"We in Germany, in Europe and the world will band together to take more decisive action than ever to confront and successfully surmount major challenges to humanity such as climate change," she told reporters.
In a strongly worded statement, Merkel said US President Donald Trump's announcement to turn his back on the Paris pact was "highly regrettable, to put it very mildly".
"This decision cannot and will not stop those of us who feel obligated to protect our Earth," she said.
She said it would be a "rocky road" toward full implementation of the agreement but called it "irreversible".
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Merkel hailed private initiatives around the world on climate protection, including in the United States, saying they would help ensure "more prosperity and opportunities for the world".
She vowed to make a success of the next round of UN climate talks in the western Germany city of Bonn in November.