Scientists can have the ability to predict future trends in climate because they may have compared the climate of the present to the climates of the past.
Reason: Meteorologist or, people who studies weather or climate patterns in a area used previous studies to predict how things may turn out.
For example in Florida, the climate is mostly hot and/or humid. This is because it is mostly hot and/or humid in Florida. Meteorologist use the previous climate of Florida (which is hot and/or humid) to predict that Florida will be in the future (which may be a literal oven0-0). Scientists also use this method to predict rain falls, storms, and even the Hurricane Seasons.
(Hope this helps you : ) )