New research published in Nature Climate Change estimates to what extent changes in atmospheric circulation explain the observed sea-ice loss of the past few decades.
Prof. Chris Rapley, Professor of Climate Science at University College London, said:
“Over the last 3 decades satellite instruments have measured a ~3% per decade loss of summer minimum sea ice extent in the Arctic. Models have consistently underestimated the loss.
“The new paper by Ding et al offers an explanation of the discrepancy. The authors provide evidence of a process by which decadal timescale natural variability of the Arctic atmospheric circulation may have contributed as much as 30-50% of the decline.
“Even so, the systematic component of loss due to human-induced climate change remains robust and significant. The possibility that the atmospheric variability in the Arctic is ‘teleconnected’ to changes in the tropical Pacific illustrates the complexity of the global coupled ocean-atmosphere-ice system and its capacity to deliver ‘surprises’. This underscores the case for responsible and prudent risk management in reducing human carbon emissions.
“Scientific uncertainty cuts both ways, and the new result demonstrates how the combination of systematic change and natural variability can result in greater impacts than anticipated from models.”
Dr Twila Moon, Lecturer in Cryospheric Sciences at the University of Bristol, said:
“This well-designed study provides the best detail yet to determine how much Arctic sea ice decline is caused by humans and how much is natural environmental change.
“Realising that humans have caused 50-70% of the decline is not good news. Continuing to put carbon dioxide and other emissions into the atmosphere is having a direct negative influence on the Arctic, including sea ice. This study is yet more evidence that serious and detrimental changes are happening now and are caused by our emissions habits.”
Prof. Andrew Shepherd, Director of the Centre for Polar Observation and Modelling at the University of Leeds, said:
“According to this new research, the dramatic decline in Arctic sea ice that we have witnessed over recent decades is primarily due to anthropogenic (man-made) climate warming.
“Although this finding may not come as a surprise, being able to separate this from the effects of natural climate variability is an important step forwards, and paves the way for an improved understanding of what we should expect in future decades.”
Dr Amber Leeson, Lecturer in Glaciology & Environmental Data Science at Lancaster University, said:
“While we tend to think of climate variability in terms of year-to-year or month-to-month fluctuations, this work shows that decade-decade changes are also important when it comes to Arctic sea-ice.
“This research helps explain why predictions of sea ice change made by climate scientists have traditionally underestimated the rate of ice loss over this period.
“But this absolutely does not mean that CO2 isn’t causing sea ice decline – or indeed, climate change. Multiple strands of evidence show that much of the Arctic sea ice decline can be attributed to human influence on the environment, in fact this study suggests that this figure could be as high as 60%.
“By partitioning the proportion of sea ice decline which can be attributed to natural and human sources, this study improves our understanding of past, present, and potentially even future sea ice extent.”
Ed Blockley, Polar Climate Manager at the Met Office Hadley Centre, said:
“It has long been known that the observed decline in Arctic sea ice, caused by global warming, is also being enhanced by the influence of natural variability in the climate system.
“This study by Ding and co-authors attempts to quantify the importance of natural variability by estimating its contribution to the observed long-term decline of summertime sea ice. They estimate that up to half of the observed decline could potentially be caused by natural variability enhancing the existing long-term decline in Arctic sea ice caused by increasing levels or carbon dioxide.
“Better knowledge of natural variability is very important for understanding changes in Arctic sea ice because natural variability could act to either reduce or to enhance the anthropogenic decline.”
Dr Ed Hawkins, Climate research scientist at the University of Reading, said:
“Recent summer Arctic sea ice extents have all been amongst the lowest on record but this is not necessarily all due to warming global temperatures – part of the sea ice decline is also because of changes in the atmospheric circulation.
“It is challenging to determine how much of the change in the circulation is itself due to warming temperatures, but this study suggests that a substantial fraction is due to natural fluctuations.
“Looking ahead, it is still a matter of when, rather than if, the Arctic will become ice-free in summer, but we expect to see periods where the ice melts rapidly and other times where it retreats less fast.”
Prof. Jeffrey Kargel, Glaciologist at the University of Arizona, said:
“This new work by Qinghua Ding and others – involving an analysis of the observational record of sea ice and numerical model testing to root out the causes of long-term decline and yearly variations in Arctic sea ice – does well to explain links between long-term weather in the Arctic and year-to-year variations in sea ice on the same time frame. In other words, weather and sea ice melting and sea ice extent are connected, and Arctic weather is connected to weather elsewhere in the world, even as far away as the tropical Pacific Ocean.
“The already well-established, if imperfectly known, many-decades-long climatic connections to human root causes of the decline in sea ice are not this paper’s focus. Even so, these authors’ and others’ work shows that burning of fossil fuels is having a large direct impact in contributing to rapid sea ice declines. This paper does well to explore the effects on Arctic sea ice of year-to-year variations in Arctic and global long-term weather. The paper does less well to explore how the ‘weather’ part of the variations are also connected indirectly– partially– to rising greenhouse gas abundances.
“If all this talk about what fraction of sea ice decline is due to this or that cause, I’ll offer this analogy. Just think about what kills any big group of HIV/AIDS patients: is it tuberculosis or hepatitis infections, or an accumulation of numerous common colds that further weakens immune defenses, or lack of sleep and depression that leads to other things that does the killing? The answer is that it is everything together. Take any other complex, oscillating and trending phenomenon within a complex system – the stock market or unemployment rate for instance. It is never one thing that causes a recession or a bull market or accounts for the day-to-day swings in the stock markets. The same is true of our interconnected climate-ocean system: you cannot change one part without impacting the others. Even sea ice-affecting changes in weather and ocean circulation as distant as the tropical Pacific Ocean are impacted by global warming and anthropogenic changes in greenhouse gases due to fossil fuel burning. Everything is interconnected, and some of these ‘teleconnections’ are relayed through the weather system. Part of the huge climatic changes affecting all of the Earth is due to processes that happened long before humans came on the scene. It does not mean that what is happening to Arctic sea ice is all normal and healthy and unperturbed by the drastic impacts humans are having on climate and weather by means of changes to the atmosphere.
“We know that humans are impacting Earth’s climate in drastic and practically important ways that affect every human being. We have known also – and now know a little more thanks to this new work – that annual scale weather is a part of the story of Arctic sea ice decline and variability.”
* ‘Influence of high-latitude atmospheric circulation changes on summertime Arctic sea ice’ by Qinghua Doing et al. will be published in Nature Climate Change on Monday 13th March.