“The role of greenhouse gases on influencing global temperatures is well understood by scientists, but it’s a complicated topic that can be difficult to communicate,” NOAA officials said in releasing the index.

That is a notable shift from last year’s release, in which NOAA declared that “human activity has increased the direct warming effect of carbon dioxide.” In 2014 the agency, which is housed in the Commerce Department, said “the warming influence from human-emitted gases continues to increase.”

The current announcement calls greenhouse gases “long-lived.” It acknowledges those emissions influence the climate, but sidesteps the scientific consensus that humans are primarily responsible for them.

Theo Stein, a NOAA spokesman, acknowledged in an email that phrasing about humans causing greenhouse gas emissions did not make it into the announcement but noted a second news release that was published on the website of the agency’s office of oceanic and atmospheric research that lists “climate change indicators.”

Scientists noted that emissions tend to rise more quickly during an El Niño weather pattern. The El Niño phenomenon, which was unusually strong in 2015-16, warms the Pacific Ocean, bringing heavy rains and droughts to different parts of the world. Scientists say the increase in sea surface temperatures that occurs during an El Niño causes less carbon dioxide to be dissolved in the oceans and, as a result, more accumulates in the atmosphere.