Actually, we do. I am quite sure that Ice core drilling allows us to look 800,000 years back into the future (we are even able to distinguish individual years in the earlier parts, and can plot the relative trends back extremely reliably. That data may not be 100% accurate, but you don't need 100% accurate data - you just need to see the trends. We have accurate information with a standard deviation of less then 1 degree and then decreasing as we come closer to today.
The science is still being developed on it. My point earlier in the thread was with the sheer age of the world we have an absolutely minuscule amount of data. Suppose we take my 25 years as data, that means we've on record 0.00000005% of data to plot out what's really going on. Now obviously it's not as black and white as that and science can tell us more, but how close are we to even having 1% correct information on what's going on now compared to say 10,000 years ago or 100,000 years etc.