Graham Readfearn: Earth's climate monsters could be unleashed as temperatures rise

As a UN panel prepares a report on 1.5 C global warming, researchers warn of the risks of ignoring ‘feedback’ effects

 Fundamental questions are being raised about the ability of governments to stop the Earth from spiraling into a ‘hothouse’. Photograph: Peter van der Sleen/University o/PA

by Graham Readfearn, The Guardian, October 6, 2018

This week, hundreds of scientists and government officials from more than 190 countries have been buzzing around a convention centre in the South Korean city of Incheon.

They are trying to agree on the first official release of a report – the bit called the Summary for Policymakers – that pulls together all of what’s known about how the world might be affected once global warming gets to 1.5 C.

What will happen to coral reefs? How will extreme weather events and droughts change? What about heatwaves? And then, what are the different “pathways” that economies could choose to keep temperatures to 1.5 C?

On Monday morning, the summary document is expected to be released, and there will be a cascade of headlines around the world.

The report, being pulled together by the United Nations Intergovernmental Panel on Climate Change, was one tiny part of the Paris climate change agreement.

As things stand, if you add up all the things that the 190-plus countries have committed to do as part of that Paris deal, global temperatures will probably go well above 3 C.

We’re already at 1 C of warming, so the extra half a degree isn’t far away – many scientists will say it’s already locked in, while others say there are plausible ways to stabilize temperatures at that level.

But in August, one of the world’s leading scientific journals – the Proceedings of the National Academy of Sciences – published a “perspective” article that has become known as the “hothouse earth” paper.

There was no new science in the paper and while it was speculative, it did raise fundamental questions about the ability of governments around the world to stop the Earth from spiraling into a “hothouse.”

One of the report’s authors, Professor Will Steffen, of Australian National University and the Stockholm Resilience Centre, talked me through it.

The problem lies with “feedbacks” – in the “supplementary information” attached to the paper, Steffen and colleagues actually listed 10 of them. With each, they include estimates of how much extra CO2 and temperature they could add once you hit about 2 C of global warming.

For example, the ability of the land and ocean to keep soaking up CO2 could weaken, giving you an extra 0.25 C of warming. Dieback of trees in the Amazon and subarctic could give us another 0.1 C.

Permafrost, which is already starting to defy its name by not being all that permanent, could release ever more methane and carbon that might add a bit more warming again (0.09 C is the estimate there).

The point is that once you add them all up, you get close to 0.5 C of warming by the end of the century. Given we’re already at 1 C of global warming, that makes the job of keeping warming “well below 2 C” or even holding it at 1.5 C much, much harder than it already is.

And there’s the rub.

While governments have the means to affect how much CO2 gets released through policies that radically cut the use of fossil fuels, it would be much harder to get a grip on thawing permafrost, mass forest collapses, or the loss of polar sea ice.

By failing to get a grip on a thing that’s feasibly under your control, we end up risking the release a whole gang of other monsters that we can’t.

This gets us to another big issue, says Steffen, because climate models don’t include some of these feedbacks. In essence, the warmer things get, the less reliable the models become. He tells me:

“I think the dominant linear, deterministic framework for assessing climate change is flawed, especially at higher levels of temperature rise.

So, yes, model projections using models that don’t include these processes indeed become less useful at higher temperature levels. Or, as my co-author John Schellnhuber says, we are making a big mistake when we think we can “park” the Earth System at any given temperature rise – say 2 C – and expect it to stay there.”

For those who understand the idea of a carbon budget – where scientists have calculated him much CO2 you could emit before hitting certain temperature rises – it looks even meaner than before if Steffen and his colleagues are right.

But as they also point out, several of these feedbacks might have “tipping points” that then set off a cascade of other issues. Steffen says:

“Even at the current level of warming of about 1 C above pre-industrial, we may have already crossed a tipping point for one of the feedback processes (Arctic summer sea ice), and we see instabilities in others – permafrost melting, Amazon forest dieback, boreal forest dieback and weakening of land and ocean physiological carbon sinks.

And we emphasize that these processes are not linear and often have built-in feedback processes that generate tipping point behavior. For example, for melting permafrost, the chemical process that decomposes the peat generates heat itself, which leads to further melting and so on.”

For the record, Steffen thinks the assumptions in climate models that cuts in fossil fuel emissions will deliver relative cuts in temperatures “is OK for perhaps lower temperature rises of 1.5 or 2 C,” but beyond that, he’s sceptical.

The paper has received a bit of pushback from scientists, largely, it appears, because of the sensational headlines it attracted.

For example, Professor Richard Betts, of the UK’s MetOffice, has a measured perspective that’s well worth a look.

Dr Glen Peters, an Australian scientist and climate modeller based at the Centre for International Climate Research in Norway, also thought some of the media coverage went too far with the doomsday vibe.

But he told me that, while it was true that many of the feedbacks in the paper were not well covered by climate models, this was partly because they were not that well understood. I’ll leave you with his thoughts:

“The hothouse earth paper conjectures that many of these feedbacks may interact like a domino effect, lead the Earth system to spiral out of control to reach a new steady state very different from today, and these processes may even start if we are successful at meeting the goals of the Paris Agreement.

“There is also an important timescale question, are we talking decades or millennia, and that is very important for how society may respond. While all the claims made in the hothouse earth paper are justified, we simply don’t have the data to verify if those claims are true. While the paper put in plenty of language to indicate its exploratory nature … many headlines and statements went too far, indicating we had already gone too far and there was no turning back.”

https://www.theguardian.com/environment/planet-oz/2018/oct/06/earths-climate-monsters-could-be-unleashed-as-temperatures-rise

Climate Code Red: What we learned about the climate system in 2017 that should send shivers down the spines of policy makers

by David Spratt, Climate Code Red, January 15, 2018

Much of what happened in 2017 was predictable: news of climate extremes became, how can I put it … almost the norm. There was record-breaking heat on several continents, California’s biggest wildfire (extraordinarily in the middle of winter), an ex-tropical cyclone hitting Ireland (yes, Ireland) in October, and the unprecedented Hurricanes Harvey, Irma and Maria that swept through the Atlantic in August. The US government agency, the NOAA, reported that there were 16 catastrophic billion-dollar weather/climate events in the USA during 2017.

And 2017 “marks the first time some of the (scientific) papers concluded that an event could not have occurred — like, at all — in a world where global warming did not exist. The studies suggested that the record-breaking global temperatures in 2016, an extreme heat wave in Asia and a patch of unusually warm water in the Alaskan Gulf were only possible because of human-caused climate change,” Reuters reported.

At both poles, the news continues to be not good. At the COP23 in Bonn, Pam Pearson, Founder and Director of the International Cryosphere Climate Initiative, warned that the cryoshere is becoming “an irreversible driver of climate change.” She said that most cryosphere thresholds are determined by peak temperature, and the length of time spent at that peak, warning that “later, decreasing temperatures after the peak are largely irrelevant, especially with higher temperatures and longer duration peaks.” Thus “overshoot scenarios,” which are now becoming the norm in policy-making circles (including all 1.5 °C scenarios) hold much greater risks.

As well, Pearson said that 2100 is a misleading and minimizing measure of cryosphere response: “When setting goals, it is important to look to new irreversible impacts and the steady state circumstances. The end of the century is too soon to show that before but inevitable response especially for sea level rises.” Pearson added that: “What keeps cryosphere scientists up at night are irreversible thresholds, particularly West Antarctica and Greenland. The consensus figure for the irreversible melting of Greenland is at 1.6 °C.”

So what did we learn about the climate system in 2017? Here’s three that stand out, that should send shivers down the spines of policy makers. 

1.  2017 was the second hottest year on record and the hottest non-El Nino year on record

Whilst not all sources have yet released data on annual warming for last year, the Copernicus Climate Change Service, the first major international weather agency to report global 2017 temperatures, said they averaged 1.2 °C above pre-industrial times. 2017 was slightly cooler than the warmest year on record, 2016, and warmer than the previous second warmest year, 2015, Reuters reported.

Other organisations have unofficial figures which either agree with this assessment, or say that 2017 has tied with 2015. And last year was Australia's third-warmest year on record.

It is no surprise that the last three years have been the hottest on the instrumental record. What is remarkable is that 2017 was as hot, or hotter than 2015, because 2015 and 2016 were both El Nino years, and the evidence shows that El Nino years are, on average, about 0.15 °C warmer than La Nina years.In fact, a remarkably hot 2017 crushed the old record for hottest non-El Niño year (2014) by an astounding 0.17 °C.

The underlying temperature trend is being driven by continuing high levels of climate pollution: The UN says carbon dioxide levels grew at record pace in 2016. The atmospheric carbon dioxide  averaged 403.3 parts per million (ppm) over the year, up from 400 ppm in 2015. The growth rate was 50% faster than the average over the past decade.

And global carbon emissions are headed up again after three years in which human-caused emissions appeared to be leveling off. A 2% increase is projected overall, with the highest rise coming in China, according to new research presented at the climate talks in Bonn.

In 2017, we also learned that there was no pause in global warming: the so-called ’slow down' in climate change between 1998 and 2012 was caused by a lack of data from the Arctic.

2. It is likely to get hotter than we think

Two significant pieces of work released towards the end of 2017 suggest that warming is likely to be greater than the projections of the Intergovernmental Panel on Climate Change (IPCC), on which climate policy-making and carbon budgets are generally based. 

This is because what is called Equilibrium Climate Sensitivity (ECS), an estimate of how much the planet will warm for a doubling in the level of greenhouse gases, is higher than the median of the IPCC’s modelling analysis. 

In “Greater future global warming inferred from Earth’s recent energy budget” published in Nature in December 2017, Brown and Caldeira compared the performance of a wide range of climate models (raw model projections) with recent observations (especially on the balance of incoming and outgoing top-of-the-atmosphere radiation that ultimately determines the Earth’s temperature), in order to assess which models perform best.

The models that best capture current conditions (the “observationally-informed” models) produce 15% more warming by 2100 than the IPCC suggests, hence reducing the “carbon budget” by around 15% for the 2C target.

 For example, they find the warming associated by the IPCC with RCP 4.5 emissions scenario would in fact “follow the trajectory previously associated with (higher emissions) RCP 6.0” scenario. 

They also find that the observationally-informed ECS prediction has a mean value of 3.7 °C (for a doubling of the atmospheric greenhouse gas level), compared to 3.1 °C used in raw models, and in the carbon budget analyses widely used by the IPCC, the UN and at climate policy conferences.

In “Well below 2C: Mitigation strategies for avoiding dangerous to catastrophic climate changes,” published in September 2017, Xu and Ramanathan look at what are called the “fat tail” risks. These are the low-probability, high-impact (LPHI) consequences (“fat tails”) of future emission scenarios; that is, events with a 5% probability at the top end of the range of possible outcomes. 

These “top end” risks are more likely to occur than we think, so “it is important to use high-end climate sensitivity because some studies have suggested that 3D climate models have underestimated three major positive climate feedbacks: positive ice albedo feedback from the retreat of Arctic sea ice, positive cloud albedo feedback from retreating storm track clouds in mid-latitudes, and positive albedo feedback by the mixed-phase (water and ice) clouds.” 

When these are taken into account, the researchers find that the ECS is more than 40% higher than the IPCC mid-figure, at 4.5-4.7 °C. And this is without taking into account carbon cycle feedbacks (such as melting permafrost and the declining efficiency of forests carbon sinks), and increase methane emissions from wetlands, which together could add another 1 °C to warming be 2100. 

This work complements other recent work which also suggests a higher climate sensitivity:

• Fasullo and Trenberth found that the climate models that most accurately capture observed relative humidity in the tropics and subtropics and associated clouds were among those with a higher sensitivity of around 4 °C.

• Zhai et al. found that seven models that are consistent with the observed seasonal variation of low-altitude marine clouds yield an ensemble-mean sensitivity of 3.9 °C. 

• Friedrich et al. show that climate models may be underestimating climate sensitivity because it is not uniform across different circumstances, but in fact higher in warmer, inter-glacial periods (such as the present) and lower in colder, glacial periods. Based on a study of glacial cycles and temperatures over the last 800,000 years, the authors conclude that in warmer periods climate sensitivity averages around 4.88 °C. Professor Michael Mann, of Penn State University, says the paper appears "sound and the conclusions quite defensible."

• Lauer et al. found that climate models that most accurately simulate recent cloud cover changes in the east Pacific point to an amplifying effect on global warming and thus a more sensitive climate. 

And the bottom line?  If this work is correct, then the pledges made under the Paris Accord would not produce warming of around 3 °C as is widely discussed, but a figure closer to and even above 4 °C. And the total carbon budget would a quarter smaller than is generally accepted, or even less.

3. Climate models under-estimate future risks

This year, the Breakthrough Centre for Climate Restoration in Melbourne, published What Lies Beneath, on the scientific understatement of climate risks. The report found that human-induced climate change is an existential risk to human civilization, yet much climate research understates climate risks and provides conservative projections. Reports from the Intergovernmental Panel on Climate Change that are crucial to climate policymaking and informing public narrative are characterized by scientific reticence, paying limited attention to lower-probability, high-risk events that are becoming increasingly likely. (Disclosure: I was a co-author of this report.) 

But don’t take my word.  At the climate policy conference in Bonn, Phil Duffy, the Director of the Woods Hole Institute, explained the scientific reticence regarding the biggest system feedback issues:

"The best example of reticence is permafrost…  It’s absolutely essential that this feedback loop not get going seriously, if it does there is simply no way to control it… The scientific failure comes in because none of this is in climate models and none of this is considered in the climate policy discussion… climate models simply omit emissions from the warming permafrost, but we know that is the wrong answer because that tacitly assumes that these emissions are zero and we know that’s not right…"

And the problems of underestimation of future climate impacts from current models was explicitly recognized by the US government in its Climate Science Special Report: Fourth National Climate Assessment. In a chapter on “Potential Surprises: Compound Extremes and Tipping Element,” two key findings were:

Positive feedbacks (self-reinforcing cycles) within the climate system have the potential to accelerate human-induced climate change and even shift the Earth’s climate system, in part or in whole, into new states that are very different from those experienced in the recent past (for example, ones with greatly diminished ice sheets or different large-scale patterns of atmosphere or ocean circulation). Some feedbacks and potential state shifts can be modeled and quantified; others can be modeled or identified but not quantified; and some are probably still unknown. (Very high confidence in the potential for state shifts and in the incompleteness of knowledge about feedbacks and potential state shifts).

• While climate models incorporate important climate processes that can be well quantified, they do not include all of the processes that can contribute to feedbacks, compound extreme events, and abrupt and/or irreversible changes. For this reason, future changes outside the range projected by climate models cannot be ruled out (very high confidence). Moreover, the systematic tendency of climate models to underestimate temperature change during warm paleoclimates suggests that climate models are more likely to underestimate than to overestimate the amount of long-term future change (medium confidence).

• The problem is that the notion that future climate changes may be faster and hotter than those projected by climate models is one rarely understood by climate policy-makers, and rarely discussed by those who do understand.

If climate policymaking is to be soundly based, a re-framing of scientific research within an existential risk-management framework is now urgently required. This must be taken up not just in the work of the IPCC, but also in the UN Framework Convention on Climate Change negotiations if we are to address the real climate challenge.

http://www.climatecodered.org/2018/01/what-we-learned-about-climate-system-in.html

Jason Box: Earth's Ice Is Melting Much Faster Than Forecast. Here's Why That's Worrying

by Jason Box, Professor of Glaciology, Geological Survey of Denmark and Greenland, Huffington Post, September 4, 2015

COPENHAGEN -- For me it was only after 8 years of studying Greenland -- installing and maintaining a network of on-ice climate stations and examining how much snow evaporates from the island -- that I suddenly realized glaciology textbooks needed a major revision. This was in 2002. Prior to the epiphany, conventional knowledge held that the ice sheet was frozen at its bed, and so the reaction time of the ice sheet to climate warming was measured in tens of thousands of years. A heck of a long time.

Climate warming had just infiltrated Greenland glaciology in earnest. Summer melt water, it turned out, drains down quickly to the bed, lubricating the glacier's flow. Suddenly we realized an expanding melt season meant the ice sheet would be sliding faster, longer. It was not to be the only time our philosophy got hit with a major surprise that connected the ice sheet with climate change and the threat of abrupt sea level rise.

The next one came in 2006.

Somehow all marine-terminating glaciers across the southern half of Greenland doubled in speed simultaneously between 2000 and 2005. [Readers, this news is what caused me to begin this blog -- I had the one and only epiphany in my life when I read about this.] We didn't yet know why.

In the meantime, scientists tried defining a plausible upper limit for the contribution to sea level rise from Greenland's ice. That was at a time when surging glacier speeds -- ice flow -- was thought to be the dominant conveyer of ice loss, and would be for the foreseeable future. Well, surprise! It became clear that for six years in a row, starting in 2007, ice loss from surface meltwater runoff took over the lead position in the competition for biggest loser.  [This was something I thought privately at the time -- that this must occur eventually, but I did not imagine that it would occur so soon. I never bought into the idea that the topography was a limit on glacial outflow and thus would restrain Greenland's contribution to sea level rise.] From 2007 to 2012, nearly each summer set higher and higher melt records, owing to persistent and unforeseen weather that by 2012 would become a signature of climate change.

The competition between how much ice is lost through glacier flows into fjords versus meltwater runoff is intimately synergistic with meltwater interacting with ice flow all along the way. Increasing melt sends more water down through the ice sheet, softening the ice so it flows faster. Once at the bed the water lubricates flow. Squirting out the front of glaciers into the sea, the meltwater drives a heat exchange that undercuts glaciers, promoting calving, loss of flow resistance and faster flow. Put it this way: in Washington, DC, to know what's happening, you follow the money; in Greenland you follow the meltwater.

Put it this way: in Washington, DC, to know what's happening, you follow the money; in Greenland you follow the meltwater.

Glaciologists became oceanographers when they realized, in 2008, the trigger effect for galloping glaciers was warm pulses of subtropical waters that undermine glaciers at great depth in the sea, at the grounding lines where this warm water can invade.

Indeed, ocean warming is arguably the climate change story. The planetary energy imbalance due to the enhanced greenhouse effect is loading far more heat into the oceans than the atmosphere or land. The world is 70% ocean-covered. after all. While there were signs of a warming hiatus in air temperatures from 1998 to 2012, the ocean continued to heat up, an equivalent of four Hiroshima bombs, per second, all day, every day. The increase is continuing as we load the atmosphere with CO2.

The fundamental climate heating issue is a problem of too much of a good thing. The natural greenhouse effect -- a good thing -- keeps temperatures tolerable at night. But it has been enhanced by more than a century of people externalizing the environmental costs of stupendous economic growth, loading the atmosphere now with 42% more carbon dioxide, 240% more methane, 20% more nitrous oxide, 42% more tropospheric ozone, etc. We have far too much gaseous carbon compounds now in our atmosphere, people. The carbon pollution is, by the way, making our oceans too acidic, threatening the base of the marine food chain. Would someone step forward and deny the changing ocean chemistry? Do I digress?

We have far too much gaseous carbon compounds now in our atmosphere, people.

The key question, as I see it, is how to project what the sea level will soon be due to ice sheet melting. But this is confounded by us not really knowing what to expect. We keep being surprised by nature being more sensitive and complex. As the science develops, we see more interconnection, where multiplying feedbacks produce surprisingly fast responses.

Will there be some saving self-regulation of human-induced climate warming and its melting land ice consequences? The enormous increase of heat in our oceans, from past decades of enhanced greenhouse effect, negates any hope that negative feedbacks or even solar output will prevent a much warmer world. The few negative feedbacks we have found for ice -- like more snow as a result of a warming climate, more reflective frost, more efficient sub-glacial water transmission -- are clearly being outdone. And at the global scale, despite some negative feedbacks like more clouds, clearly we are not seeing net cooling. Feedbacks, whether positive or negative, only do their thing after the initial effect. Negative feedbacks don't reverse the perturbation.

Seemingly the biggest issue with abrupt sea level rise comes from the now-unstoppable loss of key sectors of West Antarctic ice and the discovery of more marine instability than we thought elsewhere. Like glaciers thinning rapidly in East Antarctica. Or in Greenland, where improved bedrock maps reveal a marine connection an average of 40 kilometers further inland than previously thought. Or like how new fjord underwater mapping reveals greater fjord depths, increasing the odds that deep warm ocean water can communicate with more Greenland glaciers than previously thought. Surprise, surprise, surprise.

I'd say we are in for more surprises.

If the past decade of scientific inquiry is any indication, I'd say we are in for more surprises. That notion is further supported by the fact that the climate models used for projecting future temperatures lack key processes that likely reinforce warming or the effects of warming, not regulate it.

Despite decades of progress by many clever scientists engaged with climate modeling, climate models used to inform policymakers don't yet encode key pieces of physics that have ice melting so fast. They don't incorporate thermal collapse -- ice softening due to increasing meltwater infiltration.

Climate models also don't yet incorporate increasing forced ocean convection at the ocean fronts of glaciers that forces a heat exchange between warming water and ice at the grounding lines.

Climate models don't yet include ice algae growth that darkens the bare ice surface.

Climate models don't yet prescribe background dark bare ice from outcropping dust on Greenland from the dusty last ice age.

Climate models don't include increasing wildfire delivering more light-trapping dark particles to bright snow-covered areas, yielding earlier melt onset and more intense summer melting.

As a result of some of these factors and probably some as yet unknown others, climate models have under-predicted the loss rate of snow on land by a factor of four and the loss of sea ice by a factor of two.

Climate models also don't yet sufficiently resolve extended periods of lazy north-south extended jet streams that produce the kind of sunny summers over Greenland (2007-2012 and 2015) that resulted in melting that our models didn't foresee happening until 2100.

While individual climate models come close to observations on this or that piece of the complex big picture, what ends up in global assessment reports intended to help guide policy decisions and national discussions of climate change are very conservative averages of dozens of models that don't include the latest, higher sensitivity physics.

So, alas, when it comes to ice, how fast it can go and how fast the sea will rise, if I were a betting man, I'd put my money on it going faster than forecast.

http://www.huffingtonpost.com/jason-e-box/ice-melt-fast_b_7927186.html

Alun Hubbard, Jason Box: Heavy summer rain in Greenland speeds up ice melt

Rapid ice melt in Greenland is driven by intense late summer rainfall, making the vast ice sheet even more vulnerable as the earth warms, new research shows

Formation of lakes on the Greenland Ice Sheet after the main melt-season due to warm wet weather and heavy rain. Later this water drains to the bottom of the ice sheet enabling it to move and melt faster. This makes the ice sheet more vulnerable to such rapid melt events as the climate warms, scientists say in a new study. (Photo: Sam Doyle)

by Catherine Jex, ScienceNordic, July 13, 2015 

You may think that warmer temperatures alone were enough to drive the melting of the Greenland Ice Sheet, and you would not be alone -- many glaciologists thought roughly the same.

But a new study shows for the first time that the Greenland Ice Sheet melts rapidly, not just with warm summer temperatures, but also after intense late-summer rain.

The new study shows that these heavy rain events have occurred frequently due to the warmer, wetter weather of the last 20 years, penetrating deeper into the ice sheet, making it move and melt faster.

“We saw 10 to 15 % of the total annual surface melt occur in this unusual week of warm, wet weather in late summer 2011. When this water reached the bed, the ice sheet lifted up and moved faster,” says Sam Doyle, from Aberystwyth University, UK, lead author of the new study, which has just been published in Nature Geoscience.

According to his colleague and co-author, professor Jason Box of the Geological Survey of Denmark and Greenland, their results add more weight to scientists’ concerns of the sensitivity of the entire Greenland Ice Sheet to melting.

“We’re seeing that warm wet weather, that is increasing with climate change, is driving more melt of the Greenland Ice Sheet than we thought. And worryingly, this melt is reaching further into the ice sheet,“ says Box, speaking via satellite phone, along with Doyle, as they work together on another field trip on Greenland.

Scientist: An interesting, thorough, and novel study

Edward Hanna, professor in climate change with the University of Sheffield, UK, is impressed with the new study.

“This is an interesting, thorough and novel study based on a wealth of robust meteorological and glaciological datasets, and is based on meteorological models as well as observations,” says Hanna, who was not involved in the research.

“The study adds to an already complex story of how Greenland Ice Sheet dynamics appear to be influenced by changes in weather conditions, sometimes on a daily timescale,” he says.

Heavy rain makes the Greenland Ice Sheet move

The study began after observing exceptionally warm wet weather in late summer 2011, causing huge amounts of melt at the ice surface. At this time of the year, there was no snow on the surface of the ice to absorb and act as a buffer for all this rain and melt water, which then moved very quickly through the ice sheet.

“At first our observations of this late-August intense rainfall and melt event were met with the attitude of 'hey big deal, it’s summer, it melts,’ ” says professor Alun Hubbard from the Center for Arctic Gas Hydrate, Environment and Climate at the Arctic University of Norway, and principle investigator of the project that led to the study.

A monster moulin on the Russell Glacier, Greenland. You can see a helicopter for scale. These large gaping holes allow rain and melt water to drain quickly into the ice sheet. (Photo: Alun Hubbard)

“But this new data, shows these periods of rapid movement of the ice are in fact tied to a particular type of rainfall event, the kind of storm you would expect to see in the mid-latitudes -- UK or Scandinavia -- and not so much in Greenland where high-pressure systems are more common.”

“We can now reinterpret these big late summer melt events to get a much better understanding of what is going on -- what is affecting the ice sheet system and how it is responding,” he says.

Imagine a storm drainage system after heavy rain

Hubbard uses the analogy of a storm drainage system, to explain how the rain and melted ice moves through the ice sheet so quickly.

“Imagine a big downpour in a city. You get so much rain so quickly, on to an impermeable concrete surface that the water is immediately shunted into the city's drainage system -- which due to the huge volumes of water -- can't cope. Drains back up and it floods. Basically, the same thing happens on the bare ice sheet surface, which like the city is literally comprised of pipes, conduits and cavities,” he says.

According to him, when these pipes back up, the hydraulic pressure lifts the ice sheet up, like a gigantic iceberg. There is less friction at the bed and so it moves faster.

Melting extends far into glacier interior

But observing one event was not enough to convince some glaciologists that this process is important over longer periods of time, or that rain could cause melting over such a wide area.

Doyle and his team analysed the strange event in more detail.

They monitored the ice-movement by GPS, set up their own weather stations on the ice, and measured how much water was being discharged from the ice sheet.

According to Hubbard, the rain event covered a huge area, and melting extended deep into the interior of the ice sheet, by about 140 km. This is where you would expect it to be frozen, and for rain to fall as snow.

They also gathered rainfall data for the last 20 years and compared it with other known melt events across Greenland.

“We saw these August rainfall events were in fact quite common, a major one occurring once every couple of years, and hence could explain some of the other rapid flow acceleration events recorded by other research groups working on the ice sheet,” says Hubbard.

Timely rainfall catches the ice sheet off guard

A scientist watches melt water drain from the Russell glacier catchment in Greenland. Scientists do not yet know just how often these rapid melt events may occur in the future as the earth warms and Greenland receives more rain. (Photo: Sam Doyle)

According to Doyle, it is not only the amount of rainfall that is important, but also the precise timing of this rain.

“The late-summer timing was critical. The event occurred after the end of the melt season and the ice sheet's drainage system had started to close down,” he says.

“In this closed-down state the ice sheet's drainage system just couldn’t cope with the exceptional water inputs from melt and rain, and this is why the ice sheet reacted so dramatically, lifting off its bed and accelerating the flow of the ice towards the sea,” he says.

Scientists expect more melt due to rain

Box emphasizes how climate change will continue to drive this process in the future.

“It is rain and warmer temperatures that cause melt, and we show that this melting trend has been underway for the last two decades as the atmosphere has warmed,” he says.

According to Hubbard, a succession of these rain events would have a big impact on the ice sheet -- allowing it to melt more and move faster.

“By speeding up the ice sheet, we are effectively turning up the tap -- opening the sluice gate and hastening the speed at which the ice moves and melts and hence ends up in the ocean. But we don’t yet know for sure how frequent or how intense these late-season rainfall events will be in the future,” he says.

Hanna echoes this caution.

“The trouble is we don't really have a clear prediction of how such events will change in frequency and intensity over Greenland with ongoing climate change,” he says.

“But certainly this is another, now well-evidenced potential feedback that should be included in studies of ice sheet change during the rest of this century,” says Hanna.

Country Denmark

Related content

3D technology allows scientists to model Greenland's pastAerial photos from Greenland topple climate modelsAmerican forest fires are melting the ice on GreenlandDeclining winter sea ice near Greenland spells cooler climate for EuropeDNA drilled up from the bottom of Greenland’s ice sheet

http://sciencenordic.com/heavy-summer-rain-greenland-speeds-ice-melt

Carbon cycle feedbacks and the worst-case greenhouse gas pathway

Skeptical Science, Posted on 7 July 2015 by Andy Skuce

The worst-case emissions pathway, RCP8.5, is a scenario that burns a huge amount of fossil fuels, especially coal. The model has sometimes been criticized as implausible because of its huge resource consumption and emissions of ~1700 billion tonnes of carbon (GtC) over the century. Those emissions are based in part on carbon-cycle model assumptions, which recent work suggests may be too optimistic. New research shows that future plant growth may be restricted by nutrient availability, turning the land carbon sink into a source. Also, permafrost feedbacks (not considered in IPCC CMIP5 models) may also add significant emissions to the atmosphere under the RCP8.5 pathway. In addition, the latest research on the Amazon Basin reveals that the tropical forest carbon sinks may already be diminishing there. Together, these feedbacks suggest that the greenhouse gas concentrations in the RCP8.5 case could be achieved with ~400 GtC smaller human emissions, making the RCP8.5 worst-case scenario more plausible.

The climate models referred to  in the recent IPCC Fifth Assessment Report (AR5) are founded on one of four Representative Concentration Pathways or RCPs. The key word in RCP is concentration. In the RCPs, the concentration of greenhouse gases is fixed at different times in the future and the climate model (or general circulation model or GCM) uses those atmospheric concentrations to calculate future climate states. Underpinning the concentration pathways are socio-economic and emissions scenarios. There can be more than one underlying emissions scenario capable of producing the concentration pathway.

If you are unfamiliar with RCPs, check out the great guide that Graham Wayne wrote in August 2013 for Skeptical Science.

This way of modelling differs from previous approaches in which the starting point was a story or scenario about economic and social development that led to emissions. These emissions are run through a carbon-cycle model (which may be simple or complex) to produce atmospheric concentrations over time. 

The schematic illustrates the differences in approach. The elements in red boxes are the prescribed inputs into the models, whereas the elements in blue ellipses are outputs. The advantage of the RCP prescribed-concentration approach is that the climate model outputs do not depend to the same degree on carbon-cycle models as they did in the emissions scenario method. The disadvantage is that there is no unique link between concentrations and emissions. The schematic is simplified in that there are feedbacks and loops in the processes that are not illustrated. 

The worst-case scenario among the four Representative Concentration Pathways (RCPs) is known as RCP8.5. The number “8.5” refers to the radiative forcing level measured in W/m² in the year 2100. RCP8.5, despite it often being called “business-as usual,” has been criticized as an unlikely outcome. While true, that’s more feature than bug, since, as one of the two extreme pathways, it is designed to provide climate modellers with an unlikely, but still just plausible “how bad could it be” scenario.

Let’s look briefly at some of the underlying socio-economic assumptions behind RCP8.5, then we’ll examine how the latest research on the terrestrial carbon cycle makes the GHG concentrations in the RCP8.5 model easier to reach.

RCP8.5

The socio-economic model chosen to underpin this pathway is described in Riahi et al. (2011). The model is one in which the following all occur:

• high population growth;
• little improvement in energy efficiency;
• no new greenhouse gas mitigation policy; 
• heavy reliance on fossil fuels, especially coal.

Although economic growth is assumed to be moderate, the world economy will grow to over $200 trillion (in year 2000 dollars) and the average per-capita income will be about $20,000 per year, roughly equal to current levels in Portugal or the Czech Republic, about double today’s average world income per person.

Population and GDP: The assumption is that population will rise to 12 billion by 2100. This is higher than the UN’s medium estimate of about 11 billion, but within the range of the low and high fertility estimates (7 and 17 billion, respectively). GDP will grow, but modestly compared to some other pathways and is at the low end of the growth ranges used in the AR4 scenarios.

From Van Vuuren et al. (2011) as are the following figures. Note that Population and GDP figures are assumptions rather than outcomes of the models. The grey shaded areas are UN population models (left) and assumptions used in AR4 models (right).

Energy intensity and energy use: the assumption is that the energy intensity of the economy (the amount of energy needed to produce one dollar of output will fall, but modestly compared to projections of recent trends. This is one of the most extreme assumptions of the model and is very different from historical trends (this is hardly "business-as-usual"). This leads to a quadrupling of energy use over this century.

Fossil fuel resource use: the assumption is that the energy needs will be met mostly by fossil fuels. The graph below shows historic (since 1950) and projected supplies of primary energy. (The left-hand graph was taken from the Global Energy Assessment (Fig. SPM 3) and was squished to be at the same scales as the right-hand graph taken from Riahi et al, 2011, Fig. 5.) About half of the energy supply is provided by a gigantic increase in the use of coal: in 2100 coal consumption will be more than five times the usage in 2010. Coal will be used not only for electricity generation but for coal-to-liquids fuel processes to make up for oil production that that will peak in the 2060s at levels that are double the production rate in 2010.

 

Energy sources. Historical 1950-2008 and projected in RCP8.5 2008-2100. See text for references.

These staggering assumptions for fossil-fuel use naturally raise questions of resource adequacy. In terms of the estimates of reserves of fossil fuels, the RCP8.5 model uses (roughly, by my own calculations using the figures given in Table SPM-3 of the GEA report) twice the current coal reserves, two to three times the oil reserves and half of the gas reserves. This is not quite as unreasonable as it seems because resources are constantly converted to reserves through development. In terms of reserves + resources, the RCP model uses, by 2100, about 10% of the current coal resources, nearly all of the oil resources and around one-quarter of the gas resources. It should go without saying that this exploitation will involve aggressive development of the world’s unconventional and low-grade resources and it will have huge financial and environmental costs.

See text for data sources. Author's own calculations.

Dave Rutledge of Caltech has used logistic curve-fitting of production histories to estimate world coal resources, and he claims, contrary to the GEA, that coal resources are inadequate by a large factor to meet the demands of RCP8.5. (See, for example this PowerPoint presentation, slides 27-30.) There is a good discussions in GEA Chapter 7, pages 435-437, on the "Peak Debate" mainly focused on oil and further discussion on coal reserve and resource estimates following page 461.

Policy: The Riahi et al. model factors in no greenhouse gas mitigation policies. Despite this, the model does assume that effective action will be taken to reduce local and regional pollutants such as sulphur dioxide, NOx and black carbon, basically assuming that current practices in rich countries today will be adopted by developing countries as their economies grow.

This perfect storm of high population growth, slow improvements in energy efficiency, ruthless exploitation of fossil fuels, and non-existent climate policies leads to the dire climate outcomes of the RCP8.5 scenario. Perfect storms and worst-case scenarios like this are, by definition, unlikely.

So, can we forget about RCP8.5? Not so fast. For one thing, the economic growth forecast is relatively moderate, and a higher one could easily make up for, let’s say, actual lower population growth than assumed in the model. Secondly, and more importantly, nature could provide humans with a helping hand to reach those lofty CO2 concentration targets through the combination of natural terrestrial sinks becoming less effective, along with new sources of carbon emissions appearing as a result of rising global temperatures. Let’s briefly look at the latest research on carbon-cycle effects and see what difference they will make.

Land carbon storage

All climate models incorporate some form of carbon- cycle component. Some are simple, some complex. The most sophisticated are the Earth System General Circulation Models (ES-GCMs). When it comes to modelling terrestrial storage of carbon, the models account for the increasing effect of carbon dioxide fertilization (C), but they do not (except for two models) account for the effects of nitrogen fertilization (N) and none of them, phosphorus (P).

As any suburbanite knows, lush green grass requires not only water, carbon dioxide and sunshine, but also a supply of nutrients, among which nitrogen and phosphorous are the most important.

A recent study in Nature Geoscience by Will Wieder and three colleagues performed modelling to determine what effect limiting N and P supplies would have on plant growth in an RCP8.5 scenario. Robert McSweeney at Carbon Brief has a good summary of the findings.

New inputs of N into the terrestrial ecosystem come from fixation of atmospheric N. New inflows of P are small and come from weathering of mineral soils and rocks. The nutrients can be moved around somewhat by wind and water, but the natural supply is generally limited to what can be found locally. Once local constraints on N and P supply are factored in, the rate at which plants can grow is limited to about one-third of the rate that has been predicted in the CMIP5 models reported in the AR5 IPCC report. This is shown in “a” in the graph below as the difference between the nutrient-unconstrained growth in black and the growth limited by N and N+P nutrient supply in pink and blue.

From Wieder et al. (2015). 

There’s also a big difference in the cumulative amount of carbon stored in the terrestrial system, once N and P are limited. As shown in part “b” in Wieder et al.’s figure, the terrestrial carbon store over the 21st Century changes from a net sink of 125 GtC to a net source of 156 GtC once nutrient constraints are imposed. In other words, there’s a difference of about 280 GtC between what the AR5 models calculate will be locked up in the terrestrial biosphere compared to what might be the case if the supply of key nutrients is limited.

It is worthwhile pointing out that the uncertainties on all carbon-cycle models are very large. There are some cases where nutrient-constrained models still produce a net carbon sink and cases where the unconstrained AR5 models predict a carbon source. There is nothing hard and fast about any of these results, and a great deal more research is needed. For further reading on the research on the role of forests as carbon sinks and emerging research, I recommend "The hunt for the world’s missing carbon" by Gabriel Popkin in Nature News. Nature 523, 20–22 (02 July 2015) doi:10.1038/523020a

That 280 Gt of carbon has to go somewhere and will end up being divided between the atmosphere and oceans. An alternative way of looking at this would be, for a concentration-defined pathway, we could instead subtract that 280 GtC from the human inputs of carbon over the 21st Century to produce the same GHG forcing. The mean fossil-fuel emissions for RCP8.5 in AR5 models are 1,685 GtC, so those could be reduced to about 1,400 GtC in an N + P nutrient-limited scenario. That would significantly reduce the amount of coal we would need to mine to reach the RCP8.5 GHG forcing, making the pathway more easily achievable. That’s not good news, but it gets even worse once we incorporate permafrost feedbacks.

Permafrost

The carbon release from thawing of the Arctic permafrost are not included in the AR5 models. I wrote about the latest research in a Skeptical Science article in April 2015 "Permafrost feedback update 2015: is it good or bad news?" The review by Ted Schuur and colleagues estimated that on the RCP 8.5 scenario, some 145 ±15 GtC will be released over the rest of this century.

Some of this accelerated soil decomposition could add some N fertilization and alleviate some of the N constraints in Arctic plant growth. So, there is risk of some double counting if we were to simply add the 145 GtC from the Arctic to Wieder et al.’s 280 GtC from fertilization constraints. This would have to be calculated using a properly integrated model, but let’s assume for now that the combined effect of permafrost thaw and N and P constrained plant growth would be about 400 GtC.

That would reduce the amount of fossil fuel emissions required to produce the RCP8.5 GHG forcing to about 1,300 GtC from the AR5 figure of 1,685 GtC. For the sake of comparison, 400 GtC is approximately the total amount of carbon produced historically from fossil fuels and cement from 1750-2013.

To put it more plainly, if we follow the RCP8.5 business-as-usual pathway, nature may add to our emissions (relative to current IPCC expectations) as much additional carbon as we have emitted from fossil fuels since the Industrial Revolution began. To repeat, that carbon feedback is not factored into the latest IPCC assessment.

Alas, it doesn’t end there.

Tropical forest die-back

A recent paper by Roel Brienen and 90 or so co-authors examined the effectiveness of the Amazon rain forest as a carbon sink over the past 25 years. Again, Robert McSweeney has a good summary at Carbon Brief.

The key findings were:

• The Amazon is still acting as a net carbon sink, but its effectiveness has been diminishing over the past 25 years. Simple linear extrapolation of the rate of biomass change would predict that it could change to a net source of carbon over coming decades.
• The forest productivity measured on a per-hectare basis increased in the 1990s but levelled off in 2000-2010.
• Trees have been dying off more quickly over the 1990-2010 period.

The reasons cited (see also the accompanying Nature article by Lars Hedin) for the tree mortality are: (a) the faster the trees grow, the quicker they die; (b) drought periods, as we saw in 2010; and (c) possible limitations of N and P nutrients.

What this amounts to is an additional divergence from prevailing assumptions that the biosphere will continue to provide a strong net sink of carbon throughout the century. This effect is largely, but perhaps not completely, independent from the processes described by Wieder and will provide an additional boost to carbon-cycle feedbacks to those from nutrient limitations and permafrost thaw.

The size of this tropical-forest effect over the 21st Century is unclear. The Amazon Basin contains about 150-200 GtC in living plants and soils. It is not known how much of this carbon will be lost over coming decades. Nor is it known how much carbon will be lost from other, less-well-studied tropical rainforests in Africa and SE Asia. Brienen et al. estimate that an increase of 3.8 GtC in necromass (dead wood) produced since 1983 has yet to reach the atmosphere. That's equivalent to more than two years of current US carbon emissions.

Summing up

Compared to the existing IPCC models, terrestrial carbon-cycle processes could provide an additional net feedback of 400 GtC or more over this century following the RCP8.5 scenario. This is a quantity roughly equivalent to historical human fossil fuel emissions to date. The implication is that the gigantic fossil fuel consumption envisioned in the RCP8.5 socio-economic model could be reduced by 25% or so, and we would still achieve the 8.5 W/m2 greenhouse gas forcing required in the model. This makes this worst-case scenario much more likely to be achievable. Nevertheless, humans would still have to demonstrate ingenuity and determination in exploiting even that reduced amount of fossil fuels, while at the same time remaining oblivious to the climate consequences.

We ought, of course, to be able to do much better than that, but our recent history shows that we are perfectly capable of demonstrating short-term, resource-exploitation ingenuity at the same time as being heedless when it comes to longer-term environmental consequences. The huge and unpredicted increase in the production of unconventional oil and gas resources in North America in just a few years has shown what we can do to exploit resources once we are motivated. We certainly should not console ourselves that RCP8.5 is beyond our reach just because the present estimate of fossil fuel resources appears insufficient. The recently quantified carbon cycle feedbacks that may occur if we follow the path of no mitigation make the achievement of the RCP8.5 greenhouse gas forcing level all too plausible.

References

Brienen, R. J. W., Phillips, O. L., Feldpausch, T. R., Gloor, E., Baker, T. R., Lloyd, J., ... & Marimon, B. S. (2015). Long-term decline of the Amazon carbon sink. Nature, 519(7543), 344-348.

Hedin, L. O. (2015). Biogeochemistry: Signs of saturation in the tropical carbonsink. Nature, 519(7543), 295-296.

Johansson, T. B., & Nakićenović, N. (Eds.). (2012). Global Energy Assessment: Toward a Sustainable Future. Cambridge University Press. PDF

MacDougall, A. H., Avis, C. A., & Weaver, A. J. (2012). Significant contribution to climatewarming from the permafrost carbon feedback. Nature Geoscience,5(10), 719-721.

Popkin, G. (2015) The hunt for the world's missing carbon. Nature 523, 20–22 (02 July 2015). 

Riahi, K., Rao, S., Krey, V., Cho, C., Chirkov, V., Fischer, G., ... & Rafaj, P. (2011). RCP 8.5—A scenario of comparatively high greenhouse gas emissions.Climatic Change, 109(1-2), 33-57. PDF

Schuur, E. A. G., McGuire, A. D., Schädel, C., Grosse, G., Harden, J. W., Hayes, D. J., ... & Vonk, J. E. (2015). Climate change and the permafrost carbonfeedback. Nature, 520(7546), 171-179.

Van Vuuren, D. P., Edmonds, J., Kainuma, M., Riahi, K., Thomson, A., Hibbard, K., ... & Rose, S. K. (2011). The representative concentration pathways: an overview. Climatic change, 109, 5-31. PDF

Wieder, W. R., Cleveland, C. C., Smith, W. K., & Todd-Brown, K. (2015). Future productivity and carbon storage limited by terrestrial nutrient availability. Nature Geoscience, 8(6), 441-444.

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 Appendix: a note on the low-emission scenario

The carbon-cycle papers discussed above look only at feedbacks in the high-emissions scenario or, in the case of the Amazon paper, report recent trends. To try to estimate the effect that these phenomena will have under a lower-human-emissions scenario requires guesswork. What follows is just that, so reader beware.

According to MacDougall et al. (2012) permafrost feedbacks up to 2100 for the RCP2.6 scenario would be about 40% of those for the RCP8.5 case. Applying that percentage to Schuur's permafrost emissions for RCP8.5 gives 60 GtC of emissions from permafrost by 2100 under low human emissions.

The nutrition effect noted by Wieder will be small under a low-emissions scenario, and I will assume that it is zero for these purposes.

There are already 3.8 GtC of dead wood in the Amazon that has accumulated since 1983, according to Bienen et al. That carbon has not yet reached the atmosphere. Assuming this trend continues for several more decades, let's assume a 10 GtC feedback from the Amazon by 2100, noting also that similar processes may be playing out in other tropical forests.

The IPCC AR5 Summary for Policy Makers contains the following paragraph (my highlighting):

Using those numbers, updating them with actual emissions from 2012-2014 and subtracting the carbon cycle feedbacks gives this table:

On this estimation, the carbon-cycle feedbacks from permafrost and reduced tropical forest sinks could reduce our "safe" emissions by 20-30% (incidentally, about the same percentage as for the high-emissions scenario). The time period we have left at current emissions rates would be reduced by 6 years, to as little as 16 years, if we give ourselves a two-thirds chance of staying below two degrees, once we factor in carbon cycle feedbacks.

I should stress again that these are just my estimates, and they are intended for discussion purposes only. Properly integrated studies that incorporate all of the latest carbon-cycle feedback studies under different emissions scenarios have yet to be done.

http://www.skepticalscience.com/CCFBRCP85.html