To meet the Paris climate deal's goal of deep greenhouse gas cuts, nations appear to be relying on costly, possibly harmful large-scale projects to suck carbon dioxide from the atmosphere, says a new paper with sobering calculations of the risks.

"The Paris agreement shows where we want to go — the brave new world of a balanced carbon budget — but not how to get there," says Phil Williamson, environmental scientist at the University of East Anglia in the United Kingdom and science coordinator for the U.K. government's Natural Environment Research Council.

Williamson warned in a commentary Wednesday in Nature that even seemingly beneficial approaches like tree planting could wreak havoc if they are implemented on the massive scale required to limit the increase in average global temperature to below 2° Celsius.

"There's a lot of optimism based on the assumption