2017 Was the Warmest Year on Record for Oceans
Global warming is real, and it’s happening now. Within hours of the announcement by scientists in the US that 2017 was at least the third warmest year recorded, if not the second, over the Earth’s land and oceans, there comes a further revelation: 2017 was also the warmest year on record for the global oceans.
Both disclosures are consistent with what scientists had expected from climate change, driven by global warming as a consequence of the profligate combustion of fossil fuels that dump ever greater levels of greenhouse gases in the atmosphere.
But they add to the scientists’ sense of urgency at the need for rapid and radical action to cut greenhouse emissions. Of the US announcement, Dr Dann Mitchell, of the University of Bristol, UK, said: “The most recent global temperature observations are in line with what we expected, both from our underlying theory, but also our model projections and understanding of the climate system.
“The atmosphere is warming, by almost 1°C globally to date, and we are getting ever closer to the Paris Agreement target of 1.5°C which we are so desperately trying to avoid.”
The news that the oceans are continuing to warm to hitherto unknown levels comes in an updated ocean analysis from the Institute of Atmospheric Physics/Chinese Academy of Science (IAP/CAS). Its study was published as an early online release in the journal Advances in Atmospheric Sciences.
The authors say that in 2017 the oceans in the upper 2000-metre layer of water were warmer than the second warmest year, 2015, and above the 1981-2010 climatological reference period.
Thanks to their large heat capacity, the oceans absorb warming caused by human activities, and more than 90% of the Earth’s extra heat from global warming is absorbed by them.
The study says the global ocean heat content record robustly represents the signature of global warming, and is affected less by weather-related “noise” and climate variability such as El Niño and La Niña events.
The IAP says the last five years have been the five warmest years in the oceans, as the long-term warming trend driven by human activities continued unabated.
The rise in ocean heat in 2017 occurred in most regions of the world. Increases in ocean temperature cause the volume of seawater to expand, contributing to the global average sea level rise, which in 2017 amounted to 1.7 mm. Other consequences include a decline in ocean oxygen, the bleaching of coral reefs, and the melting of sea ice and ice shelves.
The globally averaged temperature over land and ocean surfaces for 2017 was the third highest since record keeping began in 1880, according to NOAA scientists.
There is a slight difference in the figures for 2017’s temperature. NOAA says the globally averaged temperature for the year makes it the third hottest since record-keeping began in 1880, while NASA says in a separate analysis that 2017 was the second warmest on record, behind 2016.
This minor difference is explained by the different methods used by the two agencies to analyse global temperatures, they say, though they point out that over the long term their records agree closely.
Both agree that the five warmest years on record have all occurred since 2010. The UK Met Office and the World Meteorological Organisation (WMO) also listed 2017 among the top three warmest years on record.
One striking feature of the consensus on 2017’s place in the record books is less about what did happen, and more about what didn’t. Last year was the second or third hottest after 2016, and on a level with 2015, the data show.
But those two years were affected by El Niño, the periodic natural phenomenon in the Pacific, which helps to boost temperatures worldwide. 2017 was not an El Niño year.
If it had been, the researchers say, it would probably have been the warmest year yet, outstripping the heat in 2015 and 2016.
The acting director of the UK Met Office, Professor Peter Stott, told BBC News: “It’s extraordinary that temperatures in 2017 have been so high when there’s no El Niño. In fact, we’ve been going into cooler La Niña conditions.
“It shows clearly that the biggest natural influence on the climate is being dwarfed by human activities – predominantly CO₂ emissions.”
The WMO secretary-general, Petteri Taalas, said the long-term temperature trend was far more important than the ranking of individual years: “That trend is an upward one. Seventeen of the 18 warmest years on record have all been during this century, and the degree of warming during the past three years has been exceptional.
“Arctic warmth has been especially pronounced, and this will have profound and long-lasting repercussions on sea levels, and on weather patterns in other parts of the world.”