That depends on the time-frame. It's not one number, it's an exponential sliding scale.
The thirty year average we normally use in climate studies is still far more complex than describing a climate age. When we say "in the cambrian era the climate had these attributes" absolutely nobody expresses much doubt, even though we have far less evidence for that. We derive it by looking at what sort of organisms evolved at the time and, if we're lucky, maybe an ice-core here and there. A bit of geological evidence may hold some clues too.
So how come that is almost unquestioningly accepted ? Because a description of the climate over a period of several hundred million years is exponentially simpler than over one million years, let alone over centuries or decades...get down to months and weeks and our models break down within 5 days.
All the things deniers claim against climate models today apply far more to our models of ancient climates - and we have far less evidence to support those claims. But there is hardly any questioning about those (in fact deniers keep CITING those to try and argue that climate change cannot be influenced by man - considering every OTHER organism that has ever existed has influenced climate that's a bit silly in my view - why would WE be the only one that CAN'T ? We're just not that special. Fricking algae changed the climate and atmospheric composition irrevocably - they seriously believe we can't outdo ALGAE ?!).
Why don't they doubt the oxygen levels of near 40% in the carboniferous era ? The only evidence we have for that is giant insects (which needs that level to breath) and the fact that apparently trees all fossilized instead of decomposing at the time - and produced our fossil fuels.
Compared to the thousands of pieces of evidence for climate change today - from hundreds of disparate scientific fields with no other significant contact between them...
The only difference is that there is no political gain to be made from denying the carboniferous.
Sure there are scientists who question it - investigate it and may find evidence that one day leads to us adjusting that value to say 35% or 55% instead. But there's no news debates about that, it's scientists dispasionately collaborating by questioning each other - without malice.
Why the malice from deniers today ? Why the desperate desire to call themselves sceptics (even when they decidedly are not since sceptics by definition are people who support the theory with the most evidence) ?
It's got nothing to do with the science. The science is open to question - and frequently revised with new data as it should be. Technically we're at climate change theory number 500 and something. But the core theory is unchanged. It's fine detail adjustment - much like there have been lots of fine detail adjustment with Darwin's theory but the core theory remains intact.
But we have no theory that offers a better explanation of the observations. The hypotheses that have been advanced not only lack a single shred of evidence but are flat out disproven by the evidence we do have.
All the actual sceptics are supporting climate change because a sceptic is somebody who believes the evidence over their own ideology.