Why was it so hot during the years 2014-2016?

Yin, J., Overpeck, J., Peyser, C., & Stouffer, R. (2018).  “Big jump of record warm global mean surface temperature in 2014–2016 related to unusually large oceanic heat releases.”  
Geophysical Research Letters, 45. https://doi.org/10.1002/2017GL076500


The extreme heat in recent years has led to many unprecedented events, such as global coral bleaching and reductions in Arctic sea ice.  A study by Yin, Overpeck, Peyser, and Stouffer suggests that El Nino released huge quantities of heat from the oceans, resulting in record-breaking warm temperatures during the years 2014-2016.  The rise in temperatures has been so extreme that global temperatures have now increased by one degree Celsius relative to the pre-Industrial era.  For those of you who have been following the Paris Climate Agreement, that’s already two-thirds of the 1.5 degree threshold that they’re working to avoid!


Why do the oceans store so much heat?

Firstly, the oceans are huge!  They’re over a hundred times as massive as the atmosphere and they cover about 70% of the Earth’s surface.  Secondly, the heat capacity of water is roughly 1000x that of air.  The heat capacity of a material tells you how much heat it can absorb before its temperature changes.  Water’s heat capacity is unusually high, which is why it can be very effective in cooling systems!

An example of a water cooling system.  Image Credit: Imgur.

Because of their extraordinary mass and heat capacity, the oceans absorb a lot of heat from the atmosphere.  As a result, the greatest effects of global warming are felt by the oceans.  Global warming is primarily a result of the greenhouse effect, which has been enhanced in recent years due to humans adding greenhouse gases, such as CO2, to the atmosphere.  These greenhouse gases have been preventing the oceans from radiating heat out to space, which means that the oceans have been storing more and more heat in recent years.  You can read a little bit more about this here or see the figure below.

This plot shows how the ocean heat content differs from the long-term global average.  The blue bars with negative values mean that the ocean was storing less heat than average, while the yellow bars with positive values mean that the ocean is storing more heat than average.  Image Credit: NOAA.

In summary, the oceans absorb a lot of heat from the atmosphere, and they’ve been storing more and more heat in recent years as a result of greenhouse warming.  (Remember that last part; it’ll come up again later!)  However, it turns out that heat that gets absorbed by the ocean doesn’t always stay there.  Certain weather or climatic events can release the heat stored in the ocean back to the atmosphere, with significant consequences for global temperature, as Yin and colleagues discovered when they examined the 2014-2016 El Nino.


What is El Nino?

Roughly once every two to seven years, the pattern of temperatures in the Pacific Ocean undergoes a drastic change.  When the eastern half of the Pacific Ocean is warmer than average, we call this El Nino, and when the eastern half of the Pacific Ocean is cooler than average, we call this La Nina.  During the warm El Nino phase, sea surface temperatures are high, so a lot of water can evaporate and, in doing so, release heat from the ocean to the atmosphere.

Sea surface temperatures in the Pacific Ocean during the 2014-2015 El Nino.  You can see very warm temperatures in the central and eastern Pacific Ocean – a classic signature of El Nino.  Image Credit: NOAA Environmental Visualization Laboratory.

El Nino adds huge quantities of moisture and heat to the atmosphere, so even though it starts in the Pacific Ocean, it can have significant consequences for weather all around the world.  I grew up in Florida, so to me El Nino always meant rain, but other regions experience El Nino differently (see map below).  In a recent study, Yin and colleagues found that El Nino is probably the catalyst for the record-breaking global heat that has been observed from 2014-2016.

Effects of El Nino in different parts of world, and during different times of the year.  Image Credit: NOAA/Wikimedia Commons.


How do we know that the 2014-2016 warming is related to El Nino?

Yin and colleagues examined global mean sea surface temperatures by using a variety of data sources, including satellite altimetry, which is a method for observing the height of the sea surface.  Water expands when it heats up, so the higher the sea surface, the more heat there is!

An example of sea surface heights from 1992.  Image Credit: NASA Jet Propulsion Laboratory.

The researchers found something rather striking in their data.  During the years 1993-2012, a massive amount of heat accumulated in the western Pacific, in the region east of the Philippines (plot A in the figure below).  This warm feature persisted until the El Nino event of 2014-2016, when Yin et al. observed downward trends in the sea surface heights of the western Pacific (plot B).  This drastic change in sea surface heights indicates that all the excess heat that had accumulated in the western Pacific got released during the years 2014-2016.  According to the researchers, this heat release was due to El Nino, which explains the extraordinary warming we’ve experienced in recent years.

Various measures of ocean heat content in the Pacific Ocean.  Image Credit: Yin et al. 2018.

But why was there so much heat in the western Pacific to begin with?  As we saw earlier, the oceans have been storing more and more heat in recent years because of greenhouse warming.  Yin and colleagues concluded that greenhouse warming was the primary cause for the western Pacific’s unusually large accumulation of heat during the years 1993-2012 (as you can see in plot D, where “external” mostly just means “greenhouse gases”).  Since there was so much more heat that the 2014-2016 El Nino could release back to the atmosphere, the effects on global temperatures have been correspondingly more severe.


Can this happen again?

This research demonstrates that heat accumulation due to greenhouse warming was the fundamental cause of the warming during the years 2014-2016.  El Nino played a crucial role by releasing this heat, but the reason for the heat accumulating in the first place – the greenhouse effect – remains unchanged.  So, according to the researchers, the ocean will just accumulate heat again until the next time it gets released, and record-breaking warmth like we observed in 2014-2016 will only become more and more common as global warming continues.

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Rohini Shivamoggi

I'm a PhD student studying atmospheric sciences at MIT. I study the formation of secondary eyewalls in hurricanes, which hopefully will help us improve our forecasts of hurricane intensity. Before I got to MIT, I grew up in Florida and studied Chemistry and Physics at Harvard University. My other interests include weather forecasting, photography, and encouraging diversity in STEM! You can find me on Twitter @RShivamoggi.

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