Why do most experts descirbe the rate of a fossil fuel use as unsustainable?

Answer 1

There is some debate as to whether short-term, we can continue to use fossil fuels at the same rate. We are finding new sources of fossil fuels, and these help replace the fossil fuels that we consume.

Longer term, the earth's resources are limited and as we consume our fossil fuels, we will not be able to find new fossil fuels to replace them.

See attached link on peak oil.