Environmentalist and Eco Eye presenter Duncan Stewart was speaking in Laois last weekend at a workshop on climate change.
He described a stark future unless fossil fuel use is halved by 2030.
"It's going to shock us, what's coming down the track," he said.
He said hundreds of millions of people will be forced to emigrate from climate stressed regions, and huge regions of the world will be inhospitable and uninhabitable for people and most species.
Laois is the focus for a series of public workshops led by Duncan Stewart and other experts over the coming weeks to see how it can lead the way in becoming a low carbon county.
Farmers, householders and students are all urged to take part. The project is funded by Laois Leader.