Stern spoke to reporters shortly before negotiators from nearly 200 countries were to begin their final meeting at the conference site outside Paris.
The unprecedented deal to slow global warming begins by cutting and then eliminating greenhouse gas pollution.
In the "Paris agreement," countries would commit to keeping average global temperatures from rising another degree Celsius between now and 2100, a key demand of poor countries ravaged by rising sea levels and other effects of climate change.
Still, the pact doesn't have any mechanism to punish countries that don't or can't contribute toward that goal.