When subtracting or adding two numbers in scientific notation, why do the exponents need to be the same?

This map shows how climate change might affect precipitation patterns in the great plains of the united states by the end of this century. brown shoes a decrease in percipitation, and blue shows an increase. the maps differ based on the predictive levels of green house emissions. the maps shows that northern area should get a. more pracipations regardless of emmissions b. less pracipation, regardless of emmissions levels c. more precipitation, but only when emission levels. southern areas show a. more pracipations regardless of emmissions b. less pracipation, regardless of emmissions levels c. more precipitation, but only when emission levels. to use these maps to prepare for natural disasters in the great plains, scientist need to ask? a. how many tons of emissions were used at the basis of the map. b. which types of fossil fuels are used in the great plains c. if you think the above is wrong it's probly c the question is cut off