You are the first mate on an ocean liner, when the Captain gives the following order. "You are to proceed at full speed without any deviation in course and speed unless and until you have absolute proof that there is an iceberg straight ahead. Then and only then are you permitted to take preventative action."
Or, you are an adult supervisor on a bus trip with a bunch of school children. The bus is approaching a train crossing. Yet, the driver says, "I am not slowing down unless and until I have absolute proof that the bus will not be across the tracks before the train gets here."
We know these people to be guilty of the moral crime of reckless endangerment.
If these types of people wish to take actions that risk their own lives, we may leave that up to them. They pay the costs for their own mistakes. However, when they put the lives of others at risk, then they are guilty of a moral crime.
We have good reason to condemn people like this and to condemn them in very harsh terms. They are responsible for the deaths and suffering of a great many people every year.
Now, we have a whole slew of these types of people engaging in reckless endangerment of whole cities and whole populations. They have already maimed and killed a great many people and destroyed a great deal of property, and they seem to have no qualms about continuing along the same course of action. In all cases, they behave as people who are almost if not entirely indifferent to the death and suffering of others - because they are not motivated to take any action that would avoid potential death and suffering.
The morally responsible person would not demand proof that there is an iceberg straight ahead before slowing down and taking precautions. The mere possibility of an iceberg is good enough. His responsibilities to his passengers demands that he take precautions to reduce the possibility of catastrophe - not that he act as if there is no risk until catastrophe is certain.
The responsible bus driver will not risk racing the train at the crossing. She will take precautions to protect the well-being of the children trusted to her, which means slowing down and avoiding the possibility of harm coming to them.
These same principles of moral responsibility demands that, in the face of risk of massive destruction due to climate change, that people slow down and reduce the risk. It does not require absolute proof.
Principles of rationality give us a simple formula for determining how much caution to use in the face of risk. The basic form states that the amount that it is rational to spend avoiding risk is equal to the cost times the probability.
It is worth spending up to $250 billion to avoid a 1% chance of suffering $25 trillion in harm.
It is worth spending up to $2.5 trillion in avoiding a 10% chance of suffering $25 trillion in harm.
It is worth spending $12.5 trillion to avoid a 50% chance of suffering $25 trillion in harm.
It is worth $22.5 trillion to avoid a 90% chance of suffering $25 trillion in harm.
This is what rationality and moral responsibility commands. Those who do not follow this formula are guilty of reckless endangerment. When we think of them, we should have the same reaction that we have thinking of the cruise boat captain demanding absolute proof of an iceberg or the bus driver demanding absolute proof that she will not make it across the tracks in time to avoid the train. Except, we should think of those who use the "Need for Proof" argument as willing to risk the destruction of whole cities and whole populations.
As it turns out, the "Do nothing" subset of global warming deniers are recommending exactly the same course of action as an arrogant and presumptuous person who claims, "There is a 0% chance that I am wrong when I say nothing bad will happen, and a 100% chance that anybody who says otherwise, regardless of the evidence they provide, must be mistaken."
It is worth spending $0 to avoid a 0% chance of suffering $25 billion in harm.
While these people like to crow on about the fallibility of science, they neglect to mention their own fallibility - the possibility that they are wrong and that these costs are real. They argue that the fallibility of science means that it is not permissible to take any (or only the most minimal) steps in avoiding harm. They argue that we should behave as if they, the global-warming deniers, have a greater than 99% chance in being right in claiming that no harm will come from greenhouse emissions. That all of the evidence collected to date provides a less than 1% chance that they are wrong, and that we may behave accordingly.
If you see this "Need for Proof" argument in the claims made by somebody writing on global warming policy, you know you are dealing with somebody with a moral character many times worse than that of the ship captain or the bus driver that I mentioned above. You are dealing with somebody who will risk the lives of not only a few hundred passengers or a couple dozen school children, but is willing to put whole cities at risk. You know that you are talking to somebody who lacks the moral character to act in a morally responsible manner.