What does this equation mean?

The standard output is a probability ratio, also called a risk ratio, comparing the annual probability of the event in the factual climate with its probability in the counterfactual one [ 3 ] . Writing those probabilities as the factual and counterfactual annual exceedance probabilities, $\mathrm{PR} = \frac{p_1}{p_0}, \qquad \mathrm{FAR} = 1 - \frac{p_0}{p_1} = 1 - \frac{1}{\mathrm{PR}}$. The fraction of attributable risk is a monotone transform of the ratio and contains no additional information. Its name invites a misreading that the field has warned about for two decades: it is not the probability that this event was caused by forcing, and it does not license the statement that a given fraction of the damage was anthropogenic in any individual case. It is a population-level quantity, and Lloyd and Shepherd’s central methodological point is that population-level attributions do not transfer to singular events without further argument [ 6 ] .