Climate Letter #1125

An explanation behind the increasing melt rate on the underside of polar ice shelves.  It has to do with the fact that warm water infused with salt is heavy enough to remain well below a surface covered with cooler freshwater.  Natural circulation allows it chew away at the bottoms of thick shelves, creating more cold freshwater in the process.  The entire circumference of Antarctica is exposed to that kind of circulation.  “Because ice shelves act as speed bumps for glacial ice flow and slow down the rate at which Antarctic glaciers reach the sea, an increase in ice-shelf melt would mean that glaciers could dump vast amounts of freshwater ice into the ocean unchecked. ‘If we lose (the ice shelves), the speed of the glaciers could be four to five times faster,’ said Dr. Herraiz Borreguero.”

–For a neat picture of how Antarctic ice shelves are all situated go to this site, click on the globe until you get the South Pole region, and click on the Sea Ice link.  The shelves are just at their annual minimum size right now.
A new study describes the effects of global warming on the nutritional needs of sustainable fish populations.  The temperature assumptions in this case look pretty high but having similar effects on a lesser scale remains credible.  The thawing of Antarctic sea ice, described in the story above, is part of the problem.  Overfishing and other kinds of damage were not addressed.
Tree mortality in the tropics has sharply increased due to a range of factors, many of them related to warmer temperatures.  This story explains how the rise in CO2 which has helped cause the warming is not able to help sustain the trees’ nutritional needs.  Instead, there is a process that causes trees to absorb less CO2, as a way of conserving water, and thus reduces their important ability to act as a carbon sink.  “Trees in the moist tropics play an especially important role in Earth’s ability to regulate carbon dioxide, absorbing much more carbon proportionally than all the other forests combined.”
–Correction to a story about tropical deforestation in CL#1123.  I mistakenly gave the impression that the loss of emission of certain volatile compounds was said to be responsible for “most” of the temperature increase that would result from eliminating all of the world’s forests.  A more careful reading of the study shows that only a small part of the warmup is affected in this way.  The major cause for the increase comes from the additions to CO2 that result, offset to some extent by the net cooling effect of a large increase in albedo that would occur if all boreal forests were taken out, which previous studies had already demonstrated.  My apologies for this error.  The main conclusion still stands that the loss of tropical rainforests would have catastrophic consequences for the global climate.  (The above story shows that part of the loss would be due to feedback effects that run completely out of our control.)  Here again is the link to the earlier study:
Let’s not overlook some potentially good news, hopefully very, very good:  There is a new technology that claims to achieve dramatic increases in the energy efficiency of electric motors used for many important applications.  More information should be available next week.  Having the NREL involved is a good sign.
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