‘Beyond the extreme’: Scientists marvel at ‘increasingly non-natural’ Arctic warmth

The Arctic is so warm and has been this warm for so long that scientists are struggling to explain it and are in disbelief.  Temperatures are far warmer than ever observed in modern records, and sea ice extent keeps setting record lows.

“[A]fter studying the Arctic and its climate for three and a half decades, I have concluded that what has happened over the last year goes beyond even the extreme,” wrote Mark Serreze, director of the National Snow and Ice Data Center in Boulder, Colo., in an essay for Earth magazine.

At the North Pole, the mercury has rocketed to near the melting point twice since November, and another huge flux of warmth is projected by models next week. Their simulations predict some places in the high Arctic will rise over 50 degrees above normal.

The number of freezing degree days is far lower than any other period on record. Eric Holthaus, a meteorologist and science writer who first posted the chart to Twitter, remarked it illustrated a “stunning lack of freezing power” over the Arctic. “This is happening now,” he added. “Not in 50 or 100 years — now.

”Walter Meier, a research scientist at NASA’s Goddard Space Flight Center and an expert on Arctic sea ice, explained that the recent lack of sea ice and warmer-than-normal ocean temperatures have “kept air temperatures higher than normal, especially in the Barents and Kara Seas” between Greenland and Siberia. “A series of storms tracking from the south has led to repeated influx of warm air and ocean waters into the region,” he said. Such an influx is predicted next week.

We do know that 2017 will almost certainly start with the weakest, thinnest, smallest arctic ice pack in recorded history. So we are one step closer to living with an ice-free arctic in the summer, and probably sooner than we think.”


Arctic ice melt could trigger uncontrollable climate change at global level

Arctic scientists have warned that the increasingly rapid melting of the ice cap risks triggering 19 “tipping points” in the region that could have catastrophic consequences around the globe.

The Arctic Resilience Report found that the effects of Arctic warming could be felt as far away as the Indian Ocean, in a stark warning that changes in the region could cause uncontrollable climate change at a global level.

Temperatures in the Arctic are currently about 20C above what would be expected for the time of year, which scientists describe as “off the charts”. Sea ice is at the lowest extent ever recorded for the time of year.

“The warning signals are getting louder,” said Marcus Carson of the Stockholm Environment Institute and one of the lead authors of the report. “[These developments] also make the potential for triggering [tipping points] and feedback loops much larger.”

Climate tipping points occur when a natural system, such as the polar ice cap, undergoes sudden or overwhelming change that has a profound effect on surrounding ecosystems, often irreversible.

In the Arctic, the tipping points identified in the new report, published on Friday, include: growth in vegetation on tundra, which replaces reflective snow and ice with darker vegetation, thus absorbing more heat; higher releases of methane, a potent greenhouse gas, from the tundra as it warms; shifts in snow distribution that warm the ocean, resulting in altered climate patterns as far away as Asia, where the monsoon could be effected; and the collapse of some key Arctic fisheries, with knock-on effects on ocean ecosystems around the globe.

The research, compiled by 11 organizations including the Arctic Council and six universities, comes at a critical time, not only because of the current Arctic temperature rises but in political terms.

Aides to the US president-elect, Donald Trump, this week unveiled plans to remove the budget for climate change science currently used by NASA and other US federal agencies for projects such as examining Arctic changes.

“That would be a huge mistake,” said Carson, noting that much more research needs to be done on polar tipping points before we can understand the true dangers, let alone hope to tackle them. “It would be like ripping out the airplane’s cockpit instruments while you are in mid-flight.”

He added: “These are very serious problems, very serious changes are happening, but they are still poorly understood. We need more research to understand them. A lot of the major science is done by the US.”

Scientists have speculated for some years that so-called feedback mechanisms – by which the warming of one area or type of landscape has knock-on effects for whole ecosystems – could suddenly take hold and change the dynamics of Arctic ice melting from a relatively slow to a fast-moving phenomenon with unpredictable and potentially irreversible consequences for global warming. For instance, when sea ice shrinks it leaves areas of dark ocean that absorb more heat than the reflective ice, which in turn causes further shrinkage, and so on in a spiral.

The Arctic ice cap helps to cool sea and air temperatures, by reflecting much of the sun’s radiation back into space, and acting as a global cooler when winds and ocean currents swirl over and under it. It has long been known to play a key part of the global climate system, but the difficulty and expense of close monitoring have meant that scientists have only in recent years been able to make detailed assessments.

The report, billed as the first comprehensive study of ecosystems and societies in the region, found: “The potential effects of Arctic regime shifts [or tipping points] on the rest of the world are substantial.


'Extraordinarily hot' Arctic temperatures alarm scientists

The Arctic is experiencing extraordinarily hot sea surface and air temperatures, which are stopping ice forming and could lead to record lows of sea ice at the north pole next year, according to scientists.

Danish and US researchers monitoring satellites and Arctic weather stations are surprised and alarmed by air temperatures peaking at what they say is an unheard-of 20C higher than normal for the time of year. In addition, sea temperatures averaging nearly 4C higher than usual in October and November.

This is unprecedented for November,” said research professor Jennifer Francis of Rutgers university.

Temperatures have been a few degrees [b]above[/b] freezing when -25C should be expected, according to Francis. “These temperatures are literally off the charts for where they should be at this time of year. It is pretty shocking. The Arctic has been breaking records all year. It is exciting but also scary,” she said.

Francis said the near-record low sea ice extent this summer had led to a warmer than usual autumn. That in turn had reduced the temperature difference between the Arctic and mid-latitudes.

“This helped make the jet stream wavier and allowed more heat and moisture to be driven into Arctic latitudes and perpetuate the warmth. It’s a vicious circle,” she added.

This week it has been at the lowest extent ever recorded for late November. According to the US government’s National Snow and Ice Data Centre, (NSIDC), around 2m square kilometres less ice has formed since September than average.

Rasmus Tonboe, a sea ice remote sensing expert at the Danish Meteorological Institute in Copenhagen, said: “Sea surface temperatures in the Kara and Barents seas are much warmer than usual. That makes it very difficult for sea ice to freeze.

“What we are seeing is both surprising and alarming. This is faster than the models. It is alarming because it has consequences.”

The speed at which this is happening surprises me. In the Arctic the trend has been clear for years, but the speed at which it is happening is faster than anyone thought,” said Strove. "This is all headed in the same direction and picking up speed.”


The End of Winter

"So in a matter of 1 to 2 years, there's a very large change out over the ocean, and as a I said, the only thing we can think of that happens that fast is a change in the sea ice extent ..."

Abrupt Climate Change

From Wipneus:

Guess which line is 2016. The second max peak around November is nearly completely gone. Total Terra Incognito.

From Tealight

Currently 2016 is around 2.8 million km2 below the previous record low.

The Arctic Ice growth has nearly stopped. We are approaching an extent 2 million km2 less than the next lowest for this date. This is unprecedented.

Combined polar ice is at a record low.

We are looking at a Blue Ocean event in 2017 as the Arctic becomes virtually ice free.

We have crossed a tipping point.

Holloween Heat Wave

Daily high and high min temperature records for the U.S. were broken at an alarming rate over the past week, producing a Halloween heatwave.

Over the past week alone, nearly 300 daytime high marks were broken. But the measure of record-high minimum temperatures — a key indicator of human-forced warming — is off the charts with 639 total records smashed over the past seven days.

Even more noteworthy than the degree of warmth is the lack of widespread autumn chill. For example, Minneapolis has yet to dip below 36°F as of Friday, October 28. That doesn’t look likely to happen before at least next weekend (November 5 – 6). In records going back to 1873, the latest Minneapolis has ever gone before seeing its first 35°F of the autumn is November 1, way back in 1931. The city’s latest first freeze was on Nov. 7, 1900.

In the Arctic, the ocean has been loaded up with a ridiculous amount of heat. This heat is preventing the ocean from refreezing, creating various regional barriers to ice formation as the waters ventilate this excess heat into the atmosphere. As a result, Arctic sea-ice extent record lows continue to deepen.

Yesterday, those temperatures exceeded the 6-C-above-normal mark. And later this week, temperatures for the region could approach 6.3 to 6.5 C above average.

These are the average departure ranges for the entire area above the Arctic Circle. Localities within that broader region are hitting as much as 20 C (36 Fahrenheit) or more above average on an almost daily basis, bringing temperatures more typical of the Arctic during late summer than in the middle of fall.

In Barrow, Alaska, Jonathan Erdman reports that Saturday saw the proverbial mercury hit 41 F. This temperature, at about 26 degrees above average, smashed the previous daily high and pushed the latest day Barrow has ever seen a reading above 40 F fully one week forward.

The combination of significant sea ice losses in the north and second-lowest sea ice extents in the south has resulted in a global sea-ice measure that is well below anything seen in the past for this time of year. It is also one of the largest global negative sea-ice departures seen for any part of the record for any time of year — even when compared to the extreme period of Arctic sea ice loss during September of 2012.

Combined Global Sea Ice Departure Chart

In addition to producing heatwaves, new temperature records, and ever more extreme sea ice melt, the odd Halloween warmth appears to also be generating flashes of surface melt over parts of northeastern Greenland. There, over the past few days, temperatures have exceeded the freezing point as warm winds have blown in from the heating Greenland Strait.

This heat has been enough to trip NSIDC’s Greenland melt indicators for the region of the Zachariæ Isstrøm glacier. These indicators, over the past couple of days, have shown relatively extensive melt in this sector of Greenland.

Typically isolated by sea ice from warm ocean breezes, northeast Greenland does not usually see such long-lasting periods of surface melt. This is especially true for late October as melt during this time for any portion of the Greenland Ice Sheet is practically unheard of.


Arctic methane gas emission 'significantly increased since 2014' - major new research

New expedition in Laptev Sea suggests increase in the rate of underwater permafrost degradation.

'The area of spread of methane mega-emissions has significantly increased in comparison with the data obtained in the period from 2011 to 2014,' Semiletov said. 'These observations may indicate that the rate of degradation of underwater permafrost has increased.'

Five years ago Semiletov reported:

'We found more than 100 fountains, some more than a kilometre across....These are methane fields on a scale not seen before. The emissions went directly into the atmosphere... Earlier we found torch or fountain-like structures like this...

This is the first time that we've found continuous, powerful and impressive seeping structures, more than 1,000 metres in diameter. It's amazing. Over a relatively small area, we found more than 100, but over a wider area, there should be thousands of them.'

'We have reason to believe that such emissions may change the climate. This is due to the fact that the reserves of methane under the submarine permafrost exceed the methane content in the atmosphere is many thousands of times.

'If 3-4% from underwater go into the atmosphere within 10 years, the methane concentration therein (in the atmosphere) will increase by tens to hundreds of times, and this can lead to rapid climate warming."

The new expedition was organised by the Laboratory of Arctic Research in Pacific Oceanology Institute of the Far Eastern Branch of Russian Academy of Sciences in cooperation with Tomsk Polytechnic University (TPU), the Institute of Oceanology of the Russian Academy of Sciences, Institute of Atmospheric Physics, Russian Academy of Sciences, and was  funded by the Russian Government and the Russian Science Foundation.


Now the proof: permafrost 'bubbles' are leaking methane 200 times above the norm

The swelling pockets in the permafrost - revealed this week by The Siberian Times - are leaking 'alarming' levels of ecologically dangerous gases, according to scientists who have observed this 'unique' phenomenon. Some 15 pockets have been found on the Arctic island, around one metre in diameter.

Measurements taken by researchers on expeditions to the island found that after removing grass and soil from the 'bubbling' ground, the carbon dioxide (CO2) concentration released was 20 times above the norm, while the methane(CH4) level was 200 times higher.

One account said: 'As we took off a layer of grass and soil, a fountain of gas erupted.'

A scientist, not named in video footage, warned it was a 'serious reason to be concerned if gas bubbles appear in the permafrost zone', suggesting there could be 'unpredictable' consequences.

Another scientist Alexander Sokolov has revealed he first witnessed the phenomenon last summer during an expedition to the remote island.

'We didn't know about these bubbles before the expedition,' said Sokolov who is deputy head of ecological R&D station of the Institute of Ecology of Plants and Animals, in Labytnangi, Tyumen region, Ural Department of the Russian Academy of Sciences

'Our colleagues gave us a gas analyser worth 7 million roubles ($10,850). This device measures the concentration of the two greenhouse gas, carbon dioxide and methane. Gases are typically measured in parts per million or ppm.

'The gas analyser showed that one of these gases was dozens of times higher and another was hundreds of times higher than normal.'

'I can't say if this summer has been the warmest over the past few years but simply because I'm not following it.

'Yet it is likely that that 10 days of extraordinary heat could have started some mechanisms, (and the) higher level of permafrost could have thawed and released a huge amount of gases. Geologists suppose that there might be some gas leaking from the underground but it's unlikely. There is solid permafrost under the bubbles.'

Alexander Sokolov's view is that the gas release if from thawing ground immediately below the surface to a depth of around one metre. He is concerned about the consequences of the gas release.

Scientists have warned at the potential catastrophic impact of global warming leading to the release into the atmosphere of harmful gases in the Arctic hitherto frozen in the ground or under the sea.

A possibility is that the trembling tundra on Belyy Island is this process in action.

"It is evident even to amateurs that this is a very serious alarm."

Further south, on the Yamal and Taimyr peninsulas, scientists are actively observing a number of craters that have suddenly formed in the permafrost.


Relic gas hydrates in permafrost above stability zone

Isolated gas and gas hydrates in the permafrost are serious geological danger in the process of oil and gas field development in Arctic. The particular hazard is the large gas accumulations confined to the sand and loamy sand horizons in the permafrost at depths down to 200 meters. Such gas accumulations are found in a number of Yamal gas fields and South-Tambey gas field among them. There are some indirect signs that they may be relic gas hydrates formed earlier in specific hydrate accumulation conditions. Up now they might have been preserved in the permafrost due to the effect of gas hydrate self-preservation at temperatures below zero. These gas hydrates lying above the modern gas hydrate stability zone are in a metastable state and very sensitive to various anthropogenic influences.

The aim of mathematical simulation was to understand the dynamic of the permafrost thickness and of the zone of gas hydrates stability (GHSZ) in the Late Pleistocene and Holocene. The simulation allow to present the evolution of the permafrost during which the zone of gas hydrates stability started from the earth's surface being located within permafrost. As the permafrost has never fully thawed, near surface horizons may still contain relic gas hydrates to this day.

The experiments consisted of two stages. The first part was the artificial saturation of field samples by methane hydrate. At the second stage we studied processes of self-preservation of pore gas hydrate in the frozen samples under nonequilibrium conditions close to the reservoir conditions such as temperatures-5...-6 0C and pressure of 0,6-1,3 MPa. These experiments highlignt the high stability of the pore gas hydrates in the frozen samples at the given nonequilibrium conditions. In general we can make a conclusion that the existence of relic gas hydrates on the territory of the South-Tambey gas field at depths of less than 200 m in the permafrost is possible.


The Planetary Crisis is Upon Us

Vast amounts of methane lie frozen in the Arctic. It's not news that the Arctic sea ice is melting rapidly, and that it will likely be gone for short periods during the summers starting as early as next year. Losing that ice means releasing larger amounts of previously trapped methane into the atmosphere.

Additionally, lying along the Arctic's subsea continental margins and beneath Arctic permafrost are methane hydrates, often described as methane gas surrounded by ice. In March 2010, a report in Science indicated that these cumulatively contain the equivalent of 1,000 to 10,000 gigatons of carbon.

Natalia Shakhova is a research associate professor of the University Alaska Fairbanks, International Arctic Research Center, where she focuses on the East Siberian Arctic Shelf (ESAS). Shakhova believes we should be concerned about her group's findings from the ESAS, specifically, because that area differs significantly from methane emissions happening elsewhere around the world.

The ESAS is the largest shelf in the world, encompassing more than 2 million square kilometers, or 8 percent of the world's continental shelf. Shakhova believes it holds an area-weighted contribution to the global hydrate inventory of "at least 10 to 15 percent."

"These emissions are prone to be non-gradual (massive, abrupt) for a variety of reasons," she told Truthout. "The main reason is that the nature of major processes associated with methane releases from subsea permafrost is non-gradual."

This means that methane releases from decaying frozen hydrates could result in emission rates that "could change in order of magnitude in a matter of minutes," and that there would be nothing "smooth, gradual or controlled" about it; we could be looking at non-linear releases of methane in amounts that are difficult to fathom.

She explained that the transition from the methane being frozen in the permafrost, either on land or in the shallow northern shores of the East Siberian Arctic, "is not gradual. When it comes to phase transition, it appears to be a relatively short, jump-like transformation from one state of the process to another state. The difference between the two states is like the difference between a closed valve and an open valve. This kind of a release is like the unsealing of an over-pressurized pipeline."

These immediate methane releases in the ESAS could be triggered at any moment by seismic or tectonic events, the subsiding of sediments caused by hydrate decay or sediment sliding due to permafrost degradation and thaw. The ESAS is particularly prone to these immediate shifts because it is three times shallower than the mean depth of the continental shelf of the world ocean.

"This means that probability of dissolved methane to escape from the water column to the atmosphere is from three to 10 times greater than anywhere in the world's oceans," Shakhova said. "In the ESAS, methane is predominantly transported as bubbles. Methane bubbles rise to the surface at a speed from 10 to 40 cm s-1; this means that it only takes minutes for methane to reach the water surface and escape to the atmosphere."

Even the relatively staid Intergovernmental Panel on Climate Change (IPCC) has warned of such a scenario: "The possibility of abrupt climate change and/or abrupt changes in the earth system triggered by climate change, with potentially catastrophic consequences, cannot be ruled out. Positive feedback from warming may cause the release of carbon or methane from the terrestrial biosphere and oceans."

Ira Leifer, an atmospheric and marine scientist at the University of California, Santa Barbara, and author of several Arctic methane studies, told Truthout that the scientific community has learned that methane emissions from the Arctic are already larger than previously thought, and said, "The warming trend in the Arctic is clear."

The dangers of methane-related warning are staggering, according to Leifer.

"The amount of methane trapped in submerged permafrost is vast, and if even a small fraction reaches the atmosphere on the time scale of a few decades, it would lead to a dramatic increase in warming on a global scale," he warned. "Furthermore, it could lead to a positive feedback where warming oceans release more methane which warms the Arctic more and leads to more methane release. Worse, the warming only slowly percolates to lower latitudes - and therefore it contributes to the enhanced Arctic warming."

Like Shakhova, Leifer also expressed concern about the ESAS.

"The potential is there for hydrate emissions to increase with warming oceans due to increased dissociation," he warned. He also confirmed that his recent studies of methane emissions in the Arctic even found the gas hundreds of miles from the coast. This means that the methane cannot be coming from land sources; Leifer has concluded that his recent studies "confirm a local marine source."

Meaning, the subsea hydrates are already releasing their methane very far from shore.

Paul Beckwith, a climatology and meteorology professor at the University of Ottawa, Canada, is an engineer and physicist who researches abrupt climate change in both the present day and in the paleoclimatology records of the deep past.

"It is my view that our climate system is in early stages of abrupt climate change that, unchecked, will lead to a temperature rise of 5 to 6 degrees Celsius within a decade or two," Beckwith told me. "Obviously, such a large change in the climate system will have unprecedented effects on the health and well-being of every plant and animal on our planet."

Beckwith notes that the increasing methane releases in the Arctic and the massive impact they will have on the planetary weather system mean "there will be continuing disruption and fracturing of our weather and climate systems."

He went on to issue a stark warning. "Further acceleration of these processes is very likely to lead to an 'abrupt climate change' system reorganization from a cold, snowy, ice-covered Arctic Ocean to a 'blue Arctic Ocean' regime," he said. "The final state could have a global temperature average being 5 or 6 degrees Celsius warmer and the transition to this state could occur in one to two decades, as has occurred many times in the past as recorded in paleorecords."

The advent of the "blue Arctic Ocean" Beckwith warns us of is only a matter of time, and will most likely happen before 2020
, considering that exponential decline in Arctic summer sea ice volume has already been determined by the Pan-Arctic Ice Ocean Modeling and Assimilation System data and models, which have been corroborated with recent CryoSat measurements, as well as modeling by the Naval Graduate School Regional Climate Models.

Beckwith warns that losing the Arctic sea ice will create a state that "will represent a very different planet, with a much higher global average temperature, in which snow and ice in the northern hemisphere becomes very rare or even vanishes year round."

"What happens in the Arctic does not stay in the Arctic," Beckwith explained. "The rapidly warming Arctic relative to the rest of the planet (five to eight times global average temperature rise) is decreasing the temperature gradient between the Arctic and the equator."

This decreased gradient is disrupting the jet stream, leading to further warming in the Arctic, forming a runaway feedback loop, which in turn is causing the release of more methane in the Arctic.

"As the methane concentrations increase in the Arctic from the large warming rates there in both the atmosphere and ocean, the jet streams will be greatly disrupted even more than now," Beckwith said. "Physics dictates that this will continue to increase the frequency, severity and duration of extreme weather events like torrential rains leading to widespread flooding in some regions and droughts in other regions. Needless to say, this causes enormous economic losses and poses a severe and grave threat to our global food supply. Thus, the Arctic can be considered the Achilles heel in our climate system."

British scientist John Nissen, chairman of the Arctic Methane Emergency Group, suggests that if the summer sea ice loss passes "the point of no return" and "catastrophic Arctic methane feedbacks" kick in, we'll be in an "instant planetary emergency."


Arctic heat wave; sea ice at record February low

It’s an upside down weather pattern at the top of the world.

A rare February Arctic heat wave has boosted temperatures to as high as 20 to 30 degrees warmer than average in the north polar regions.

The rare Arctic warmth is already causing a decline in Arctic sea ice coverage, at a time of the year when ice cover is historically still increasing toward peak coverage in March. In an eye opening development, current Arctic sea ice coverage is at record lows for February and is tracking well below the previous record year of 2012.

The unprecedented low Arctic sea ice cover has opened the eyes of many climate watchers around the planet.

The National Oceanic and Atmospheric Administration’s Global Forecast System model forecasts continued warmth in the Arctic into early March.

Temperature anomalies March 1 are forecast to remain as much as 20 degrees Fahrenheit above average over parts of the Arctic Ocean.

The persistent Arctic heat wave may push Arctic sea ice levels to even more unprecedented lows in the coming weeks.


Arctic ozone could hit all-time low this spring

Scientists who study the earth’s atmosphere have noticed a significant decrease in ozone levels over the Arctic recently and, with the sun’s return in spring, they predict that ozone hole will get a lot bigger.

“At certain altitudes, we’ve already seen about 25 per cent of the ozone being destroyed. That’s the amount we’re sure of,” said Gloria Manney, an expert in stratospheric dynamics and transport at Northwest Research Associates in New Mexico.

“The big question is: what happens next? Because since these reactions depend on sunlight, as you can imagine, as we go into the spring, toward the equinox… these reactions will get much more rapid.”

Scientists are predicting ozone destruction over the Arctic this spring will exceed the highest recorded depletion, which took place in 2011. That year, 80 per cent of the ozone over the Arctic was destroyed.

The science behind this phenomenon is interesting, albeit complicated, and we can expect it to happen more frequently as the earth’s climate warms, said Manney. Here’s why.

There is a fixed amount of heat radiating from the sun. As greenhouse gases, such as carbon dioxide and methane, collect in the earth’s atmosphere, these gases trap more heat closer to the earth, warming the climate. But as a result of that entrapment, less heat leaks out into the atmosphere’s higher altitudes in the stratosphere.

And so the stratosphere is getting colder. When that happens, water vapour and nitric acid condense more frequently at high altitudes to form what are called polar stratospheric clouds — beautifully colourful, but foreboding in nature.

When a harmless form of chlorine gas, which is naturally present in the stratosphere, comes into contact with those unusual clouds, the chlorine is transformed into its “active” form. And chlorine, in its active form, destroys ozone.

Scientists who study stratospheric meteorology, such as Manney, have noted the stratosphere is colder than normal and has been that way since mid-December.

Satellite-based Instruments which measure gases in the stratosphere have also shown that the amount of “inert” or harmless chlorine in the stratosphere is diminishing while active or harmful chlorine is fast increasing.

The final ingredient in this equation is sunlight, Manney said. Sunlight is what causes active chlorine to destroy ozone and right now, there’s little sunlight in the Arctic.

But the polar vortex — that large mass of low pressure and cold air that contains this chemical soup and naturally swirls around the poles — is unusually large in the Arctic this year. Parts of it have expanded down into southern regions where there is sunlight in winter.

In fact, even with the limited amount of sunlight penetrating the polar vortex this year, there is already more ozone depletion than at this time during the record-breaking year of 2011.

And so scientists predict most of the ozone over the Arctic could be destroyed this spring.


The formation of Type II PSCs may be linked to tropospheric methane concentrations because oxidation of methane in the troposphere is a significant source of stratospheric water vapor (Lelieveld et al., 1993). Methane oxidizes to water vapor in the troposphere, and current observations suggest that approximately 10% of that water vapor diffuses to the stratosphere (Lelieveld et al., 1993).

Increased tropospheric methane concentrations may have led to greater amounts of stratospheric water vapor, creating greater areal extents of PSCs, and/or higher emissitivity values of the clouds during the Eocene.

The PSC mechanism is a very appealing explanation for high latitude warming in a greenhouse world for several reasons.

First, PSCs produce a greater magnitude of high latitude warming than any other climate forcing factor tested to date. Second, the warming is concentrated at high latitudes, and tropical temperatures are not affected. Third the warming is preferential in the winter season, corresponding more closely to proxy records of high latitude Eocene temperature (se .g., Zachose t al., 1994; Greenwood and Wing, 1995). Fourth, the mechanism of PSC warming is clearly defined, unlike other hypotheses for high-latitude warming (e.g., poleward oceanic heat transport (Sloan et al, 1995)). Lastly, a positive feedback mechanism amplifies the warming, through the connection between tropospheric warming and stratospheric cooling. As the troposphere warms in a greenhouse world, the corresponding increased cooling of the stratosphere could lead to larger areas with temperatures below ~ -190 K, leading to more extensive PSCs (Shine, 1988; Austin et al., 1992). This feedback makes PSCs an even more plausible explanation for high latitude warming during times of warm climate and high greenhouse gas concentrations, such as the middle Cretaceous (e .g., Huber et al., 1995), provided that atmospheric methane exists in sufficient concentrations.

The methane source for the PSCs originally was hypothesized to have been the greater area of wetlands that existed in the ancient greenhouse world (Sloan et al., 1992). Recent work has suggested that the release of methane clathrates to the atmosphere could provide another source of methane at times (Dickens et al., 1997; Bralower et al., 1997). Latest Paleocene marine and isotopic records suggest that an extreme warm "event" occurred, lasting less than 500,000 years and concentrated at high latitudes (Bralower et al., 1997).
Dickens et al. (1997) and Bralower et al. (1997) have suggested that the catastrophic release of methane clathrates could explain the latest Paleocene warming, via tropospheric greenhouse effects.

Not only would the clathrate-released methane have provided more of a source for PSCs, but a pulse of methane also might have increased either the residence time (Lelieveld et al., 1993; Lelieveld and Crutzen, 1992) and/or the amount of methane that diffused to the stratosphere to create PSCs. If so, extensive PSCs might have been an important climate modifier during such methanogenic "events".

Therefore we conclude that PSCs may have been an important climatic forcing factor during past warm climates, and that considering only the tropospheric effects of methane upon climate in paleoclimate studies omits significant mechanisms for potential modification of surface temperatures.