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Stefan Rahmstorf

Filed under: — stefan @ 6 December 2004

A physicist and oceanographer by training, Stefan Rahmstorf has moved from early work in general relativity theory to working on climate issues.

He has done research at the New Zealand Oceanographic Institute, at the Institute of Marine Science in Kiel and since 1996 at the Potsdam Institute for Climate Impact Research in Germany (in Potsdam near Berlin).

His work focuses on the role of ocean currents in climate change, past and present.

In 1999 Rahmstorf was awarded the $ 1 million Centennial Fellowship Award of the US-based James S. McDonnell foundation.

Since 2000 he teaches physics of the oceans as a professor at Potsdam University.

Rahmstorf is a member of the Academia Europaea and served from 2004-2013 in the German Advisory Council on Global Change (WBGU). He was also one of the lead authors of the 4th Assessment Report of the IPCC. In 2007 he became an Honorary Fellow of the University of Wales and in 2010 a Fellow of the American Geophysical Union.

More information about his research and publication record can be found here.

All posts by stefan.

Climate Change and Extreme Summer Weather Events – The Future is still in Our Hands


Summer 2018 saw an unprecedented spate of extreme weather events, from the floods in Japan, to the record heat waves across North America, Europe and Asia, to wildfires that threatened Greece and even parts of the Arctic. The heat and drought in the western U.S. culminated in the worst California wildfire on record. This is the face of climate change, I commented at the time.

Some of the connections with climate change here are pretty straightforward. One of the simplest relationships in all of atmospheric science tells us that the atmosphere holds exponentially more moisture as temperatures increase. Increased moisture means potentially for greater amounts of rainfall in short periods of time, i.e. worse floods. The same thermodynamic relationship, ironically, also explains why soils evaporate exponentially more moisture as ground temperatures increase, favoring more extreme drought in many regions. Summer heat waves increase in frequency and intensity with even modest (e.g. the observed roughly 2F) overall warming owing to the behavior of the positive “tail” of the bell curve when you shift the center of the curve even a small amount. Combine extreme heat and drought and you get more massive, faster-spreading wildfires. It’s not rocket science.

But there is more to the story. Because what made these events so devastating was not just the extreme nature of the meteorological episodes but their persistence. When a low-pressure center stalls and lingers over the same location for days at a time, you get record accumulation of rainfall and unprecedented flooding. That’s what happened with Hurricane Harvey last year and Hurricane Florence this year. It is also what happened with the floods in Japan earlier this summer and the record summer rainfall we experienced this summer here in Pennsylvania. Conversely, when a high-pressure center stalls over the same location, as happened in California, Europe, Asia and even up into the European Arctic this past summer, you get record heat, drought and wildfires.

Scientists such as Jennifer Francis have linked climate change to an increase in extreme weather events, especially during the winter season when the jet stream and “polar vortex” are relatively strong and energetic. The northern hemisphere jet stream owes its existence to the steep contrast in temperature in the middle latitudes (centered around 45N) between the warm equator and the cold Arctic. Since the Arctic is warming faster than the rest of the planet due to the melting of ice and other factors that amplify polar warming, that contrast is decreasing and the jet stream is getting slower. Just like a river traveling over gently sloping territory tends to exhibit wide meanders as it snakes its way toward the ocean, so too do the eastward-migrating wiggles in the jet stream (known as Rossby waves) tend to get larger in amplitude when the temperature contrast decreases. The larger the wiggles in the jet stream the more extreme the weather, with the peaks corresponding to high pressure at the surface and the troughs low pressure at the surface. The slower the jet stream, the longer these extremes in weather linger in the same locations, giving us more persistent weather extremes.

Something else happens in addition during summer, when the poleward temperature contrast is especially weak. The atmosphere can behave like a “wave guide”, trapping the shorter wavelength Rossby waves (those that that can fit 6 to 8 full wavelengths in a complete circuit around the Northern Hemisphere) to a relatively narrow range of latitudes centered in the mid-latitudes, preventing them from radiating energy away toward lower and higher latitudes. That allows the generally weak disturbances in this wavelength range to intensify through the physical process of resonance, yielding very large peaks and troughs at the sub-continental scale, i.e. unusually extreme regional weather anomalies. The phenomenon is known as Quasi-Resonant Amplification or “QRA”, and (see Figure below).

Many of the most damaging extreme summer weather events in recent decades have been associated with QRA, including the 2003 European heatwave, the 2010 Russian heatwave and wildfires and Pakistan floods (see below), and the 2011 Texas/Oklahoma droughts. More recent examples include the 2013 European floods, the 2015 California wildfires, the 2016 Alberta wildfires and, indeed, the unprecedented array of extreme summer weather events we witnessed this past summer.

The increase in the frequency of these events over time is seen to coincide with an index of Arctic amplification (the difference between warming in the Arctic and the rest of the Northern Hemisphere), suggestive of a connection (see Figure below).

Last year we (me and a team of collaborators including RealClimate colleague Stefan Rahmstorf) published an article in the Nature journal Scientific Reports demonstrating that the same pattern of amplified Arctic warming (“Arctic Amplification”) that is slowing down the jet stream is indeed also increasing the frequency of QRA episodes. That means regional weather extremes that persist longer during summer when the jet stream is already at its weakest. Based on an analysis of climate observations and historical climate simulations, we concluded that the “signal” of human influence on QRA has likely emerged from the “noise” of natural variability over the past decade and a half. In summer 2018, I would argue, that signal was no longer subtle. 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.

In a follow-up article just published in the AAAS journal Science Advances, we look at future projections of QRA using state-of-the-art climate model simulations. It is important to note that that one cannot directly analyze QRA behavior in a climate model simulation for technical reasons. Most climate models are run at grid resolutions of a degree in latitude or more. The physics that characterizes QRA behavior of Rossby Waves faces a stiff challenge when it comes to climate models because it involves the second mathematical derivative of the jet stream wind with respect to latitude. Errors increase dramatically when you calculate a numerical first derivative from gridded fields and even more so when you calculate a second derivative. Our calculations show that the critical term mentioned above suffers from an average climate model error of more than 300% relative to observations. By contrast, the average error of the models is less than a percent when it comes to latitudinal temperature averages and still only about 30% when it comes to the latitudinal derivative of temperature.

That last quantity is especially relevant because QRA events have been shown to have a well-defined signature in terms of the latitudinal variation in temperature in the lower atmosphere. Through a well-established meteorological relationship known as the thermal wind, the magnitude of the jet stream winds is in fact largely determined by the average of that quantity over the lower atmosphere. And as we have seen above, this quantity is well captured by the models (in large part because the change in temperature with latitude and how it responds to increasing greenhouse gas concentrations depends on physics that are well understood and well represented by the climate models).

These findings, incidentally have broader implications. First of all, climate model-based studies used to assess the degree to which current extreme weather events can be attributed to climate change are likely underestimating the climate change influence. One model-based study for example suggested that climate change only doubled the likelihood of the extreme European heat wave this summer. As I commented at the time, that estimate is likely too low for it doesn’t account for the role that we happen to know, in this case, that QRA played in that event. Similarly, climate models used to project future changes in extreme weather behavior likely underestimate the impact that future climate changes could have on the incidence of persistent summer weather extremes like those we witnessed this past summer.

So what does our study have to say about the future? We find that the incidence of QRA events would likely continue to increase at the same rate it has in recent decades if we continue to simply add carbon dioxide to the atmosphere. But there’s a catch: The future emissions scenarios used in making future climate projections must also account for factors other than greenhouse gases. Historically, for example, the use of old coal technology that predates the clean air acts produced sulphur dioxide gas which escapes into the atmosphere where it reacts with other atmospheric constituents to form what are known as aerosols.

These aerosols caused acid rain and other environmental problems in the U.S. before factories in the 1970s were required to install “scrubbers” to remove the sulphur dioxide before it leaves factory smokestacks. These aerosols also reflect incoming sunlight and so have a cooling effect on the surface in the industrial middle-latitudes where they are produced. Some countries, like China, are still engaged in the older, dirtier-form of coal burning. If we continue with business-as-usual burning of fossil fuels, but countries like China transition to more modern “cleaner” coal burning to avoid air pollution problems, we are likely to see a substantial drop in aerosols over the next half century. Such an assumption is made in the Intergovernmental Panel on Climate Change (IPCC)’s “RCP 8.5” scenario—basically, a “business as usual” future emissions scenario which results in more than a tripling of carbon dioxide concentrations relative to pre-industrial levels (280 parts per million) and roughly 4-5C (7-9F) of planetary warming by the end of the century.

As a result, the projected disappearance of cooling aerosols in the decades ahead produces an especially large amount of warming in middle-latitudes in summer (when there is the most incoming sunlight to begin with, and, thus, the most sunlight to reflect back to space). Averaged across the various IPCC climate models there is even more warming in mid-latitudes than in the Arctic—in other words, the opposite of Arctic Amplification i.e. Arctic De-amplification (see Figure below). Later in the century after the aerosols disappear greenhouse warming once again dominates and we again see an increase in QRA events.

So, is there any hope to avoid future summers like the summer of 2018? Probably not. But in the scenario where we rapidly move away from fossil fuels and stabilize greenhouse gas concentrations below 450 parts per million, giving us a roughly 50% chance of averting 2C/3.6F planetary warming (the so-called “RCP 2.6” IPCC scenario) we find that the frequency of QRA events remains roughly constant at current levels.

While we will presumably have to contend with many more summers like 2018 in the future, we could likely prevent any further increase in persistent summer weather extremes. In other words, the future is still very much in our hands when it comes to dangerous and damaging summer weather extremes. It’s simply a matter of our willpower to transition quickly from fossil fuels to renewable energy.

Does a slow AMOC increase the rate of global warming?

Filed under: — stefan @ 18 July 2018

Established understanding of the AMOC (sometimes popularly called Gulf Stream System) says that a weaker AMOC leads to a slightly cooler global mean surface temperature due to changes in ocean heat storage. But now, a new paper in Nature claims the opposite and even predicts a phase of rapid global warming. What’s the story?

By Stefan Rahmstorf and Michael Mann

In 1751, the captain of an English slave-trading ship made a historic discovery. While sailing at latitude 25°N in the subtropical North Atlantic Ocean, Captain Henry Ellis lowered a “bucket sea-gauge” down through the warm surface waters into the deep. By means of a long rope and a system of valves, water from various depths could be brought up to the deck, where its temperature was read from a built-in thermometer. To his surprise Captain Ellis found that the deep water was icy cold.

These were the first ever recorded temperature measurements of the deep ocean. And they revealed what is now known to be a fundamental feature of all the world oceans: deep water is always cold. The warm waters of the tropics and subtropics are confined to a thin layer at the surface; the heat of the sun does not slowly warm up the depths as might be expected. Ellis wrote:

“This experiment, which seem’d at first but mere food for curiosity, became in the interim very useful to us. By its means we supplied our cold bath, and cooled our wines or water at pleasure; which is vastly agreeable to us in this burning climate.”

More »

Does global warming make tropical cyclones stronger?

Filed under: — stefan @ 30 May 2018

By Stefan Rahmstorf, Kerry Emanuel, Mike Mann and Jim Kossin

Friday marks the official start of the Atlantic hurricane season, which will be watched with interest after last year’s season broke a number of records and e.g. devastated Puerto Rico’s power grid, causing serious problems that persist today. One of us (Mike) is part of a team that has issued a seasonal forecast (see Kozar et al 2012) calling for a roughly average season in terms of overall activity (10 +/- 3 named storms), with tropical Atlantic warmth constituting a favorable factor, but predicted El Nino conditions an unfavorable factor.  Meanwhile, the first named storm, Alberto, has gone ahead without waiting for the official start of the season.

In the long term, whether we will see fewer or more tropical cyclones in the Atlantic or in other basins as a consequence of anthropogenic climate change is still much-debated. There is a mounting consensus, however, that we will see more intense hurricanes. So let us revisit the question of whether global warming is leading to more intense tropical storms. Let’s take a step back and look at this issue globally, not just for the Atlantic. More »

Why global emissions must peak by 2020

Filed under: — stefan @ 2 June 2017

(by Stefan Rahmstorf and Anders Levermann)

In the landmark Paris Climate Agreement, the world’s nations have committed to “holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels”. This goal is deemed necessary to avoid incalculable risks to humanity, and it is feasible – but realistically only if global emissions peak by the year 2020 at the latest.

Let us first address the importance of remaining well below 2°C of global warming, and as close to 1.5°C as possible. The World Meteorological Organization climate report[i] for the past year has highlighted that global temperature and sea levels keep rising, reaching record highs once again in 2016. Global sea ice cover reached a record low, and mountain glaciers and the huge ice sheets in Greenland and Antarctica are on a trajectory of accelerating mass loss. More and more people are suffering from increasing and often unprecedented extreme weather events[ii], both in terms of casualties and financial losses. This is the situation after about 1°C global warming since the late 19th Century. More »

The NASA data conspiracy theory and the cold sun

When climate deniers are desperate because the measurements don’t fit their claims, some of them take the final straw: they try to deny and discredit the data.

The years 2014 and 2015 reached new records in the global temperature, and 2016 has done so again. Some don’t like this because it doesn’t fit their political message, so they try to spread doubt about the observational records of global surface temperatures. A favorite target are the adjustments that occur as these observational records are gradually being vetted and improved by adding new data and eliminating artifacts that arise e.g. from changing measurement practices or the urban heat island effect. More about this is explained in this blog article by Victor Venema from Bonn University, a leading expert on homogenization of climate data. And of course the new paper by Hausfather et al, that made quite a bit of news recently, documents how meticulously scientists work to eliminate bias in sea surface temperature data, in this case arising from a changing proportion of ship versus buoy observations. More »

Record heat despite a cold sun

Filed under: — stefan @ 14 November 2016

Global temperature goes from heat record to heat record, yet the sun is at its dimmest for half a century.

For a while, 2010 was the hottest year on record globally. But then it got overtopped by 2014. And 2014 was beaten again by 2015. And now 2016 is so warm that it is certain to be once again a record year. Three record years in a row – that is unprecedented even in all those decades of global warming.

Strangely, one aspect of this gets barely mentioned: all those heat records occur despite a cold sun (Figs. 1 and 2). The last solar minimum (2008-2010) was the lowest since at least 1950, while the last solar maximum (2013-2015) can hardly be described as such. This is shown, among others, by the sunspot data (Fig. 1) as well as measurements of the solar luminosity from satellites (Fig. 2). Other indicators of solar activity indicate cooling as well (Lockwood and Fröhlich, Proc. Royal Society 2007).

herdsoftwidget

Fig. 1 Time evolution of global temperature, CO2 concentration and solar activity. Temperature and CO2 are scaled relative to each other according to the physically expected CO2 effect on climate (i.e. the best estimate of transient climate sensitivity). The amplitude of the solar curve is scaled to correspond to the observed correlation of solar and temperature data. (Details are explained here.) You can generate and adapt this graph to your taste here, where you can also copy a code with which the graph can be embedded as a widget on your own website (as on my home page). Thus it will be automatically updated each year with the latest data. Thanks to our reader Bernd Herd who programmed this. More »

Don’t make a choice that your children will regret

Filed under: — group @ 4 November 2016

Dear US voters,

the world is holding its breath. The stakes are high in the upcoming US elections. At stake is a million times more than which email server one candidate used, or how another treated women. The future of humanity will be profoundly affected by your choice, for many generations to come.

The coming four years is the last term during which a US government still has the chance, jointly with the rest of the world, to do what is needed to stop global warming well below 2°C and closer to 1.5°C, as was unanimously decided by 195 nations in the Paris Agreement last December. The total amount of carbon dioxide the world can still emit in order to have at least a 50% chance to stop warming at 1.5 °C will, at the current rate of emissions, be all used up in under ten years! This time can only be stretched out by making emissions fall rapidly.

Even 2°C of global warming is very likely to spell the end of most coral reefs on Earth. 2°C would mean a largely ice-free Arctic ocean in summer, right up to the North Pole. Even 2°C of warming is likely to destabilize continental ice sheets and commit the world to many meters of sea-level rise, lasting for millennia. Further global warming will likely lead to increasing extreme weather, droughts, harvest failures, and the risk of armed conflict and mass migration.

greenland00037small

Meltwater on the Greenland Ice Sheet. Photo with kind permission by Ragnar Axelsson.

In case you have any doubts about the science: in the scientific community there is a long-standing consensus that humans are causing dangerous global warming, reflected in the clear statements of many scientific academies and societies from around the world. None of the 195 governments that signed the Paris Agreement saw any reasons for doubting the underlying scientific facts; doubts about the science that you see in some media are largely manufactured by interest groups trying to fool you.

You have a fateful choice to make. The policies of candidates and parties on climate change could hardly be more different. Hillary Clinton would continue to work with the international community to tackle the global warming crisis and help the transition to modern clean and renewable energies. Donald Trump denies that the problem even exists and has promised to go back to coal and to undo the Paris Agreement, which comes into force today, the 4th of November 2016, as culmination of over twenty years of negotiations.

Please consider this carefully. This is not an election about personalities, it is about policies that will determine our future for a long time to come. While the presidential race has gotten the most attention, voters should consider climate not just at the ‘top of the ticket’, but all the way down the ballot. Don’t make a choice that you, your children and your children’s children will regret forever.

David Archer, Rasmus Benestad, Ray Bradley, Michael Mann, Ray Pierrehumbert, Stefan Rahmstorf and Eric Steig

How long does it take Antarctica to notice the Northern Hemisphere is warming?

Filed under: — eric @ 29 April 2015

Eric Steig

A series of large and abrupt climate changes occurred during the last ice age, most clearly expressed in ice cores from Greenland and other paleoclimate data from the circum-North-Atlantic region. Since the discovery of these events, we’ve been trying to pin down the timing of abrupt climate changes elsewhere on the globe. Were there corresponding events in the Southern Hemisphere? And did they occur at the same time? A new paper published this week in Nature (April 30th, 2015) provide a significantly updated answer to these questions. Many in the climate research community — both modern climate and paleoclimate — will find the results quite interesting.

The new paper is the culmination of a huge effort to develop the best-dated long ice core record from Antarctica, rivaling the GISP2 and GRIP ice cores obtained from central Greenland in the early 1990s, and the more recent NGRIP and NEEM cores from North Greenland (e.g. NEEM Community Members, 2014). The core was obtained at the West Antarctic Ice Sheet divide (WAIS Divide), led by the Desert Research Institute and the University of Nevada, with the University of New Hampshire. The new paper was written by a consortium of postdocs, faculty, and students at the Oregon State University and University of Washington: Christo Buizert, myself, and Joel Pedro (now at University of Copenhagen), with Brad Markle, Ed Brook, Jeff Severinghaus and Ken Taylor. We have more than 70 other co-authors — faculty colleagues, students, postdocs, logistics coordinators, and ice-drillers — who all made substantial contributions as well. These deep ice cores are a lot of work!

We published records from the WAIS Divide ice core in 2013, covering the last two millennia and the last 30 thousand years (Steig et al., 2013, WAIS Divide Project Members, 2013). Our new work, WAIS Divide Project Members, 2015, extends the record to the bottom of the core (nearly the bottom of the ice sheet at 3400 m depth), and an age of 68 thousand years. Details on the timescale for the core are given in the open-access paper in Climate of the Past (Buizert et al., 2015). The new paper in Nature provides a comparison of the timing of changes in Antarctic temperature with the timing of the abrupt warming and cooling events that characterize the Greenland ice core records. Note that the comparison is actually made between the records of oxygen isotope ratios (δ18O), but we have very strong evidence that these track temperature quite faithfully on the relevant timescales, so I’ll use “temperature” here for simplicity.

The abrupt events in Greenland, characterized by rapid transitions from cold “stadial” to warm “interstadial” conditions and back, and commonly known as Dansgaard-Oeschger (D-O) events, were felt across the Northern Hemisphere almost immediately, as far as we can tell. But the impact of D-O events in Antarctica has been ambiguous. We’ve known for some time that temperatures in Antarctica change more slowly, and with much smaller amplitude, than in the Northern Hemisphere. In general, the Antarctic temperatures begin to decline when Greenland warms abruptly, and to increase when Greenland cools abruptly (Blunier et al., 1998). This relationship is often called the “bipolar seesaw” and is commonly attributed to the redistribution of heat between the Northern and Southern Hemispheres via changes in the Atlantic meridional overturning circulation (AMOC). We’ve also been pretty sure that each of the D-O interstadials has a corresponding warm peak in Antarctica, referred to as the “Antarctic Isotope Maximum” (AIM) events (EPICA Community Members, 2006, and Stefan Rahmstorf’s write-up in an earlier RealClimate post). But the exact phase relationship has been unknown, making it problematic to validate model simulations with confidence (see e.g. Roe and Steig, 2004, Steig, 2006).
WAIS_Divide_record

The first paper to really start to pin down the phase relationship was that of Pedro et al., 2011, who showed that the most recent of the major abrupt events in Greenland cores (the Bølling warming about 14,700 years ago and the Younger Dryas cooling about 12,880 years ago), the direction of Antarctic temperatures changed at almost exactly the same time. (Note that the Antarctic temperatures don’t change abruptly — their slow trends simply reverse sign, as shown in the figure). But the uncertainties estimated by Pedro et al. were about 200 years on either side of zero. That’s impressively good precision for something that happened more than 10,000 years ago, but not quite good enough. Our new work firms up these numbers a lot, and shows that Antarctic temperatures did not really change at the same time as the abrupt events in Greenland. Instead, when an abrupt D-O warming occurs in Greenland, it takes about 200 years until the concomitant cooling begins in Antarctica. Similarly, when an abrupt cooling occurs in Greenland, it takes about 200 years until Antarctic starts warming up. Our uncertainties are much smaller, +/-95 years*, and it is very unlikely that our numbers overlap with zero. Antarctica, in other words, almost certainly takes a century or two to notice what is happening in the Greenland.

WAIS_Divide_timing
The 200-year timescale is fascinating, because it is longer than suggested by a number of modeling studies, such as the simple “bipolar seesaw” model of Stocker and Johnson, 2003, as well as the fully-coupled transient run of a general circulation model by He et al., 2013: both show an essentially instantaneous response between the Northern Hemisphere and the Antarctic. Yet it’s also shorter than implied by many discussions of the relationship; for example, we (Steig and Alley, 2002) made the case for a 400-year lag between Greenland Antarctica, which Schmittner et al., 2003 reproduced in a climate model in which the AMOC is perturbed.

So what does the intermediate timescale of ~200 years tell us? It’s important to recognize that there are reasons why one might expect either a “fast” propagation or a “slow” propagation of the Greenland climate signal to the Antarctic. The very large sea ice changes in the North Atlantic associated with D-O events would have an impact on the atmosphere, and this should propagate global signals almost instantaneously. Indeed, this must have occurred, or we wouldn’t have the evidence that we do for abrupt changes in places as far flung as China, India, or the tropical Pacific that are in phase with Greenland temperature change. Methane variations, which are probably of tropical origin, are in phase with Greenland temperature within about 20 years (e.g., Rosen et al., 2014).

The ocean itself can propagate signals very fast, via adjustment of the upper ocean by fast Kelvin waves propagating along the ocean boundaries (e.g. Johnson and Marshall, 2002). For example, Schmittner et al. (2003), and Rind et al. (2001) both found that the North Atlantic signal in their models propagates to the South Atlantic region very quickly, appearing in subsurface waters with a time lag of only about a few decades. However, they also find a century or multi-century delay in the further southward propagation from the South Atlantic to the Antarctic across the Antarctic Circumpolar Current (ACC). Furthermore, the propagation time varies with the strength of the ACC as imposed in the model.

There are, in short, multiple parts of the ocean and atmosphere system involved, and these have different timescales. It appears that our results are right about where the physics suggests they should be. A key factor in capturing this physics properly in models seems to be how (or how well) the ACC is simulated. That’s interesting, and highly relevant to modern climate studies. How the ACC is changing now, or may change in the future, is a topic of significant interest (Fyfe and Saeko, 2005; Böning et al., 2008). And the question of how long it will take the Antarctic to catch up with the rest of the globe is of critical importance to long term projections of the response of the ice sheet to climate change — and hence the response of sea level. The D-O events and the current anthropogenic global warming are of course very different beasts, but the long timescales indicated by our results are certainly in keeping with climate model projections of the future, showing that most of Antarctic should lag the rest of the planet (recent rapid warming on the Antarctic Peninsula and West Antarctica not withstanding).

Note that our results should not be taken as demonstrating anything very specific in terms of the cause of Dansgaard-O events. To be sure, the results demonstrate a clear north-to-south direction in the propagation of the climate signal associated with abrupt D-O warming and cooling events. But that doesn’t tell us what the driving “trigger” is. To use an analogy suggested by co-author Severinghaus, suppose we didn’t know anything about the physics relating lightning to thunder. Careful measurements of their relative timing would reveal that thunder always occurs very soon (or immediately) after lightning. But hearing thunder gives you only a general prediction of when the next lightning will be observed. One would correctly deduce that lightning causes thunder. That’s progress. But it would not tell you the cause of the lightning in the first place.

For my part, I’ve long been a skeptic about the old idea that the D-O variations are ultimately driven by meltwater and/or iceberg fluxes into the North Atlantic (the Day after Tomorrow scenario, if you will). There are only 6 clearly-identified Heinrich events (that is, layers of terrestrial sediment in ocean sediment cores from the North Atlantic, evidence for massive iceberg discharges from the Laurentide ice sheet) but there are at least 23 D-O events. It more likely that there is intrinsic variability in the coupled ocean-atmosphere system, as found for example in a long simulation with the climate model CCSM4 by Peltier and Vettoretti (2014 and 2015) (though there is debate about the validity of the very low values for ocean vertical mixing used in those simulations). Iceberg discharges are then just the consequence, not the cause, of changes in ocean circulation, as argued recently by Alvarez Solaz et al. (2013) and also suggested by Barker et al. (2015) who found that on average, evidence for icebergs in the North Atlantic follow, rather than precede, the abrupt coolings at the end of some D-O events. That doesn’t mean that the huge ice and meltwater fluxes associated with Heinrich events don’t have an impact; most modeling work suggests that they would. But it may be important in this context that our results show no dependence of the ~200-year lag on whether or not a Heinrich event has occurred: that is, there is no evidence that “Heinrich stadials” (the cold periods during which Heinrich events occur) are unusual with respect to ocean “seesaw” dynamics. The role of these events in millennial scale variability therefore remains an important, and open, research question (see e.g., Margari et al., 2010).

In the meantime our precise observations of the phasing of D-O and AIM events provide an important new constraint against which to validate model simulations designed to capture the dynamics of these interesting features of the climate system.

UPDATE: The News & Views article about our article, by Tas van Ommen is worth a read. Available here (subscription based only, I’m afraid).



Notes. *The ability to obtain such small uncertainties is owing to four main things. First, we have very high resolution measurements of methane in both the WAIS Divide and the Greenland cores; methane is globally well mixed, and so abrupt changes in methane must happen at the same time (within a year) in cores from both regions. This means that we can synchronize the age of the gas trapped in the bubbles within the cores very precisely. Second, we have very high resolution measurements of the nitrogen isotope ratio (15N/14N in atmospheric N2), also trapped in the bubbles in the cores. This isotope ratio provides information on the age difference between the gas and the ice, because gravitational settling increases the 15N/14N ratio; this depends on the thickness of the firn (the permeable ice between the surface and the impermeable ice at depth where bubbles are trapped). The deeper the firn, the longer it takes to trap gases, and the larger the age difference. It’s the age of the ice that we’re actually interested in, because this, not the gas trapped within the ice, is what the δ18O measurements are made on. Fourth, this age difference is much smaller at WAIS Divide than in any other long Antarctic record; it is at most ~500 years, compared with e.g., ~4000 years at Vostok. Finally, we also have unprecedentedly high resolution measurements of δ18O, and very high quality borehole temperature measurements, which together provide a very robust measure of the temperature variations through time.

Data: The data from the paper are all available in the Supplement to the paper. The timescale and the oxygen isotope data from our lab — what most people will be interested in — are available at the National Snow and Ice Data Center, at doi:10.7265/N5GT5K41.

References:

Barker, S., J. Chen, X. Gong, L. Jonkers, G. Knorr, D. Thornalley. Icebergs not the trigger for North Atlantic cold events. Nature 520, 333–336, 2015. http://dx.doi.org/10.1038/nature14330.

Blunier, T., J. Chappellaz, J. Schwander, A. Dällenbach, B. Stauffer, T.F. Stocker, D. Raynaud, J. Jouzel, H.B. Clausen, C.U. Hammer, and S.J. Johnsen, “”, Nature, vol. 394, pp. 739-743, 1998. http://dx.doi.org/10.1038/29447.

Böning, C.W., A. Dispert, M. Visbeck, S. R. Rintoul, F. U. Schwarzkop. The response of the Antarctic Circumpolar Current to recent climate change. Nature Geoscience 1, 864-869 (2008).
http://dx.doi.org/10.1038/ngeo362.

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Ten years of RealClimate: Thanks

Filed under: — group @ 10 December 2014

rc10 As well as the current core team – David Archer, Eric Steig, Gavin Schmidt, Mike Mann, Rasmus Benestad, Ray Bradley, Ray Pierrehumbert, Stefan Rahmstorf – this blog has had input from many others over the years:

The 90+ guest contributors and previous team members who bring a necessary diversity of experience and expertise to the blog: Abby Swann, Alan Robock, Anders Levermann, Andrew Monaghan, Andy Baker, Andy Dessler, Axel Schweiger, Barry Bickmore, Bart Strengers, Bart Verheggen, Beate Liepert, Ben Santer, Brian Helmuth, Brian Soden, Brigitte Knopf, Caspar Ammann, Cecilia Bitz, Chris Colose, Christopher Hennon, Corrine LeQuere, Darrell Kaufman, David Briske, David Karoly, David Ritson, David Vaughan, Dim Coumou, Dirk Notz, Dorothy Koch, Drew Shindell, Ed Hawkins, Eugenie Scott, Figen Mekik, Francisco Doblas-Reyes, Frank Zeman, Geert Jan van Oldenborgh, Georg Feulner, Georg Hoffmann, George Tselioudis, Jacob Harold, Jared Rennie, Jason West, Jeffrey Pierce, Jim Bouldin, Jim Prall, John Fasullo, Joy Shumake-Guillemot, Juliane Fry, Karen Shell, Keith Briffa, Kelly Levin, Kevin Brown, Kevin Trenberth, Kim Cobb, Kyle Swanson, Loretta Mickley, Marco Tedesco, Mark Boslough, Martin Manning, Martin Vermeer, Matt King, Matthew England, Mauri Pelto, Michael Bentley, Michael Oppenheimer, Michael Tobis, Michelle L’Heureux, Natassa Romanou, Paul Higgins, Peter Minnett, Phil Jones, Pippa Whitehouse, PubPeer, Raimund Muscheler, Rein Haarsma, Richard Millar, Robert Rohde, Ron Lindsay, Ron Miller, Russell Seitz, Sarah Feakins, Scott Mandia, Scott Saleska, Simon Lewis, Spencer Weart, Stephen Schneider, Steve Ghan, Steve Sherwood, Sybren Drijfhout, Tad Pfeffer, Tamino, Terry Gerlach, Thibault de Garidel, Thomas Crowley, Tim Osborn, Tom Melvin, Urs Neu, Vicky Slonosky, William Anderegg, William Connolley and Zeke Hausfather;

The thousands of commenters that have enlivened the conversation and explored many issues in more depth than is possible in the main posts;

The translators of hundreds of posts into Polish, French, Czech, German, Italian, Spanish, Turkish, Mandarin etc;

Miloslav Nic for his “Guide to RC” which provides a comprehensive set of indexes to the content here;

Ryan and the internet service providers at Peer, and now Webfaction, that have helped deal with the many technical challenges and to Environmental Media Services and later, the Science Communication Network, for covering some of those costs;

A sincere thanks to all.