New data indicates the continuation of a long-term decline in summer ice underway since at least 1979. Researchers say roughly half the decline can be attributed to global warming.

On Thursday, the extent of summer sea ice in the Arctic Ocean fell to its lowest level for any Sept. 8 since satellites first began to monitor conditions there in 1979, according to researchers at the University of Bremen’s Institute of Environmental Physics.

Recent work by researchers at the US National Center for Atmospheric Research (NCAR) suggests that roughly half the decline can be attributed to the effects of global warming. The other half they lay at the doorstep of natural swings in regional wind patterns that can drive ice out of the Arctic and into the North Atlantic.

In addition, several researchers see a contribution coming from black-carbon soot. Winds bring the soot up from lower latitudes and deposit it on the ice, where it absorbs sunlight and re-radiates it as heat.