"The average surface temperature of the world's seas has increased by around 0.9C compared to preindustrial levels, with 0.6C coming in the last 40 years alone.
This is less than increases in air temperatures over the land - which have risen by more than 1.5C since preindustrial times. This is because much more energy is needed to heat water than land, and because oceans absorb heat far below their surface.
Even this seemingly small average increase has significant real-world consequences.
- Loss of species: more frequent and intense marine heatwaves lead to mass mortality of sea life. This is particularly damaging for coral reefs.
- More extreme weather: increased heat in the upper ocean surface means hurricanes and cyclones can pick up more energy. This means they become more intense and longer-lasting.
- Sea-level rise: warmer waters take up more space - known as thermal expansion - and can greatly accelerate the melting of glaciers from Greenland and Antarctica that flow into the oceans. This raises global sea levels, increasing risks of coastal flooding.
- Less ability to absorb CO2: the oceans currently take up about a quarter of greenhouse gas emissions. Warmer waters have less ability to absorb CO2. If the oceans take up less CO2 in future, more would accumulate in the atmosphere - further warming the air and oceans."
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