Global warming is the increase in average temperature of the oceans and air near the earth's surface occurred. (slideplayer.com)
The error of each estimate of global average temperature of an ocean layer remains closer to that of the instruments, or about 0.1 C. (joannenova.com.au)
I assume average temperature of the oceans is 5 C, and the GMST is 15 C. (judithcurry.com)