........

 I read an extract in : http://72.14.235.132/search?q=cache:J_wzdTdbW2IJ:iramis.cea.fr/en/Phocea/file.php%3Fclass%3Dstd%26file%3DNews/ice-feb-2005.pdf+melting+point+of+ice+is+-1degree+C+when+pressure+is&hl=en&ct=clnk&cd=9&gl=in

the temperature at which icemelts falls by about 20 degrees when

thepressure is increased to about 2000 atmo-spheres (2kbar),

and then rises again as thepressure is increased further. 

so going as per the extract for melting point to change to -1degree celsius pressure required would be : 100 atm

the nearest answer seems to be 133 atm.

dear JPS,

ankit is right . there is something for which there is a very complex mathematics. we can only observe for ther occurace. so, no mathematcal step ths time. hope you understand.