By Henry Fountain / Source: nytimes

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The world is on pace to set another high temperature benchmark, with 2016 becoming the third year in a row of record heat.

NASA scientists announced on Tuesday that global temperatures so far this year were much higher than in the first half of 2015.

Gavin Schmidt, the director of NASA’s Goddard Institute of Space Studies, said that while the first six months of 2015 made it the hottest half-year ever recorded, “2016 really has blown that out of the water.”

He said calculations showed there was a 99 percent probability that the full year would be hotter than 2015.

Dr. Schmidt said the world was now “dancing” with the temperature targets set last year in the Paris climate treaty for nations to limit climate change.

He attributed part of the rise in temperatures this year to El Niño, in which warming waters in the equatorial Pacific Ocean pump a lot of heat into the atmosphere.

El Niño is now ending, and water temperatures in the Pacific are dropping, which should lead in 2017 to lower but still historically high temperatures.

Average temperatures for the first six months of this year were about 1.3 degrees Celsius, or 2.3 degrees Fahrenheit, above the average in 1880, when global record-keeping began, and “quite close” to 1.5 degrees Celsius above preindustrial levels, Dr. Schmidt said in a conference call with other NASA scientists.

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.”

Dr. Schmidt said that although NASA did not usually offer midyear updates to its global surface temperature analysis, it had decided to do so now “because average temperatures for the first half of this year are so in excess of any first part of a year we’ve seen.”

January 2016 was the hottest January since 1880, and that distinction continued for each month through June, NASA said.

Dr. Schmidt said that although El Niño contributed to some of the increase in temperatures from last year, almost all of the increase since the 1960s was because of human-caused emissions of carbon dioxide and other greenhouse gases.

Forecasters now expect that later this year, sea-surface temperatures in the Pacific will become lower than normal, a condition called La Niña. That should result in somewhat lower global temperatures next year, he said.

The NASA announcement reflects long-term trends in a climate affected by rising concentrations of greenhouse gases in the atmosphere.

That is distinct from short-term weather trends like the heat waves that the National Oceanic and Atmospheric Administration issued warnings about this week for much of the eastern half of the United States.

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