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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 »

The underestimated danger of a breakdown of the Gulf Stream System

Filed under: — stefan @ 4 January 2017

A new model simulation of the Gulf Stream System shows a breakdown of the gigantic overturning circulating in the Atlantic after a CO2 doubling.

A new study in Science Advances by Wei Liu and colleagues at the Scripps Institution of Oceanography in San Diego and the University of Wisconsin-Madison has important implications for the future stability of the overturning circulation in the Atlantic Ocean. They applied a correction to the freshwater fluxes in the Atlantic, in order to better reproduce the salt concentration of ocean waters there. This correction changes the overall salt budget for the Atlantic, also changing the stability of the model’s ocean circulation in future climate change. The Atlantic ocean circulation is relatively stable in the uncorrected model, only declining by about 20% in response to a CO2 doubling, but in the corrected model version it breaks down completely in the centuries following a CO2 doubling, with dramatic consequences for the climate of the Northern Hemisphere. 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).


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.


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

Q & A about the Gulf Stream System slowdown and the Atlantic ‘cold blob’

Last weekend, in Reykjavik the Arctic Circle Assembly was held, the large annual conference on all aspects of the Arctic. A topic of this year was: What’s going on in the North Atlantic? This referred to the conspicuous ‘cold blob’ in the subpolar Atlantic, on which there were lectures and a panel discussion (Reykjavik University had invited me to give one of the talks). Here I want to provide a brief overview of the issues discussed.

What is the ‘cold blob’?

This refers to exceptionally cold water in the subpolar Atlantic south of Greenland. In our paper last year we have shown it like this (see also our RealClimate post about it):


Fig. 1 Linear temperature trends from 1901 to 2013 according to NASA data. Source: Rahmstorf et al, Nature Climate Change 2015.

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AMOC slowdown: Connecting the dots

Filed under: — stefan @ 19 May 2016

I want to revisit a fascinating study that recently came from (mainly) the Geophysical Fluid Dynamics Lab in Princeton. It looks at the response of the Atlantic Ocean circulation to global warming, in the highest model resolution that I have seen so far. That is in the CM2.6 coupled climate model, with 0.1° x 0.1° degrees ocean resolution, roughly 10km x 10km. Here is a really cool animation.

When this model is run with a standard, idealised global warming scenario you get the following result for global sea surface temperature changes.


Fig. 1. Sea surface temperature change after doubling of atmospheric CO2 concentration in a scenario where CO2 increases by 1% every year. From Saba et al. 2016.

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What drives uncertainties in adapting to sea-level rise?

Filed under: — stefan @ 17 March 2016

Guest article by Sally Brown, University of Southampton

Let me get this off my chest – I sometimes get frustrated at climate scientists as they love to talk about uncertainties! To be sure, their work thrives on it. I’m someone who researches the projected impacts and adaptation to sea-level rise and gets passed ‘uncertain’ climate data projections to add to other ‘uncertain’ data projections in my impact modellers work bag. But climate scientists do a good job. Without exploring uncertainties, science loses robustness, but uncertainties in combination can become unbounded and unhelpful to end users.

Let’s take an adaptation to sea-level rise as an example: With increasing scientific knowledge, acceptance and mechanisms that would allow adaptation to potentially occur, one would think that adaptation would be straight forward to implement. Not so. Instead of hard and fast numbers, policy makers are faced with wide ranges of uncertainties from different sources, making decision making challenging. So what uncertainties are there in the drivers of change, and can understanding these uncertainties enable better decisions for adaptation?

Prior to considering adaptation in global or regional models, or implementation at local level, drivers of change and their impacts (and thus uncertainties) require analysis – here are a few examples. More »

And the winner is…

Filed under: — group @ 17 November 2015

Remember the forecast of a temporary global cooling which made headlines around the world in 2008? We didn’t think it was reliable and offered a bet. The forecast period is now over: we were right, the forecast was not skillful.

Back around 2007/8, two high-profile papers claimed to produce, for the first time, skilful predictions of decadal climate change, based on new techniques of ocean state initialization in climate models. Both papers made forecasts of the future evolution of global mean and regional temperatures. The first paper, Smith et al. (2007), predicted “that internal variability will partially offset the anthropogenic global warming signal for the next few years. However, climate will continue to warm, with at least half of the years after 2009 predicted to exceed the warmest year currently on record.” The second, Keenlyside et al., (2008), forecast in contrast that “global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.”

This month marks the end of the forecast period for Keenlyside et al and so their forecasts can now be cleanly compared to what actually happened. This is particularly interesting to RealClimate, since we offered a bet to the authors on whether the results would be accurate based on our assessment of their methodology. They ignored our offer but now the time period of the bet has passed, it’s worth checking how it would have gone.

More »


  1. D.M. Smith, S. Cusack, A.W. Colman, C.K. Folland, G.R. Harris, and J.M. Murphy, "Improved Surface Temperature Prediction for the Coming Decade from a Global Climate Model", Science, vol. 317, pp. 796-799, 2007.
  2. N.S. Keenlyside, M. Latif, J. Jungclaus, L. Kornblueh, and E. Roeckner, "Advancing decadal-scale climate prediction in the North Atlantic sector", Nature, vol. 453, pp. 84-88, 2008.

Bjørn Lomborg, just a scientist with a different opinion?

Filed under: — stefan @ 31 August 2015 - (Español)

Bjørn Lomborg is a well-known media personality who argues that there are more important priorities than reducing emissions to limit global warming. In a recent controversy centering on him, the Australian government (known for its contradictory position on climate change) offered the University of Western Australia (UWA) $4 million to make Lomborg professor – which UWA first accepted, but then after massive protest from its staff and students refused. The Australian government was quick to label it a “freedom of speech” issue that Lomborg should get a university position, and vowed to find another university that would host him. However, free speech doesn’t guarantee everyone a university position; there are also academic qualifications required.

More »

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).

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.

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.


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