State of the Climate 2019: Strong wake-up call on ocean heat and oxygen

14 - Life Below Water

Madrid, 3 of December - In the framework of the 25th Conference of Parties to the United Nations Framework Convention on Climate Change (UNFCCC COP25), the World Meteorological Organization launched its annual report on the State of Climate. This Report was produced with key contributions from UNESCO‘s Intergovernmental Oceanographic Commission, via the Global Ocean Acidification Observing Network (GOA-ON) and The Global Ocean Oxygen Network (GO2NE).

Key take-away: 2019 concludes a decade of exceptional global heat and high-impact weather and the Ocean - which acts as a buffer by absorbing heat and carbon dioxide - is paying a heavy price.

The year 2019 concludes a decade of exceptional global heat, retreating ice and record sea levels driven by greenhouse gases from human activities. Average temperatures for the five-year (2015-2019) and ten-year (2010-2019) periods are almost certain to be the highest on record. 2019 is on course to be the second or third warmest year on record, according to the World Meteorological Organization.To date, oceans have been highly impacted by this tendency. Climate change raises ocean temperature, alters ocean circulation, reduces sea-ice extent and thickness, changes salinity, and leads to ocean acidification, sea level rise and changes in extreme weather events.

Some highlights from the Report:

Acceleration of global mean sea level rise

Sea level has increased throughout the satellite altimeter record, but the rate has increased over that time, due partly to melting of ice sheets on Greenland and Antarctica. In October 2019, the global mean sea level reached its highest value since the beginning of the high-precision altimetry record (January 1993).

Ocean heat

More than 90% of the excess energy accumulating in the climate system as a result of increased concentrations of greenhouse gases goes into the ocean. In 2019, ocean heat content in the upper 700m (in a series starting in the 1950s) and upper 2000m (in a series starting in 2005) continued at record or near-record levels, with the average for the year so far exceeding the previous record highs set in 2018.

Satellite retrievals of sea-surface temperature can be used to monitor marine heatwaves. So far in 2019, the ocean has on average experienced around 1.5 months of unusually warm temperatures. More of the ocean had a marine heatwave classified as "Strong" (38%) than "Moderate" (28%). In the north-east Pacific, large areas reached a marine heatwave category of “Severe”.

Continued ocean acidification

In the decade 2009-2018, the ocean absorbed around 22% of the annual emissions of CO2, which helps to attenuate climate change. However, increasing atmospheric CO2 concentrations affect the chemistry of the ocean.
Ocean observations have shown a decrease in the average global surface ocean pH at a rate of 0.017–0.027 pH units per decade since the late 1980s, as reported in the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate, which is equivalent to an increase in acidity of 26% since the beginning of the industrial revolution.


Observations and numerical models indicate that oxygen is declining in the modern open and coastal oceans, including estuaries and semi-enclosed seas. Since the middle of the last century, there has been an estimated 1%–2% decrease (that is, 2.4–4.8 Pmol or 77 billion–145 billion tons) in the global ocean oxygen inventory.

The projected expansion of the pre-industrial area of oxygen minima (< 80 µmol kg-1) by 7% through 2100 is expected to alter the diversity, composition, abundance, and distribution of marine life . New studies further identified deoxygenation alongside ocean warming and ocean acidification as a major threat to ocean ecosystems and human well-being. Even coral reefs are now recognized as vulnerable to major oxygen loss .

Decline of sea ice

The continued long term decline of Arctic Sea Ice was confirmed in 2019. The September monthly average extent (usually the lowest of the year) was the third lowest on record with the daily minimum extent tied for second lowest until 2016, Antarctic sea ice extent had shown a small long-term increase. In late 2016 this was interrupted by a sudden drop in extent to extreme values. Since then, Antarctic sea-ice extent has remained at relatively low levels.

With the insight of the previous Global Climate Indicators, the need for climate adaptation, informed by science, is now more urgent than ever. Since 1960, UNESCO’s IOC has conducted ocean observation activities that researchers and scientists rely on to measure and predict the major challenges facing the health of our ocean and planet.


For more details, please contact:

Salvatore Arico (