Trust data that is manually manipulated, incomplete, inaccurate, disparate, and only goes back a blink of the eye in terms of the planet's history.
800,000 years may be a blink of an eye compared to the life of the planet, but the changes on C02 since the industrial revolution, are certainly more than has been seen in a long time. This video does a nice job of visualizing the recent changes.
higher temperatures are when life flourished
Plant life, insects, microorganisms, and dinosaurs flourished, although they also had higher concentrations of oxygen. With the huge numbers of people who live within a few feet of sea level, an increasing warming trend, resulting in rising sea levels, will kill many people, and seriously f*ck up the lives of those above the high water mark. Sea ports underwater, huge numbers of refugees, major pollution caused when low lying industrial areas are flooded, will certainly not cause life to flourish, at least not human life.
The bottom line is that acting as if the changes were brought about by humans and may result in bad things happening, will have good effects in the long run, even if the predictions are all wrong. Ignoring them until it may be too late could cost millions of lives. I prefer the idea of erring on the side of the issue that will have good effects either way.