Atmospheric concentrations of CO2 will stay permanently above 400 parts per million (ppm) this year due to El Nino—and will likely not drop below that number again “within our lifetimes,” according to a study published this week in the journal Nature The milestone represents a symbolic threshold that scientists and environmentalists had long sought to avoid. Greenhouse gases have jumped 48 percent from the pre-industrial era, and 29 percent in just the past 60 years, from 315 ppm to 407 ppm today. CO2 concentrations tend to ebb and flow with the seasons, dipping as vegetation grows in summer and increasing during winter. But in the study published in Nature, scientists at the U.K.’s Met Office and University of California, San Diego warned that because of the recent El Nino, CO2 concentrations wouldn’t fall below 400 ppm this year, or any year into the distant future.