NASA/Peter Griffith
Wildfires blazed along the Arctic Circle this summer, incinerating tundra, blanketing Siberian cities in smoke and capping the second extraordinary fire season in a row. By the time the fire season waned at the end of last month, the blazes had emitted a record 244 megatonnes of carbon dioxide. That’s 35% more than last year, which also set records. One culprit, scientists say, could be peatlands that are burning as the top of the world melts….Continue reading...
By: Alexandra Witze
Source: Nature
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Critics:
Arctic sea ice maintains the cool temperature of the polar regions and it has an important albedo effect on the climate. Its bright shiny surface reflects sunlight during the Arctic summer; dark ocean surface exposed by the melting ice absorbs more sunlight and becomes warmer, which increases the total ocean heat content and helps to drive further sea ice loss during the melting season, as well as potentially delaying its recovery during the polar night.
Arctic ice decline between 1979 and 2011 is estimated to have been responsible for as much radiative forcing as a quarter of CO2 emissions the same period., which is equivalent to around 10% of the cumulative CO2 increase since the start of the Industrial Revolution. When compared to the other greenhouse gases, it has had the same impact as the cumulative increase in nitrous oxide, and nearly half of the cumulative increase in methane concentrations.
The effect of Arctic sea ice decline on global warming will intensify in the future as more and more ice is lost. This feedback has been accounted for by all CMIP5 and CMIP6 models, and it is included in all warming projections they make, such as the estimated warming by 2100 under each Representative Concentration Pathway and Shared Socioeconomic Pathway.
They are also capable of resolving the second-order effects of sea ice loss, such as the effect on lapse rate feedback, the changes in water vapor concentrations and regional cloud feedbacks. In 2021, the IPCC Sixth Assessment Report said with high confidence that there is no hysteresis and no tipping point in the loss of Arctic summer sea ice. This can be explained by the increased influence of stabilizing feedback compared to the ice albedo feedback.
Specifically, thinner sea ice leads to increased heat loss in the winter, creating a negative feedback loop. This counteracts the positive ice albedo feedback. As such, sea ice would recover even from a true ice-free summer during the winter, and if the next Arctic summer is less warm, it may avoid another ice-free episode until another similarly warm year down the line. However, higher levels of global warming would delay the recovery from ice-free episodes and make them occur more often and earlier in the summer.
A 2018 paper estimated that an ice-free September would occur once in every 40 years under a global warming of 1.5 degrees Celsius, but once in every 8 years under 2 degrees and once in every 1.5 years under 3 degrees. Very high levels of global warming could eventually prevent Arctic sea ice from reforming during the Arctic winter. This is known as an ice-free winter, and it ultimately amounts to a total of loss of Arctic ice throughout the year.
A 2022 assessment found that unlike an ice-free summer, it may represent an irreversible tipping point. It estimated that it is most likely to occur at around 6.3 degrees Celsius, though it could potentially occur as early as 4.5 °C or as late as 8.7 °C. Relative to today’s climate, an ice-free winter would add 0.6 degrees, with a regional warming between 0.6 and 1.2 degrees.
Arctic amplification and its acceleration is strongly tied to declining Arctic sea ice: modelling studies show that strong Arctic amplification only occurs during the months when significant sea ice loss occurs, and that it largely disappears when the simulated ice cover is held fixed.
Conversely, the high stability of ice cover in Antarctica, where the thickness of the East Antarctic ice sheet allows it to rise nearly 4 km (2.5 mi) above the sea level, means that this continent has not experienced any net warming over the past seven decades: ice loss in the Antarctic and its contribution to sea level rise is instead driven entirely by the warming of the Southern Ocean, which had absorbed 35–43% of the total heat taken up by all oceans between 1970 and 2017.
Since the early 2000s, climate models have consistently identified that global warming will gradually push jet streams poleward. In 2008, this was confirmed by observational evidence, which proved that from 1979 to 2001, the northern jet stream moved northward at an average rate of 2.01 kilometres (1.25 mi) per year, with a similar trend in the Southern Hemisphere jet stream.
Climate scientists have hypothesized that the jet stream will also gradually weaken as a result of global warming. Trends such as Arctic sea ice decline, reduced snow cover, evapotranspiration patterns, and other weather anomalies have caused the Arctic to heat up faster than other parts of the globe, in what is known as the Arctic amplification.
In 2021-2022, it was found that since 1979, the warming within the Arctic Circle has been nearly four times faster than the global average, and some hotspots in the Barents Sea area warmed up to seven times faster than the global average.While the Arctic remains one of the coldest places on Earth today, the temperature gradient between it and the warmer parts of the globe will continue to diminish with every decade of global warming as the result of this amplification.
If this gradient has a strong influence on the jet stream, then it will eventually become weaker and more variable in its course, which would allow more cold air from the polar vortex to leak mid-latitudes and slow the progression of Rossby Waves, leading to more persistent and more extreme weather.
Barents Sea is the fastest-warming part of the Arctic, and some assessments now treat Barents sea ice as a separate tipping point from the rest of the Arctic sea ice, suggesting that it could permanently disappear once the global warming exceeds 1.5 degrees. This rapid warming also makes it easier to detect any potential connections between the state of sea ice and weather conditions elsewhere than in any other area.
The first study proposing a connection between floating ice decline in the Barents Sea and the neighbouring Kara Sea and more intense winters in Europe was published in 2010, and there has been extensive research into this subject since then. For instance, a 2019 paper holds BKS ice decline responsible for 44% of the 1995–2014 central Eurasian cooling trend, far more than indicated by the models, while another study from that year suggests that the decline in BKS ice reduces snow cover in the North Eurasia but increases it in central Europe.
There are also potential links to summer precipitation: a connection has been proposed between the reduced BKS ice extent in November–December and greater June rainfall over South China. One paper even identified a connection between Kara Sea ice extent and the ice cover of Lake Qinghai on the Tibetan Plateau. However, BKS ice research is often subject to the same uncertainty as the broader research into Arctic amplification/whole-Arctic sea ice loss and the jet stream, and is often challenged by the same data.
Nevertheless, the most recent research still finds connections which are statistically robust, yet non-linear in nature: two separate studies published in 2021 indicate that while autumn BKS ice loss results in cooler Eurasian winters, ice loss during winter makes Eurasian winters warmer: as BKS ice loss accelerates, the risk of more severe Eurasian winter extremes diminishes while heatwave risk in the spring and summer is magnified.
In 2019, it was proposed that the reduced sea ice around Greenland in autumn affects snow cover during the Eurasian winter, and this intensifies Korean summer monsoon, and indirectly affects the Indian summer monsoon. 2021 research suggested that autumn ice loss in the East Siberian Sea, Chukchi Sea and Beaufort Sea can affect spring Eurasian temperature.
Autumn sea ice decline of one standard deviation in that region would reduce mean spring temperature over central Russia by nearly 0.8 °C, while increasing the probability of cold anomalies by nearly a third. Cracks in sea ice can expose the food chain to greater amounts of atmospheric mercury. A 2015 study concluded that Arctic sea ice decline accelerates methane emissions from the Arctic tundra.
With the emissions for 2005-2010 being around 1.7 million tonnes higher than they would have been with the sea ice at 1981–1990 levels. One of the researchers noted, “The expectation is that with further sea ice decline, temperatures in the Arctic will continue to rise, and so will methane emissions from northern wetlands.”
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