The trees will ride through floods and reduce water flows, improve species diversity for insects, birds, small mammals, improve temperatures and ameliorate winds, provide shelter for farm animals, can enhance grazing. And there's wood to harvest in due course.

There's a lot more reasons to plant trees than direct AGW offset.

A huge amount of farmland is now surplus to production. Grasses and fields of weeds aren't always ideal. Taking land out of production can also attract offset funding for farmers (-yes, this is a secondary economic outcome and may also incur other costs)

People are healthier around trees. People like trees. Even Bill Gates may actually like trees.