I think it's better to make forecasts that are later proved wrong, then acknowledge that they are wrong, make new forecasts and appropriately calibrate the new forecasts based on the lesson learned in humility from the first wrong forecast.
Trenberth in particular seems to be a fairly honest and open climate scientist, in that he made an explicit forecast, then later admitted a change of mind. He's also the person who admitted (within an email leaked by Climategate) that there was a problem with balancing the energy budget, and he later publicly noted the same, and tried to come up with an explanation.
The problem isn't with people making wrong forecasts, it's with people (a) refusing to make forecasts while still implicity doing so by claiming near-certainty about the future and seeking action based on that, or (b) making forecasts and insisting on the forecasts being treated as correct without an external test of validity or a past record of forecasting expertise.
See also:
http://econlog.econlib.org/archives/2012/05/the_bettors_oat.html
Yes, of course it's better to try and fail, and try again, and fail better...
people (a) refusing to make forecasts while still implicity doing so by claiming near-certainty about the future and seeking action based on that
The situation with global warming reminds me very much of a recent Yvain post on his blog about the "motte-and-bailey doctrine". I think the AGW proponents use this technique extensively.
Note: Please see this post of mine for more on the project, my sources, and potential sources for bias.
I have written a couple of blog posts on my understanding of climate forecasting, climate change, and the Anthropogenic Global Warming (AGW) hypothesis (here and here). I also laid down the sources I was using to inform myself here.
I think one question that a number of readers may have had is: given my lack of knowledge (and unwillingness to undertake extensive study) of the subject, why am I investigating it at all, rather than relying on the expert consensus, as documented by the IPCC that, even if we're not sure is correct, is still the best bet humanity has for getting things right? I intend to elaborate on the reasons for taking a closer look at the matter, while still refraining from making the study of atmospheric science a full-time goal, in a future post.
Right now, I'm curious to hear how you formed your views on climate change. In particular, I'm interested in answers to questions such as these (not necessarily answers to all of them, or even to only these questions).