Abrupt physical ocean changes due to marine heatwaves are expected with very high likelihood and high confidence concerning negative impacts on ecosystems. Increased heatwave occurrences are not reversible on short time scales and would persist from decades to centuries. The physical−chemical−biological ocean systems are at the verge of tipping into another state in many oceanic regions. Integrated over the world ocean, this adds up to a global issue of concern.
Here is a striking summary graphic that I want to pass on that shows candidates for high-probability high-impact marine tipping elements that concern warming, deoxygenation, and ocean acidification as well as their impacts. (Click to enlarge).
AND, I want to point you to a striking New York Times article that has amazing graphic animations showing the current dangerous weakening of the Gulf Stream that is generating a "cold blob" in the North Atlantic, threatening the normal warming effect of the Gulf Stream on the British Isles and northern Europe.