Thermohaline and Sea Ice. Tipping points become blurry,

Reaching a tipping point within the thermohaline would be manifested as a continual decrease on the strength of the overturning current. Arctic sea ice once more plays a part in the perpetuation of a slowing circulation. Not only does it drive the slowing but is also advanced by the slowing. This positive feedback loop can be manifested in many ways but most notably by a decline in ocean heat distribution as well as the previously noted increase in sea ice and Greenland ice sheet melt.

Firstly, it is important to acknowledge a noticeable increase, during the past century, in the overall temperature of the planets oceans. Given the intrinsic properties of the thermohaline circulation an overall and generalised increase will not fundamentally adjust the system.

However, our interest lies in a shift in the distribution of heat within the global oceans. As visible in Figure 2, the North Atlantic has traditionally been warmer than other areas as a result of the gulf stream carrying heat energy north. Contradictory to the idea that thermohaline is slowing as a result of climate change, the image below shows a consistent trend in North Atlantic warmth ceetaintly not weaking up intil 2005. However, it does also reinforce the general warming trend of the last century.

The last time arctic sea surface temperatures were at a similar level to today, 2016, was during the Eemian Period or the last inter-glacial. However during this period a vast melting occurred causing extreme storms and sea levels, 5 to 9 m higher than today. There is one massive difference between that period and today however, and that is carbon diodixe levels. During the last inter-glacial CO2 levels we significantly under 300ppm yet in 2016 for the first time in history CO2 levels rose above 400ppm without dipping below, and will probably not for the foreseeable future. The last inter-glacial the colder winters hindered the massive ocean temperature rise. However signifcant sea ice melt experienced at present as well as black carbon deposits covering the remaining ice sheets may well increase the temperature to an unsustainable point.

In NASA’s temperature data is embedded a polynomial trendline which points at temperature anomolies of over 4^oC by 2060. A worse case arises when analysing the polynomial trendline in the Arctic which predicts temperature anomolies of over 4°C by 2020, 6°C by 2030 and 15°C by 2050. It is in this scenario where feedback mechanisms could easily bring about abrupt climate change.

Questions posed by a slowing therrmohaline include a cooling of the Arctic as a result of slowing. Disregarding the global climate changes and potential extreme methane releases, a reduction in heat flowing north would restrict the warming of arctic waters and hence reduce ice melt, therefore restricting the decrease in salinity and the slowing of the meridional overtunring circulation. This is where the holistic nature of climate systems begin to blur the simplicity of a tipping point. The major tipping point is yet to be found but any combination of these may trigger an extreme event and abrupt change.

Update on Arctic Sea Ice and a brief introduction to the Thermohaline circulation

Sea Ice Update

November 2016 set a record low for average ice extent. This reflects this years’ unusually high temperatures, winds from the south and a warm ocean. This November’s extent was the lowest on satellite record. The temperature was so warm that for a period in November extent decreases by 50,000km2 per day as opposed to the long-term average of 69,600km2 growth (NSIDC). Ice growth was less extensive in the Kara, Barents, East Greenland and Chukchi seas and larger in the Beaufort and East Siberian as well as Baffin Bay (NSIDC). The exceptional warmth this year could signal a step beyond the tipping point only time will tell. If a similar pattern of ocean and surface warming in the Arctic continues next year, the sea ice as well as the Greenland Ice sheet could experience rapid deterioration, flooding the global climate network with changes.

Continual and current updates can be tracked here: http://nsidc.org/arcticseaicenews/.

Background to the Thermohaline

The thermohaline system is a part of the large-scale ocean circulation that re-distributes heat across the globe. It relies on temperature and salinity as drivers with the Arctic ocean and waters of Antarctica being major areas of sinking. These are supported by warm surface currents such as the Gulf Stream which transfer heat away from the equator in the Atlantic but normally upwell in the Southern Ocean (Rahmstorf, 2003). The major benefit of such an intricate system is the mixing that is allowed transforming the world’s oceans into a single global system. Although its greatest asset, this also means it is its greatest vulnerability. Failings along any point of the system can causes a shutdown of the circulation inducing rapid shifts in regional climates.

Arctic Sea Ice: Summary and Feedbacks

2007 – Average extent, 4.28 million square km. Minima, September 16^th. An unusual atmospheric pattern with high pressures over the Central Arctic Ocean and lower pressures over Siberia caused clear skies in spring and summer, intensifying the melt. However, it is worth noting NSIDC Research Scientist Julienne Stroeve noted that at this point the seasonal ice cover was thinner than usual anyway.

2012 – Average extent, 3.41 million square km of sea ice. Minima, September 13^th. In contrast to 2007 when climate conditions caused the rate of melt, the conditions in this case were much more mild. In fact, 2012 was a stark representation of the declining multi-year ice present in the Arctic Ocean. This means that the seasonal ice becomes the dominant coverage. Although the extent was larger than 2007 in some parts of the central Arctic ocean there was a notable lack of ice in the marginal seas. This is a result of warming sea surface temperatures inhibiting the formation of seasonal ice in these areas.

2016 – Average extent, 2.55 million square km. Minima, September 10^th, increased melt occurring all the way up to this point. Although the ice extent minima in 2016 was higher than that experienced in 2012, as visible in Figure 1, its current trajectory implies that the 2017 maxima will be the lowest on record. Early October daily ice growth was significantly the lowest documented, October 20^th set the new low for daily ice growth. It began to recover towards normal averages later in October however, ice extent continues to be noticeably lower. The NSIDC identified higher sea-surface temperatures as well as unusually high well into the atmosphere limiting the October growth. Lowest October area coverage on satellite record, 400,000km2 less than the previous lowest extent in 2007.

Figure 1: cesm.ucar.edu/models/ccsm4.0/

From the above data and Figure 1, courtesy of the NSIDC, it is clearly evident that sea ice cover is following a decreasing trajectory. In 2012, the NSIDC noted that the seasonal ice cover forming in spring and winter was becoming so thin that it no longer required extreme persistent weather to melt the ice as in 2007. However, the years following returned to a more expected level of cover representing the continual reduction in multi-year ice, as opposed to extreme formation drop. However, 2016 is different. 2015-2016 was the year of El Nino. This means that we should have experienced a disruption in the pattern, yet this never came. The low levels of seasonal ice coverage currently occurring the Arctic ocean will result in rapid ice cover loss come March 2017 and therefore a greater reduction in multi-year ice. It is clear then that this is not a natural occurrence and the impact we, as humans, are having on the planet is driving the climate to unchartered territory. This also provides difficulties when scientists look to predict. The extreme interconnectedness and apposing feedback cycles make determining a conclusive list of consequences is near impossible.

Now, on to the important stuff. The Arctic Sea Ice has long been identified as sensitive to global climate changes. However, more importantly, changes in the Arctic sea ice is amplified throughout the global system. The main feedback mechanism involved within the Arctic ocean is the ice albedo effect. Albedo refers the reflectivity of a surface. The white ice reflects as much as 90% of solar energy (NSIDC) therefore the approximate 60% loss in ice cover in the past decade is a significant loss in reflectivity. The increased energy warms surface temperatures and exaggerates the ice melt. This excess warmth is fed back into the global climate system increasing the global temperature. Evidence of this is provided by Screen and Simmonds (2010). Using ERA-interim they assessed the vertical profile of warming in the Arctic concluding that arctic amplification as a result of sea ice and snow cover is a key feature in the warming since 1989. They also noted that winter and autumn surface temperatures are increasing by 1.6 °C per decade between 1989 and 2008. A trend that if continued could mean the arctic ocean becomes warmer than the freezing temperature of sea water (2°C) during winter, meaning no formation.

The results of this consist of a variety of global impacts. Firstly, and the most direct is the enhanced melting of the Greenland ice sheet and Antarctic sea ice. Subsequently causing a large sea level rise permanently flooding low lying land. A secondary yet possibly extremely significant impact of this reduced ice formation is reduced salinity in the arctic ocean. The arctic ocean plays a vital role in the continuation of the thermohaline circulation however salinity is the driver of the water sink that creates Atlantic deep waters. As water freezes salt is deposited into the remaining water, increasing the salinity and causing it to sink. A slowing of the thermohaline would result in less heat being carried to Arctic waters therefore hindering ice reform. However, it will also hinder the redistribution of heat around the planet. Higher latitudes would become colder and drier with mid-latitudes becoming hotter and wetter. As well as causing more coming extreme weather events. A direct impact of rising sea temperature is the inhibiting the role of oceans as a CO2 sink. As the temperature of the water increases its ability to absorb CO2 decreases. Due to the interconnectedness of the global climate system it is easy to get carried away when discussing the impacts so I have limited the depth of the impact discussion.

The question is then; can we change our trajectory? Without the immediate cooling of our oceans, this feedback will continue to enhance. And with the Paris 2015 conference aiming to limit global temperature rise by 2°C this immediate change will come. It is therefore difficult to see is reverse this trend within the foreseeable future.