I'm assuming you're using Beyond the Sword 3.17? If you aren't, it shouldn't be too different, but for that version you have to find BtS's GlobalDefines.xml, which will be located in the game's folder.
On my computer, this is found at C:\Program Files\2K Games\Firaxis Games\Sid Meier's Civilization 4 Gold\Beyond the Sword\Assets\XML. Just dig down there.
Copy GlobalDefines.xml and go to C:\...\My Games\Beyond the Sword\CustomAssets\xml and copy it there, right click and choose "edit" which should bring it up in notepad. Do I search for 'Global' or something until you find the line
Code:
<Define>
<DefineName>GLOBAL_WARMING_PROB</DefineName>
<iDefineIntVal>20</iDefineIntVal>
</Define>
Change the 20 to 0 and save, and you should be good, but to be honest I haven't fully tested this yet (my game hasn't gone far enough!) but it makes sense and others said it works.