How much has the earth warmed in the past 100 years and is that a big deal?

In the past 100 years there have been three rates of temperature rise:

1. From around 1910 to 1940 there was a rise of 0.35 degrees Celsius.
   
2. From around 1970 to 2015 there was a rise of 0.55 degrees Celsius.
   
3. Between 1990 and the present there has been an increased rate of warming, and 11 of the hottest years on record have occurred in the last 12 years.
   

This is a big deal. Almost one degree Celsius already. Scientists are hoping that the amount of greenhouse gases already in the atmosphere will not raise the temperature by 2 degrees. They agree that 2 degrees of warming is about all we can tolerate without serious deaths and difficulties. However many scientists say we are too far advanced and there's no way we can NOT reach 2 degrees and even higher.

There is no one thermometer that measures the world's temperature. Every day thousands of thermometers all over the earth take measurements. This happens on land, on the oceans, and from satellites. These temperatures are all averaged out to produce an estimate of a global monthly average temperature.