You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query-continue>
    <allpages gapcontinue="Terrestrial_biology_over_the_next_100_years" />
  </query-continue>
  <query>
    <pages>
      <page pageid="271" ns="0" title="Temperature changes in the instrumental period" contentmodel="wikitext" pagelanguage="en-gb" touched="2014-08-22T17:04:57Z" lastrevid="366" counter="9407" length="30545" />
      <page pageid="272" ns="0" title="Temperature changes over the 21st century" contentmodel="wikitext" pagelanguage="en-gb" touched="2014-08-06T14:34:02Z" lastrevid="275" counter="3991" length="7900" new="" />
      <page pageid="273" ns="0" title="Terrestrial biological responses to climate change" contentmodel="wikitext" pagelanguage="en-gb" touched="2014-08-22T17:14:10Z" lastrevid="367" counter="11844" length="21098" />
      <page pageid="274" ns="0" title="Terrestrial biology in the instrumental period" contentmodel="wikitext" pagelanguage="en-gb" touched="2014-08-06T15:17:43Z" lastrevid="319" counter="9788" length="32762" />
    </pages>
  </query>
</api>