"The problem with injecting so much CO2 into the atmosphere is much more complicated and has many more effects than just a higher temperature in the world."

That is precisely the problem the scientific community has to deal with every day.   Among themselves there is an understanding that there is no such thing as knowing for sure, that as the data slowly accumulates it is never possible to make absolute statements, that everything they do is an estimation.  They are prepared to accept that.   They are perfectly aware of the complexity of the relationship between CO2 and climate, in fact, they are the ones who developed what we do know.   They feel almost certain that the temperature will continue to climb but know full well that the climb will probably be irregular, in fits and starts, with a long term upward trend.  

The problem is that the words they must use as scientists to acknowledge uncertainty do not provoke a response among the public to act.   There is an incredible number of people who think that a 5 degree increase in global temperature means simply that the afternoon will be 75 degrees instead of 70, so what's the big deal?  That kind of simplistic attitude is exploited by the other side, and it's a hell of a tough problem.  At the same time, there is an increasing realization that there may well be a limited amount of time left for a successful response to the problem.   They feel a need to convey urgency but must do so hobbled by their instincts to use language with words that in the political world are considered weasel words, like "maybe", or "possibly", or "speculative".

The people who spend all of the money to try to discredit the science have no corresponding ethical imperative to moderate their language.   They speak freely of "conspiracies" by scientists to "scare us" and how this is "the greatest hoax ever perpetrated" and claim without qualification and with resolute certainty that it has been "proven" that the sun is to blame, or water vapor, or volcanoes, or cosmic rays, or that the earth is cooling, and on and on.  

Take the idiot argument that earth has been cooling, not warming, for the last decade.   I call it an idiot argument because so many idiots proclaim this "fact" without having any idea whatsoever that it's false.   The closest thing to truth that you can say about it is that the last 10 years have been warming, just not quite as fast as the 10 before.   Even so, 11 of the hottest years on record have been in the past 14 years, and the 2000s are expected to be the hottest decade ever reported.   When a large majority of the people who patrol these conservative "science" sites come away with the impression that the hottest decade on record was actually a cooling period we've got a serious disinformation problem.

So how the hell are scientists expected to combat this?   I can imagine the discussions that must go on.   The emails reflect some of that frustration.   They were saying in effect:

"Okay, we know the last 10 years have not warmed as fast as predicted.   We feel we ought to know why that is but we don't, which is often the case in complex systems.   Still we know that it doesn't change the overall picture because the melting is still going on within predicted ranges and all the detailed science keeps pointing back to the greenhouse effect.  We also know that the industries who pay big money to deliberately mislead the public will use this minor change and our admitted uncertainties about it to pretend that warming is no longer happening and to argue for continued inaction."

Is it any wonder that they sometimes try to fight fire with fire?   How would you defend yourself against that kind of attack from entities outside of the scientific community?  

Some do so by citing the worst case scenarios.  Jim Hansen of NASA is a case in point.   I have seen numerous climate scientists who admit freely in their online writings that Hansen tends to predict on the high side of current estimates, and don't necessarily approve, but neither can they come right out and say he's wrong because it can certainly happen that way, especially if certain positive feedback loops come into play.   Most scientists don't feel entitled to include such uncertain possibilities in their predictions, even though the constitute a real possibility.   The other side has no problem doing so.