Climate Feedback

Australia: The Land Where Time Began

A biography of the Australian continent

Climatic Feedback

1 Ice-albedo feedback

2 Snowline retreat feedback

3 Water vapour feedback

4 Ice sheet melt feedback

5 Arctic river feedback

6 Black carbon feedback

7 Ocean acidification feedback

According to Wadhams the indirect effects of the retreat of sea ice in the Arctic are overwhelmingly negative for the entire Earth, not just areas surrounding the Arctic.

Positive feedbacks are the processes that are responsible for the exacerbation of the global warming towards disastrous levels for the entire planet. These feedbacks and connections are present in all parts of the climate system.

Also, an impact of ice retreat in the Arctic that had not been predicted has recently become apparent. This is the position of the jet stream, which has led to new extreme weather patterns at critical times in the agricultural areas of the Northern Hemisphere, and therefore has a large influence on the production of food in those areas.

1. Ice-albedo feedback

The albedo of open water is about 0.1, the fraction of the incident solar radiation that is reflected back directly to space. The albedo of sea ice can vary between 0.5 and 0.9, while for fresh snow on sea ice the albedo is 0.9. The albedo drops to 0.8 if there are any ridges or other angular surface on the ice, or as the snow weathers or is piled by the wind into sastrugi, undulating hummocks. The albedo drops even more in spring: whenever the temperature rises above 0^oC and there is minor melting of the superficial snow, it becomes a duller white which drops the albedo. When the snow actively melts into a soggy slush, which can also include some black carbon that was deposited over winter when it is hidden from sight by successive snow falls, the albedo drops even further. When the surface consists of bare ice that is riddled with melt pools the final limit is reached. The melt pools now preferentially absorb solar radiation as they have dark surfaces and therefore melt more deeply into the ice, giving the ice the appearance of Swiss cheese, which also has very little mechanical strength. At this stage the area-averaged albedo can be 0.5 or less, though the largest change occurs when the albedo drops to 0.1 as the ice disappears completely.

It has always been a problem to measure and model the summer albedo. The albedo of fresh snow, 0.9, is known very accurately, though it does not have much of an effect on the heat balance of the Arctic and there is not much solar radiation in winter. The highest level of solar radiation occurs in summer, a time when the average albedo must be estimated over a composite surface of melting ice and melt pools, which can change its nature within hours as its surface temperature varies. Gary Maykut and Norbert Untersteiner were the first to successfully model thermodynamics of the Arctic in 1971 (Maykut & Untersteiner, 1971). Their original model needed to use some values for summer albedo that were fairly arbitrary because a lack of accurate data. More recent studies have had a strong emphasis on careful field observations which have shown just how much variability there is. There are therefore 2 aspects associated with climate change. As well as, and exceeding it, the simple change associated with sea ice retreat from a summer surface covered with ice. The albedo dropped from 0.5 to 0.1, therefore the details of how the decline of the albedo during summer is less important than the obtaining good values for the total area of ice cover so that the area of water that has replaced what used to be ice is known.

It has been estimated in a study (Pistone, Eisenman & Ramanathan, 2014) that between the 1970s and 2012s the area of summer sea ice lost has caused a decrease in the global albedo that is equivalent to adding the warming that would result from adding 25 % of the amount of carbon dioxide that was added to the atmosphere by human activity over the same period. Wadhams describes this as a ‘fast feedback’ as its effect is immediate. The global temperatures increase because of a reduction of short-wave reflected energy leads to an extra radiative forcing on a worldwide scale. The CERES satellite measures radiation values directly, which solved the problem of measuring actual ground-based albedos all over the Arctic. The results of the study showed that between 1979 and 2011 the albedo, averaged over the Arctic and over the year, dropped from 0.52 to 0.48.

2 Snowline Retreat Feedback

Snowline retreat is caused by warm air over an ice-free Arctic. As sea ice recedes in arctic coastal lands the sea ice-albedo feedback is enhanced by spring snow melting faster, which Wadhams says is probably due to warmed air masses moving over the coastal areas from the ice-free sea. By June 2012, the time of year when the solar radiation is at its maximum, a negative area anomaly had developed that covered an area of 6 million km²‑ compared with the situation in 1980. Put another way, compared with the late 20^th century, the extent of the midsummer snow is now 6 million km² less. This is the of the same magnitude as the sea ice negative anomaly during the same period, and the albedo change between the land that is snow-covered and the tundra that is snow-free is roughly the same as the albedo change is between the sea ice and the open water. Wadhams says this means that the snowline retreat and the sea ice retreat are each adding about the same amount of global warming, though calculations for the tundra have not yet been published, as it has for sea ice, which Pistone and her colleagues have for sea ice. The overall ice/snow-albedo feedback is therefore adding not 25 % to the direct global heating effect due to the addition of CO₂; it is actually adding 50 % to the direct global heating effect, which shows how the Arctic can become a driver of, and not just a responder to, global change.

Though it is not generally realised, this if of the utmost importance: as Wadhams says when the global feedback due to the retreats of Arctic snow and ice totals 50 % of the warming resulting from the addition of atmospheric CO₂, the point has been reached it should not be said adding CO₂ to the atmosphere is warming the planet. It should be said that the CO₂ that has already been added to the atmosphere has already warmed the planet to the point where ice/snow feedback processes are actually increasing the effects by an extra 50 %. Wadhams warns that it is not far to go to the point where it is the feedback that will be driving the changing climate. At that point adding atmospheric CO₂ will no longer matter, global warming will happen regardless of what is done about CO₂ emissions, the warming will still happen, runaway warming, which is suggested to have been the process that made Venus the planet it is.

3 Water vapour feedback

Water vapour depends entirely on the change in temperature. About 7 % of the extra water vapour content of the atmosphere per degree of warming in the temperature of the air, and in turn that adds about 1.5 Watts/m² of radiative forcing, as water vapour is a greenhouse gas. The Arctic with its amplification factor is rapidly warming, while globally air temperatures have been warming slowly in the last decade, probably as a result of heat being absorbed by the deep ocean. Therefore the Arctic offers a major water vapour feedback that severely inhibits the outgoing long-wave radiation, holding the heat closer to the surface of the ice and ocean. In recent years the local Arctic temperature has increased by 3^oC and this has increased the water vapour concentration by 20 %, which adds about 4.5 W/m² to the heating of the pole over the Arctic basin. Though this heating is locally specific to the Arctic, it must be added to the total warming effect.

4 Ice sheet melt feedback and sea level rise

Until about 1980 sea level researchers believed that there were 2 factors, each of equal magnitude, contributing to global sea level rise. Sea water expands and stands higher as the sea warms as a result the transfer from the warmer atmosphere and greater downwards radiation flux from the greenhouse gas blocking the outgoing radiation. The surface water was the only water to be warmed, but now the warming is spreading to the deeper ocean layers. This is known as the steric sea level rise, in which no water is entering the ocean. A rise due to water entering the ocean from terrestrial sources were subpolar glaciers, the Alps, the Himalayas, glaciers in Alaska and Chile and Norway, and even a few low latitude high altitude glaciers such as the top of Mt Kilimanjaro, is the second factor. As glaciologists have made a point of mapping these glaciers for many decades, initially using traditional methods such sticking stakes into the glacier to measure how fast it was decaying, and more recently, satellite methods were used to measure the elevation of the surface of the ice, all of which resulted in a good knowledge of how fast the glaciers were losing mass. In the 1980s it was already the case the almost all of the world’s glaciers were retreating – as seen in dramatic pictures of Alpine glaciers which are close to disappearing. Wadhams says he visited the Columbia Icefield in the Rocky Mountains in the 1970s and in 2008. On the 1970 visit the snout of the glacier was up against the Trans-Canadian Highway, though by 2008 it required a long bus ride to reach it. At present there isn’t a glacier system in the world that isn’t in retreat. In the 1980s there were a few glaciers that were advancing and these were themselves products of global warming – warmer, wetter winds over the coastal glaciers of Norway caused them to gain mass. But now they are also in retreat.

The eustatic component of sea level rise, in which water is added to the ocean, is contributed to by glacier retreat, as well as contributions from dams and hydroelectric schemes. By now it is exceeded by run-off from the 2 great polar ice sheets of the world, Greenland and Antarctica. Wadhams warns the threat is real. The Greenland ice sheet, that is at high latitudes and has an elevation of 2-3 km, has always been frozen solidly all year round, apart from a small amount of melt around the edges. In the mid-198s meltwater began to appear on top of the ice sheet for a brief period in summer. This area of melt began increasing and the melt area expanded. In 2012 was the biggest melt so far, surface melt spread across 97 % of the surface of the ice sheet in the period 1-11 July. The modellers still didn’t worry. They expected most of the meltwater to refreeze at the end of summer which would mean that loss from the ice sheet would be small, and they were still thinking it would take several thousand years for the ice sheet to melt and dump its water into the sea, which would cause a sea level rise of 7.2 m. An unexpected phenomenon, moulins, developed. These are huge drain holes in the surface of the ice sheet, in many cases dropping water to bedrock 3 km below. Wadhams describes the draining of meltwater from the surface through these moulins as a frightening surge. The water deposits its heat at different levels on the way down through the ice sheet, in the process warming the entire ice sheet towards the melting point. Once on the bedrock the water drains along channels under the ice to the sea, and this flowing water also lubricates the lower surface of the ice sheet, especially its outlet glaciers, which allows the glaciers to flow faster to the sea. Satellite imagery was used by Eric Rignot of NASA to find that many glaciers on Greenland were flowing twice as fast as they had previously (Rignot & Kanagaratnam, 2006). Therefore they are dumping twice as much fresh water, in the form of icebergs, into the ocean. The result of this is becoming apparent in the shrinking of the ice sheet. The mass of the ice sheet can now be measured accurately by using a pair of NASA satellites, GRACE, Gravity Recovery and Climate Experiment), which measure to a very exact degree how the mass below them is changing by slight changes in gravity. It has been shown by these satellites that the ice sheet is now losing 300 km³ of water equivalent every year, and that rate is increasing and is already as much as the combined loss of all other glaciers.

There are also some other minor factors involved in the eustatic sea level rise. One of these is the transfer of fossil aquifer water in the hydrology cycle. Ground water has not been accessible to the atmosphere for many thousands of years and when it is pumped out of the underground aquafers it is used then drains off into rivers, evaporates into the atmosphere and eventually it finds its way to the ocean where it adds its mass to the rising sea level. Other manmade effects such as building increasing numbers of dams are holding back water; and it has a net negative effect as the number of dams around the world is constantly increasing.

The change in the altitude of the ice caps is another minor feedback. The overall elevation of the Greenland ice sheet is decreasing slowly, and the lower it gets the higher the surface temperature of the ice sheet goes, as temperatures are lower at higher altitudes, and this leads to more melting in summer. In itself, this causes the surface elevation to decline faster, which leads to more warming, and so on in a feedback loop. Wadhams says this effect is probably small, though could become more significant in later stages of decline of the ice sheet, which would speed its final loss.

It has been assumed until recently that the Antarctic ice sheet was in approximately in neutral mass balance, as snowfall was offsetting any melt that was occurring, especially on mountains around the coastlines of the Antarctic. Since GRACE has been applied to Antarctica it also has been shown to be definitely in retreat, though this has not yet reached the amplitude of the Greenland loss of ice (McMillan et al., 2014). According to the latest estimates the Antarctic ice sheet is losing 84 km³ per year, compared to the 300 km³ for the Greenland ice sheet. As there is much more ice in the Antarctic to melt, this is alarming as this is the equivalent of 60 m of sea level rise. It has been calculated that a particular part of the Antarctic ice sheet, the West Antarctic ice sheet in the area of the Antarctic Peninsula, is not as stable as previously believed, and following a substantial melt could break free of its base. This alone would result in a sudden lea level rise of several metres.

Wadhams suggests the IPCC has been complacent in the face of these worrying threats. In its Fourth Assessment Report (AR4), it was seriously so. The IPCC authors gave only the steric rise, as they had difficulty calculating the eustatic sea level rise, and extrapolated this to the end of the century to give a mere 30 cm rise of sea level by 2100. Though they pointed out that this was a partial figure that didn’t include the rise from glacier melt, so most non-scientists and policymakers didn’t read the small print and some very serious underestimates of sea level rise have been used by national bodies that are responsible for things such as flood defences, e.g., the city authorities of Shanghai.

5 Arctic River Feedback

The warmer temperature of the run-off of rivers flowing north into the Arctic Ocean is another feedback. As the snowline on land retreats the albedo of the land surface declines dramatically. The result of this is much greater of the northern tundra, which leads to the snow melt water running off and flowing through warmer land areas which then discharges into the continental part of the ocean that is now ice free, where it warms it even further. In turn, this accelerates the decline in albedo, which then increases the heating of the coastal zone, driving the snow line further back, which accelerates the change in temperature of the tundra, which then increases the heat of the run-off of the river, and on and on. The effect is a classic case of a positive feedback which develops itself through a sequence of stages, though the effect is probably less than many of the other feedbacks that have been discussed here.

6 Black carbon feedback

Recently it has been found there is a new feedback that turned out to be somewhat more significant than originally believed, the deposition of black carbon, soot, originating from fires in agriculture and forests, and the use of diesel fuels and industrial activity on the reflectivity and melting of snow and ice (Quinn et al., 2011). According to Wadhams it is common for glaciologists to see dirt on the surface of glaciers that has blown in from the surrounding mountains, and they had often seen it form amazing little self-contained ecosystem. In these cases a patch of dirt that had gathered on the surface of a glacier in early summer absorbs solar radiation preferentially, increasing its temperature above that of the surrounding ice, eventually melting a small hole in the ice that it sinks into. At this stage bacteria form a mat of vegetation, as the meltwater dissolves salts from the dirt to provide the nutrients. The end product is a cryoconite, a good example of how life can establish itself almost anywhere. These cryoconites can cause a glacier to have a black, green or pink tinge.

Whether or not cryoconites are present sea ice accumulates dirt during the beginning of the melt season, at which time the snow on the surface melts and the sum of all the dirt that has been deposited on the ice during winter appears as a single patch. There was a tendency for this to be either disregarded or included in the calculations of the summer albedo, until recently. When an attempt is made to isolate black carbon its global effects appear to be quite small. It has been estimated by the IPCC that the radiative forcing due to black carbon is 0.04 W/m², and observational studies have shown that since its Arctic Atmospheric concentrations appears to have dropped, that Wadhams suggests is possibly as a result of the worst of the atmospheric polluters, such as China, are starting to clean up their operations.

7 Ocean Acidification Feedback

It is known that the ocean is becoming acidic as a result of the dissolution of excessive CO₂ in the ocean where it forms carbonic acid,

CO₂ + H₂O ^←/_→ H₂CO₃

H₂CO₃ ^←/_→ H⁺ + HCO₃^-

HCO₃^- ^←/_→ H⁺ +CO₃^2-

and a complex equilibrium is set up between the various ions. The H⁺ is the acidic hydrogen ion. As more CO₂ is emitted to the atmosphere a proportion of it is absorbed into the ocean which acts as a buffer to reduce the rate of global warming. As CO₂ continues to dissolve into the ocean it takes part in the above reactions which makes the ocean more acidic, which if it continues long enough will eventually have serious consequences such as dissolving the shells of marine organisms that use calcium carbonate in their shells, especially the shells of the foraminifera (forams) which are widely distributed throughout the ocean. In an ocean with natural acidity the shells of the forams slowly sink to the sea floor when the animals die and form a type of sediment called ooze. While this system is working and is not overloaded this is a way of taking carbon dioxide that is removed from the atmosphere and takes it out of the system permanently as it is added by the burning of fossil fuels, about 41 % of this CO₂ is dissolved in the ocean. As the ocean continues to dissolve CO₂ it becomes more acidic over time and eventually reaches a point at which the foram shells dissolve as they sink and are completely dissolved before they reach the floor of the sea. The carbon released by this mechanism then remains in the Earth system. There are also animals such as pteropods that use carbon in their shells and when their shells are dissolved they are left with no defence against predators. Once acidification of the ocean progresses far enough the proportion of the atmospheric CO₂ dissolving in the ocean will begin to decline. According to the latest available estimates the proportion of CO₂ dissolving in the ocean has declined from 41 % to 40 % over the past 30 years. The diminishing cover of sea ice is affecting the acidification of the ocean by exposing the ocean surface to the atmosphere with its increasing concentration of CO₂, and in the case of oceans like the Arctic Ocean that has until recently been protected from the atmospheric CO₂ by the ice cover has not previously dissolved CO₂ from the atmosphere with the result that it enhances the CO₂ sink. For the atmospheric CO₂ this is a negative feedback, at the price of increased acidification of the Arctic. Wadhams says this is a rare case of a negative feedback, though if the extra acidification that is occurring, with the consequent loss of carbon sink, is included the feedback might be positive in the long term.

Which Feedbacks are the Most Serious

Wadhams suggests that when considering the 7 types of feedback he has listed in his book the albedo feedback that is associated with the retreat of both the snow line (snow line retreat from coastal lands surrounding the Arctic is also in part a consequence of the retreat of sea ice and warming winds) and sea ice. If the 2 albedo changes are added together and the albedo of black carbon is added to the albedo calculation, the effect described by Pistone is about doubled, adding 50 % to the radiative forcing effect of the CO₂ that is being released into the atmosphere. As Wadhams says it is equivalent to a case of delivering 2 climate-changing molecules and getting another one free.

The acceleration of the melting of the Greenland ice sheet is also associated directly with the retreat of sea ice, leading to a rise in global sea level which is accelerating and will exceed 1 m by 2100.

Sources & Further reading