Ice cores provide direct information about how greenhouse gas concentrations have changed in the past, and they also provide direct evidence that the climate can change abruptly under some circumstances. However, they provide no direct analogue for the future because the ice core era contains no periods with concentrations of carbon dioxide comparable to those of the next century.
-What are ice cores?-Ice cores are cylinders of ice drilled out of an ice sheet or glacier. The longest ice core extended to 3km in depth. The oldest continuous ice core records to date extend 800,000 years in Antarctica. Ice cores contain information about past temperature, and about many other aspects of the environment. The ice encloses small bubbles of air that contain a sample of the atmosphere, from these it is possible to measure directly the past concentration of gases (carbon dioxide and methane) in the atmosphere.
-What do the gases mean?-
Ice core measurements allow us to extend way back into the past based upon measurements of carbon dioxide in the atmosphere. It is possible to measure concentrations in air from as recently as the 1980s that is already enclosed in bubbles within the ice. The ice cores act as a faithful recorder of atmospheric concentration.
Antarctic ice cores show the concentration of carbon dioxide was stable over the last millennium until the early 19th century. It then started to rise, and its concentration is now nearly 40% higher than it was before the industrial revolution. Other measurements confirm that the increase must be due to emissions of carbon dioxide from fossil fuel usage and deforestation. Measurements from other older ice cores confirm that both the magnitude and rate of recent increase are almost certainly unprecedented over the last 800,000 years. The fastest large natural increase measured in older ice cores is around 20ppmv (parts per million by volume) in 1,000 years. Carbon dioxide concentration increased by the same amount, 20ppmv, in the last 10 years. Methane, another important greenhouse gas, also shows a huge and unprecedented increase in concentration over the last two centuries. Its concentration is now much more than double its pre-industrial level. This is mainly due to the increase in emissions from sources such as rice fields, ruminant animals and landfills, that come on top of natural emissions from wetlands and other sources.
-How are ice cores related to climate change?-
By measuring the ratios of different water isotopes in polar ice cores, we can determine the temperature in Antarctica has changed in the past. The oldest ice core that extends back to 800,000 years, shows a succession of long, cold 'glacial' periods, interspersed roughly every 100,000 years by warm 'interglacial' periods. This succession of events is well-known from other records, and the coldest periods in Antarctica are the tomes when we had ice ages.
-Whats the connection between greenhouse gases and glacial-interglacial cycles?-
From the air trapped in the oldest Antarctic ice core, we can see that carbon dioxide changed in a remarkably similar way to Antarctic climate, with low concentrations during cold times, and high concentrations during warm periods. This is entirely consistent with the idea that temperature and carbon dioxide are intimately linked, and each acts to amplify changes in the other. It is believed that warmings out of glacial periods are paced by changes in Earths orbit around the Sun, but the tiny changes in climate this should cause are amplified, mainly by the resulting increase in carbon dioxide, and by the retreat of sea ice and ice sheets. Looking at the warming out of the last glacial period in detail, we can see how remarkably closely Antarctica temperature and carbon dioxide tracked each other. It is often said that the temperature leads the carbon dioxide during the warming out of a glacial period. On the most recent records, there is a hint that the temperature started to rise slightly before the carbon dioxide, as expected if changes in Earth's orbit cause an initial small warming. But for most of the 6,000 year long transition, Antarctica temperature and carbon dioxide rose together, consistent with the role of carbon dioxide as an important amplifier of climate change. In our modern era, of course, it is human emissions of carbon dioxide that are expected to kick-start the sequence of events. We see no examples in the ice core record of a major increase in carbon dioxide that was not accompanied by an increase in temperature. Methane concentration also tracks the glacial-interglacial changes, probably because there were less wetlands in the colder, drier glacial periods.
-What does this mean for climate change?-
The climate changes described above were huge, but relatively gradual. However, ice cores have provided us with evidence that abrupt changes are also possible. Other records show us that major changes in atmospheric circulation and climate were experienced all around the northern hemisphere. Antarctica and the Southern Ocean experienced a different pattern, consistent with the idea that these rapid jumps were caused by sudden changers in the transport of heat to the ocean. At this time, there was a huge ice sheet over northern North Atlantic. Freshwater delivered from the ice sheet to the North Atlantic was able periodically to disrupt the overturning of the ocean, causing the transport of tropical heat to the north to reduce and then suddenly increase again. While the mechanism cannot occur in the same way in today's world, it does show us that, at least regionally, the climate is capable of extraordinary changes within a human lifetime - rapid switches we certainly want to avoid experiencing.
http://www.antarctica.ac.uk/bas_research/science_briefings/icecorebriefing.php
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