If someone doesn't believe in global warming because they have been monitoring the temperatures through a local weather station and the temperatures has stayed steady or the past few years how would I develop an argument to convince the person that their evidence does not mean global warming does not exist

The major argument is the identification of a climate related phenomenon on basis of weather data of a local region.

Explanation:

The major difference is regarding the difference between the basic definitions of the term weather and climate. The observations made by the protagonist are constrained both by the location as well as the time duration. Where as the global warming is linked to climate of the planet and these observations are to be general inclusive of all the regions across the globe as well as comparative historical data for large amount may be decades.