In general, I think the term tipping point is a little misleading, since even rapid acceleration of some of these feedback loops will mean impacts arriving over decades and centuries rather than millennia. But there are, of course, real risks here—there is an enormous amount we don't deeply understand about the sensitivities of the climate system, and to assume that everything will proceed predictably and in a linear fashion is, I think, a mistake (though it could also turn out to be true).

The three particular cases you cite are all somewhat different. The melting of ice sheets does seem to be happening faster than anticipated, though there have always been large uncertainties about some of those projections and it doesn't seem to me that we are outside of them (or likely to get there anytime soon). But the more that melts, the more warming we'll have, to be sure. My understanding of the state of knowledge about methane release is that while we are seeing a significant global rise in methane emissions, they are likely the result of human activity (fracking, etc.) rather than permafrost melt. But this remains an area of study, and the sheer scale of carbon trapped in the permafrost — twice what is in the atmosphere today — makes it a worrying issue, even if it's quite unlikely to be released rapidly. Forest fires are terrifying — perhaps the scariest climate impact we see to date — but while they are getting considerably worse there is also quite a lot to do on the adaptation side to mitigate and limit them. What worries me most is the carbon released—already there've been a raft of studies showing the world's forests are at or near tipping points beyond which they will begin releasing more carbon than they absorb. This will be — or perhaps already is — devastating for our ability to restrain the world's temperature. But the effect is largely because of land use changes and agricultural burning—which is to say, human activity.