Ocean heat wave a sign of worsening climate change, study says, page-15

they do have records for cities going back that far.
but a temperature reading taken in 1910 cant be compared to a one taken today in the same city.

They are trying to measure changes at the first and second decimal point.
Different instruments, different measuring procedure even the different time of day will have an effect at the first and second decimal point level.

In reality you cant compare the 2 , but they do so by homogenising the data.
Some people believe this is to fudge the figures as part of a AGW conspiracy others believe it is their best efforts to make a comparison.

They cant put their hand on their heart and say 100% this is the temperature difference, they can say I believe this is the most accurate comparison between the two.

I've never been one for conspiracy theories, so for now I am happy to go with NOAA rather than the bloggers.