A country's birth rate and death rate (per 1000 population) over five decades are:

Decade: 1970 1980 1990 2000 2010

Birth rate: 40 34 28 22 18

Death rate: 20 17 14 11 9

Based on this data, what can be inferred about the country's population growth rate (birth rate minus death rate) from 1970 to 2010?