In the two years between June 2014 and May 2016, world leaders came together and adopted the landmark Paris Agreement, and the global average sea level rose by a staggering 15 millimeters or 0.6 inches.

Although that rise isn't much more than the length of a fingernail, it was actually a frightening leap on long-term trends that a new study suggests was driven by a particularly extreme and unusual climate period.

Global average sea levels have risen by over 9 centimeters (about 3.5 inches) since 1993, the first year that NASA satellites were whizzing around our planet monitoring changes in sea levels.

From those long-term datasets, scientists estimated global mean sea levels should rise about 4 millimeters per year over the 2014-2016 period – yet global sea levels rose, on average, nearly twice as much as expected in that time: 15 millimeters instead of the expected 8 (0.3 inches).

A new study from a team of oceanographers at the French National Centre for Scientific Research (CNRS) has linked that sudden jump to back-to-back El Niño events in the Pacific Ocean, which appears to have sped up sea-level rise by shifting rainfall patterns and drying out the Amazon Basin.

El Niño is one phase of the El Niño Southern Oscillation (ENSO), an irregular shift in wind patterns and sea surface temperatures that swings back and forth across the tropical Pacific Ocean. The change in trade winds pushes warm water either towards the west coast of the Americas (as so happens in El Niño) or towards Asia (in La Niña, the opposite phase).

Two consecutive El Niño events in 2014-2015 and 2015-2016 coincided with the 15-millimeter rise in global mean sea levels. The 2015-2016 El Niño event was particularly extreme, with 2016 quickly surpassing 2015 as the hottest year on record, according to the World Meteorological Organization (WMO).

"Those unusual El Niño events affected the precipitation pattern worldwide, decreasing the terrestrial water storage in the Amazon basin and therefore leading to an increase of the global mean ocean mass," William Llovel and colleagues explain in their paper.

To come to this finding, the researchers combined data from multiple satellites and a global array of floating ocean sensors, and analyzed the changes in ocean temperatures, ocean mass, and the amount of water stored on land, in river systems like the Amazon Basin.

The Amazon Basin covers about 35 percent of the South American continent, a huge water catchment consisting of the Amazon River and its capillary-like tributaries.

Llovel and colleagues found that 80 percent of the 15-millimeter rise in 2014-2016 was due to the world's oceans increasing in mass as El Niño changed rainfall patterns: the Amazon Basin held less water, and more rain fell over the eastern tropical Pacific Ocean and Argentina.

Ice sheets melting and other land water changes were also a factor, but the drying out Amazon Basin alone contributed 5 millimeters to rising seas during this period.

The remaining 20 percent or 3 millimeters of the 2014-2016 rise was attributed to the expansion of the ocean as it warmed up, the analysis showed.

No two El Niño events are the same. So investigating the imprint of past El Niño – and La Niña – events on global sea levels will help refine climate model projections of future sea-level rise, particularly in the next few decades, the researchers say.

Earlier this year, in July, the WMO declared El Niño conditions had developed in the tropical Pacific straight off the back of a 'triple-dip' La Niña which began in 2020. Extra hot temperatures are expected to follow.

The study has been published in Geophysical Research Letters.