Continuous records of the atmospheric greenhouse gases CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;, and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O and their radiative forcing since the penultimate glacial maximum