount of solar radiation that hits the earth is closely related to the sunspot number, as well as to other parameters that can be recorded from ground level. Since some of these provide records stretching back several hundred years, it is possible to reconstruct solar activity with quite high resolution over these time spans. An overview of some of these is shown on this page. Solar activity further back in time can be estimated from the amount of beryllium isotopes trapped in ice cores and elsewhere. Using these isotope data, it can be shown that solar brightness increased in the early part of the 20th century, to its highest level for the past 1000 years. These reconstructions of solar activity have allowed scientists to investigate the relative impact of solar activity and greenhouse gases on current climate change. Several studies have investigated the relationship between changes in solar activity, volcanoes and temperatures over the past 1000 years and have concluded that, although changes in solar activity can explain much of the temperature changes, there is a 'residual' unexplained temperature increase in the 20th century. For example, Mann et al concludes that "While the natural (solar and volcanic) forcings appear to be important factors governing the natural variations of temperatures in past centuries, only human greenhouse gas forcing alone ... can statistically explain the unusual warmth of the past few decades". A similar analysis by Crowley et al concludes that there is "a very large late 20th century warming that closely agrees with the response predicted from greenhouse gas forcing.” The temperature changes in this century have been studied using sophisticated statistical methods. Such an analysis is presented by Thomas et al (1997), who show that changes CO2 over the last century have had around three times the impact on temperature as have changes in solar irradiance, and that there is no evidence in the statistics of any major unidentified source of natural variation. Another, more recent, analysis (Kaufmann & Stern, 2002) found that temperature changes could only be explained by taking into account changes in solar activity, sulphate aerosols and greenhouse gases. However, the only way to fully understand this complex relationship is by using climate models (see also Are climate models accurate?). The detailed causes of the recent warming trend have been investigated by the UK Meteorological Office, and are presented here (see also Stott et al, 2000). They found that about half of the warming is caused by solar variability but that, in the second half of the century, these effects have been countered by sulphate emissions from dirty fuel and from volcanoes (which contributed to a global cooling observed in the 1960s and 1970s). The overall effect of all the natural causes (sun and volcanoes combined) has been quite small. They conclude that there is "very large late 20th century warming that closely agrees with the response predicted from greenhouse gas forcing.” Similarly, two recent studies of ocean temperatures have found that the observed increase is best explained by the effect of greenhouse gases.
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| Is the recent warming caused by volcanic activity? | |
| In the short term, volcanoes exert a net cooling
effect at the surface due to their emissions of sulphur dioxide, because particles in
the stratosphere absorb the sun's energy before it can reach the surface. The cooling
effect depends on the composition of the volcanic emissions (particularly sulphur content)
and on the location of the volcanoes (high latitude volcanoes - nearer to the poles - tend
to have a greater effect because more of their emissions reach the stratosphere). The
cooling effect of some of the most important recent volcanoes is provided by Volcano
World. In the atmosphere, the effect is more complicated. In the short term (for a few years) the stratosphere is warmed by the energy that is blocked from reaching the surface. However, volcanic dust also provides a surface for ozone depletion. Ozone normally absorbs sunlight in the stratosphere. In the long term, the effect of this ozone depletion is to cool the stratosphere somewhat. These complicated effects partly explain the discrepancy between surface and satellite measurements of the earth's temperature (see above). Volcanoes do also emit CO2, and massive eruptions in the past may emitted enough CO2 to cause long-term climate change. However, in the recent climatic record, volcanic emissions have been much lower. Gerlach (1991) estimated a total global release of 3-4 x 1012 mol/yr from volcanoes. Man-made (anthropogenic) CO2 emissions overwhelm this estimate by at least 150 times. Analyses of temperature changes over the past 1000 years also show that the rise in temperature this century can't be explained by solar or volcanic activity (see below).
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| What caused the global temperature changes of the 20th century? | |
 In
addition to greenhouse gases (GHG), changes in solar activity and
volcanoes (i.e. stratospheric aerosols), numerous other factors are
believed to have affected global temperatures over the past century. For
example, black carbon (soot) has increased the amount of sunlight
absorbed, and so caused increased warmth. Ozone and stratospheric water,
in part a product of the breakdown of methane, have acted as greenhouse
gases. On the other hand changes in land use (mainly deforestation) and
sulphate pollution (tropospheric aerosols) are believed to have had a
significant cooling effect.
The effect of these individual factors (radiative forcings) can be calculated using climate models. The results of one such model (GISS SI 2002) are shown in the figure on the right. A positive forcing indicates a warming effect, whereas a negative forcing indicates a cooling effect. The effects of volcanic activity are represented as 'stratospheric aerosols). It can be seen that the major positive effect is due to GHGs, whereas the major negative effect is due to sulphate pollution (reflective tropospheric aerosols and indirect aerosol effect). When all these effects are added together, the result is that shown in the bottom graph. From this, it can be seen that there was negative forcing before 1900 (i.e. cooling), followed by warming up until 1950. Then there was a marked increased in tropospheric and stratospheric aerosols, resulting in a slight cooling until the warming trend resumed in around 1980. These effects explain the trend in 20th century temperatures (see Have surface temperatures risen?). It should be remembered that although this is an explanation, it is not the only possible one. Different models, for example, provide different results for the effects of the different forcings. One of the most important unknown factors is the effects of tropospheric aerosols. It is believed that they have masked a great deal of the 20th century warming, but it is not known exactly how much. If they did not have much effect, then that means that the warming due to other factors must also have been quite small, and this means that future effects of GHG are also likely to be relatively small. On the other hand, if they have had larger effects, then that means that the effects of GHG must also be larger than usually thought. This means that the amount of warming in the future is likely to be larger (especially as fuels around the world become cleaner, and so emit fewer sulphates). This complex subject is explained in more detail on the homepage of Stephen Schwartz, a leading expert in the effects o aerosols on climate.
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Last updated 29/07/05. By Tom Rees. Contact the author |
