evidence: However, there is also some uncertainty on how sensitive Arctic sea ice itself is to warming. For a given amount of warming, different climate models predict different amounts of sea ice loss. Fortunately, we have now over 40 years of satellite-based measurements of both Arctic sea ice and temperature, allowing scientists to estimate this temperature sensitivity from observations directly: for every 1°C (1.8°F) of warming, the Arctic sea ice shrunk by about 800,000 km2 (about 300,000 square miles or the combined size of Texas and New Mexico). It turns out that most climate models show a weaker temperature sensitivity than observed – in other words, they underestimate how vulnerable sea ice is to warming. Two recent studies, by Bonan et al. (reported on here) and Kim et al., have used this insight to provide updated predictions of when the Arctic will be ice-free: both predict it to happen about 10 years earlier than we thought based on the climate models. The Kim study, published this year, made headlines for concluding that an ice-free Arctic is now inevitable, even under the most optimistic emissions scenario. It’s important to understand that they found individual years to dip below the "ice-free" threshold of 1 million km2, not that the sea ice disappears completely to never return. Still, this is an alarming result suggesting we have less time than we thought to prevent this profound transformation of the Arctic ecosystem.