PHP: Variable/Value only available inside loop - using it outside?

I've already asked this question on wordpress.stackexchange.com but they told me this is a plain PHP question and might have nothing to do with wordpress.

I have a fairly big problem with a function i wrote …

The function lists all posts of a custom-post-type for "events".

function get_event_list( $latest = true, $order = 'ASC' ) {
    echo '<ul class="event-items">';

    $yesterday = time() - 24*60*60;
    $compare = $latest ? '>' : '<';
    $current_year = '';

    $args = array(
        'post_type' => 'wr_event',
        'posts_per_page' => -1, // show all posts
        'meta_key' => 'event_date',
        'orderby' => 'meta_value_num',
        'order' => $order,
        'meta_value' => $yesterday,
        'meta_compare' => $compare
    );

    $loop = new WP_Query( $args );
    while ( $loop->have_posts() ) : $loop->the_post();
        global $post;
        $this_year = get_post_meta( $post->ID, 'event_date', true );
        $this_year = date('Y', $this_year);

        if ( !$latest && ( $this_year != $current_year ) ) :
            // new year, output the year header
            // and reset current year to this new value
            echo '<li class="wrapper year"><h2>' . $this_year . '</h2></li>';
            $current_year = $this_year;
        endif;

        get_template_part( 'inc/event', 'item' ); // creates the actual li-item
    endwhile;
    wp_reset_postdata();
    echo '</ul>';
}

As you can see I compare the "event-date" against the current year, so I can split my list in different years. The problem I have is that get_post_meta( $post->ID, 'event_date', true ); this is only available inside the while loop.

Currently the function above outputs this …

<ul class="event-items">
   <li class="year">2012</li>
   <li>Event 05</li>
   <li>Event 04</li>
   <li>Event 03</li>
   <li>Event 02</li>
   <li>Event 01</li>
   <li class="year">2011</li>
   <li>Event 03</li>
   <li>Event 02</li>
   <li>Event 01</li>
   <li class="year">2010</li>
   <li>Event 04</li>
   <li>Event 03</li>
   <li>Event 02</li>
   <li>Event 01</li>
</ul>

However this structure is fairly complicated to use and semantically not really correct and useful.

I'd like to wrap each year into a new <ul>.

Any idea how to do so? The post_meta "event_date" is only available inside the loop. How could I compare the "event_date" with the current year and wrap the queried results into a new ul

Like so …

<ul class="event-items">
    <li class="year"><div>2012</div>
        <ul>
            <li>Event 05</li>
            <li>Event 04</li>
            <li>Event 03</li>
            <li>Event 02</li>
            <li>Event 01</li>
        </ul>
    </li>
    …
</ul>

Maybe there are some people who know a good and logical solution to this. Maybe I need to use get_posts() instead of new WP_Query however I'm not that good in PHP and have no idea how I could handle this.

Please, if anybody has an idea let me know! Thank you in advance!