Ed, Josh: As suggested by Ed, a return to a Miocene-like climate is one (and the most likely) possiblity for a high end global warming scenario.
However, a runaway greenhouse effect is also possible. These are the 4x and 6x CO2 scenarios where CO2 increases to 1200-1800 ppm. Scientists run them just for fun and for sensitivity analysis, and such scenario have gotten little attention by the public. An increase in greenhouse forcing of between 10 and 20 watts per square meter (compared to the time just before the industrial revolution) could cause a runaway greenhouse effect. The total effect of human greenhouse gases and black carbon (soot) at this point is about 3.7 watts per square meter, and that has been partially offset by increases in aerosol pollution. If all fossil fuels were burned, including the tar sands, and most methane and CO2 from permafrost was also released into the atmosphere, it is possible to reach 10-20 Watts per square meter of forcing. To get a runaway greenhouse effect, there would have to be very strong rapid global warming of 10 degrees C, and a substantial positive feedback loop caused by water vapor from evaporating oceans. The earth has seen atmospheric conditions with this level of forcing before without causing a runaway greenhouse effect (100s of millions of years ago), but there was less energy output from the sun then, and also when greenhouse forcing was that high, the forcing increased slowly, over 1000s or 1000000s of years, so that weathering processes had time to create a negative feedback, preventing a runaway greenhouse effect. The same amount of greenhouse gases now would warm the earth much more due to the now significantly higher energy output from the sun, and could happen too fast to be counterbalanced by weathering processes. This scenario was first suggested in the 1980s, when we talked about it in a class on climate change I had at the University of Wisconsin taught by two of the top climate scientists in the world, Reid Bryson and John Kutzbach. There was little interest in such scenarios, however, until the last year or two, and it is now once again an active area of research, after James Hansen chose to talk about it in his keynote address at the American Geophysical Union annual meeting last December. I did have a 4xCO2 scenario in one of my climate change presentations, that showed shortgrass prairie replacing the boreal forests of northern Ontario and Manitoba, including the shore of Hudsons Bay, but people in the audience aren't capable of processing changes that large, so I took that out. Lee Edward Frank wrote: > Life After Warming > > Really even looking at the warmest predictions of climate change, 5 to > 7 degrees centigrade, it should not be any warmer than it was in the > Miocene Epoch 23.8 to 5.3 Million years ago. The Eocene Epoch, > Paleocene Epoch, and Cretaceous Period were each respectively both > older and warmer. It is nothing that has not been seen before. > Global warming will certainly screw things up for people and our > existing ecosystem distributions, but will not be the end of everything. > > http://www.ucmp.berkeley.edu/tertiary/mio/miolife.html > > The overall pattern of biological change for the Miocene is one of > expanding open vegetation systems (such as deserts, tundra, and > grasslands) at the expense of diminishing closed vegetation (such > as forests). This led to a rediversification of temperate > ecosystems and many morphological changes in animals. Mammals and > birds in particular developed new forms, whether as fast-running > herbivores, large predatory mammals and birds, or small quick > birds and rodents. Plant studies of the Miocene have focused > primarily on spores and pollen. Such studies show that by the end > of the Miocene 95% of modern seed plant families existed, and that > no such families have gone extinct since the middle of the > Miocene. A mid-Miocene warming, followed by a cooling is > considered responsible for the retreat of tropical ecosystems, the > expansion of northern coniferous forests, and increased > seasonality. With this change came the diversification of modern > graminoids, especially grasses and sedges. > > The climate change from global warming is likely to make the human > influenced extinctions worse. There will be problems in that the > change will be more rapid than many species can migrate, but likely > (in my opinion) some populations of most of the major tree and plant > species will survive and potentially spread out again once the climate > stabilizes. The distribution of various ecosystems will change, and > some new associations will develop. This type of change has happened > many times in the geologic history, the only difference is that this > change MAY be more rapid. There are many predictions, but honestly I > don't think anyone really knows exactly what will happen. > > Ed Frank > > > Check out my new Blog: http://nature-web-network.blogspot.com/ (and > click on some of the ads) > > > --~--~---------~--~----~------------~-------~--~----~ Eastern Native Tree Society http://www.nativetreesociety.org Send email to [email protected] Visit this group at http://groups.google.com/group/entstrees?hl=en To unsubscribe send email to [email protected] -~----------~----~----~----~------~----~------~--~---
