Ancient records confirm Arctic warming due to man

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If Arctic warming continues at its current rate, the Arctic Ocean could have ice-free summers by 2040 or even earlier, modelling studies suggest. The last time the ocean may have had ice-free seasons was around 10,000 years ago, when the region was getting much more sunlight than today due to Earth’s orbital fluctuations. By using geological records to piece together the history of Arctic sea ice over the last 50 million years, scientists have shown that the combined magnitude and abruptness of the recent ice loss is likely higher than ever before and can’t be explained by any known natural variables.

Leonid Polyak, from the Byrd Polar Research Center of Ohio State University, US, and colleagues employed marine sediment cores and ice-core and terrestrial Arctic temperature records. Palaeoclimate proxies found in these sediments, such as ice-rafted debris, microscopic organisms, driftwood, whalebone, and plant material, indicate the presence or absence of sea ice in a particular region. Historical records and satellite data complete the picture for modern times.

The proxy records show that around 50 million years ago the Arctic was a balmy place, with summer temperatures as high as 24 °C and subtropical aquatic ferns basking in the warm waters. Then around 47 million years ago sea ice started to form, most probably encouraged by a fall in atmospheric carbon dioxide and an accompanying drop in temperatures.

Atmospheric carbon dioxide continued to decrease – caused in part by weathering of rocks as the Earth reorganised its continents – and temperatures fell. Then around 3 million years ago the carbon dioxide decline slowed and regular glacial cycles started to dominate temperature changes, driven by orbital variations which alter the amount of solar radiation reaching Earth. Since then the Earth has swung predictably from glacial to interglacial and back again, every few tens of thousands of years. Emerging data suggest that Arctic sea-ice was probably much reduced during the major interglacials.

For the last 11,000 years or so we have been enjoying a relatively warm, low-ice interglacial period, with a gentle cooling as we head towards the next glacial. “From orbital variations, we’d expect the Arctic to continue to slowly cool as it has done so for the past several thousand years, eventually slipping into a new ice age,” said Mark Serreze director of the US National Snow and Ice Data Center at the University of Colorado.

But the last 100 years have bucked the trend in a big way. “We’ve lost about 30% of the summer ice extent and as much as 85% of the multi-year ice volume since the 1970s,” Serreze told environmentalresearchweb. And this decrease can’t be explained by natural variations alone. “If you ignored our recent atmospheric carbon dioxide rise, the recent reduction in sea ice in the Arctic would look highly anomalous, because it comes at a time when orbits favour extensive sea ice,” said Richard Alley from Pennsylvania State University.

Publishing their findings in Quaternary Science Reviews, Polyak and his colleagues conclude that the recent decrease in Arctic sea ice doesn’t fit any of the natural variabilities known from existing paleoclimatic data. This conclusion implies that the most plausible trigger for this warming is rising atmospheric carbon dioxide levels coming from human activities. “Orbital variations, which are currently slowly cooling the Arctic, are still there, it’s just that climate warming due to human activities is now dominating and operating on a much shorter timescale – about 100 years – than orbital variations – [which operate over] thousands of years,” explained Serreze.

The implications of ice-free summers in the Arctic within a few decades are of great concern. Coastal erosion will likely increase and many ice-adapted species will struggle, which will inevitably affect the human inhabitants of the Arctic. Out beyond the Arctic, weather systems will alter as atmospheric circulation patterns adjust to the effect of an ice-free Arctic Ocean.

As the geological record shows, the Arctic has occasionally been ice-free in the past. However, the current speed of on-going change is exceptional. “In the past, one went from heavier ice to milder, or ice-free, conditions over the span of thousands of years,” said Serreze. “Now we are talking about doing it in 100 years, or less. Can species like polar bears adapt to such rapid change? We’ll see.”

About the author

Kate Ravilious is a contributing editor to environmentalresearchweb.

Original article here

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