Tesla CEO Elon Musk hit back at billionaire Bill Gates on Twitter after the Microsoft founder downplayed his attempt to short Tesla stocks Saturday.

Gates made the comments during an interview with French YouTuber HugoDécrypte. Musk has recently accused Gates of not being serious about fighting climate change because he made a “half-billion dollar” bet against Tesla.

Gates argued he had put more money toward climate change than Musk or anyone else, to which Musk responded “Sigh.”

“I give a lot more to climate change than Elon or anyone else,” Gates said. “I give a lot of philanthropic dollars, I back companies – you know electric cars are about 16% of emissions, so we also need to solve that other 84%.”

The exchange comes more than a month after Musk confirmed he rejected an opportunity to work with Gates on philanthropy in late April. Musk revealed a text conversation in which he confronted Gates over his purchase of $500 million in shorting stock against Tesla.

The billionaire pair appeared set to meet before Musk asked Gates if he still had a “half billion dollar short position against Tesla.”

“Sorry to say I haven’t closed it yet,” Gates responded. “I would like to discuss philanthropy possibilities.”

Musk then blew him off, saying he couldn’t take Gates’ offer seriously “when you have a massive short position against Tesla, the company doing the most to solve climate change.”