The warming in the first half of this year extended across all parts of the planet except for most of Antarctica, Dr. Schmidt said.

Warming was especially strong in the Arctic, where it had an effect on sea ice coverage.

Walt Meier, a scientist at NASA’s Goddard Space Flight Center, said that the geographical extent of the Arctic ice so far this year was the lowest for any half-year since satellite record-keeping began in 1979, largely because warm temperatures caused melting to begin as much as two months earlier than usual.

“It’s been an extreme beginning to the year for sea ice,” he said. It is not yet clear if this year will exceed the record for the lowest sea-ice extent, set in 2012, because most of the melting takes place later in the summer.

Warming of 1.5 degrees Celsius has special significance because at the Paris climate treaty in December, the world agreed to aim to limit the increase in average global temperatures to that amount above preindustrial levels.

Dr. Schmidt said that the Paris target referred to sustained temperatures over the long term.

“I certainly would not say that we have now gotten to that initial Paris number and are going to stay there,” he said. “But I think it’s fair to say that we are dancing with that lower target.”