Background
The authors note that "almost every summer, there is a heat wave somewhere in the United States that garners popular media attention," and that it is only natural to wonder if global warming played a role in producing it.

What was done
Driven by this same curiosity, Redner and Petersen investigated "how systematic climatic changes, such as global warming, affect the magnitude and frequency of record-breaking temperatures," after which they assessed the potential of global warming to produce such temperatures by comparing their predictions to a set of Monte Carlo simulation results and to 126 years of real-world temperature data from the city of Philadelphia.

What was learned
At the end of their mathematical analysis, the two researchers concluded that "the current warming rate is insufficient to measurably influence the frequency of record temperature events, a conclusion that is supported by numerical simulations and by the Philadelphia data." Hence, they state that they "cannot yet distinguish between the effects of random fluctuations and long-term systematic trends on the frequency of record-breaking temperatures," even with 126 years of real-world data.

What it means
In light of the fact that confident attribution of record-breaking temperatures in Philadelphia to generic global warming over the past 126 years cannot yet be made, it would appear that the much more difficult task of attributing such temperatures to CO2-induced global warming must be far from being achieved.