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.
The report comes as the Trump administration dismantles many of former President Barack Obama’s climate change policies and scrubs most mentions of global warming from government websites. Within hours of President Trump taking the oath of office on Jan. 20, a White House website on climate change was replaced by a promise to eliminate key portions of Mr. Obama’s environmental agenda.
The Environmental Protection Agency has removed climate change-related pages on subjects from ice core samples to tide gauges. Last month, the Energy Department said it was closing the Office of International Climate Change Technology.
NOAA, however, continues to maintain social media accounts devoted to climate change and its website on Thursday says “U.S. saw second warmest year to date on record and warmer-than-average June.” The greenhouse gas index says the growth in greenhouse gases in the atmosphere since the Industrial Revolution is “mainly the result of human activity.”
NOAA’s greenhouse gas index was intended to provide a straightforward way to understand the warming influence of greenhouse gases and how it changed year to year. The steady increase since 1990 underscores how little society has done to reduce the planet-warming pollutants.
Calculated using air samples from the nearly 80 remote sites NOAA operates around the world, the measurement accounts for the five major greenhouse gases accompanying the index. “As expected, CO² dominates the total forcing,” scientists wrote in the index, referring to the effect of higher concentrations of warming gases in the atmosphere. Methane and ozone-depleting gases known as chloroform carbons, or CFCs, are becoming relatively smaller contributors, the report states.
Stephen A. Montzka, a NOAA research chemist who worked on the index, said the spike between 2015 and 2016 — the largest incremental increase in greenhouse gases since 1988 — was largely caused by El Niño. The jump between 1987 and 1988, which came in at a slightly higher 2.8 percent, was also linked to an El Niño.
Neither Mr. Montzka nor Jim Butler, director of NOAA’s global monitoring division, would discuss the agency’s decision not to highlight human activity as the main driver of greenhouse gases. But they said the report itself stated specifically that carbon dioxide emissions were overwhelmingly generated by humans.
Mr. Butler said that the 40 percent growth in atmospheric greenhouse gases since 1990 was a noteworthy sign of human impact on the climate.
“If you start at 1750, most of the increase is due to human activity,” Mr. Butler said. “What this means is 40 percent of what has been emitted since 1750 has happened since 1990.”
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