New study: Freak summer weather and wild jet-stream patterns are on the rise because of global warming

Simulation of jet stream pattern July 22, 2018. (VentuSky.com)

by Jason Samenow, "The Capitol Weather Gang," The Washington Post, October 31, 2018

In many ways, the summer of 2018 marked a turning point, when the effects of climate change — perhaps previously on the periphery of public consciousness — suddenly took center stage. Record high temperatures spread all over the Northern Hemisphere. Wildfires raged out of control. And devastating floods were frequent.

Michael Mann, climate scientist at Pennsylvania State University, along with colleagues, has published a new study that connects these disruptive weather extremes with a fundamental change in how the jet stream is behaving during the summer. Linked to the warming climate, the study suggests this change in the atmosphere’s steering current is making these extremes occur more frequently, with greater intensity, and for longer periods of time.

The study projects this erratic jet-stream behavior will increase in the future, leading to more severe heat waves, droughts, fires and floods.

[Climate change supercharged a hot and dangerous summer]

The jet stream is changing not only because the planet is warming up but also because the Arctic is warming faster than the mid-latitudes, the study says. The jet stream is driven by temperature contrasts, and these contrasts are shrinking. The result is a slower jet stream with more wavy peaks and troughs that Mann and his study co-authors ascribe to a process known as “quasi-resonant amplification.”

The altered jet-stream behavior is important because when it takes deep excursions to the south in the summer, it sets up a collision between cool air from the north and the summer’s torrid heat, often spurring excessive rain. But when the jet stream retreats to the north, bulging heat domes form underneath it, leading to record heat and dry spells.

[‘Extreme’ jet stream pattern spurred a week of wild weather in U.S. this past summer]

If the excursions in the jet stream endure long enough, it can then set the stage for floods where the jet dips, and wildfires and drought where it ascends.

“What made these events [in the summer of 2018] so devastating was not just the extreme nature of the meteorological episodes but their persistence,” Mann said in a blog post discussing the implications of the new study.

The study, published Wednesday in Science Advances, finds that these quasi-resonant amplification events — in which the jet stream exhibits this extreme behavior during the summer — are predicted to increase by 50 percent this century if emissions of carbon dioxide and other greenhouse gases continue unchecked.

Whereas previous work conducted by Mann and others had identified a signal for an increase in these events, this study for the first time examined how they may change in the future using climate model simulations.

“Looking at a large number of different computer models, we found interesting differences,” said Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research and a co-author of the study, in a news release. “Distinct climate models provide quite diverging forecasts for future climate resonance events. However, on average they show a clear increase in such events.”

In an email, Mann said climate models aren’t fully capturing the phenomenon, and, for this reason, we should expect weather extremes “beyond what is typically projected” into the future.

Mann added the existing analyses that attempt to uncover the role of climate change in recent extreme events “are under-attributing the role that climate change is having … because they are not capturing the key mechanism responsible.”

Mann said in his blog commentary that he was particularly struck by the jet-stream behavior in the summer. “In summer 2018, I would argue, that signal was no longer subtle,” he said. “It played out in real time on our television screens and newspaper headlines in the form of an unprecedented hemisphere-wide pattern of extreme floods, droughts, heat waves and wildfires.”

Although model projections suggest these extreme jet-stream patterns will increase as the climate warms, the study concluded that their increase can be slowed if greenhouse gas emissions are reduced along with particulate pollution in developing countries. “[T]he future is still very much in our hands when it comes to dangerous and damaging summer weather extremes,” Mann said. “It’s simply a matter of our willpower to transition quickly from fossil fuels to renewable energy.”

Dr. Jennifer Francis, a climate researcher at Rutgers University who has published work exhibiting changing jet-stream behavior because of climate change, found the results of this new study compelling. “This work takes a big step toward understanding the spate of deadly extreme weather events during recent summers — heat waves, floods and droughts,” she said in an email.

https://www.washingtonpost.com/weather/2018/10/31/study-freak-summer-weather-wild-jet-stream-patterns-are-rise-due-global-warming/?noredirect=on&utm_term=.7afba03610ea

Stefan Rahmstorf's letter to the editor of the New York Times cancelling his subscription due to their hiring Bret Stephens

To the Executive Editor

The New York Times

27 April 2017, via email

Dear Editor, 

I am a climate researcher, professor for physics of the oceans and have worked for eight years as advisor to the German government on global change issues. I regret to have to tell you that hereby I cancel my subscription to the New York Times in the wake of you hiring columnist Bret Stephens. Let me explain my reasons.

When Stephens was hired I wrote to you in protest about his spreading of untruths about climate change, saying “I enjoy reading different opinions from my own, but this is not a matter of different opinions.” I did not cancel then but decided to wait and see. However, the subsequent public defense by the New York Times of the hiring of Stephens has convinced me that the problem at the Times goes much deeper than a single error of judgement. It concerns its attitude towards seeking the truth.

The Times argued that “millions agree with Stephens.” It made me wonder what’s next – when are you hiring a columnist claiming that the sun and the stars revolve around the Earth, because millions agree with that? My heroes are Copernicus, Galilei and Kepler, who sought the scientific truth based on observational evidence and defended it against the powerful authority of the church in Rome, at great personal cost. Had the New York Times existed then – would you have seen it as part of your mission to insult and denigrate these scientists, as Stephens has done with climate scientists?

The Times has denounced the critics of its decision as “left-leaning.” This is an insult to me and was the final straw to cancel my subscription. There is no left-leaning or right-leaning climate science, just as there is no republican or democrat theory of gravity. I have several good climate scientist friends who have been lifelong republicans. Their understanding of climate change does not differ from mine, because it is informed by the evidence.

Quite unlike Stephens’ views on climate change, which run counter to all evidence. He is simply repeating falsehoods spread by various “think tanks” funded by the fossil fuel industry.

In December 2015, Stephens called global warming “imperceptible” and the Paris climate summit a “meeting to combat a notional enemy in the same place where a real enemy just inflicted so much mortal damage.” My colleagues and I have analysed 150,000 temperature time series from around the world, finding that monthly heat records occur five times more often now as a result of global warming than in an unchanging climate (Coumou et al., published in Climatic Change 2013). One of those record-hot months was August 2003 in western Europe. 70,000 people died due to this heat wave. Was global warming “imperceptible” to these people and the ones they left behind? On 15 August 2003, the New York Times reported: “So many bodies were delivered in recent weeks to the Paris morgue that refrigerated tents had to be erected outside the city to accommodate them all.” Was that just a “notional” problem?

Stephens doubts that global warming will continue, claiming that in hundred years “temperatures will be about the same.” That is a shockingly ignorant statement, ignoring over a century of climate science. Our emissions increase the level of CO2 in the atmosphere, it is higher now than in at least 3 million years. CO2 is a greenhouse gas, as demonstrated first in the year 1859 by physicist John Tyndall. CO2 traps heat – more CO2 means a warmer climate. That is basic physics, borne out by the history of climate. Denying these well-established facts is about as smart as claiming the Earth is flat, and best left to cranks, ideologues and fossil fuel lobbyists.

Stephens has claimed that “in the 1970s we were supposed to believe in global cooling.” That’s an age-old climate denier myth. It would have cost Stephens just 60 seconds with Google to find out it is wrong. (Try and google “Did scientists predict an ice age in the 1970s.”) But Stephens is clearly not interested in evidence or seeking the truth about matters.

Last Friday, you sent me an email with the subject: “The truth is more important now than ever.” It made me cringe seeing this in my inbox. It said “thank you for supporting news without fear or favor.” The hypocrisy of that is unbearable, and I will support your newspaper no more. Instead, I will give the money to ClimateFeedback.org, a worldwide network of scientists sorting fact from fiction in climate change media coverage. It is much better invested there.

Best regards,

Prof. Stefan Rahmstorf 

Head of Earth System Analysis, PIK

February’s global temperature spike is a wake-up cal

by Steve Sherwood, UNSW Australia and Stefan Rahmstorf, Potsdam Institute for Climate Impact Research

Global temperatures for February showed a disturbing and unprecedented upward spike. It was 1.35 ℃ warmer than the average February during the usual baseline period of 1951-1980, according to NASA data.

This is the largest warm anomaly of any month since records began in 1880. It far exceeds the records set in 2014 and again in 2015 (the first year when the 1 ℃ mark was breached).

In the same month, Arctic sea ice cover reached its lowest February value ever recorded. And last year carbon dioxide concentration in our atmosphere increased by more than 3 parts per million, another record.

What is going on? Are we facing a climate emergency?

February temperatures from 1880 to 2016 from NASA GISS data. Values are deviations from the base period of 1951-1980. Stefan Rahmstorf

El Niño plus climate change

Two things are combining to produce the record warmth: the well-known global warming trend caused by our greenhouse gas emissions, and an El Niño in the tropical Pacific.

The record shows that global surface warming has always been overlaid by natural climate variability. The biggest single cause of this variability is the natural cycle between El Niño and La Niña conditions. The El Niño in 1998 was a record-breaker, but now we have one that looks even bigger by some measures.

The pattern of warmth in February shows typical signatures of both long-term global warming and El Niño. The latter is very evident in the tropics.

Further north, the pattern looks similar to other Februaries since the year 2000: particularly strong warming in the Arctic, Alaska, Canada and the northern Eurasian continent. Another notable feature is a cold blob in the northern Atlantic, which has been attributed to a slowdown in the Gulf Stream.

The February warming spike brought us at least 1.6 ℃ above pre-industrial global average temperatures. This means that, for the first time, we have passed the 1.5 ℃ international aspirational goal agreed in December in Paris. We are coming uncomfortably close to 2 ℃.

Fortunately, this is temporary: the El Niño is beginning to subside.

Emissions still increasing

Unfortunately, we have done little about the underlying warming. If unchecked, this will cause these breaches to happen more and more often, with a greater than 2 ℃ breach perhaps only a couple of decades away.

The greenhouse gases slowly heating the Earth are still increasing in concentration. The 12-month average surpassed 400 parts per million roughly a year ago – the highest level for at least a million years. The average rose even faster in 2015 than previous years (probably also due to the El Niño, as this tends to bring drought to many parts of the globe, meaning less carbon is stored in plant growth).

A glimmer of hope is that our carbon dioxide emissions from fossil fuels have, for the first time in decades, stopped increasing. This trend has been evident over the past couple of years, mainly due to a decline of coal use in China, which recently announced the closure of around 1,000 coal mines.

Have we underestimated global warming?

Does the “spike” change our understanding of global warming? In thinking about climate change, it is important to take the long view. A predominant La Niña-like situation over recent years did not mean global warming had “stopped” as a few public figures were (and probably still are) claiming.

Likewise, a hot spike due to a major El Niño event – even though it is surprisingly hot – doesn’t mean global warming was underestimated. In the longer run the global warming trend agrees very well with longstanding predictions. But these predictions nevertheless paint a picture of a very warm future if emissions are not brought down soon.

The situation is similar to that of a serious illness like cancer: the patient usually does not get slightly worse each day, but has weeks when the family thinks he may be recovering, followed by terrible days of relapse. The doctors do not change their diagnosis each time this happens, because they know this is all a part of the disease.

Although the current El-Niño-driven spike is temporary, it will last long enough to have some severe consequences. For example, a massive coral bleaching event now appears likely on the Great Barrier Reef.

Here in Australia we have been breaking heat records in the past few months, including 39 straight days in Sydney above 26 ℃ (double the previous record). News reports seem to be focusing on the role of El Niño, but El Niño does not explain why oceans to the south of Australia, and in the Arctic, are at record high temperatures.

The other half of the story is global warming. This is boosting each successive El Niño, along with all its other effects on ice sheets and sea level, the global ecosystem and extreme weather events.

This is the true climate emergency: it is getting more difficult with each passing year for humanity to prevent temperatures from rising above 2 ℃. February should remind us how pressing the situation is.

Steve Sherwood, Director and ARC Laureate Fellow, Climate Change Research Centre, UNSW Australia, and Stefan Rahmstorf, Professor of Physics of the Oceans, Potsdam Institute for Climate Impact Research.

This article was originally published on The Conversation. Read the original article.

Stefan Rahmstorf, SMH: Australia must step up on emission cuts to rejoin world's enlightened countries

The climate change goal posts have long been changing everywhere, except Australia

A polar bear on the ice floe in Baffin Bay above the Arctic circle, where the ice is melting.

by Stefan Rahmstorf, The Sydney Morning Herald, November 5, 2015

As an oceanographer and climate researcher, I have mapped plenty of alarming trends over the past few decades. But I am confident that humanity has the capability, capacity and means to keep the increase in global temperatures below the potentially catastrophic threshold of 2 degrees. And I am also cautiously optimistic that a meaningful global agreement to limit greenhouse gas emissions will emerge from the upcoming Paris talks.

From an Australian perspective this might seem fanciful. But virtually everywhere else in the world, the climate change goal posts have long been shifting.

It's now half a century since the first official scientific report, published in November 1965 by scientific advisors to US President Lyndon Johnson, warned that rising greenhouse gas emissions would cause global warming that "could be deleterious from the point of view of human beings." The predictions of that report, based on sound physical science, have turned out to be true. And despite myriad entrenched vested interests worldwide we are finally making headway in moving away from fossil fuels, the root cause of the unfolding climate crisis.

Professor Stefan Rahmstorf.

My own country, Germany, moved early and wagered much public, private and political capital on a transformation to a modern sustainable economy. Today, our emissions are 27% below 1990 levels while our GDP has almost doubled. The European Union as a whole has likewise grown its economy while reducing its emissions by about a fifth. We've effectively debunked the myth that reducing emissions harms the economy. Innovation and ingenuity seldom do.

And it's not just a European story. Worldwide, 19% of our energy now comes from renewable sources and growth in these industries is exponential, not linear. Last year half of all global energy investments were in renewables, led by China. This momentum suggests that investments in fossil fuels will collapse worldwide in the coming years as investors realise they'd risk massive stranded assets.

The numbers are also in on the true costs of transitioning to renewable energy worldwide. We can afford it. About US$500 billion a year is spent looking for new fossil fuel deposits. That's about the same amount of money we need to invest in renewable energy to keep global warming below 2 degrees. On top of that there's some US$500 billion in various government subsidies to fossil fuels.

The many economic studies reviewed by the Intergovernmental Panel on Climate Change put it clearly: the cost of keeping global warming to below 2 degrees would reduce global economic growth by just 0.06% a year. The expected 8-fold increase in global wealth by the year 2100 achieved by assuming a typical 2.3% annual growth would be reached just two years later. Against this, the human and economic costs of "business as usual" as oceans acidify, weather extremes intensify, crops fail, and sea levels rise are virtually incalculable.

If the technology and economics are in place, that leave politics standing in our way. But as a German, I've seen consensus achieved across the political spectrum. This cross-party consensus is important for German businesses: most enterprises are less worried by climate policy than by a politically unpredictable investment climate.

When the delegates of more than 190 countries meet in Paris next month the emissions reduction targets they put on the table will probably not suffice to keep global warming below 2 degrees. We know this because most nations have already declared their hand. This need not be a fatal flaw of a new global agreement on emissions reductions, as long as it provides a structure on which much more can be built.

We went down this same path when the growing hole in the ozone layer, and the resulting increase in UV radiation, precipitated global co-operation to phase out chlorofluorocarbons. The original Montreal Protocol on CFCs was far too weak, but it was subsequently tightened and proved to be one of our most effective international environmental instruments.

I also believe we now have a critical global mass of decarbonisation pioneers, both nations and businesses, that won't wait around for the laggards. China's lead is very important here. It is the world's largest emitter, but its government is well aware how threatened the country's future is by increasing drought, rising sea levels and the risk of stronger cyclones.

As a visitor to Australia, it feels like the country has moved backwards.

Back in the 1980s Australia was among the world's most enlightened countries regarding climate change. In 1990, its government decided to lower greenhouse gas emissions by 20% by 2005. But a concerted effort led by fossil fuel interests and aided by some media managed to sow doubt about man made climate change, and Australian emissions have kept rising.

Now, Australia is nearly the only industrial nation that for Paris has pledged emissions reductions that wouldn't even bring down its emissions significantly below the 1990 level by 2030. And it does not have the policies in place that would deliver even those modest reductions. Australia thus has much scope to improve – and it has fantastic renewable energy resources that make every German envious.

Professor Stefan Rahmstorf, of the Potsdam Institute for Climate Impact Research, will speak at Climate for Change, presented by UNSW and the Herald, at UNSW on Tuesday, November 10, 2015.

http://www.smh.com.au/comment/australia-must-step-up-on-emission-cuts-to-rejoin-worlds-enlightened-countries-20151103-gkpk5a.html

Most Comprehensive Paleoclimate Reconstruction Confirms Hockey Stick

by Stefan Rahmstorf (via Scilogs), by Climate Guest Contributor, Climate Progress, July 8, 2013

The past 2,000 years of climate change have now been reconstructed in more detail than ever before by the PAGES 2k project. The results reveal interesting regional differences between the different continents, but also important common trends. The global average of the new reconstruction looks like a twin of the original “hockey stick,” the first such reconstruction published 15 years ago.

Green dots show the 30-year average of the new PAGES 2k reconstruction. The red curve shows the global mean temperature, according HadCRUT4 data from 1850 onwards. In blue is the original hockey stick of Mann, Bradley and Hughes (1999 ) with its uncertainty range (light blue). Graph by Klaus Bitterman.

78 researchers from 24 countries, together with many other colleagues, worked for 7 years in the PAGES 2k project on the new climate reconstruction. “2k” stands for the last 2,000 years, while PAGES stands for the Past Global Changes program launched in 1991. Recently, their new study was published in Nature Geoscience. It is based on 511 climate archives from around the world, from sediments, ice cores, tree rings, corals, stalagmites, pollen or historical documents and measurements (Fig. 1). All data are freely available.

Figure 1. The map gives an overview of the studied continental areas and the particular combination of the proxies used for each. Source: Nature Geoscience.

The climate history of the past one to two thousand years was reconstructed for 7 continental regions in 30-year intervals (Figure 2).

Figure 2. Temperature evolution of the individual continental regions (30-year average). Red means hot, blue is cold. Source: Nature Geoscience.

Regional climate evolution

The data show, as expected, significant regional differences. Such regional patterns are an important clue to the causes and mechanisms of climate change. Significant local variations in climate can occur through changes in the atmospheric or oceanic circulation patterns, with very little effect on global mean temperature, because heat is only distributed differently (a recent short-term example is the record cold March in parts of Europe while Greenland was extremely warm – nothing unusual happened in the global mean). Changes in global mean temperature, however, occur due to changes in radiative forcing (e.g., solar fluctuations). This forcing can be globally uniform (e.g., well-mixed greenhouse gases) or can have a regional pattern (for example, volcanic eruptions and orbital cycles). So in the individual continents one expects a response to the forcings, superimposed by internal fluctuations.

The data shown in Figure 2 reflect this: they show some coherent signals, especially a long-term cooling trend that leads to increasingly cooler climate conditions from the relatively warm Middle Ages until this is turned around in the late 19th Century (see the next section). But, as expected, some regional variability is superimposed, especially on shorter time scales of decades to a century, where particularly warm and cold phases do not coincide in their timing on different continents, as the authors emphasize in the abstract:

There were no globally synchronous multi-decadal hot or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age.

But they identify some shorter intervals where extremely cold conditions coincide with major volcanic eruptions and / or solar minima (as already known from previous studies).

Global Trends

The global mean temperature is of particular interest because it is a direct response to the global radiative forcing, buffered by the thermal inertia of the oceans. This follows from the first law of thermodynamics, i.e. the law of conservation of energy. So the globally coherent signals indicate globally effective forcings. The authors write in the abstract:

The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. (…) Recent warming reversed the long-term cooling; during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.

The following Figure 3 therefore compares the area-weighted mean over the continents (b) with some previous northern hemisphere reconstructions (a) and the forcings (f, g, h). The basic pattern – a long-term slow cooling which in the late 19th Century turns into a rapid warming – has been known for 15 years and is often compared to a hockey stick: the long cooling trend is the handle, the modern warming the angled blade.

Figure 3. Temperature evolution according to some previous studies (a) and from the new PAGES 2k reconstruction (b). The panels f, g and h show the radiative forcing, see text. Source: Nature Geoscience. (Click here for enlarged image.)

Comparison with the forcings shows that this blade is due to the radiative forcing from the increasing amount of greenhouse gases in the atmosphere (green line in g). The solar and volcanic forcing can be reconstructed only with some uncertainty, therefore two variants are shown. For solar forcing, it is less the shape of the curve but mainly the amplitude which is controversial (a constant conversion factor) – the very high amplitude  adopted by Shapiro (dotted line) is widely regarded as highly questionable (see, e.g., Feulner and Judge et al.). But even with this extreme assumption, the solar forcing in the 20th Century cannot compete by far with the greenhouse gases, and it also does not match the temperature evolution.

Interesting especially on long time scales of thousands of years is the orbital forcing (the well-known  Milanković  cycles; here is an online calculator for the radiative forcing). In the north, especially the summer insolation is important (green curve in panel h), because it is greatly amplified by the albedo feedback (i.e., by changes in the ice and snow-covered area). This may explain part of the cooling trend in the Northern Hemisphere. In Antarctica the situation is different: in summer the ice-covered area is relatively constant at about the size of the Antarctic continent because there is little sea ice and the trend of solar radiation (blue curve in panel h) is relatively weak. Therefore, the summer insolation change is much less important. But the annual average insolation in Antarctica (not shown in the PAGES graph) has declined strongly due to the orbital cycles over the past two millennia. This could be the explanation for the long-term cooling in Antarctica, which (unlike in the global average) has not yet been made up for by the modern anthropogenic warming.

Climate models are computer programs that calculate climate evolution from such forcings, based on the equations of thermo-and hydrodynamics. In the last IPCC report 18 such model calculations for the last millennium by different research groups are listed, almost all of which reproduce the reconstructed climate history from proxy data reasonably well (see IPCC, Figs. 6.13 and 6.14: the red-dashed outlier in 6.13 is the drift-affected model used by von Storch et al. in their failed attempt to discredit Mann et al.). The models also show that without the anthropogenic forcing there would have been no warming in the last 150 years.

The first comparable hockey stick curve was published in 1998 and 1999 by Mann et al. – at that time based on data only from the Northern Hemisphere. Figure 4 compares the new hockey stick (from panel b in Figure 3) with this original hockey stick. (The data for the original hockey stick have also been available since 1999 on the NOAA Paleoclimatology website.)

Figure 4. Green dots show the 30-year average (area-weighted mean over the continents) of the new PAGES 2k reconstruction, as shown in Figure 3b. The red curve shows the global mean temperature, according HadCRUT4 data from 1850 onwards (also in Figure 3b, smoothed with a 30-year window). In blue is the original hockey stick of man, Bradley and Hughes (1999 ) with its uncertainty range (light blue). Graph by Klaus Bitterman.

For the scientific community, the confirmation of the old hockey stick is no surprise (except perhaps for the closeness of the match); many other climate reconstructions with a similar time evolution have already appeared since. Mann et al. at the time cautiously assumed a wide margin of uncertainty (light blue) because of their limited data base and a possible underestimation of the variance by their method; later reconstructions run largely within this margin. The work of Mann and colleagues has gained the highest recognition. For example, Bradley was honoured in 2007 with the Oeschger Medal of the European Geosciences Union and Mann likewise in 2012, and both were (as well as Hughes) elected as fellows of the American Geophysical Union. Politically motivated attacks on their work were immense, however; both Bradley and Mann have published books about that experience:

• The Hockey Stick and the Climate Wars: Dispatches from the Front Lines by Michael E. Mann
• Global Warming and Political Intimidation: How Politicians Cracked Down on Scientists As the Earth Heated Up by Raymond S. Bradley

Stefan Rahmstorf is Co-Chair of Earth System Analysis, Potsdam Institute for Climate Impact Research. This article was originally published in German and translated was provided by the author.

Peter Sinclair: A Nasty Surprise in the Greenhouse

by Peter Sinclair, This Is Not Cool, March 23, 2015

The disaster movie “The Day After Tomorrow” was based on long-term scientific concerns about global warming’s impact on the North Atlantic Current, also called the Atlantic Meridional Overturning Circulation – what most people think of as “The Gulf Stream” – although that is a simplification.

The movie was obviously over the top in terms of the projected impacts, but after a decade in which science has downplayed the possibility of such an event, a new paper shows that the circulation is indeed slowing down.

This could signal potential impacts on weather, the food chain, and circulation of oxygen and nutrients throughout the ocean.

I’ve been interviewing key authors of the paper: lead author Stefan Rahmstorf, as well as paleoclimate expert Mike Mann, and glaciologist Jason Box.

This is a paper that could have substantial impact, and might very well be distorted or sensationalized,  – so bookmark this post as a damper for overhyped speculation, as well as a warning about real impacts.

Lead Author Stefan Rahmstorf in RealClimate:

The North Atlantic between Newfoundland and Ireland is practically the only region of the world that has defied global warming and even cooled. Last winter there even was the coldest on record – while globally it was the hottest on record. Our recent study (Rahmstorf et al., 2015) attributes this to a weakening of the Gulf Stream System, which is apparently unique in the last thousand years. 

The whole world is warming. The whole world? No! A region in the subpolar Atlantic has cooled over the past century – unique in the world for an area with reasonable data coverage (Figure 1). So what’s so special about this region between Newfoundland and Ireland?

Figure 1. Linear temperature trend from 1900 to 2013. The cooling in the subpolar North Atlantic is remarkable and well documented by numerous measurements – unlike the cold spot in central Africa, which on closer inspection apparently is an artifact of incomplete and inhomogeneous weather station data.

It happens to be just that area for which climate models predict a cooling when the Gulf Stream System weakens (experts speak of the Atlantic meridional overturning circulation or AMOC, as part of the global thermohaline circulation). That this might happen as a result of global warming is discussed in the scientific community since the 1980s – since Wally Broecker’s classical Nature article “Unpleasant surprises in the greenhouse?” Meanwhile evidence is mounting that the long-feared circulation decline is already well underway.

Difficult to measure 

Climate models have long predicted such a slowdown – both the current 5th and the previous 4th IPCC report call a slowdown in this century “very likely,” which means at least 90% probability. When emissions continue unabated (RCP8.5 scenario), the IPCC expects 12% to 54% decline by 2100 (see also the current probabilistic projections of Schleussner et al., 2014). But the actual past evolution of the flow is difficult to reconstruct owing to the scarcity of direct measurements. Therefore, in our study we use data on sea surface temperatures in order to infer the strength of the flow: we use the temperature difference between the region most strongly influenced by the AMOC and the rest of the Northern Hemisphere.

Figure 2. Schematic of the Atlantic circulation. Surface currents in red, deep currents in blue, sea-ice cover in winter in white. (Source: Rahmstorf, Nature, 1997.) 

Now we are not the first to have inferred from temperature data that the flow must have weakened. Evidence for this was already presented by Dima and Lohmann, 2010, or Drijfhout et al., 2012, among others (for further references see the introduction of our paper). 

What is new is that we have used proxy reconstructions of large-scale surface temperature (Mann et al., 2009) previously published by one of us (study co-author and RealClimate co-founder Mike Mann) that extend back to 900 AD (see “What we can learn from studying the last millennium (or so)”) to estimate the circulation (AMOC) intensity over the entire last 1100 years (Fig. 3). This shows that despite the substantial uncertainties in the proxy reconstruction, the weakness of the flow after 1975 is unique in more than a thousand years, with at least 99% probability. This strongly suggests that the weak overturning is not due to natural variability but rather a result of global warming.

Figure 3. Time series of the temperature difference between the subpolar North Atlantic and the entire northern hemisphere, which can be interpreted as an indicator of the strength of the Atlantic circulation.

Also in 2014 we again find a remarkable cold bubble over the northern Atlantic – as a look at the NASA website shows. 2014 was globally the warmest year on record, 1 °C warmer than the average for 1880-1920. But the subpolar Atlantic was 1-2 °C colder than that baseline! And even more recently, NOAA last week released the stunning temperature analysis for the past winter shown in Figure 4. That winter was globally the warmest since records began in 1880. But in the subpolar North Atlantic, it was the coldest on record! That suggests the decline of the circulation has progressed even further now than we documented in the paper.

Jason Box’s Meltfactor blog:

While global surface temperatures are increasingly dominated by warm anomalies, a conspicuous area of cold has persisted south of Greenland and Iceland visible at the ocean surface in sea surface temperature observations. The abnormal cold there has been more anomalous than the US northeast winter. While the most recent northern winter was the warmest on record globally, the ocean surface area south of Greenland & Iceland had the lowest temperatures in the 136-year record. How could this be? 

A new study estimates the Atlantic Meridional Overturning Circulation (AMOC) using the sea surface temperature difference at that cold spot south of Greenland/Iceland with the Northern Hemispheric temperature from NASA-GISS instrumental records since the 1880s (Hansen and others, 1999) and from coral-based proxies after Sherwood and others (2011) that span years since 500 AD. 

Based on this AMOC reconstruction, the study finds that the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) after 1975. 

1. appears unprecedented in the past millennium;
2. is expected to continue, even intensify through year 2100, as simulated with the MPI-ESM-MR global climate model of the Max Planck Institute in Hamburg (Jungclaus and others, 2013)
3. may result to a large degree from Greenland melting.
My contribution was my work of 6 years, a 172-year Greenland mass balance
reconstruction published in a 3 part series in the Journal of Climate (Box and others, 2013; Box, 2013; Box and Colgan, 2013), enabling Greenland melting to be brought more into context of its ocean thermohaline perturbation.

Mass Balance terms of the Greenland Ice Sheet. Data from Box and Colgan. Cumulative anomaly relative to the mean over 1840-1900, a pre-industrial period during which the Greenland Ice Sheet was approximately in balance.

Melt from Greenland produces water that is lighter and colder than the sea surface waters. The meltwater is light enough to float above the saltier sea surface waters. Because Atlantic surface waters flow northward (see map below), an increasing ice melt freshwater supply (blue line in the figure above) may pile up near the sea surface, capping or backing up the Gulf Stream North Atlantic Drift current that (a) delivers warmth to northwestern Europe and (b) is part of a global ocean heat conveyer. While Bamber and others (2012) set the stage with “Recent large increases in fresh- water fluxes from Greenland into the North Atlantic,” the new study more directly quantifies the possible impact from Greenland on the ocean thermohaline circulation.

Why should we care?
The North Atlantic ocean circulation is an important part of a global ocean circulation that exchanges heat from the equatorial surplus to the poles where the energy is lost by thermal radiation to space. A slowed global oceanic ‘conveyer belt’ may further destabilize our changing global climate. We expect no new ice-age – but major negative effects are possible. The effects could be on global climate, fisheries, or also for example storminess. 

Influence on weather?
The study will stimulate discussion and research on how the large area of negative sea surface temperatures anomalies south of Greenland and Iceland may influence European and downstream weather. Given some heat exchange between a warm air mass with an anomalously cold North Atlantic sea surface, some strengthening of winds, lowering of central pressure of cyclonic systems, should result from increased “baroclinic instability” (see, for example, Holton et al., 1992) arising from an increased temperature difference between sea surface and atmosphere. As the subpolar North Atlantic cools and the atmosphere warms, the physics are set to strengthen cyclonic circulation in warm air masses. Conversely, cold air masses drifting off of North America would be less prone to baroclinic deepening. In any case, the perturbation may be felt not just in that part of the world, but downstream, and like the proverbial butterfly (seagull) flapping its wings, alters atmospheric and oceanic flow, with certain though hard to predict downstream consequences.

UPDATE: Washington Post now covering another aspect of this story.

However, there are many other effects, ranging from dramatic impacts on fisheries to, perhaps most troubling of all, the potential for extra sea level rise in the North Atlantic region. 

That may sound surprising, but here’s how it works. We’re starting out from a situation in which sea level is “anomalously low” off the U.S. east coast due to the motion of the Gulf Stream. This is for at least two reasons. First, explains Rahmstorf’s co-author Michael Mann of Penn State University, there’s the matter of temperature contrast: waters to the right or east of the Gulf Stream, in the direction of Europe, are warmer than those on its left or west. Warm water expands and takes up more area than denser cold water, so sea level is also higher to the right side of the current, and lower off our coast. 

“So if you weaken the ‘Gulf Stream’ and weaken that temperature contrast…sea level off the U.S. east coast will actually rise!” explains Mann by e-mail. 

But there’s another factor, too, involving what is called the “geostrophic balance of forces” in the ocean. This gets wonky, but the bottom line result is that “sea surface slope perpendicular to any current flow, like the Gulf Stream, has a higher sea level on its right hand side, and the lower sea level on the left hand slide,” says Rahmstorf. 

We’re on the left hand side of the Gulf Stream. So weaken the flow, and you also raise the sea level. (For further explanation, see here and here.) 

Indeed, researchers recently found a sudden, 4-inch, sea-level rise of the U.S. East Coast in 2009 and 2010, which they attributed to a slowdown of the Atlantic overturning circulation. Rahmstorf says that “for a big breakdown of the circulation, [sea level rise] could amount to one meter, in addition to the global sea level rise that we’re expecting from global warming.”

http://climatecrocks.com/2015/03/23/a-nasty-surprise-in-the-greenhouse-new-paper-new-video/