"Our planet cannot be saved unless we leave fossil fuels in the ground, where they belong," DiCaprio, the U.N. Messenger of Peace, warned the officials gathered for the signing ceremony . "An upheaval and massive change is required, now. One that leads to a new collective consciousness. A new collective evolution of the human race, inspired and enabled by a sense of urgency from all of you."Using his dramatic skills, the Academy Award winner and l ongtime environmental activist quoted Abraham Lincoln and compared climate change to slavery as the "defining crisis of our time.""We can congratulate each other today, but it will mean nothing if you return to your countries and fail to push beyond the promises of this historic agreement," DiCaprio said. "Now is the time for bold, unprecedented action...The world is now watching. You will either be lauded by future generations or vilified by them."The agreement, which was made during the U.N. COP21 Climate Change Summit last December, aims to keep the global temperatures below 2 degrees Celsius over pre-industrial levels. After signing, each country still has to ratify its pledge back at home. Secretary of State John Kerry (who brought his 2-year-old granddaughter, Isabel Dobbs-Higginson, to the podium for the signing) said that he hopes the U.S. will be able to ratify by executive action this year.