Paleoclimate: The deep and modern time perspective
Eystein Jansen, Bjerknes Centre for Climate Research and Dept. of Earth Science, University of Bergen, N5007, Bergen, Norway. Email: eystein.jansen@geo.uib.no
Abstract
The ongoing global warming and the increasing content of atmospheric greenhouse gases which will give continued warming of our planet, call for knowledge about the operation of the climate system on a number of timescales. We need to identify the true sensitivity of the climate system to forcings, we need to identify mechanisms and feedbacks through which climate forcing is transformed into climate change, and we need to understand how natural and man-made changes interact. Improved projections of the future state of our planet will hinge on better understanding of the climate system and its interactions, but also from how climates changed in the past unrelated to human forcings. Thus paleoclimate knowledge is clearly relevant for climate policy, but there are also restrictions to the relevance as stated in the 4th Assessment Report of the IPCC. Climate change is both driven by internal processes of the climate system, but also originate to a large extent from external forcings such as atmospheric trace gas content, solar variability, volcanism, changes in the Earth´s orbit and on longer time scales tectonic changes and orogeny. In terms of deep time perspectives, it is difficult to quantify and model past changes, yet there appear to be a clear correspondance between major changes in the mean state of the climate and changes in forcings such as atmospheric greenhouse gas content. The orbital parameters are major drivers of climate change, and enables us to identify both forcings and critical aspects of the feedbacks of the system that translate small changes in radiative forcings into major changes such as growth and demise of ice sheets. The background state of the climate system, has great importance for these factors, exemplified by the emergence of an active cryosphere on the planet in the form of snow and ice, which sets into operation powerful feedback mechanisms. The background state also appears to be important for the possibility for abrupt changes to occur. Abrupt changes appear mostly to have regional and hemispheric extent and can be triggered in situations when the climate system operates in situations which are unstable, often intermediate between warm and cold states, during ice ages. The recent millennia provide the best detailed information of climate change under boundary conditions akin to those that prevail now, except for the fact that the current level of greenhouse gas forcing probably is unprecedented for the last million years. The recent millennia tell us a story of regional and hemispheric changes that to a large extent were a response to a combination of stochastic procesess and volcanic and solar forcing, creating multidecadal to century scale changes. The knowledge we have from the past supports our understanding of an ongoing man made global warming. Thus there is little conflict between the perspective and understanding we have from the past, and the projections we have for the future.
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