The National Oceanic and Atmospheric Administration announced last week that 2012 was the warmest year on record in the contiguous United States. By far a whole degree Fahrenheit.
Scientists can't yet know to what extent man-made emissions influenced the heat and calamitous drought. But the result is nevertheless ominous, "a huge exclamation point on the end of several decades of fairly consistent warming," as NOAA's Deke Arndt put it. The year offers a vision of what will happen more often on a planet that is heating slowly and fitfully, not every year warmer than the last, but inexorably.
There is still uncertainty. Though they have a range of estimates, scientists still do not know exactly how sensitive the global climate system is to human carbon emissions and exactly how steep the long-term temperature line will be. Predicting the consequences of a given temperature rise is also difficult.