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As an indicator of the impacts of climate change, Arctic sea ice is hard to beat. Scientists have observed the advance and retreat of the frozen polar ocean in this very sensitive region of the Earth over decades, to better understand the potential effects on natural systems: global ocean circulation, habitats and surrounding ecosystems, food sources, sea level, etc.
Despite efforts to have model simulations more accurately reflect actual observations of sea ice melt in the Arctic, a ditch has been dug: ground reports indicate that ice is melting much further. faster than predicted by global climate models.
"On the basis of this phenomenon, people have differing opinions," said Qinghua Ding, a UCSB climatology scientist, an assistant professor at the campus Earth Research Institute.
The consensus of the community of climate scientists, he said, is that the difference is due to wrong modeling. "It's a bit like the model was prejudiced; it is insensitive to anthropogenic forcing, "he said.
Ding and his group do not agree. In a study titled "Fingerprints of Internal Factors of Arctic Sea Ice Loss in Observations and Model Simulations," published in Nature Geoscience, the group stated that the models were perfect .
According to them, about 40-50% of the loss of sea ice over the last three decades is due to important but still poorly understood internal factors, some of which are partly as far afield as the tropics.
"In fact, we are comparing apples to oranges," said Ding about the difference between real-time observation and melting of simulated Arctic ice, caused by anthropogenic forcing.
The average of the models, he explained, only accounts for the effects resulting from historical radiative forcing (calculations based mainly on greenhouse gas levels), but does not rely, for example, on the variations in short-term sea surface temperatures, humidity, atmospheric pressure and other local factors and related to other phenomena elsewhere on Earth.
Such higher frequency events often occur as noise in individual simulation repetitions, while scientists search for long-term trends.
"Any run of a model will produce random noise," said Bradley Markle, a postdoctoral researcher at Ding's research group. "If you perform 20 or 30 model runs, they will each have their own random noise, but they will cancel one another."
The resulting value is the average of all simulations without the random variability. But this random variability can also impact what is observed on the ice, in addition to the forced signal.
Due to their nature, internal variability may also result in a slowdown or even reversal of Arctic ice melt, but overall, climatologists still see the total melting of Arctic sea ice during part of the year. .
"There are so many reasons why we are focusing on the Arctic sea ice, but one of the main concerns of people is the timing of the ice-free summer," Ding said, pointing to the moment when the North Pole will no longer be the frozen border, it was still in summer.
"At the present time, it is expected that by 20 years, the summer will be free of ice," said Ding.
More than a climatic problem, he continued, the summer without ice is also a societal problem, given its effects on fishing and other food sources, as well as on natural resources and habitats benefiting from a frozen polar ocean.
This discrepancy between simulation and observation indicates, he said, that the forecast of when this summer without ice will occur should be tempered with some recognition of the effects of internal variability.
"There is a great deal of uncertainty associated with this window of time," Ding said. "When we take into account internal variability and CO2 forcing, we should be more cautious about the timing of the ice-free summer."
For Markle, this situation highlights the lag that often occurs when long-term climate trends are discussed in relation to short-term observations.
Over our human time scales from a few hours to a few days, we see changes in the temperature of the atmosphere by several degrees. An average increase in global temperature of one or two degrees does not seem so significant.
"Similarly, the annual variability of temperature, such as that associated with these internal tropical variations, can reach several degrees of average annual temperature in a given area, so close to the same magnitude as the secular global warming signal." he declared. I said.
An example of this relatively short-term climatic variability is the well-known ENSO (El Niño Southern Oscillation), the constant shift between the weather systems of El Niño and La Niña, which brings both drought and rain, scarcity and abundance to the world.
Extremely extreme weather behavior is expected in ENSO, as the Earth's climate seeks to balance itself against an average increase in global temperature of a few degrees.
"For reference, 20,000 years ago, there was an ice cover covering most of Canada during the height of the last ice age – it was an average temperature change." annual four or five degrees, "said Markle," but that's a huge difference. "
Ding's research group continues its research into the mysterious and complex internal factors that affect Arctic sea ice, especially those from the warm, humid tropics.
"We are primarily interested in the period from the early 2000s to today, where we are witnessing such a melting," said Ian Baxter, a graduate student, who also works with Ding.
It is known, he added, that the effects of changes in the Arctic are no longer confined to the region but actually extend to mid-latitudes – often in the form of cold spells. The group is interested in how the effects in the tropics could spread beyond this region and affect the Arctic.
"We are trying to put in place a mechanism by which this happens," said Baxter.
– Sonia Fernandez for UCSB.
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