In Assets\XML\Terrain Find CIV4TerrainInfos file. Look for this line: <iHealthPercent>-15</iHealthPercent>
Iirc, Tundra, Snow, Desert, and Ice has this line. But check the whole file to be sure.
Adjust the >-15< to the value (% of damage you want). Must be -x to give damage.
JosEPh