Accelerating the Heat: Arctic Amplification

As I briefly mentioned in my previous post, the Arctic region has been considered a "canary in the coal mine", serving as an example of the real-time effects of climate change. Warming is happening twice as fast compared to other regions of the planet, a phenomenon dubbed "Arctic amplification" (Miller et al. 2010; Screen and Simmonds 2010). Perhaps even more alarming is that models have been underestimating the rate at which these changes are taking place, indicating that there are feedback mechanisms in place that are stronger than initially thought.

In this post, I want to explain the scientific processes underlying Arctic amplification. As I discussed in my last post, changes in the Arctic may have far-reaching effects, and even exacerbate global warming. Some authors have even gone as far as identifying the region as a so-called "tipping point" (Duarte et al. 2012) - all the more reason to understand what is going on, and what we might expect.

Key to the process of Arctic Amplification is the fact that the Arctic is a sea, covered by ice, surrounded by land. Antarctica, oppositely, is a continent surrounded by ocean. The ice plays a central role in autumn, when the air starts to cool, but the water is still relatively warm. By shielding the lower parts of the atmosphere, the ice actually supports further cooling. Without the ice, the water keeps putting in heat in the atmosphere, keeping it warmer and delaying sea ice growth in winter. Any ice that does form is less extensive, and thinner, making it more susceptible to melt the following summer.

Furthermore, the ice has a much higher albedo than sea ice, meaning that it reflects more of the incoming solar radiation (and therefore, heat). It thereby reduces the heat that enters the system to begin with. Without the ice, the ocean absorbs more heat, becoming warmer, leading to warmer ocean and atmosphere in fall. Not to mention that more open patches mean that there is more interaction between ocean and atmosphere, both strengthening the feedback process I described in the paragraph above.

To visualise these processes, I drew a conceptual diagram, to give an indication of the interactions and feedback loops. It is a (very) simplified overview, in which I try to highlight some of the processes described in Serreze and Barry (2011) - any mistakes, errors and/or omissions are entirely my own. It only serves here to outline the most basic effects taking place - in reality, there are also other processes occurring that may effect sea ice extent and melt.

An overview of the most important processes in the ice-feedback systems in the Arctic. The most important effects are circled in red. Each feature has been moved to the appropriate season, except for 'Warmer Ocean' and 'More Open Water', which are of importance throughout the year (after Serreze & Barry 2011). 

As can be seen in the diagram, there are some processes with multiple effects, some that may have direct and indirect effects, and some that only effect other processes in later seasons. For example, a longer summer season means more ice melts overall, but it also means that the ocean has a longer time to absorb heat (and thus absorbs more heat in total), which leads to a warmer ocean, which leads to more ice melt. A warmer ocean also means that ice formed in winter will be thinner, which makes the ice more susceptible to melt the next year.

The recent observations of Arctic warming, declining summer sea ice extent, and a reduction in multi-year ice (which is typically thicker than first-year ice) indicate that these processes are taking place in realtime, and that we should not underestimate global warming - it is taking place before our eyes.

Further Reading

• "The Melting North" - An article in The Economist from this summer, about the current melting in the Arctic and its implications

References

• Duarte, C.M., Lenton, T.M., Wadhams, P., and Wassmann, P., 2012. Abrupt climate change in the Arctic. Nature Climate Change, 2, 60-62.
• Miller, G.H., Alley, R.B., Brigham-Grette, J., Fitzpatrick, J.J., Polyak, L., Serreze, M.C., and White, J.W.C., 2010. Arctic amplification: can the past constrain the future? Quaternary Science Reviews, 29, 1779-1790. 
• Screen, J.A., and Simmonds, I., 2010. The central role of diminishing sea ice in recent Arctic temperature amplification. Nature, 464, 1334-1337. 
• Serreze, M.C., and Barry, R.G., 2011. Processes and impacts of Arctic amplification: A research synthesis. Global and Planetary Change, 77, 85-96. 

Global Warming, Colder Winters: Arctic Sea Ice Melt and Cold Spells in Europe

It has been quiet for a while but I'm back after a load of coarse work. In my last post I mentioned geo-engineering, but I wanted to use this first post of 2013 to discuss the recent cold spell in Western Europe, North America and northern Asia. The icy conditions have led some people to dismiss current climate change ("if it's getting colder in winter, we can't possibly cause global warming!").

Climate change is a complicated issue to study for scientists precisely because the effects are not the same everywhere. It is a bit rash to conclude that global warming is a hoax based on a week of snow, just as one swallow doesn't make summer. Moreover, research suggests that global warming may actually cause colder winters in Western Europe, and other continents in the Northern Hemisphere, and it's got everything to do with the Arctic, and Arctic sea ice melt. Which ties nicely in with some earlier topics, and I thought it would therefore be interesting to discuss.

It does seem counterintuitive, a warm Arctic and colder continents. For clarity's sake, with colder continents, I mean relatively colder continents compared to the current average temperatures, and a warmer Arctic is an Arctic with higher than average temperatures there, not that Europe would become warmer than the Arctic. The crux is the anomaly, the difference between the present state and the average state over a longer period of time. 

Temperature anomalies for the Arctic region. The period of 2001-2010 is compared to the period of 1971-2000, the difference between them is the anomaly. As can be seen, warming in the Arctic is happening faster than in the surrounding regions. 

From: Overland et al. 2011. 

It is important to note that the Arctic, at the present, is the fastest warming region on the planet, and has often been dubbed the canary in the coal mine when it comes to real time observations of climate change (also see the figure above).

The Arctic is warming so quickly due to a process called Arctic amplification; something I will go into further detail in during a later post, but is somewhat out of the scope of this one. For now, just know that the warming is mainly caused by the enhanced albedo-feedbacks from melting sea ice (see this post for a quick refresher on those processes). The more sea ice melts in summer, the more the Arctic warms, leading to more sea ice melt, etc. 

So how does that tie in with colder winters in, say, Europe? Normally, cold polar air stays in the Arctic, trapped by strong polar vortex winds. This is called a positive Arctic Oscillation (AO). The polar vortex is a circulation pattern high in the atmosphere over the polar regions. 

Normally, the polar vortex is strong during winter, and the AO is positive. This means that there is low pressure over the Arctic region. As air moves from high pressure areas to lower pressure areas, the air flows toward the Arctic, and Europe and North America have mild winter temperatures. Strong winds circle the vortex from west to east and trap the cold polar air (see the left part of the figure below). If the polar vortex is weak, so the AO is negative, the pressure in the polar region is higher, the jet stream winds weaker and cold air is able to move to the surrounding continents, causing a sudden cold spell. The opposite can also occur, with warm air extending northward (see the right part of the figure).

A schematic representation of what happens when the polar vortex is strong (positive phase, left) and weak (negative phase, right). As can be seen in the right part, the jet stream meanders far more when the polar vortex is weaker, leading to colder air extending further south and warmer air going up north (Source: Wikipedia)

Melting sea ice in summer increases the temperature of the upper ocean layers, as the water absorbs the incoming solar heat. When the sun goes down in autumn, the air starts to cool, but the ocean is still warm, and releases its heat to the air. This increases atmospheric pressure over the pole, making it more likely for the polar vortex to be weak in the following winter, leading to cold spells on the surrounding continents (Greene & Monger 2012).

Even so, there is not a one on one relation between weaker polar vortexes and cold winters - the interaction between pressure gradients in the polar regions and in lower latitudes is complicated and much research is being done in understanding how the polar vortex might affect Europe, Asia and North America. Other weather indexes such as the El Niño system in the Southern Hemisphere also affect meteorological conditions around the globe (Greene & Monger 2012).

It is an interesting phenomenon though, and an example of the complex dynamics of the climate system. It also shows how sea ice melt in the Arctic affects weather here, and is another reason why it is important to study the Arctic. By being able to make better predictions of winter weather, society can prepare better for extreme conditions, minimising the surprise factor (Greene & Monger 2012). 

Next time, I'll write a quick post on Arctic amplification to explain some of the feedbacks that have caused the Arctic to warm so dramatically, and have astonished scientists (yes, astonished!).

Further Reading

• "Colder winters may be new normal due to melting Arctic Ice" - Article about the relation between sea ice and colder winters via Ars Technica. 
• "Arctic Oscillation" - A short summary explaining Arctic Oscillation via the website of the National Snow and Ice Data Center (NSIDC). 
• "Polar vortex impact on winter weather" - Explanation of the polar vortex, with animation, by Atmosphere and Environment Research. 

References

• Greene, C.H., and B.C. Monger, 2012. An Arctic wild card in the weather. Oceanography, 25, 5-7, http://dx.doi.org10.5670/oceanog.2012.58
• Overland, J.E., Wood, K.R., and Wang, M., 2011. Warm Arctic - cold continents: climate impacts of the newly open Arctic Sea. Polar Research, 30, 15787, doi:10.3402/polar.v30i0.15787