A study published recently in Nature, used measurements of O₂ and CO₂ emissions from the ocean to estimate warming. The warmer the water, the more gas emitted. The approach differs from the more commonly used water temperature measurements, which are considered limited and heterogeneous. Although the new data also have drawbacks, they show that between 1991 and 2016, the ocean may have warmed 60% more than we had previously thought!

A recent UNESCO-IOC-GO2NE report for policymakers outlines the impact of global warming and nutrient inputs on ocean deoxygenation and stresses that this is in addition to overfishing, habitat degradation and toxic pollution. See the UNESCO web page for more details.
A global increase in temperature of 1.5 °C is expected by 2030, beyond which, the severest consequences will be felt in tropical areas. The Intergovernmental Panel on Climate Change (IPCC) has released a report which says that limiting global warming to 1.5 °C

“… would require rapid and far-reaching transitions in energy, land, urban and infrastructure … and industrial systems … These systems transitions are unprecedented in terms of scale, but not necessarily in terms of speed, and imply deep emissions reductions in all sectors, a wide portfolio of mitigation options and a significant upscaling of investments in those options…”

There is, however, optimism for keeping on track with the 1.5 °C target. See how you can help here!

Update: 07 Feb 2020 – – A new study shows that ocean circulation has accelerated by 36% since the 1990s, a statistically significant increase in oceanic kinetic energy. In the tropics, increased ocean circulation is especially pronounced, even reaching 2,000 meters deep. Ocean circulation redistributes water masses and heat, affecting continental climate and weather conditions all over the planet. The current acceleration is attributed to increased intensity of surface winds since the 1990s, caused in part by natural variation, but appears mostly linked to a long term trend in greenhouse gas emissions associated with increases in global mean wind speed and eventually accelerated ocean circulation. The authors caution that more intensive monitoring of deep global ocean circulation is required to understand past conditions but also to make more robust future projections of the global ocean circulation.

Shijian Hu et al. 2020 Deep-reaching acceleration of global mean ocean circulation over the past two decades. Science Advances 6(6): eaax7727. https://advances.sciencemag.org/content/6/6/eaax7727