Causes of Ocean Extremes: Certainty and Uncertainty

Summarise and explain why Ocean extremes are becoming more severe. What is certain vs. uncertain, and what are the main reasons for uncertainty.

The ocean plays a critical role for life on Earth, with many species relying on it to survive. It produces over half of the world’s oxygen and absorbs 50 times more carbon dioxide than the Earth’s atmosphere [1]. The ocean’s surface layer absorbs over half of the sun’s radiation that reaches the Earth [2], with ocean currents distributing large amounts of heat and water around the world. This plays a key role in regulating climate and weather patterns and crucially, it allows the ocean to act as a heat sink to delay the full effects of climate change. However, the Intergovernmental Panel on Climate Change (IPCC) reported in their fifth assessment (AR5) that no past climate change event perfectly parallels future projections of anthropogenic climate change [3, p. 423]. This is unprecedented in evolutionary history and brings into question how capable of adapting to this change our ocean will be. We are already starting to see potentially irreversible changes to the ocean system due to climate change and other forces.

The El Niño-Southern Oscillation (ENSO) is a recurring climate pattern involving oscillations between warming and cooling of waters in the central and eastern tropical Pacific Ocean. This cycle directly impacts the rainfall distribution in the tropics and can have a strong effect on weather across different parts of the world. ENSO has two extreme phases called El Niño and La Niña, in El Niño the ocean experiences a warming and above-average sea surface temperatures whereas, in La Niña, there is a cooling resulting in below-average sea surface temperatures. There has been a statistically significant increase in ENSO variance in recent decades with a large El Niño event occurring in 2015/16, which may be viewed as the first extreme emergence in the 21^st century [4, p. 35]. Extreme El Niño events result in a rainfall increase of greater than 5mm per day during December-February in the equatorial eastern Pacific, where it is usually cold and dry [4, p. 35]. These extreme and potentially devastating weather events that occur as a result of extreme ENSO can have vast impacts on natural systems and communities that rely on regular weather patterns for their habitats and agriculture. The frequency of El Niño is predicted to double in the 21^st century under 1.5°C of global warming, from about 1 event occurring every 20 years, to one event every 10 years. Even after global mean temperatures stabilise at 1.5°C the increase in frequency is predicted to continue for up to a century [4, p. 35], which challenges the ability of ecosystems to adapt. Extreme ENSO events are rare, with only three occurring since 1950 [4, p. 35]. So, along with the quality of data before 1950 being low, the limited amount of data available has made it difficult to draw firm conclusions regarding the impact of climate change on these extreme events. However, swings from extreme El Niño to extreme La Niña have been projected to increase in frequency under greenhouse warming [5]. Particularly concerning is the fact that there is medium confidence that this risk will remain high for the entire 21^st century, even if substantial reductions in greenhouse gas emissions are made globally [4, p. 35]. The dynamics of ENSO are complex, but it is clear if these extreme events are predicted to occur more frequently in the future then it seems essential for there to be a greater global understanding of precisely how global warming impacts ENSO. 

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Past and current greenhouse gas emissions have resulted in global warming already reaching 1°C above the pre-industrial level [6]. As we experience increased rates of global warming, the ocean seems to take the biggest hit. It is virtually certain that the ocean has warmed since the 1970s and has taken up more than 90 percent of the excess heat in the climate system [7, p. 8]. Along with long-term ocean warming, there are additional short-term extreme warming events, known as marine heatwaves (MHWs), which are now becoming more frequent, extensive and intense. Marine heatwaves are periods of extremely high ocean temperatures (when the daily sea surface temperature exceeds the 99^th percentile), lasting for days to months, that can extend up to thousands of kilometres and penetrate multiple hundreds of metres deep into the ocean [4, p. 26]. One of the most important global drivers of MHWs are El Niño events, during which the sea surface temperatures are exceptionally warm [4, p. 26]. MHWs are just one example of the extreme events that are shown to increase in frequency as a direct result of increased ENSO variance. Over the period of 1982-2016 the number of MHW days doubled globally from about 2.5 to 5 days per year, maximum intensity increased by 0.15°C and the spatial extent by 66 percent, with about a quarter of the surface ocean experiencing either the longest or most intense MHW in 2016 [4, p. 29]. This observed trend towards more frequent, intense and extensive MHWs is predicted to continue under future global warming as it is very likely due to the long-term anthropogenic increase in average ocean temperatures and cannot be explained by natural climate variability [4, p. 29]. However, these observations and predictions are based on sea-surface temperature measurements; it is still currently unknown if marine heatwaves have changed over time in deep waters, and how big this possible change is. This uncertainty over MHWs in deeper waters is due to the scarcity of below surface temperature data with high temporal and spatial resolution [4, p. 29]. Some studies have taken place to investigate the impact of anthropogenic warming on the likelihood of individual MHWs occurring. Although not all major individual MHWs have been assessed, and so it is unknown whether there is anthropogenic forcing for these events, there is a clear human-induced signal on the individual analysed MHW events [4, p. 29]. In other words, these individual MHW events could not have occurred without global warming since pre-industrial times. The IPCC concluded that it is very likely that along with most marine heatwaves increasing in frequency, duration and intensity, between 2006 and 2015 most marine heatwaves (84%-90%) occurred as a result of global warming [4, p. 30].

Global mean sea level is rising, with acceleration due to increased rates of ice loss, continued glacier mass loss and ocean thermal expansion [7, p. 10] (increase in ocean volume due to the higher temperature of the water), all of which are direct effects of global warming. Extreme sea levels occur as a result of a combination of different factors including variability in sea levels, wind waves and swell, and storm surges. Storm surges are caused by decreasing atmospheric pressures and wind force associated with severe storms, such as tropical and extratropical cyclones [3, p. 370]. Extreme sea-level events, therefore, may be impacted if storms are affected by climate change however, observed trends in extreme sea levels are mainly consistent with mean sea level trends. This indicates that increases in mean sea level, rather than changing weather patterns, is responsible for increased extreme sea levels [3, p. 370]. Extreme wave heights, another contributor to extreme sea-level events, have been observed to increase from 1985 to 2018 in the Southern and North Atlantic oceans by around 1 cm/yr and 0.8 cm/yr [7, p. 11] with increased sea ice loss in the Arctic due to climate change projected to further increase wave heights [4, p. 22]. Global wave power represents the transfer of energy from the wind into sea surface motion and therefore includes wave height. Global wave power has increased at a rate of 0.41% per year between 1948 and 2008 with strong correlations found between increases in wave power and sea surface temperature [4, p. 22], signifying that global warming is also having a significant impact on waves and extreme wave heights. However, there is low confidence in predicted changes in extreme winds and waves due to the limited studies available [3, p. 371]. It is clear that although there are many regional factors that will impact the severity of extreme sea levels, all of which are continuously changing due to global warming, the main contributor is the global increase mean in sea level.

There is high confidence in that the role of climate change in ocean extreme events (events that are rare for that particular place and time of year) is increasingly driving extreme climate and weather events across the globe [4, p. 10]. Therefore, understanding ocean extremes and how to possibly prevent them from worsening is likely to reduce the probability of worsening climate events globally. Overall, it is obvious that global warming is having a severe impact on our ocean system. All the ocean extremes that have been reviewed have global warming as the main contributor as to why they are becoming more severe. Limiting global warming will reduce the risk of impacts of extremes such as marine heatwaves, however, some ecosystems (e.g. coral reefs) will still reach critical limits even at relatively low levels of future global warming [4, p. 4]. Additionally, even at low levels of future global warming,  extreme ENSO events are likely to occur more frequently and intensify existing impacts [4, p. 4]. Drastic and global changes need to be made to reduce the level of climate change in order to prevent further changes in ocean extremes, which may exceed the levels of resilience of ecosystems leading to irreversible loss and damage.

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