This chart nicely summarizes ... one of the best arguments against anthropogenic global warming: that temperatures are not, in fact, at alarming levels (or headed there). Click the chart to embiggen; go here for the supporting article. The chart covers the entire span of human civilization, and it's obvious at a glance that we're not in a particularly warm period right now (at right on the graph) – in fact, quite the reverse.
If you're wondering why the temperatures are reading negative °C, it's because (a) that's the temperature of the ice core drilled from Greenland ice sheets, and (b) scientists use Celsius temperature scales. Just for your reference, in °F that right-hand scale would read from -19°F to -27°F.
There's debate about how good a proxy Greenland ice sheets are for global temperatures (in other words, how well they correlate). There's no debate, though, on whether the ice cores from those sheets are a better proxy than tree rings: they are, most certainly.