Climate change has largely been defined as an environmental issue, with the worst effects decades or centuries away. But a sobering new report from a commission convened by the medical journal The Lancet, released Monday evening, could change that assessment.

The new report says that climate change is already harming human health on a vast scale. “Climate change is happening, and it’s a health issue today for millions worldwide,” said Anthony Costello, a co-chairman of the commission that produced the report, called The Lancet Countdown.

The Lancet is one of the world’s most prestigious medical journals. It isn’t an environmental advocacy group, which is why this document could galvanize the public health community in ways that other reports on the consequences of the warming planet have not.

The journal has played an important role in bringing public attention to other medical issues, especially on the link between smoking and lung cancer. We take it for granted today that smoking causes that disease. But that was not always the case. The Lancet helped validate those connections for the medical community and, eventually, for the public.