The process of assigning causes to detected climate change.
As opposed to detection, which is the process of demonstrating that an observed change is significantly different (in a statistical sense) than that which can be explained by natural variability.
Example Usages -
1. "Investigating the cause of 20th Century warming is done in so-called detection and attribution studies, which analyze the various forcings (e.g., solar variations, greenhouse gases or volcanic activity) and the observed time and space patterns of climate change in detail. These studies, with a range of different techniques, have invariably concluded that the dominant cause of 20th Century warming is man-made greenhouse gases."
(Source: Real Climate, What If … the “Hockey Stick” Were Wrong? )