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Global Warming
Answers to Frequently Asked Questions
| How does the current temperature compare with the past 1000 years? | |
 Fact sheets
on the paleoclimate
are provided by the US National Ocean and Atmospheric Administration Paleoclimatology Program
and the UK Climatic
Research Institute . There are a variety of climate proxies that can be used to estimate past temperatures. These include tree rings, ice cores, corals and historical records. For the past 100 years or so, the instrument record provided by weather stations has overlapped with the record provided by climate proxies. This enables the climate proxies to be calibrated, and also allows scientists to check their accuracy. The figure on the right (provided by Wikipedia) shows the various different reconstructions of temperature over the past millennium. They use different methods and different data. Although they differ in the specifics, all these reconstructions agree that the global mean temperatures have declined gradually over the past 1000 years, before increasing rapidly more recently. In the northern hemisphere, there seems to have been a more pronounced warm period early in the millennium and cool period in the latter part of the millennium (the 'Medieval Warm Period' and 'Little Ice Age', respectively). These do not appear to have been global events (see this review), and instead are likely to reflect changes in the North Atlantic Oscillation and linked to changes in solar activity (see "Is the recent warming caused by changes in solar radiation?"). For more information on the Medieval Warm Period, see this article from the NOAA Paleoclimatology Program. It has been argued that tree ring data are misleading, because increased levels of CO2 may increase the growth rate of trees (which would be interpreted as increased temperature). However, studies of the tree rings from this century show that any such 'fertilisation effect' is negligible. This doesn't rule out the possibility that there may be some other, unidentified, bias. However, for the periods where sufficient non-tree data are available to make a comparison, the record reconstructed solely from the tree ring data is essentially the same as that constructed from non-tree ring data (see this link).
Before the onset of the industrial revolution, and the resultant increase in greenhouse gases, global temperature changes are thought to be largely a result of changes in solar activity, volcanic activity, and deforestation (which caused a gradual cooling, especially in the last few hundred years), overlaid onto a background of internal climate 'noise'.
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| How has temperature and CO2 changed since the last ice age? | |
 Further back in time,
temperatures and atmospheric carbon dioxide levels can be measured using ice cores. The
figure on the right shows the records from the Vostok and Law Dome ice cores, although
similar records are available from other ice cores obtained from Antarctica and Greenland.Twenty thousand years ago, the earth was in the grip of an ice age. Around fifteen thousand years ago, the temperature started to warm (probably as a result of variations in the earth's orbit, see below), triggering natural CO2 release and a feedback greenhouse effect. The temperature increase, around 8 deg C, reflects temperatures around the South Pole. The temperature change in tropical regions was smaller. For the past ten thousand years, however, the earth's temperature and atmospheric CO2 has been relatively stable – although temperatures have varied over a range of 4ºC (although some of this variation is probably an artifact due to inevitable measurement errors). The causes of these fluctuations are not known with certainty, but are likely to be due to combinations of variation in solar activity and periodic changes in ocean currents. The near-vertical red line at the far left marks the rise in atmospheric CO2 since the start of the industrial revolution.
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| How does the current temperature compare with the past 400,000 years? | |
 A record of temperature and
atmospheric CO2 over the past 400,000 years is preserved in the Vostok Ice
Core and is shown in the figure on the right. It can be seen that there have been a
series of large fluctuations in temperature (the Ice Ages),
accompanied by large changes in atmospheric CO2. It is thought that these large
temperature fluctuations are triggered by Milankovitch
cycles - variations in the earth's orbit that change the amount of energy from the sun
that reaches us. However, on their own, these cycles are not enough to explain the changes
in temperature. The full explanation seems to be that the small change in temperature
caused by the changing orbit are amplified by natural processes on earth. These cause CO2
to be released from the oceans and the biosphere, causing an increased greenhouse effect.
This is described more fully in this article
from the New Scientist (see also Shackleton 2000).
For more details on the timing of changes in CO2 and
temperature, click on the figure.
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| How does the current temperature compare with the past 500 million years? | |
 Further back in
time, estimates of temperature and atmospheric CO2 become increasingly
uncertain. The current best estimates, along with predicted global temperatures (based on
CO2 concentrations and solar activity) and estimated actual tropical
temperatures, are shown in this figure, based on the paper by Royer
et al, 2004. Previous analyses have found that there was not a
good correspondence between
CO2 concentrations and temperature measured using chemical
isotopes in sea shells (see Nature Science Reviews and BBC News articles).
However, Royer et al showed that this was because CO2
in the atmosphere made the sea water more acidic. The temperature results
therefore needed to be corrected for atmospheric CO2 (see commentary).Although there is a good overall correlation between CO2 and temperature over the past 500 million years, and it is likely that other factors besides CO2 play an important role over these very long time scales. A prime candidate factor is continental drift. In the case of the late Ordovician, a huge supercontinent known as Gondwanaland lay over the south pole, and climate models suggest that this would result in glacial conditions even with very high atmospheric CO2. The mid-Mesozoic warming may also be related to continental drift, although in a more complicated way – it may be that there was an increased difference between the temperatures of the tropics and the poles . Whatever the explanation it can be seen that, on time scales of millions of years, there are many factors other than CO2 that exert a powerful influence on temperature. For more information on the factors that determined the climate of the mid-Cretaceous era (90–120 million years ago), see this article from the NOAA Paleoclimatology Program. For more general information on the powerful influence that continental drift exerts on global temperatures, see this presentation. |
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| Has a sudden rise in greenhouse gases ever caused climate change? | |
| As the above discussion shows, greenhouse gases are just one
of many factors that can cause climate change, and climate change in the past appears to
be very different to that of modern times. CO2 dissolved in
rainwater contributes to erosion, and the conversion of CO2 to insoluble
carbonates. The Co2 is returned to the atmosphere by volcanic activity,
and this cycle can fluctuate, changing the amount of CO2 in the
atmosphere. In addition, the small temperature rises associated with the earth's orbital 'wobble' appear to
have triggered the release of CO2, resulting in a climate feedback effect and a
rapidly warming earth. The complicated nature of past climate has lead some people to suggest that changing levels of greenhouse gases have only a small effect on temperature, especially when compared with other causes of temperature change. To test whether this is so, it is important to know whether a rapid increase in greenhouse gases, such as is now occurring, can actually cause significant climate change. Two events in the past suggest that this is, indeed, the case. KT Boundary (65 million years ago) The KT Boundary is the name given to the a thin geological layer that marks the impact of a gigantic meteorite – the same meteorite that caused the extinction of the dinosaurs. In the short term, this meteorite caused great clouds of dust that obscured the sun. However, it also released huge amounts of CO2 by vaporising carbonate-rich rocks. This lead to a massive increase in the greenhouse effect, as described in this BBC News article and, for more detail, the scientific paper (Beerling et al, 2002). Late Palaeocene Thermal Maximum (LPTM, 55 million years ago) Scientists have long known that there was a short, but dramatic period of warming at the end of the Palaeocene era. This is now believed to have resulted from several massive releases of methane from the sea floor, probably as a result of continental drift, but possibly as a result of volcanic activity. For more information, see Earth 's ancient heat wave gives a taste of things to come, this NASA story and the paper by Bains et al, Science 1999. The changes in sea surface temperatures fit well with predictions from climate models, according to research published in 2003. Although there are several similarities to today's warming, one major difference is the timescale: the release of methane took place over a period of several thousand years. There was no mass extinction, probably because plants had sufficient time to 'migrate' northwards to more hospitable climates. |
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Last updated 02/02/06. By Tom Rees. Contact the author |
