Brunhes and Matuyama found ways to measure geological time using the magnetism of rocks.
Schott, Kullenburg, and Arrhenius developed methods of studying sediments raised from the bottoms of the oceans.
Harold Urey set out a theory for the oxygen isotope method of dating deep sea sediments, a theory that was confirmed by
Cesare Emiliani in 1955 and 1966. Nicholas Shackleton confirmed the work of Emiliani and extended the dating to include the Pliocene. (See: Time periods and
USGS).
Cores have now been raised from the depths of all the oceans providing results that resolve dates with great precision (Imbrie and Imbrie).
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Relative change in sea level
Relative sea levels change both world-wide and regionally. There are many causes. Land moves vertically (isostatically) as weight is added or removed. Ocean basins change their size and shape as continents move about. Similar continental movements cause land to rise and fall. The shape of the earth, the geoid, changes under the influence of gravity.
Finally, glaciers either remove water from the oceans as water freezes or add water as glaciers melt.
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Earth systems
With the development of theories to account for ice ages, earth scientists began to approach the study of earth by combining the study of astronomy, geology, geophysics, oceanography, and climatology. This approach is necessary for two reasons. First, since so many factors are at work, it is very difficult to determine cause and effect relationships. More cooperation among scientists is needed. Second, the earth does not respond in a simple way to any single factor or combination of factors. For example, astronomical factors do not simply force climate change. There are complex lags and feedbacks in earth systems.
Some earth systems appear to be linear up to a point when they become non-linear or chaotic. For this reason certain stages in earth's glacial history are extremely interesting for what they may tell us of the potential for chaotic change(Broecker, 1997).
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Key issues
Today global warming is in the news and is a topic of conversation.
Central Canada and United States may look forward to warmer winters and less rainfall during summer holidays,
while Florida and other parts of the Sunbelt may expect a decline in winter tourism.
Some farmers look forward to a longer growing season while others expect less summer rainfall and crop failures.
Most discussions of climate change focus on the direct effects of global warming as patterns of temperature and rainfall change.
Lack of agreement arises in part because the winners in this grim game appear to balance or outnumber the losers.
If people view global warming as a zero sum game or a positive sum game then our political traditions lead us to prescribe a policy of "business as usual". But not all regions and not all countries accept such a policy.
In America the evening news features storm-struck coastal areas when the sea washes away homes.
In the Netherlands severe storms and coastal flooding raise questions about national survival,
for in their struggle with the sea the Dutch have been united for centuries.
In Bangladesh the monsoon is a matter of life and death, life in the form of rainfall,
and death when overflowing rivers and high sea levels drown almost half the country every year.
Recognizing that there are exceptional situations and exceptional nations, we sympathize with people who live near the sea and in far-away countries. At the same time we know that sea level is rising too slowly to affect our lives.
A rise in sea level of about 3.5 mm (one-eighth inch) per year means that children born today will witness sea level only 10 inches (25 cm) higher than now. Sea level changes at a geological rate but we live in human time, thus there seems to be no need for concern. But can we be certain sea level will continue to rise at the same slow rate as now?
Archeologists and geologists have found evidence for a great flood that drowned seacoasts about 5,000 years ago.
In many coastal areas of the world, sand, silt and clay deposited in that flood indicate that sea level was higher than now. What if sea level were to rise 5 to 7.5 cm (two to three inches) per year in this century as it did the previous interglacial, about 110,000 ago? Children born today would live to witness sea level as much as two meters (six feet) higher than now. Even a low probablility of a rapid rise in sea level makes climate change an urgent issue because the land that would be drowned by the sea is now home to millions of people.
However, not all scientists agree about the causes of global warming and about the role of human activity.
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Dissenting views
Some scientists do not agree that the evidence clearly indicates a warming trend. Others agree about the warming, but do not agree that man is the sole agent or even the most important agent of change. There are many viewpoints, but one in particular has been around for a long time. In 1952 H.W. Willet of MIT presented a paper to the Climatic Change Symposium in which he discussed a short-term 1,850-year cycle. Various authors have since correlated such cycles and with historical events such as the rise and fall of states in the Mediterranean and the Near East and with the Little Ice Age (Perry & Hsu; Bryson & Bryson, and H.H. Lamb).
Other scientists have examined more fundamental aspects of current theories of global warming,
questioning the role of carbon dioxide (Pearson et al and Paganini et al, cited by Kerr).
The most conservative view is that the output of energy from the sun is cyclical and that the
earth is still emerging from the Little Ice Age, the cool period that began around the time of Shakespeare, improved until about 1800, and then worsened in the first quarter of the 1800's. Though the warming began around 1825 the climate was still cool enough to cause, among other catastrophes, the famines in Ireland. Tree-ring evidence shows that the warming trend that began about 175 years ago is still continuing (Esper, et al). The point is that the warming began too early for it to be explained by man-made changes in the atmosphere. Coal was not burned in significant quantity before 1875, petroleum not before 1900, and gas not before 1950 (Graedel & Crutzen).
What makes it so difficult to distinguish between the natural and human agents of climatic change is partly that so many inter-connected factors form the climatic system and partly that the sytem appears to be non-linear and even chaotic (See also: Lags and Feedback).
Uncertainty concerning causes of global warming gives rise to objections about how to deal with it.
One objection is that mitigation measures may already be too late because lags in the system mean that no matter what is done, things will get worse before they will get better.
Another is that the expense of mitigation measures might be wasted because rapid rise in sea level might never occur.
Like the homeowner whose house did not burn down, we might regret paying for insurance.
Resolving these uncertainties occupies many earth scientists around the world.
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