9. A system for temporal content selection via a computer network, comprising: an interface executing on one or more processors configured to receive, responsive to a search query input into a search engine, a request for a content item to be displayed alongside search results, the request associated with a first keyword corresponding to the search query; a keyword lookup module executing on the one or more processors configured to parse a data structure storing historically linked keywords to select a second keyword linked to the first keyword via a historical link, the historical link generated based on a frequency of co-occurrence of the first keyword and the second keyword in historical content; a content selector executing on the one or more processors configured to select a first content item for display alongside the first search results based on the second keyword; the interface module configured to receive a second request for a content item to be displayed alongside second search results, the second request associated with the first keyword; the keyword lookup module further configured to parse a second data structure storing temporally linked keywords to select a third keyword temporally linked to the first keyword in the second data structure based on a frequency of co-occurrence of the first keyword and the third keyword in online documents published within a temporal threshold satisfying a frequency of co-occurrence threshold, wherein the temporal link between the first keyword and the third keyword is based on comparing the frequency of co-occurrence of the first keyword and the third keyword in the online documents with a historical frequency of co-occurrence of the first keyword and the third keyword; the content selector configured to identify a plurality of content items matching the third keyword, determine that a content provider of a second content item of the plurality of content items provided an indication to use temporal links for content selection, and select, based on the indication, the second content item as a candidate for display alongside the second search results.
9. A system for temporal content selection via a computer network, comprising: an interface executing on one or more processors configured to receive, responsive to a search query input into a search engine, a request for a content item to be displayed alongside search results, the request associated with a first keyword corresponding to the search query; a keyword lookup module executing on the one or more processors configured to parse a data structure storing historically linked keywords to select a second keyword linked to the first keyword via a historical link, the historical link generated based on a frequency of co-occurrence of the first keyword and the second keyword in historical content; a content selector executing on the one or more processors configured to select a first content item for display alongside the first search results based on the second keyword; the interface module configured to receive a second request for a content item to be displayed alongside second search results, the second request associated with the first keyword; the keyword lookup module further configured to parse a second data structure storing temporally linked keywords to select a third keyword temporally linked to the first keyword in the second data structure based on a frequency of co-occurrence of the first keyword and the third keyword in online documents published within a temporal threshold satisfying a frequency of co-occurrence threshold, wherein the temporal link between the first keyword and the third keyword is based on comparing the frequency of co-occurrence of the first keyword and the third keyword in the online documents with a historical frequency of co-occurrence of the first keyword and the third keyword; the content selector configured to identify a plurality of content items matching the third keyword, determine that a content provider of a second content item of the plurality of content items provided an indication to use temporal links for content selection, and select, based on the indication, the second content item as a candidate for display alongside the second search results. 13. The system of claim 9 , wherein: the keyword lookup module is further configured to identify, via the second data structure, a geographic scope attribute of the temporal link; and the content selector is further configured to determine that a geographic location associated with the second request corresponds to the geographic scope attribute, and select at least one content item of the plurality of content items corresponding to the geographic scope attribute.