If you look at a temperature graph all would become clear. It's not like the data they had was fake or misrepresented, it's just that the trend is noisy and became clearer later. Now that it is clear, it makes no sense to say "we could still be wrong". It's like a stock market graph; you don't know if a few days ago was the peak, but in a year you do. Or in case of climate, 30-40 years.