Brexit
The United Kingdom will vote June 23 on whether to leave the European Union. We're tracking public opinion.
He's not the first prominent scientist to call out the presumptive GOP nominee.
WHAT'S HAPPENING
"More unity is a path toward greater prosperity," the Canadian prime minister said.
"You would certainly not be back of the queue, that I can tell you," the presidential hopeful said.
Johnson is leading the campaign to leave the European Union.
Most world leaders have taken the opposite stance on Britain's upcoming referendum on Europe.
The "Brexit" vote could have global ramifications.