There is a very good trend calculator here.

It gives you error bars that include allowance for autocorrelation. Temperature data is autocorrelated, i.e. a warm year will tend to be followed by a warm year because the surface cannot cool fast enough (ditto for colder years). Therefore the independent observations assumption behind Ordinary Least Squares is not true .... but that is not a disaster.

The major effect is to inflate the variance of the trend estimate. Which is why this calculator is so useful. Not sure if Nick Stokes' trend calculator does that.

Temperature trends