The conditions hitting Europe this week are "at the edge of the envelope" of what the Intergovernmental Panel on Climate Change has long projected, IPCC chairman Jim Skea said.
But some impacts at the regional level and in the ocean have exceeded expectations, Skea told journalists.
"Some things... have gone outside the envelope in terms of what we are experiencing," he said.
France recorded its hottest day on record on Tuesday while several other European countries, from Britain to Hungary, have issued high-level heat warnings.
At least 94 million people in Europe are expected to experience temperatures above 35C on Wednesday, most of them in France and Spain, according to AFP calculations.
"Inevitably, we're going to experience more of what we've been seeing over the last few days," Skea said.
He noted that the hottest days warm much faster than average days, increasing by a factor of 50 to 100 percent.
In a world where temperatures would rise by 2C on average, the warmest day of the year would be 3C to 3.5C hotter.
The IPCC assesses global climate research and issues comprehensive reports every five to seven years to inform policymakers and guide climate negotiations. Its next report is due in 2028 or 2029.
