Similarly it's impossible to have an accurate model of how the human body works. By your logic we should abandon all medicine 
There seems to be confusion about the time scales involved. Medicines can be effective within days, weeks and months. We can experiment with animals such as mice and rats which have a much shorter lifespans than ours, but which share a large percentage of our genome.
Before a drug or medicine is approved, many studies have to be carried out, including comparisons with placebos. Such experiments take place within a relatively short time frame which is irrelevant in relation to the long periods for changes in climate to become certain.
Even after such studies, long term adverse effects of a particular drug are sometimes discovered to have devastating consequences.
I'm not suggesting that we abandon research into climate change issues. I'm suggesting that we pay more attention, and more money, to more certain issues which we know can harm human beings and our environment.
For example, the certainty that the severity of natural, extreme weather events of the past will continue into the future, is more certain than the projection that such weather events will become more extreme in the future.
If we can't be bothered to protect ourselves from natural weather events, such as hurricanes, droughts and floods, how are reductions in CO2 levels going to help?
We should spend our resources fixing the immediate problems of extreme weather events first, and then consider the possible effects of rising CO2 levels.
For example, if one builds a house in an area that has experienced a number of category 4 cyclones in the past, then one should build one's house to withstand category 5 cyclones, just in case the climate, for whatever reason, changes for the worse.
Likewise, if one builds a house in an area which has been flooded on several occasions in the past, one should build one's house above the level of those previous floods, but not just a few inches above, but perhaps a whole metre above in order to protect oneself from the uncertain event that a future flood, influenced by anthropogenic climate change, might be the worst on record.