What is the yearly average temperature for the years 1915-2015?

Here's a reputable data source for the yearly average temperature for the years 1915-2015;

NASA's globally-averaged annual temperatures by month from JAN-1880 through JUN-2015 @ http://data.giss.nasa.gov/gistemp/

Data ...

http://data.giss.nasa.gov/gistemp/tabledata_v3/GLB...

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It'll take a little work to get the actual annual temperature.

Scroll to the bottom of the page to see the methodology.

► Use the data in the column 'AnnMean' 'J-D'

► These data are the annual departure from the average global temperature * 100 between 1951-1980 (14°C). Divide each data point by 100 to get the change in C°, eg. 22 ==> 0.22°C.

► Add 14°C to each data point in this column to get the average annual temperature.

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IMAGE: Average annual temperature (blue bars), average temperature for the 1880-2014 period-of-record (horizontal grey line), percent change from period-of-record average (red line; secondary Y-axis).

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The Standard Atmosphere (SA) that is used to calibrate all instruments, use 15 degrees Celsius as the yearly average temperature on earth at sea level. But an increase has been registered without changing the SA yet.

From what I read, in the past 30 years, the average temperature has increased by about one degree Celsius in the northern hemisphere, half a degree in the southern one and nothing really noticeable between the tropics.

Sincerely there are two questions in your question :

1-yearly average temperatures?

2-century average temperatures?

So which is which?