The Arctic has been unusually warm since the beginning of 2018. In the past week air temperatures have hovered around 20℃ above normal or even higher. On February 25, the Cape Morris Jesup weather station in northern Greenland recorded 6.1℃, despite the fact that at this time of year, when the sun is still below the horizon, temperatures are typically around -30℃.
A surprising feature of this warming event was how far into (and beyond) the Arctic it has penetrated. Warm air migrated north from the Atlantic Ocean, over the North Pole and towards the Pacific Ocean, bringing above-freezing air temperatures to large areas of the Arctic Ocean for more than 24 hours.
We have not seen a warm intrusion from the Atlantic Ocean on this scale since at least 1980.
Is this unprecedented?
Warm events in the middle of the northern winter are not unheard of. Large winter storms can bring strong winds that pump warm air into the Arctic from lower latitudes.
For example, during the Norwegian explorer Fridtjof Nansen’s 1896 expedition aboard the icebreaker Fram, the crew observed temperatures of -3℃ on one midwinter’s day. More recently, in December 2015, an Arctic warming event brought temperatures of 2℃ to the North Pole, and the warm weather continued into early 2016.
But, crucially, this type of event is becoming more common and longer in duration, with higher peak temperatures.
Record low sea ice extent
February 26 brought a new record low for sea ice extent: maximum sea ice extent on that day was 14.20 million square kilometres, which is 1.29 million km2 below the 1981-2010 average for that day. This follows several years with record low winter maximum sea ice extents in 2015, 2016 and 2017.
The current warm conditions in the Arctic have implications for sea ice year-round. Sea ice grows in winter and melts in summer. The warm air temperatures will slow down sea ice growth, and strong winds will push it around, breaking it up in places – as happened north of Greenland earlier this week.
Open water where the ice is broken will release extra heat into the atmosphere. By the time the spring sun comes around, the sea ice pack is thinned and weakened, and may melt more easily.
Cold weather in Europe
While the Arctic has been hot, Europe has been bitterly cold this week: London recorded -6℃; Berlin reached -14℃; and the Alps plunged to -27℃. Rome received 5-15cm of snow on Monday, and up to 40cm of snow fell in Britain on Wednesday.
It might sound counter-intuitive, but this cold weather is directly linked to the recent warming event in the Arctic.
Normally, the cold air above the polar region is contained in the Arctic by a ring-like band of strong winds called the polar vortex. But in the middle of February this year, the polar vortex split into two vortices: one over Eurasia and the other over North America.
Between these two features, a strong high-pressure system gradually formed. As a result, warm air was pumped up into the Arctic on the west side of the high, while cold air was channelled southwards to the east of it. Hence the exceptionally warm air in the Arctic and the cold snap in Europe.
Is the polar vortex changing?
The polar vortex is driven by the strong temperature differences between the warm mid-latitudes and the cold Arctic. With the Arctic warming more rapidly than the mid-latitudes, this temperature difference is decreasing and some scientists believe that the polar vortex is weakening.
Research suggests that the polar vortex has become “wavier” as a result of this weakening. A wavier jet stream would lead to more frequent cold outbreaks of polar air at lower latitudes, and at the same time cause warm air to intrude into the Arctic. However, other researchers have argued that “large uncertainties regarding the magnitude of such an influence remain”.
Generally speaking, warming at every latitude makes cold spells at low latitudes less likely, and warm intrusions at high latitudes more likely, unless the Arctic warming leads to a fundamental change in the dynamics of the atmosphere.
Since 1979, Arctic warming events have grown more frequent. However, climate projections indicate that there will be fewer Arctic storms in the latter part of this century, and thus fewer Arctic warming events.
As scientists, we were startled by the extent of this week’s Arctic warming, and will be working hard to understand the short- and long-term implications. All eyes will be on the upcoming maximum winter Arctic sea ice extent, which is likely to happen in the next few weeks and could possibly set a new record low.
Amelie Meyer, Postdoctoral Researcher, Physical Oceanography, Norwegian Polar Institute; Erik W. Kolstad, Research professor, Uni Research; Mats Granskog, Senior research scientist, Norwegian Polar Institute, and Robert Graham, Postdoctoral Researcher, Climate Modelling, Norwegian Polar Institute
For the Arctic, like the globe as a whole, 2016 has been exceptionally warm. For much of the year, Arctic temperatures have been much higher than normal, and sea ice concentrations have been at record low levels.
The Arctic’s seasonal cycle means that the lowest sea ice concentrations occur in September each year. But while September 2012 had less ice than September 2016, this year the ice coverage has not increased as expected as we moved into the northern winter. As a result, since late October, Arctic sea ice extent has been at record low levels for the time of year.
These record low sea ice levels have been associated with exceptionally high temperatures for the Arctic region. November and December (so far) have seen record warm temperatures. At the same time Siberia, and very recently North America, have experienced conditions that are slightly cooler than normal.
Extreme Arctic warmth and low ice coverage affect the migration patterns of marine mammals and have been linked with mass starvation and deaths among reindeer, as well as affecting polar bear habitats.
Given these severe ecological impacts and the potential influence of the Arctic on the climates of North America and Europe, it is important that we try to understand whether and how human-induced climate change has played a role in this event.
Our World Weather Attribution group, led by Climate Central and including researchers at the University of Melbourne, the University of Oxford and the Dutch Meteorological Service (KNMI), used three different methods to assess the role of the human climate influence on record Arctic warmth over November and December.
We used forecast temperatures and heat persistence models to predict what will happen for the rest of December. But even with 10 days still to go, it is clear that November-December 2016 will certainly be record-breakingly warm for the Arctic.
Next, I investigated whether human-caused climate change has altered the likelihood of extremely warm Arctic temperatures, using state-of-the-art climate models. By comparing climate model simulations that include human influences, such as increased greenhouse gas concentrations, with ones without these human effects, we can estimate the role of climate change in this event.
To put it simply, the record November-December temperatures in the Arctic do not happen in the simulations that leave out human-driven climate factors. In fact, even with human effects included, the models suggest that this Arctic hot spell is a 1-in-200-year event. So this is a freak event even by the standards of today’s world, which humans have warmed by roughly 1℃ on average since pre-industrial times.
But in the future, as we continue to emit greenhouse gases and further warm the planet, events like this won’t be freaks any more. If we do not reduce our greenhouse gas emissions, we estimate that by the late 2040s this event will occur on average once every two years.
Watching the trend
The group at KNMI used observational data (not a straightforward task in an area where very few observations are taken) to examine whether the probability of extreme warmth in the Arctic has changed over the past 100 years. To do this, temperatures slightly further south of the North Pole were incorporated into the analysis (to make up for the lack of data around the North Pole), and these indicated that the current Arctic heat is unprecedented in more than a century.
The observational analysis reached a similar conclusion to the model study: that a century ago this event would be extremely unlikely to occur, and now it is somewhat more likely (the observational analysis puts it at about a 1-in-50-year event).
The Oxford group used the very large ensemble of Weather@Home climate model simulations to compare Arctic heat like 2016 in the world of today with a year like 2016 without human influences. They also found a substantial human influence in this event.
All of our analysis points the finger at human-induced climate change for this event. Without it, Arctic warmth like this is extremely unlikely to occur. And while it’s still an extreme event in today’s climate, in the future it won’t be that unusual, unless we drastically curtail our greenhouse gas emissions.
As we have already seen, the consequences of more frequent extreme warmth in the future could be devastating for the animals and other species that call the Arctic home.
Geert Jan van Oldenborgh, Marc Macias-Fauria, Peter Uhe, Sjoukje Philip, Sarah Kew, David Karoly, Friederike Otto, Myles Allen and Heidi Cullen all contributed to the research on which this article is based.
You can find more details on all the analysis techniques here. Each of the methods used has been peer-reviewed, although as with the Great Barrier Reef bleaching study, we will submit a research manuscript for peer review and publication in 2017.
There is no doubt that 2016 has been a record-breaking year for Earth’s climate.
We will have to wait another couple of months for the final tally, but 2016 will be the hottest year in recorded history globally. Average temperatures are well above 1℃ warmer than a century ago.
Global average temperatures, and “global warming”, often give the impression of a gradual change in Earth’s climate occurring uniformly across the planet. This is far from the truth – particularly at the ends of the Earth. The Arctic and Antarctic are behaving very differently from the global picture.
One particular polar change that has caught the attention of scientists and the media this year has been the state of sea ice. The seasonal growth and decay of sea ice over the Arctic and Southern oceans is one of the most visible changes on Earth.
But in the past few months its seasonal progression has stalled, plunging Earth’s sea ice cover off the charts to the lowest levels on record for November. Explaining what has caused this unexpectedly dramatic downturn in sea ice is a tale of two poles.
The northern polar region is an epicentre for change in our warming world.
On average, the Arctic is warming at around twice the global average rate. This is due to several environmental processes in the Arctic that amplify the warming caused by rising atmospheric greenhouse gas levels.
One of these amplifiers is the sea ice itself.
As the climate warms, it’s no surprise that ice melts. What is less obvious is that when bright, white ice melts it is replaced with a dark surface (the ocean or land). Just as a black car parked in the sun will warm up faster than a white one, so the dark surface absorbs more heat from the sun than ice. This extra heat promotes more ice loss, and so the cycle goes.
This can explain the marked long-term decline of Arctic sea ice. But it can’t explain why the past month has seen such a sudden and dramatic change. For this we need to look to the weather.
Arctic climate is characterised by very large natural swings – so much so that in the past few weeks some regions of the Arctic have been a whopping 20℃ warmer than expected for this time of year.
The polar regions are separated from milder equatorial climates by a belt of westerly winds. In the northern hemisphere these winds are commonly referred to as the jet stream.
The strength of the jet stream is related to the north-to-south (cold-to-warm) gradient in northern hemisphere climate. The amplification of warming in the Arctic has reduced this gradient, and some scientists believe that this is allowing the northern jet stream to develop a more meandering path as it travels around the globe.
A weaving jet stream allows warm air to penetrate further northwards over the Arctic (the flip side is that extremely cold polar air can also be pulled south over the northern hemisphere continents, causing extreme cold snaps). This appears to be responsible for the current extremely warm temperatures over the Arctic Ocean, which have caused the normal advance of winter sea ice to stall.
In effect, what we are seeing in the Arctic is the combined effect of long-term climate change and an extreme short-term weather event (which itself is probably becoming more common because of climate change).
The southern story
It’s a different story when we look at the ocean-dominated southern hemisphere.
Antarctic climate records point to a delay in some of the effects of “global warming”. The reasons are still debated, partly because of the much shorter climate records that scientists have to work with in the Antarctic.
But it is likely that the expansive Southern Ocean is an important climate change dampener that is able to “hide” some of the extra heat being absorbed by our planet beneath the ocean surface where we don’t feel it – yet.
Unlike the dramatic declines in Arctic sea ice over recent decades, the sea ice that surrounds Antarctica has been increasing slightly over the past three-and-a-half decades and 2014 set records for the most extensive Antarctic sea ice on record. So the decline in Antarctic sea ice since August this year to record low levels has come as somewhat of a surprise.
Again, the weather may hold part of the answer.
The westerly winds that circle the Southern Ocean (analogous to the northern hemisphere’s jet stream) have strengthened and moved closer to Antarctica over the past few decades. One of the effects of this has been to push sea ice away from the Antarctic continent, making for a more expansive coverage across the surrounding ocean.
But the westerly winds are fickle. They are able to change their path across the Southern Ocean very quickly. And so while the southward march in their average position over many years is clear, predicting their behaviour from month to month remains difficult. This spring the westerly winds have tended to sit closer to Australia and out of reach of Antarctica’s sea ice.
What Antarctica’s sea ice will do in the future is still an open question. Climate models indicate that Antarctica won’t remain protected from global warming forever, but just if and when this might cause Antarctica’s sea ice to replicate the Arctic sea ice loss is still anyone’s guess.
Lessons in the madness
Extreme years, such as 2016, are important as they provide glimpses of what the new normal of our climate system may look like in the not-too-distant future.
But these pointers to where we are going also need to be assessed in terms of where we have come from. For sea ice, logbooks from the age of heroic exploration suggest that the Antarctic system is mostly still operating within its normal bounds.
The same cannot be said for the Arctic. The decline of sea ice there has been likened to a ball bouncing down a bumpy hill – some years it will bounce higher than others, but eventually the ball will reach the bottom.
When it does, the Arctic Ocean will be ice-free in summer. That’s a boon for shipping, but don’t expect to see any polar bears on those Arctic cruises.