Nobody actually waits until all the evidence is in. The simple reason is that all the evidence will never be in. For that to occur one would have to know about everything going on in the universe right now and where those things would lead in the future. For Earth-bound residents, perhaps it would be sufficiently rigorous to know everything that is going on in the solar system and its future.
What do people mean when they say they want to wait until all the evidence is in? Most often they mean they want to wait for more information. But, sometimes they mean nothing of the sort. Sometimes they mean they want you to wait until all the evidence is in before you proceed to do something they don't like. In other words it's a stalling technique, one used quite effectively by the fossil fuel industry to prevent meaningful action to control greenhouse gasses.
The implication behind this ruse is that public policy is made based on all the evidence. But this has never been the case and never will be the case. Public policy is made based on incomplete information and perceived probabilities of gains and threats. (It's also made based on the influence of powerful interest groups; but that would require a discussion all by itself.) This is why modeling has become such an important tool for those involved in climate and energy supply research. If we had all the information, we wouldn't need models that announce possible outcomes based on incomplete information.
So, if we can't wait until all the evidence is in, what standards should we use to guide our decision-making, both in public policy and in our daily lives? I would propose two that we already use. First, unlike the standard for criminal court cases which rightly requires jurors to judge someone guilty beyond a reasonable doubt, civil cases require only a preponderance of evidence for a party to prevail. When it comes to the issue of climate change, the preponderance of evidence is clearly on the side of those advocating for a swift and dramatic reduction in greenhouse gas emissions. Are there bits of information which are inconclusive when it comes to the dangers of climate change? Certainly, there are. But the vast bulk of the information, the preponderance of the evidence we now have, points to huge risks ahead.
This leads me to my second principle for decision-making: the precautionary principle. The precautionary principle is actually a very conservative principle in the original meaning of the word "conservative." The main thrust of this principle is that actions which have the potential to pose severe risks or irreversible harm to society should be prohibited or seriously restricted until evidence accumulates that suggests that the risks are below a threshold acceptable to society. This is a slippery principle. But its usefulness rests on the same foundation as the first principle: an evaluation of risks and uncertainties. It is axiomatic that the more uncertainty there is associated with a given policy or course of action, the greater the risk. And, so the burden is on those proposing a policy or action to show that the risks and uncertainties are at acceptable levels.
Now you might think that the second principle contradicts the first. After all, those advocating for dramatic reductions in greenhouse gas emissions are suggesting a radical departure from current practice. But this is where the severity of the risks of two courses of action must be weighed against one another. Those advocating for severe curbs on greenhouse gasses have complex models suggesting risks to the very foundations of civilization: to food and water supplies; to public health because of the possible spread of diseases formerly restricted to the tropics and because of a significant increase in heat-related deaths; to public order as refugees flee low-lying regions inundated by rising sea levels; to peace as nations vie over increasingly scarce water and agricultural resources. The list goes on. On the other side of the argument is the threat to economic growth posed by expenditures needed to move away from fossil fuels. There is also the possibility of reduced opportunities for a better standard of living for the poor. (This argument cleverly avoids all mention of redistribution or enhanced public services as a means to assist the poor.)
The problem is that the decision to put the world on a fossil-fuel diet took place more than two centuries ago with no understanding of where it would lead. The initial levels of fossil fuel combustion might not have posed much of a threat at all for a very long time. But fossil fuel consumption and population have been moving targets (mostly up) since the dawn of the industrial revolution. And, that means our threat assessment needs to move with it. Not only is all the evidence never going to be in, but the evidence is actually changing over time as population rises, as technology and infrastructure change and expand, and as the biogeochemical processes of the Earth react to that change. Witness the increasingly rapid melting of the world's major ice sheets.
We need to recognize the phrase "until all the evidence is in" for what it really is: 1) a stalling technique, 2) a reflection of the ignorance of the speaker about the limits of our knowledge or 3) a colloquialism signaling the desire to wait for more information. Having sorted that out, we can move on to more prudent and efficacious ways of making public policy and personal decisions.