Losing sea ice has knock on effects for climate. One of the most direct consequences is that losing sea ice changes something scientists call albedo. Albedo is a measure of how well the earth's surface reflects sunlight.
Snow-covered sea ice has a high albedo, reflecting up to 85 per cent of sunlight. As the area covered by ice and snow gets smaller, sunlight that would have been reflected is absorbed by open water instead, warming it up.