In the debate about whether or not past climates have been warmer than the present we have to distinguish between 'detection' of anthropogenic climate change and 'impacts' of climate change. That the Earth's climate 6000 years ago seemed to have been warmer than now bears no relevance for the 'detection problem' but I think it is quite important for possible impacts.
Has solar radiation any influence on the temperature of the room I am now in ? It sees obvious to every one that it has. It is warmer when it is sunnier and cooler when it is cloudy. However, I could put forward the argument that the solar radiation has no influence because when I once set up the heating really high, it was warmer than at any other time I can think of. Surely this argument is illogical. If we want to argue whether or not solar radiation has an impact on room temperature we should compare alike with alike situations.This means situations in which the heating was set exactly at the same level.
This is, however, the type of argument that is sometimes used to refute the effect of greenhouse gases in the atmosphere because 'thousand years ago or six thousand years ago" it was warmer than today. Well, at those times, the 'external heating' was different than today and thus we cannot compare directly both situations. It has to be always underlined that the basic tool, although no the only one, to detect and attribute climate change is the comparison of climate model simulations, in which models are driven with and without anthropogenic external drivers like CO2.
This being said, the possibility that Earth's climate may have been warmer than today is relevant for the discussion about climate impacts. Continuing with the example of my room, I can indeed argue that the warmer temperatures caused by a stronger solar insolation may be perhaps not so much damaging for my canary bird because when I had the heating turned on it seemed to feel quite happy. And this seems to be what happened with polar bears during the Holocene period that started about 10 thousand years ago. Two papers published recently, Funder et al. and Jakobsson et al. (with supplementary material including an interview with the leading author) have reconstructed the extent of summer sea-ice in the Arctic over most of the Holocene period, and both come to the conclusion that in extended periods of the Mid-Holocene the Arctic ocean was ice-free in summer. This conclusion is interestingly based on the analysis of ancient wood. Drifting ice in the Arctic ocean transports wood from the American and Siberian shorelines to Greenland. The species to which the driftwood belongs is an indicator of its origin. The wood can be also dated by standard radiocarbon methods so that a picture of the evolving sea-ice cover through the Holocene can be reconstructed. The absence of driftwood is thus an indication of ice-free or near ice-free conditions. It turns out that over a period of 2 thousand years, the limit of sea-ice minimum was located about one thousand kilometers to the north of its present position.
Obviously polar bears survived the Holocene. To my knowledge -and I may be ere completely wrong here - there are no geological records of mass extinctions of Arctic species during the mid-Holocene. This does not say any thing important for or against the influence on climate of anthropogenic emissions of greenhouse gases, but it does say something about the resilience of ecosystems to climate change