On the possibility of rapid climate change

Over the course of geologic history, the environment on Earth has been far from static. Geologic evidence suggests that 600 million years ago the atmosphere lacked sufficient oxygen to support animal life. More recently, as indicated by sediments recording conditions over the past 500,000 years, the climate of the planet varied between at least two different states. The record from the past 150,000 years is particularly well-preserved, offering details concerning repeated climate changes. Between approximately 131,000 and 114,000 years ago, a warm period similar to the climate of today occurred. This was followed by what is called the "Wisconsin ice age", which ended approximately 12,000 years ago when the current relatively warm *Holocene period began. ... ... Kendrick Taylor (Desert Research Institute, US) presents a review of the research of a large project to develop a climate record for the past 110,000 years, the author making the following points:

  1. The layerings of glacial ice record seasonal variations of temperature, snowfall, concentrations of atmospheric gases, and atmospheric circulation patterns. In general, the weight of accumulating snow compresses the snow below it, trapping atmospheric gases, dust, and chemicals, and a deep ice core thus provides a sequential record amenable to analysis.
  2. The author reports that by examining ice cores from Greenland, he and his colleagues have determined that climate changes large enough to have extensive impacts on our society have occurred in a time-frame of less than 10 years. The author suggests that the climate of Earth could change significantly during a lifetime, that we are still a long way from being able to predict such a change, but we are getting closer to an understanding of how it might occur. A pressing concern is whether anthropogenic changes in the atmosphere of the planet might perturb climate stability.
  3. The author points out that climate is the result of the exchange of heat and mass between the land, ocean, atmosphere, ice sheets, and space. As long as changes to the land, ocean, atmosphere, and ice sheets stay below certain thresholds, climate changes will occur slowly. But climate will change rapidly if those thresholds are crossed. *Greenhouse warming, for example, by altering ocean circulation and the flow of tropical heat to the North Atlantic, could lead to rapid cooling in eastern North America, Europe and Scandinavia. Altered ocean circulation could lead to much larger changes. We have no experience predicting climate switches between stable modes.
  4. The author suggests human ingenuity would most likely allow us to adapt to a rapid change in climate, but we would pay a larger price than our civilization has ever known. The author poses a scenario: "Imagine the economic and social cost of moving, in a 20-year period, most of our agricultural activities 500 miles south of their current locations. Imagine the social cost and famine if agriculture could not be relocated quickly enough."
  5. Although we do not know the critical level of greenhouse gas concentration that would trigger a rapid climate change, we do know that reducing the rate of greenhouse emissions would help in two ways. First, the atmospheric concentration of greenhouse gases would increase more slowly. Second, numerical models predict that the climate threshold will occur at a higher concentration of greenhouse gases if the concentration of greenhouses increases slowly.
  6. The author suggests it will be another 20 years before the climate changes that are predicted to be associated with the greenhouse effect becomes large enough to be unambiguously differentiated from naturally occurring variations in climate. As a society we have the choice of ignoring the warning signs or taking some action.

Kendrick Taylor: Rapid climate change.
(American Scientist Jul/Aug 1999 87:320)
QY: Kendrick Taylor [kendrick@dri.edu]
Text Notes:
... ... *Holocene period: The most recent epoch of the geologic time scale, from approximately 10,000 years ago to the present.
... ... *Greenhouse warming: See notes to report #1, this issue.
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 13Aug99
Related Background:
A "climate forcing" is an imposed perturbation of the Earth's energy balance with space, for example, a change of the solar radiation incident on the planet, or a change of carbon dioxide in the Earth's atmosphere. The unit of measure of climate forcing is Watts per square meter. Thus, the forcing due to the increase of atmospheric carbon dioxide since pre-Industrial times is approximately 1.5 Watts per square meter. Climate change is combination of deterministic response to forcings and *chaotic fluctuations -- the chaos a consequence of the nonlinear equations governing the dynamics of the system. Quantitative knowledge of all significant climate forcings is needed to establish the contribution of deterministic factors in observed climate change and to predict future climate. J.E. Hansen et al, in a review of current considerations concerning climate forcings in the Industrial era, make the following points:

  1. The forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change.
  2. Anthropogenic greenhouse gases, which are well-measured, cause a strong positive (warming) force. But other, poorly measured, anthropogenic forcings, especially changes of atmospheric aerosols, clouds, and land-use patterns, cause a negative forcing that tends to offset greenhouse warming.
  3. One consequence of this partial balance is that the natural forcing due to solar irradiance changes may play a larger role in long-term climate change than inferred from comparison with greenhouse gases alone.

Current trends in greenhouse gas climate forcings are smaller than in popular "business as usual" or 1 percent per year carbon dioxide growth scenarios. The authors suggest that a summary implication of their considerations is a paradigm change for long-term climate projections: uncertainties in climate forcings have supplanted global climate sensitivity as the predominant issue. The authors further suggest that climate forcing scenarios are essential for climate predictions, but if only one forcing scenario is used in climate simulations, as has been a recent tendency, the scenario itself is likely to be taken as a prediction, as well as the calculated climate change. The authors recommend that the use of multiple scenarios will aid objective analysis of climate change as it unfolds in coming years.
J.E. Hansen et al (6 authors at National Aeronautics and Space Administration, US)
Climate forcings in the Industrial era.
(Proc. Natl. Acad. Sci. US 27 Oct 98 95:12753)
QY: James E. Hansen [jhansen@giss.nasa.gov]
Text Notes:
... ... *chaotic fluctuations: The term "chaotic", in this context, is specific. In the study of physical systems, the term "chaotic behavior" has a specific meaning: the behavior of a system is said to be "chaotic" if its final state is so sensitive to the system's precise initial conditions that the behavior of the system is in effect unpredictable and cannot be distinguished from a random process, even though the behavior of the system is strictly determinate in a mathematical sense. In other words, a deterministic system characterized by extremely sensitive instabilities, despite the system being determinate, can exhibit behavior that is unpredictable, and the system is then called "chaotic". During the past several decades, the analysis of such chaotic systems has intrigued both physicists and mathematicians.
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 4Dec98

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