by G.T. Davidson, PhD.


Climate Change and Global Warming have been much in the news in recent years.  Global Warming has become a "hot" political issue, since it threatens massive social and economic change.  The extent and timing of that change are the subject of both scientific and political debate.  These notes may not direct those debates in any particular direction, but I hope they will offer some elementary background, regarding climate change and the possibility of climate cycles in the past.

Former Vice President Al Gore has entered the poltical debate with great eclat.  One of the particular points that he has hammered at is the rapidity of the expected warming.  Generally he has done his homework well, and presented an accurate and compelling picture.  However he has been criticized for his statements to the effect that the warming in the 21st century will be the hottest episode in history.  Yet we know that the Earth has gone through many cycles of climate change; and has at times been very warm—perhaps even warmer than than expected by the end of this century.  That should not be a source of comfort.  The world was a very different place then, with plants, animals, and an environment that would be difficult to recognize today.

The biggest short-term worry is that a climate change could lead to more extreme weather and more intense storms.  In the past there have been intensely cold ice ages and there have been extraordinarily warm times when the ice at the poles disappeared.  Some scientists have concluded that the climate for the past 30 million years has been extremely variable, and that the present mild climate is highly exceptional.  We have been the beneficiaries of an unusually stable warm period beginning about 8,000 to 6,000 years ago, which began with temperatures as much as 3 degrees higher than today.  It has been suggested that this stability is what made agriculture, cities and great civilizations possible.

See also the essential facts on the present episode of global warming

A note on units of units of measurement:
Measured quantities are generally given here in International Metric units, so temperatures are in Centigrade units, and distances in meters and kilometers.  When the standard U.S. units are useful, those units are given in parentheses.  Dates are given in years before the present, which can be reckoned to be the year 2000.  This may lead to some confusion, because ages arrived at by radiochemical or other non-historical means are usually stated relative to the year 1950.

Also note that many scientific papers and books cite dates derived from Carbon-14 analysis.  These dates have been found to underestimate the actual ages of sample materials. By 10,000 years ago the Carbon-14 dates are almost 2000 years too late.


Weather and climate fluctuations at scales from days to millions of years have been recognized since the nineteenth century.  Whether cycles lasting many years are repetitive or periodic is vital to our current understanding and modeling of the climate.  It is of particular importance whether climate variations can be related to known slow periodic phenomena, such as the 11.5 year sunspot cycle and the 18.6 year lunar tidal cycle.

Climate changes have occured on all time scales, from years to millions of years.  These may be cyclic, irregular, or monotonic.  They may involve gradual changes, or they can lead to sudden "tipping points," when the circulation or physical state of the atmosphere-ocean system changes from one state to another totally different state.  This is a serious concern because the system is highly nonlinear, and small perturbations in nonlinear systems can cause sudden large changes, from which it would be nearly impossible to recover.  One of the worries that attends the present debate on Global Warming is whether we might be facing such a tipping point, with consequences beyond our ability to make predictions about the future.  Indeed, our present theoretical understanding leaves open the possibility that the climate could flip to a new state from which it could not recover for hundreds or thousands of years.  Could the Gulf Stream be diverted, resulting in extremely cold weather in northern Europe?  Could the polar ice caps suddenly melt, raising the sea level by as much as 100 meters?  These are well grounded fears, and have been the subject of intense debate at recent meetings of the American Geophysical Meeting.

Nonlinear systems:
"Nonlinear" is more than just a "buzz" word tossed off to make a scientific statement sound more impressive.  It is a precise mathematical term. which indicates that a change in one measured quantity leads to a disproportionate change in another quantity.  Thus, for example, a 1% increase in the temperature of the Sun would lead to a 4% increase in the energy it emits.


If we are to fully to understand the climate changes that are always occuring, we must have a better comprehension of the cyclic effects.  This is absolutely essential for the refinement of the predictions of Global Warming.  It is also necessary to meet the objections, both well intended and spurious, that have been raised regarding the need for actions to mitigate Global Warming.

The search for climate variations and cyclic periods is complicated because data sets extending into the remote past are spatially inhomogeneous. Consequently specific cyclic periods may show up in the analysis of one data set, while a different set of periods may appear in another data set. The comments here are intended to summarize reported periodic phenomena in climate observations, and perhaps to eventually become a repository of newly reported findings.

Several astonomical cycles are well known, such as the 11.5 year sunspot cycle, and various cycles related to the motion of the moon.  The influence of these on the weather and climate is not yet understood.  Other than the obvious annual cycle, few repetitive cycles have been recognized in the weather and climate.  One potential weather cycle of particular interest is the occurence of drought, because drought has strong impacts on the production of food.  The search for precipitation cycles generally involves the accumulation of measurements over many years from weather observing stations, and extending the records back many years by the use of tree ring and other "proxy" data.  Mathematical analysis of such data has led to many claims of weather cycles at periods from days to hundreds of years; the lack of general agreement makes most such claims suspect.

  1. A time-line for climate change over the history of the Earth
  2. Astronomical cycles and the climate
  3. The terminology of geological ages
  4. The big events of Earth's history
  5. A Note on Climate Models
  6. A technical note on the determination of periodic cycles from long trains of data.

Further Reading and Bibliography

This reading list is rather short on original papers; but consists mainly of review works.  This is perhaps appropriate, because these pages are aimed at a synthesis of current knowledge, without being picky about the validity or strength of the the current arguments.

  • Benestad, Rasmus E., Solar Activity and Earth's Climate, Springer-Verlag, New York, 2002, 287pp
  • Benton, Michael J., Verterbrate Palaeontology, Unwin Hyman Ltd., Cambridge, MA, 1990, 377pp
  • Burroughs, William James, Climate Change in Prehistory, The End of the Reign of Chaos, Cambridge Univ. Press, Cambridge, UK, 2005, 356pp
  • Burroughs, William James, Climate Change, A Multidisciplinary Approach, Cambridge Univ. Press, Cambridge, UK, 2001, 298pp
  • Burroughs, William James, Weather Cycles, Real or Imaginary?, Cambridge Univ. Press, Cambridge, UK, 2003, 317pp
  • Committee on Abrupt Climate Change, Abrupt Climate Change, Inevitable Surprises, National Academy Press, Washington, DC, 2002, 230pp
  • Erwin, Douglas H., The Great Paleozoic Crisis: Life and Death in the Permian, Columbia Univ. Press, New York, 1989, 2328pp
  • Dawson, Alastair G., Ice Age Earth, Late Quaternary Geology and Climate, Routledge, New York, 1992, 293pp
  • Dillehay, Thomas D., The Settlement of the Americas, A New Prehistory, Basic Books, Inc., New York, 2000, 371pp
  • Donovan, Stephen K., Mass Extinctions: Processes and Evidence, Columbia Univ. Press, New York, 1989, 266pp
  • Fagan, Brian, The Little Ice Age, How Climate Made History 1300 - 1850, Basic Books, Inc., New York, 2000, 246pp
  • Gould, Stephen J., ed., The Book of Life, W.W. Norton & Co. Inc., New York, 2001
    (Although this is a large format book, suitable mainly for the student, it does contain lots and lots of useful information.)
  • Martin, Paul S., Twilight of the Mammoths, Ice Age Extinctions and the Rewilding of America, Univ. of California Press, Berkeley, CA, 2005, 250pp
  • Pielou, E. C., After the Ice Age, The Return of Life to Glaciated North America , Univ. of Chicago Press, Chicago, IL, 1991, 366pp
  • Roberts, Neil, The Holocene, An Environmental History, Basil Blackwell Inc., New York, 1989, 227pp

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This web site has been created and maintained by Gerald Davidson, PhD.
Most recent revision April, 2010.

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