$args = array(
    'post_type' => 'publication',
    'post_status' => 'publish',
    'category_name' => 'my-category',
    'posts_per_page' => 20,
    'orderby' => 'publication_year',
    'order' => 'DESC',
    'paged' => $paged
);
$loop1 = new WP_Query($args);

$args = array(
    'post_type' => 'publication',
    'post_status' => 'publish',
    'category_name' => 'my-category',
    'posts_per_page' => 20,
    'orderby' => 'meta_value_num',
    'meta_key' => 'publication_year',
    'order' => 'DESC',
    'paged' => $paged
);