Is this a fresh water question, or a "global warming" question?
both are extremely complex and attempting to grasp all the factors involved individually is an impossible task. Trying to understand them together is impossible squared.
Ask any real scientist if he or anyone else fully understands all the factors, interactions and feedback mechanisms that determine long term temperature trends (that's what climate is, by definition) and if he is competent and honest, his answer will be: NO. No one and no group knows!
That is why not IPCC reports are comic books in disguise. The Science is done by scientists, but the final report (the Executive Summary) is done by politicians. Politicians then order the scientific findings and expressed uncertainties modified to agree with the political conclusions.
Just think about all the computer models trotted out over the last 30 years. Every single one fails the simplest test. They fail accurately to predict a past 10 year period trend, having been fed the temperature data from the previous ten or 20 years.
At the bottom line, it is quite simple.
Unless you are a democrat. And I know you are not.