Simulation versus observation EurekAlert! Science News

(Santa Barbara, California) – As an indicator of the effects of climate change, Arctic sea ice is hard to beat. Scientists have observed the frozen polar ocean and retreated to this most sensitive part of the earth for decades to understand the potential ripple effects on various natural systems: global ocean circulation, surrounding habitats and ecosystems, food sources, sea levels and more.

Despite attempts to bring model simulations closer to the actual observations of the melting ocean, a hole in the air is melting: reports on the ground indicate that the ice is melting much faster than predicted by global climate models.

"Based on this phenomenon, people have different opinions," said UC Santa Barbara climate scientist Qinghua Ding, an assistant professor at the Earth Research Institute on campus. The consensus of the climate-scientific community, he says, tends towards the idea that the discrepancy is due to faulty modeling. "It is something like the model has a bias, it has a low sensitivity to anthropogenic forcing," he explained.

Ding and his group do not agree with that. In a study entitled "Fingerprints of internal factors of Arctic sea ice loss in observations and model simulations", published in the journal Nature Geoscience, the group says that the models are fine. Approximately 40 to 50 percent of the loss of sea ice in the past three decades, they argue, is due to important but hitherto little understood internal factors – including effects that are partly as far away as the tropics.

"We actually compare apples with oranges," Ding said about the discrepancy between real-time observation and simulated polar ice melt, driven by anthropogenic forcing. The average of the models, he explained, only takes into account which effects are the result of historical radiation forcing – calculations mainly based on the level of greenhouse gases – but do not rely on, for example, the short-term variations in the sea surface temperatures, humidity, atmospheric pressure and other factors, both local and connected with other phenomena elsewhere on earth. Such events with a higher frequency often appear as noise in the repeated, individual runs of the simulations, because scientists are looking for general long-term trends.

"Every run of a model will have random noise," says Bradley Markle, a postdoctoral researcher in Ding's research group. "If you take 20 or 30 runs of a model, they each have their own random noise, but they will cancel each other out." The resulting value is the average of all simulation runs without the random variability. But that random variability can also affect what is observed on the ice, in addition to the forced signal.

Because of their nature, internal variations will probably also result in periods when the melting of polar ice seems to slow down or even reverse, but in the larger picture climate scientists still see the eventual complete melting of Arctic sea ice during part of the year.

"There are so many reasons that we focus on Arctic sea ice, but one of the most important things that people really care about is the timing of the ice-free summer," said Ding, referring to a time when the North Pole no longer has the frozen border. it has even been in the summer.

"At the moment the prediction is that we will see an ice-free summer in 20 years," Ding said. More than just a climate problem, he continued, the ice-free summer is also a social problem, given the effects on fisheries and other food sources, as well as natural resources and habitats that benefit from a frozen polar ocean. One of the things that indicates this discrepancy between simulation and observation, he said, is that predictions about when this ice-free summer takes place will have to be tempered with some recognition of the effects of internal variabilities.

"There is a great uncertainty associated with this time window," Ding said. "If we take into account internal variables, plus CO2 imperative, we should be more careful about the timing of the ice-free summer. "

For Markle, this situation emphasizes the broken connection that often occurs when we talk about long-term trends of the climate versus short-term observations. In the course of our human time scales from hours to days, we experience atmospheric temperature changes over several degrees, so an average global temperature rise of one or two degrees does not seem so important.

"Similarly, the year-to-year temperature variability, such as that associated with these tropical internal variations, in degrees in the annual average temperature in a specific area can be about the same as the centuries-long greenhouse effect," he said.

An example of this relatively short climate variability is the well-known El Niño Southern Oscillation (ENSO), the constant transition between the El Niño and counterpart of La Niña weather systems that cover drought as well as rain, scarcity and abundance to different parts of the world. More extreme ENSO driven weather behavior is expected if the earth's climate balances in the light of an average global temperature rise of even a few degrees.

"For reference only, 20,000 years ago there was an ice cap across most of Canada during the peak of the last ice age – that was an annual average temperature change of four or five degrees," Markle said, "but it's a huge difference . "

The research group of Ding continues to investigate the mysterious and complex internal factors that influence Arctic sea ice, especially those from the warm, wet tropics.

"We are particularly interested in the period from the beginning of the years 2000 to the present, where we see such a strong melting", said graduate student Ian Baxter, who also works with Ding. It is well known, he added, that the effects of changes in the Arctic are no longer limited to the region and even spread to the middle latitudes – often in the form of outbreaks of cold weather. The group is interested in how the effects in the tropics can spread outside that region and influence the Arctic.


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