Heat that hit Europe exceptionally early and hard this year appears to have led to a spike in deaths, with well over 10,000 more people dying at the height of the heat wave than would normally have been expected, according to figures that are still emerging across the continent.

Measures of what researchers call “excess mortality” — the difference between the normally expected number of deaths and the actual number — spiked in late June, when parts of Europe experienced record temperatures. Experts caution that it takes a while for a full picture to emerge, and that many heat-related deaths will never formally be recorded as such. For example, a heart attack, which can be triggered by extreme heat exposure, especially in people who are older or have underlying health issues, may be listed on a death certificate simply as a heart attack.