Climate change—perspectives from the past


Judith Totman Parrish, Dept. of Geological Sciences, Mines 322, University of Idaho, Moscow, Idaho 83844-3022, USA.


Current concerns about global climate change are built around very short-term observations of climate variability.  Although the social and political consequences of the results of climate change cannot be ignored, often lost in the discussion is the fact that climate has in the deep past varied on time scales as fast and with amplitudes much larger than we have observed in the recent past.  This natural variability of climate should be part of the discussion in order to provide context for any proposed actions.  Geoscientists uniquely contribute to understanding the past, pre-human behavior of climate.


Perhaps the most dramatic example of high-amplitude, though longer-time scale evolution of climate is that of the supercontinent Pangea.  As Pangea was assembled and moved north, the large sizes of the land masses on either side of the equator created a strong alternating flow equivalent to the Asian monsoon, but fully developed in both winter and summer hemispheres, unlike the Asian monsoon.  This system eventually influenced all but the highest latitudes, bringing seasonal rainfall to nearly the entire continent and, coupled with what were apparently relatively warm global temperatures, highly seasonal semi-arid to semi-humid climates worldwide (Parrish 1993). 


Although the evolution of the Pangean climate system as a whole took place over tectonic time scales, recent information suggests that sub-tectonic-scale variations also occurred.  Most of these variations have been attributed to Milankovitch cyclicity, that is, variability on the time scale of variations of the Earth’s orbit.  The significance of this is that this type of cyclicity has been expressed throughout the geologic record regardless of the overall climate that prevailed—it is observed in marine and terrestrial systems, and in both humid and arid climates.


However, not all small-scale variability can be attributed to Milankovitch cyclicity, emphasizing the dynamic nature of climate behavior.




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