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November 27, 2013 / compassioninpolitics

Global Warming/Climate and Feedback Loops

( ) Feedbacks are net positive-assumes their defense

Homer-Dixon ‘7

[Thomas – Centre for International Governance Innovation Chair of Global Systems at the Balsillie School of International Affairs, Professor in the Centre for Environment and Business in the Faculty of Environment at the University of Waterloo, PhD in IR from MIT — 11/14, Address to the conference for a Globally Integrated Climate Policy for Canada, “Positive Feedbacks, Dynamic Ice Sheets, and the Recarbonization of the Global Fuel Supply: The New Sense of Urgency about Global Warming”, http://www.homerdixo…omer-dixon.pdf%5D

Let me now say a little bit more about some other feedbacks. This is one of the punch lines of my presentation today. I mentioned earlier that there are two general kinds of feedback: those that operate more- or-less directly on temperature, such as the ice-albedo feedback, and those that operate on Earth’s carbon cycle, where warming produces a change in the amount of carbon in the atmosphere. We have a fairly good understanding of the former and not such a good understanding of the latter. One carbon feedback that worries scientists involves the melting of the permafrost in Siberia, Alaska, and Northern Canada. As the permafrost melts it releases large quantities of methane – a very powerful greenhouse gas that, in turn, causes more warming. Scientists are also concerned about the potential release of more carbon dioxide from forests: just yesterday researchers reported evidence that, as the climate has warmed, the Canadian boreal forest has gone from being a carbon sink to a slight carbon emitter. And then there’s the matter of pine bark beetles. As you likely know, we’ve lost wide swaths of pine forest in British Columbia and Alaska – huge areas of trees – to bark-beetle infestation. As the climate warms, bark-beetle populations reproduce through two generations during the summer, and beetle mortality is lower during the winter. Both these changes mean that beetle populations become much larger overall. If these larger populations cross the Rockies and get into the boreal forest that stretches from Alberta to Newfoundland, and if they kill that forest, the forest will be susceptible to fire that could release astounding quantities of carbon dioxide. I asked Stephen Schneider, a leading cli- mate scientist at Stanford, about the implications of such a develop- ment. He just shrugged and said, ‘well, we’re talking about billions of tonnes of carbon.’ Other potentially destabilizing carbon-cycle feedbacks include the drying of the Amazon and the possibility that if it dries it will burn; the drying of peat bogs in Indonesia, which have already been susceptible to wide-spread burning; and the saturation of ocean carbon sinks. The Southern Ocean around Antarctica is no longer absorbing carbon diox- ide to the extent it did in the past. Warming has produced much more vigorous winds closer to Antarctica. These winds have churned up the sea and brought to the surface deep carbon-rich water, which absorbs less carbon from the atmosphere. Also, higher levels of carbon dioxide in the atmosphere are acidifying the oceans, a change could reduce populations of molluscs and phytoplankton that absorb carbon into the calcium carbonate of their shells. Our climate has both positive and negative feedbacks. The positive ones are self-reinforcing, and the negative ones equilibrate the climate and counteract the tendency towards self-reinforcing climate change. The big question for climate scientists then is: What is the balance is between the positive and negative feedbacks? A consensus has emerged over the last two years – a consensus again not reflected in the recent IPCC reports – that the positive feedbacks in the climate system are much stronger and more numerous than the negative feedbacks. In a paper published last year in Geophysical Research Letters, Scheffer, Brovkin, and Cox carried out a comprehensive assessment of the feed- back situation.7 They wrote, ‘[we] produce an independent estimate of the potential implications of the positive feedback between global tem- peratures and greenhouse gasses.’ In other words, these researchers focused specifically on carbon cycle feedbacks. They went on, ‘we sug- gest that feedback of global temperature and atmosphere CO2 will promote warming by an extra 15% to 78% on a century scale over and above the IPCC estimates.’ Let’s turn to the issue of dynamic ice sheets. The Greenland ice sheet is the second largest mass of ice in the world, after that in Antarctica. If we melt Greenland entirely, we get seven metres of sea-level rise. If we melt the West Antarctic ice sheet, we get another five metres. If we melt the rest of Antarctica, we get an additional fifty or so metres. The Greenland ice sheet will probably be the first to melt, because it’s the most vulnerable. During the last interglacial period 125,000 years ago, when temperatures were roughly what they’re going to be at the end of this century, much of Greenland melted, and sea levels were four to six metres higher than they are right now.


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