I don't mean to ally myself with the denialists, but I feel I should point out that one year above 1.5°C does not indicate that we have crossed that threshold. Climate is usually defined as a thirty-year average, so we will first be able to say that the threshold has been reached about 15 years from now, give or take, when the moving average passes that point.

That does leave us with the problem of the physical effects of heat over a shorter timescale, however. One or two years of warmth could suffice to trigger a tipping point and render warming irreversible. We could spring the trap before it can ever be definitively said that we have passed a given level of warming. I have a concerned eye on the UAH temperature graph, waiting for the recent el-Nino warming to pass its peak. So far it's holding strong.