On the action page results.cfm:
<cfset session.dd_year = form.dd_year>
This will make the year value:
1) available to all pages;
2) specific to a particular user session.
Consider passing the value as a url variable on each page request. Session variables are convenient but the way browsers work these days they can lead to unexpected results.
In your case, this could happen. The user selects 2002 on the form and goes into the site. He then opens a new tab in his browser, goes to your form and submits 2004. Guess what happened to the session variable on tab where he selected 2002.