Is it better to let the Greenland ice sheet collapse or use geoengineering?

Two leading climate academics – Ken Caldeira and Clive Hamilton – debate the issue of geoengineering and whether it should be allowed to go-ahead
Geoengineering sceptic Clive Hamilton said it was "irresponsible" of scientist Ken Caldeira to ask whether it was better to let the Greenland ice sheet melt or use geoengineering.
"Is he saying we do it no matter what? Do we do it if we’ve got evidence showing there’s a huge risk involved? Do we allow ExxonMobil to have the patent on that aerosol spraying so that they’re the only ones who can do it? Is it done by Iran unilaterally? Or by a UN group of countries?" He said in a recent article on chinadialogue
We decided to follow up this debate with both academics. Below, you can find the response given first by Ken Caldeira to the questions raised by Clive Hamilton, and then following them in italics are Hamilton’s responses back again.
You can read also the original article: China could move first on geoengineering the planet
CH: Do we only do it [try to protect the Greenland ice sheet with a solar geoengineering effort] if we’ve got evidence showing there’s a huge risk involved? 
KC: I would think that society would want to engage in such a risky undertaking if we are clear that the risks of acting were less than the risk of not acting, and so I believe the answer is ‘yes, solar geoengineering would be something that would be undertaken, but only if huge risks were involved.’
CH: Dr Caldeira had posed a simple binary choice, phrased in a way that downplayed the risks of solar geoengineering. Now some of his conditions have been flushed out. First he says that it should only be done if there are huge risks from a melting ice-sheet. OK. In the situation he wants us to imagine, how big a risk? Risks of what? Who makes the judgment? 
CH: Do we allow ExxonMobil to have the patent on that aerosol spraying so that they’re the only ones who can do it? 
KC: Spraying things out of the back of airplanes has been done for crop dusting and many other applications and is not easily patentable, so I do not think there is a significant risk that only one party will hold the relevant patents and prevent the rest of the world from doing what it wants to do.
National governments can require a transfer of intellectual property to the government’s use, much as a government can require the transfer of land to build a highway. So, control of these technologies by a single company through the patent laws is not a real concern.
CH: Dr Caldeira is not worried whether the research is done by ExxonMobil; nor is he worried about a private company having a patent on geoengineering technologies. Yet he is associated with a company, Intellectual Ventures, that has taken out a patent on a method of spraying sulphate aerosol precursors into the stratosphere. The ‘Strato Shield’ is marketed as ‘a practical, low-cost way to reverse catastrophic warming of the Arctic  or the entire planet’. And he is listed, along with Bill Gates and various others, as the owner of a patent for a method of pumping cold sea-water to the surface as a form of geoengineering.
There has been a flurry of patents taken out in recent years on geoengineering technologies. Dr Caldeira says ‘don’t worry, the government will just expropriate the intellectual property’. His understanding of political economy is naive.
CH: Is it done by Iran unilaterally? Or by a UN group of countries?
KC: Obviously, the broadest possible participation in the decision-making process would be best. I would think something like the United Nations or some similar body might be appropriate. 
CH: And now he states another of his conditions, that any solar geoengineering intervention should be carried out by the United Nations ‘or some similar body’ (although I am not sure what they might be). So he would oppose China doing it, or the United States, or some ‘coalition of the willing’. That is reassuring, although I have the sense that if he believed the risks were high enough he might sacrifice ‘the broadest participation’ on the argument that desperate times call for desperate measures.
So questions I posed, off the top of my head, were designed to expose the simple-mindedness of the: Are you for geoengineering or against it? I know of no-one who has thought about it who is unconditionally for it or unconditionally against it. Yet Dr Caldeira says we must say one way or another, without considering the conditions in which geoengineering technologies might be deployed. He has now given us a few clues as to the conditions he would want to see prevail before deployment. There must be others. It would aid public discussion if he would set out clearly the circumstances in which he would advocate solar geoengineering, instead of leaving them for us to work out.