2021 Saw Record Sea Levels And Ocean Warming And ‘Huge Swings’ in Antarctic Sea Ice, Latest Climate Report Confirms

Despite ongoing pandemic disruptions, climate change has continued unabated and reached new record levels of warming in the atmosphere and the ocean in 2021, according to the latest State of the Climate report.

The report is an annual and comprehensive update on Earth’s climate indicators, notable weather events and climate data collected on land, water, ice and in space. It shows that concentrations of the three most dominant greenhouse gases (carbon dioxide, methane and nitrous oxide) reached new record highs, as has ocean warming and sea-level rise.

Kyle Clem, a climate scientist at Te Herenga Waka—Victoria University of Wellington and the lead editor of the report’s chapter on Antarctica and the Southern Ocean, describes 2021 as a year of extremes and rapid swings.

Record ocean warming and mixing to depth

The world’s oceans are taking up more than 90% of the excess heat energy produced by greenhouse gas emissions as well as abound a third of the carbon dioxide we emit from burning fossil fuels. The chilly waters of the Southern Ocean are the largest carbon sink.

But the Southern Ocean is also one of the stormiest places on Earth and its turbulent wind and ocean currents pull down heat to deeper layers than elsewhere. “The Southern Ocean is quite a unique environment for ocean heat uptake compared to other parts of the world,” Clem says.

“We have a [belt of] westerlies whipping around Antarctica over the Southern Ocean. That contributes to most of the heat being mixed down to depth. We don’t see strong sea-surface warming and that was similar in 2021, because we had a very, very intense storm track of strong westerly winds wrapped around Antarctica.”

The westerly winds vary from year to year and are strongly influenced by the ozone hole. But the primary factor that governs their strength is the pressure gradient between Antarctica in the middle latitudes, which in turn changes as the atmosphere heats up.

The tropics and mid-latitudes are warming more than Antarctica and the ozone hole also helps to cool Antarctica – and both factors strengthen the westerlies. Clem says what we saw in 2021 is consistent with the long-term trend of increasingly stronger westerlies and mixing of heat to deeper ocean layers.

Sea levels highest on record and accelerating

As the world’s oceans warm, they expand (because warmer water takes up more space) and raise sea levels. For the 10th consecutive year, global average sea level reached a new record high, and in 2021 alone, the seas rose by 4.9 millimeters. This is more than double the rate of sea-level rise during earlier decades.

The earliest records of sea-level rise go back to the 1880s, when tide gauges were set up in ports. “Since then we’ve seen over 25 centimeters [of sea-level rise]. But through much of the 19th and 20th century, sea level was rising by about two millimeters a year. That’s been increasing and increasing up to 4.9 millimeters now.”

During 2021, the biggest contribution to sea-level rise came from the thermal expansion of the global ocean, Clem says. The ongoing melting of ice sheets in Greenland and Antarctica also contributed, but only about half a millimeter.

Antarctica lost 50 billion tons of ice in 2021

Antarctica has been losing 140-160 billion tons of ice each year since 1993. In 2021, it only lost 50 billion tons because the continent also experienced an unusually high number of extreme weather events that brought large dumps of snow to inland areas in October and December.

“These heavy precipitation events, which occur sporadically when atmospheric rivers in middle latitudes are guided to Antarctica, are likely to become more intense in a warming atmosphere that can hold more moisture.”

Most of the ice loss came from West Antarctica, Clem says. “That’s because the warming ocean is eating away at the protective ice shelves from below. We lost a lot of ice mass there and the glaciers thinned. But in other parts of Antarctica, we saw these huge snowfall dumps that helped to balance that out. But the overwhelming signal still was a mass loss from West Antarctica.”

A similar, moisture-laden atmospheric river, the strongest yet to make landfall on the frozen continent, triggered a heatwave in East Antarctica in March 2022, but the data will be part of the next state of the climate report.

“We’re seeing more extreme precipitation events, which counters ice loss. However, mass loss is still dominating that signal, and we would expect even with increasing precipitation in coming decades, that melting is going to win.”

Long-lived ozone hole, coldest winter at the South Pole

As the strong circumpolar westerlies and polar vortex gripped Antarctica through much of 2021, the ozone hole lasted longer than in previous years, making it the second longest lived after 2020.

During the continent’s long polar night between April and September, the South Pole experienced its coldest winter on record, with June temperatures of −63.9°C (5.8°C below average). But it nevertheless remains one of the fastest warming regions on the planet.

“The long-term warming trend that we’ve seen at the South Pole over the past 30 plus years was not substantially reduced. Even with that cold winter, which resulted in an overall cold year at the South Pole, the rate of warming went from 0.6°C per decade, which is three times the global average, to 0.4°C, which is still more than twice as fast as the world average.”

See-sawing sea ice

Antarctica’s size doubles each southern autumn when an apron of sea ice freezes around it. The annual freeze-thaw cycle of sea ice, Antarctica’s slow breathing in and out, has presented climate scientists with a conundrum.

“It used to be quite stable, with low year-to-year variability, from the start of our observation record in 1979 to 2014. It was just a slow, steady and gradual increase, but what we’ve seen since 2015 is a brand new transition to a new regime of extreme variability.”

2021 was consistent with what we’ve seen since 2015, with huge swings bringing both record highs and lows, Clem says. “Sea-ice extent began well below average during January and February 2021. It then grew rapidly in March and remained above average through much of autumn and winter, with near record-high daily sea ice in August. Just as quickly as it grew, it rapidly diminished in September and October, plummeting to new daily record lows.”

This is alarming, he says, because sea ice plays a critical role in the global climate system. “When it freezes, it rejects salts, and it creates very dense water that sinks … then it melts and adds freshwater and that plays a big role in ocean circulation.”