Predict the average temperature for next 30 years
Do you have context where this problem originates from? You seem to be mixing up a few things.

Q1: depends on what dataset do you have available, or can you find? It can be just year and average temperature of the year, and then you'd apply a regression or ARIMA model.
But maybe you have average temperatures for various countries and regions, and then you could predict average temperatures per region.

Q2: again, depends on what dataset and columns you have. Typical independent variables are variables whose variation does not impact the temperature. Those typically have near 0 correlation with the temperature.

Q3: depending on the dataset, you probably should not use machine learning at all. Maybe regression/ARIMA is enough. And regression to predict values you should not do on partial data.

When you train a model with 30%, you use 70% for validation or scoring the model.

Your prediction could be just the extrapolation on the fitted curves on your data set.
You also have some information on the variations (R2), that can help give probability scores.
A typical outcome could be a 95% confidence interval that the temperature will be Tx +/- dTx.