The Wrong Question

This article by Kevin Trenberth appears in Climatic Change today, released under a Creative Commons-Attribution license.

Dr. Trenberth argues that the inevitable question “is this or that event caused by global warming” is ill-posed and can have no reasonable answer. The atmosphere as a whole is non-negligibly changed, and all atmospheric events occur in a changed context.

Dr. Trenberth retains copyright. We reproduce the article in full herewith.

Image: Pakistan flooding in 2010.


Climatic Change
DOI 10.1007/s10584-012-0441-5

 

Framing the way to relate climate extremes to climate change

Kevin E. Trenberth

Received: 17 October 2011 / Accepted: 6 March 2012
# The Author(s) 2012. This article is published with open access at Springerlink.com

Abstract: The atmospheric and ocean environment has changed from human activities in ways that affect storms and extreme climate events. The main way climate change is perceived is through changes in extremes because those are outside the bounds of previous weather. The average anthropogenic climate change effect is not negligible, but nor is it large, although a small shift in the mean can lead to very large percentage changes in extremes. Anthropogenic global warming inherently has decadal time scales and can be readily masked by natural variability on short time scales. To the extent that interactions are linear, even places that feature below normal temperatures are still warmer than they otherwise would be. It is when natural variability and climate change develop in the same direction that records get broken. For instance, the rapid transition from El Niño prior to May 2010 to La Niña by July 2010 along with global warming contributed to the record high sea surface temperatures in the tropical Indian and Atlantic Oceans and in close proximity to places where record flooding subsequently occurred. A commentary is provided on recent climate extremes. The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question. All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.

1 Introduction

How big is the human influence on climate? Is it big enough that a question such as “Is this event due to global warming?” even makes sense? Here these questions are addressed along with improved ways to frame the questions that inevitably arise when new climate extremes occur, and there have been many over the past 2 years. Clearly natural variability plays a major role. Accordingly a brief commentary on some of these extremes and how they relate to both natural variability and climate change is provided.

Climate change from human influences is difficult to perceive and detect because natural weather-related variability is large. Even with a significant climate change, most of the time, the weather is within previous bounds. However, human-induced climate change is persistent and tends to be in one direction, at least insofar as the increases in greenhouse gases are concerned (IPCC 2007). So one way of detecting such an influence is through long-term changes in mean conditions, preferably guided by climate model studies as to which variables and how they should change. This requires long averages to overcome the effects of natural variability (climate noise), and for quantities such as global temperatures, about 17 years is needed (Santer et al. 2011). With global warming, the thermodynamic variables have much stronger signal-to-noise ratios than dynamic variables (Deser et al. 2010).

Accordingly, changes in temperature and the water holding capacity of the atmosphere are more robust than changes that depend on winds in any way.

If the problem is generalized to look at the entire probability distribution function (pdf) of the climate variables, then the biggest changes percentagewise occur in the tails of the distribution, where they can easily exceed several hundred percent (Trenberth 2011b). Accordingly, a change in climate is most likely to be perceived by encountering new “weather” and breaking records: changes in the extremes. Changes in certain extremes, such as higher temperatures and increases in heavy rains and droughts are expected with climate change (IPCC 2007; Trenberth 2011a).

Attribution (IPCC 2007; Stott et al. 2010) of the extremes requires a model to separate out the human influence from natural variability using numerical experimentation. This requires considerable integrity in the model’s ability to simulate both, but models typically have great difficulty in simulating extremes well (Lin et al. 2006; Kharin et al. 2007) especially throughout the tropics for precipitation. In many model studies, the metric of Fraction of Attributable Risk (FAR) (e.g., Allen 2003) is used to express the fraction of risk of a particular threshold being exceeded. This is a relative rather than absolute metric.

However, methodological issues arise about the null hypothesis and where to assign the errors (Trenberth 2011b). The issue is whether the benefit of doubt errs on the side of natural variability (as has been the case) or on the side of a human influence.

Extremes are always expected to happen as the climate record gets longer, but certain extremes related to heating are becoming more evident. For example in the United States, extremes of high temperatures have been occurring at a rate of twice those of cold extremes (Meehl et al. 2009), and this has accelerated considerably since June 2010 to a factor of 2.7, and in the summer of 2011 to a factor of over 8 (Skolnik 2011). Texas, Oklahoma, New Mexico and Louisiana all suffered their hottest June-July-August (JJA) 2011 since 1895 (average temperature over 30 °C in Oklahoma and Texas), according to NOAA. Texas also
experienced the driest JJA on record.

Climate extremes are typically treated individually, but many are not unrelated. The clustering of extremes occurs when natural variability creates anomalies that are in the same direction as global warming. This occurs especially in association with the dominant mode of natural variability: El Niño-Southern Oscillation (ENSO) during and following the warm El Niño phase (Trenberth et al. 2002) as heat leaves the ocean. During ENSO, large regional changes occur in Sea Surface Temperature (SST) throughout the tropics. Large positive SST anomalies in the central and eastern Pacific during El Niño tend to focus convective activity (thunderstorms, tropical storms, etc.) into those regions while suppressing activity elsewhere via both changes in atmospheric stability and wind shear. Meanwhile lighter winds and decreased evaporative cooling, and sunny skies in the tropical Atlantic and Indian oceans result in higher than normal SSTs 3–7 months after the peak SSTs in the Niño 3.4 region
(Trenberth et al. 2002). As noted below, this happened in 2010 following the end of the El Niño in May 2010.

As climate varies or changes, several direct influences alter precipitation amount, intensity, frequency, and type (Trenberth et al. 2003; Trenberth 2011a). Warming accelerates land-surface drying as heat goes into evaporation of moisture, and this increases the potential incidence and severity of droughts, which has been observed in many places worldwide (Dai 2011). The moisture in the atmosphere, which has been widely observed to be increasing in association with increased SSTs, then gets carried around by atmospheric winds to where storms are favored. Typical storms reach out a distance of about three to five times the radius of the rain dimension, and gather in the water vapor, to produce precipitation. In weather systems, convergence of increased water vapor leads to more intense precipitation and the risk of heavy rain and snow events, but may also lead to reductions in duration and/or frequency of rain events, given that total amounts do not change much. The result is longer dry spells, as observed in the United States (Groisman and Knight 2008). Basic theory, climate model simulations, and empirical evidence all confirm that warmer climates, owing to increased water vapor, lead to more intense precipitation events even when the total annual precipitation is reduced slightly (Trenberth et al. 2007). A warmer climate therefore increases risks of both drought—where it is not raining—and floods—where it is—but at different times and/or places.

2 Is this extreme due to global warming?

Changes in atmospheric composition from human activities are the main cause of anthropogenic climate change by enhancing the greenhouse effect, although with important regional effects from aerosol particulates (IPCC 2007). Anthropogenic global warming inherently has decadal time scales and can be readily masked by natural variability over periods less than a decade or so. To the extent that interactions are linear, below normal temperatures can be fully consistent with climate change but are likely warmer than they otherwise would have been.

Globally on a day-to-day basis the climate change effects are 1–2 % of the natural energy flow, as elaborated on below. However, because global warming is always of one sign, a much bigger impact is from the cumulative effects of these radiative perturbations on the climate. The main memory is through the warming of the oceans, manifested in part through the ongoing rise in sea level, and the loss of Arctic sea ice and glacier mass. SSTs have risen by 0.5–0.6 °C since the 1950s, and over the oceans this has led to 4 % more water vapor in the atmosphere since the 1970s (Trenberth et al. 2007). As a result, the air is on average warmer and moister than it was prior to about 1970 and in turn has likely led to a 5–10 % effect on precipitation and storms that is greatly amplified in extremes. The warm moist air is readily advected onto land and caught up in weather systems as part of the hydrological cycle, where it contributes to more intense precipitation events that are widely observed to be occurring (IPCC 2007; Trenberth 2011a; Groisman and Knight 2008; Min et al. 2011; Pall et al. 2011).

The rationale for these numbers is as follows. The radiative forcing (IPCC 2007) is about 1.6 Wm−2 for both carbon dioxide increases alone and also the total with all other effects included (0.6–2.4 as 95 % confidence limits), and the net energy imbalance of the planet is estimated (Trenberth et al. 2009) to be 0.9±0.5 Wm−2. The net energy flow through the climate system is equivalent to about 240 Wm−2. The difference between the net imbalance and the radiative forcing is because of the response of the climate system to the forcing, namely the warming of the planet and moistening of the atmosphere (Murphy et al. 2009). Water vapor is a powerful greenhouse gas. The increased water vapor roughly doubles the direct radiative forcing, giving the 1–2 % value, although this will vary from day to day. However, the average 4 % increase in water vapor becomes amplified in weather systems because it adds buoyancy to the air flowing into all storms, promoting them to become more intense and multiplying the effect (Trenberth et al. 2003; Trenberth 2011a). Instabilities can magnify effects further, although changes in wind shear and atmospheric stability as a consequence of the enhanced vertical motion may have reverse effects elsewhere. These lead to the approximate 5–10 % effect overall. For major droughts that last a month or longer, cumulative effects again become important as the absence of moisture means that all heating goes into sensible heating, creating higher temperatures, that in turn desiccate plants, and promote heat waves and wild fires. Lau and Kim (2012) quantify these effects for the Russian heat wave in 2010. During drought the memory stems from the changes in soil moisture.

Whether or not these values are accepted, the key point is that the anthropogenic climate change effect is not zero or negligible, nor is it large relative to the mean, but it is systematic. While natural variability clearly plays a major role in all events, such as those detailed below in 2010 and 2011, the record high SSTs did as well. In part the high SSTs were a consequence of the previous El Niño (Trenberth et al. 2002) but there is surely a significant global warming component (Gillett et al. 2008). Hence anthropogenic global warming has an identifiable role in the extreme weather (Trenberth and Fasullo 2012; henceforth TF12).

3 Some examples of recent climate extremes

3.1 SSTs

ENSO played a major role in climate extremes in 2010 and 2011 (TF12). El Niño conditions persisted through April 2010 but rapidly gave way to La Niña conditions by June. The [sea surface temperature -mt] anomalies for the northern summer (JJA) of 2010 (Fig. 1) reveals the La Niña conditions in the Pacific and hence the cooler than normal conditions mean that this was the region where the thunderstorms, tropical storms, and other convective activity were not occurring. However, as shown in TF12, very high SST anomalies from 0.5 to 1.5 °C, indeed record high SSTs in many instances (Fig. 1), occurred in the Indian Ocean and Indonesian region as well as throughout the tropical Atlantic (relative to a 1951–1970 normal that precedes most anthropogenic warming), regions that are normally very warm anyway (TF12). The total SSTs exceeded 29 °C over broad regions and were at an all time high in May 2010 (30.4 °C) in the northern Indian Ocean encompassing the Arabian Sea and Bay of Bengal (TF12). SSTs were also very high (second highest on record) north of Australia for September to November 2010, and by December they were the highest on record for that month. SST anomalies were also highest on record in the Gulf of Mexico in August 2010 and in the Caribbean in September 2010 (TF12). In 2011, SSTs were well above normal in the Gulf of Mexico in April but had cooled off by May. However, SSTs were still very high in the tropical Atlantic.

Because the water holding capacity of the atmosphere increases exponentially with temperature (e.g., Trenberth et al. 2003), a positive anomaly on top of already high SSTs has much greater effect than if located elsewhere. Indeed, the high SSTs were accompanied by very high water vapor amounts. The high SSTs provide ample moisture to the atmosphere and the resulting evaporative cooling of the ocean dropped the subsequent SST values down, but meanwhile heavy rains, often record breaking in intensity, occurred nearby to where the winds carried the moisture. This happened in China, India and Pakistan (June to early August 2010); Queensland, Australia (December 2010 and January 2011), and Colombia (October to December 2010) (Fig. 1). It also seems to have been a factor from 19 to 25 April 2011 when exceptionally heavy rains, exceeding 300 mm, occurred over southern Missouri, parts of Arkansas, eastern Oklahoma, and southern Illinois, and extended along the Ohio River Valley http://earthobservatory.nasa.gov/IOTD/view.php?id050243, as a prelude to the flooding in the Mississippi.

3.2 La Niña and the Americas

La Niña conditions are well known to be associated with major anomalies in the Americas, and precipitation and flooding risk increase substantially in northern South America, such as in Colombia (Poveda et al. 2011). In La Niña summer and autumn the hurricane season is more active owing to a more favorable tropical circulation that allows storms to form in an environment of reduced wind shear and stability (Vecchi et al. 2008).

The SSTs (Fig. 1) in the Atlantic sector throughout the region north of Colombia were above 29 °C from July to September, and August 2010 was the warmest on record in both the Caribbean and in the Gulf of Mexico: anomalies exceeded 0.5–1.5 °C relative to the 1971–2000 base period (Fig. 1) (TF12). SST anomalies were especially large off the Colombian coast. The much cooler conditions to the west of the Central American isthmus both in absolute and anomaly terms understandably focused convective activity as a whole into the Atlantic and away from the Pacific. North of the equator, the result was a much above normal Atlantic hurricane season, in which there were 19 named storms, and 12 hurricanes, of which 4 were category 4 or 5, likely making it the second most active year after 2005. These aspects related to specific extremes are documented in TF12, including links between the heavy rains and the Russian heat wave of 2010, and the Colombian rains and the drought in the Amazon.

When La Niña is present, it strongly influences where the storms track across the United States, and the storms track in such a way as to miss the South. Consequently, Texas and surrounding areas (especially parts of Arizona, New Mexico and Oklahoma) suffered severe drought, and subsequently heat waves and wild fires in the northern spring and summer 2011. Nevertheless in spring, the storms crossing the central Midwest were able to link up with the warm moist air from the Gulf of Mexico, creating extra instability and buoyancy for the air that was entrained into the storms. This led to extensive heavy rains, flooding and tornado outbreaks. The pattern of rainfall in the spring is characteristic of La Niña although the extreme nature of the changes is not. The intense heat wave and “exceptional drought” continued in Texas through August. Many of these events are described in detail on line at the NOAA National Climatic Data Center, State of the Climate, Global Hazards site: http://www.ncdc.noaa.gov/sotc/hazards/2010/m (or 2011/m) where m is the month.

In spring, when land-sea contrasts transition to zero, strong westerly winds blow from the Pacific Ocean across the United States. Because the Rockies block the wind at low levels, the result is a strong westerly jet stream aloft while at low levels the air east of the Rockies comes from elsewhere including the Gulf of Mexico when there is a pronounced southerly component ahead of cold fronts. Both the change in wind speed and direction with height (southerlies at low levels, strong westerlies aloft) create wind shear, which sets the stage for super-cell thunderstorms to form tornadoes as the shear gets converted into rotation. According to NOAA, there were 539 deaths from over 1075 (actual count) tornadoes in April and May 2011 in the United States, the most deadly on record. Trends in the tornado record are not reliable, as increases in population over previously rural areas lead to more reporting of tornadoes, but the exceptional nature of the 2011 spring is not in doubt.

Global warming does not contribute directly to tornadoes themselves, but it does contribute to the vigor of the thunderstorms that host them through the increased warmth and moisture content (moist static energy) of the low level air flow. The increase in buoyancy of the air flowing through the Gulf of Mexico helps fuel the storms. Similarly, the extra moisture provided incremental amounts to the heavy rains that ultimately led to flooding along the Mississippi and later, farther north, heavy rains and melting snows contributed to extensive flooding of the Missouri River.

3.3 The Asian sector

The heavy rains and flooding in China, India, and Pakistan in JJA 2010 were associated with the very high SSTs to the south (Fig. 1) that provided extra moisture for the monsoon rains. The strong monsoon circulation then played a role in the Russian heat wave from mid-June to mid-August 2010 (Barriopedro et al. 2011; TF12), perhaps not unlike that in 2003 (Black and Sutton 2007) although influences from the Atlantic likely also played a role. The drought and famine in East Africa was also related to the high Indian Ocean SSTs (Williams and Funk 2011). Very large anomalies also existed at this time in Arctic sea ice and, in conjunction with positive Arabian Sea SST anomalies, connections to the events in Eurasia are suggested (Sedláček et al. 2011).

In the Asian sector, as the northern monsoon faded in late August of 2010, activity began to pick up in Australia, which switched to become very wet in September, continent wide, again reflecting the very high SSTs to the north (second highest on record), abundant moisture and the La Niña conditions. This was a fore-runner to the exceptionally heavy rains in Queensland in December 2010, and January 2011 where the southern monsoon rains kicked in with the presence of record high SSTs. Category 5 hurricane Yasi made landfall in Queensland in early February 2011.

4 Conclusions

The above commentary describes how natural variability in the presence of record high SSTs led to exceptional flooding events and extremes in 2010–11; see TF12 for details. Note that the La Niña in 2011–12 has different character owing to the absence of the high SSTs in the Indian and Atlantic Oceans. The SST changes feature contributions from climate change as well as strong regional contributions from ENSO.

The climate has changed; global warming is unequivocal (IPCC 2007) and human activities have undoubtedly changed the composition of the atmosphere and produced
warming. Moreover there is no other plausible explanation for the warming. The human- induced changes are inherently multi-decadal and provide a warmer and moister environment for most weather events, even in the presence of large natural variability. In attribution studies, changing the null hypothesis from “there is no anthropogenic global warming effect” to one that recognizes the changed environment can completely change the outcome (Trenberth 2011b). In Bayesian statistics, this change might be thought of as a “prior”. Scientists are frequently asked about an event “Is it caused by climate change?” The answer is that no events are “caused by climate change” or global warming, but all events have a contribution. Moreover, a small shift in the mean can still lead to very large percentage changes in extremes. In reality the wrong question is being asked: the question is poorly posed and has no satisfactory answer. The answer is that all weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.

Acknowledgments Thanks to Dennis Shea for Fig. 1. This research is partially sponsored by NASA grant NNX09AH89G.


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Comments:

  1. Dr Trenberth, Thanks for this helpful article. I see that you didn't need the "climate system" concept although high SSTs must be related to increasing ocean heat content. You put a great deal into plain words very nicely. You set out to give a good set of examples, not reconstruct the world, but might a little more of the globe profitably be covered? The Thailand area got pretty soggy, but I'm thinking especially of the West African floods.
    http://blogs.worldbank.org/africacan/benin-under-water
    http://en.wikipedia.org/wiki/2010_West_African_floods
    That area gets lost at the map edges.

    Just lately we have had the march heat wave in the US & Canada, and at about the same time Arctic temperatures plummeted:
    http://ocean.dmi.dk/arctic/meant80n.uk.php
    There is only so much energy to go around, and it looks like that big southern loop in the jet stream reduced heat transfer to the Arctic.

    It occurs to me that it would be quite handy to have globes, at least beach ball size, with anomalies, weather and other outputs projected from the inside. (liquid crystal display perhaps) This could really help establish the idea that in planetary physics one must think globally and locally and seasonally and ENSO-ly. Maybe some big company would make them to show off their technological prowess.

  2. Pete, the spherical-display digital globe things exist. Indeed there's one in the lobby of the building where Dr. Trenberth works. But they are damnably expensive.

    I agree that I'd like better real-time displays of jet stream, frontal boundaries, anomalies, etc. A flat screen with a few controls can go a long way. This is something we could consider doing rather than just demanding. I absolutely agree with your statement that most people really don't understand that the changes are in a sense top-down, and that without a global perspective we cannot understand large scale anomalies. I'll have more to say on that subject soon.

    Regarding Dr. Trenberth reading this, I am not sure he will participate here but I'll pass your comment along.

  3. There's been a few important papers out, over that last year or so, that have a similar conclusion to this one.

    just a couple of examples:
    http://www.noaanews.noaa.gov/stories2011/20111027_drought.html

    http://www.realclimate.org/index.php/archives/2012/03/extremely-hot

    Basically all saying that the natural variation in climate can no longer explain the extremes we are experiencing in intensity and number.

    As notable as this growing body if work is, just as notable is the climate media (which seems to have denigrated into a group of commentary police) seems to have no idea how to handle it. If one of the reasons that climate gets little coverage is that nothing "newzy" is out there, then I have to call BS. This stuff is worth covering. This, in combination with the IPCC report on extremes should be like a giant blaring fog horn throughout the media world. This attribution science turns over prior thinking on when extreme events should begin to occur. It turns out the alarmists were very likely right!

    Now watch how Revkin responds to a commenter (Roddy, whom is a pain, so I'm not defending him) who brings up the RC article:
    http://dotearth.blogs.nytimes.com/2012/03/25/managing-a-planet-under-pressure/?comments#permid=7:1

    Andrew Revkin
    Dot Earth blogger
    That's an viewpoint piece, and well countered by one of the country's most eminent climatologists, John M. Wallace, writing in EOS a couple of weeks ago. It's behind a paywall but I can send it to you if you're open-minded.

    The paper in question, of course, doesn't counter the RC paper, but that's not what is important. Here's the last line of the abstract.

    The problem with this approach is that the attribution of extreme events to human-induced climate change is often viewed as gratuitous and labeled as fear mongering. A more effective communications strategy, in my view, is to use these events to illuminate society's increasing vulnerability to natural disasters in the face of our deteriorating planetary life-support system.

    Here's the 1 page opinion piece:
    http://www.csp.rutgers.edu/component/docman/doc_download/4-wallace-teachable-moments

    And here is David Appell:
    http://davidappell.blogspot.com/2012/03/ive-read-most-of-new-paper-by-coumou.html

    But these are all anecdotal evidence, if you will, and the paper doesn't contain a single equation. I suspect you can always find regions that have extremes

    Have we entered a new era of climate journalism that bypasses the balance problem and moves directly into distorting reality? Or is this just a new version of balance that serves the same purpose as the original? I believe coverage of this deserves its own coverage.

  4. Thanks for that material, grypo.

    As you say, by no means is Wallace opposing R+C. He does not suggest that extreme event attribution is wrong or scientifically inappropriate, rather that a different approach would be more effective with the public.

    He is, however and IMHO, entirely naive to think so. *Any* message will be "viewed as gratuitous and labeled as fear mongering" by the usual suspects for the same reasons as the current one, plus Wallace's runs the risk of supporting too heavy of a tilt toward adaptation rather than mitigation. He also neglects to address the issue of why his preferred approach has failed to catch fire despite having been promulgated several years ago. He says:

    "More recently, a group of 28 environmental scientists [Rockstrom et al., 2009] listed climate among nine planetary life-support systems that are being stressed by human activities. When viewed in this broader context, weather- and climate-related
    extreme events take on greater significance because they serve as early warning signs, not only of climate change but also of increasing societal vulnerability to naturally occurring disruptions."

    How exactly do they take on greater significance? To the contrary, while the Rockstrom et al. paradigm is very appealing to the scientifically literate, the fact that it's much more complicated than one focused directly on extremes makes it harder to communicate to the general public, not easier.

    Wallace also speaks of extreme events being predicted to get a lot worse in the latter part of the century, too long in his opinion to make for an effective current argument. I have a rather different impression from the literature. Things seem to be moving relatively fast.

    Re Appell, it's sad that he attacks a "perspective" article for itself lacking equations, even though as appropriate for such a piece it instead references and synthesizes papers that do. I'm afraid that he, like Revkin, despite good early work, has found his physical intuition outstripped by reality and the science, but chooses instead to take comfort in and defend that past understanding from assaults by the immanent future.

  5. http://www.ipcc-wg2.gov/SREX/

    The complete SREX is out.

    The most important and vital information, according to Revkin, is from a Pielke Jr. hobby horse.

    “There is medium evidence and high agreement that long-term trends in normalized losses have not been attributed to natural or anthropogenic climate change”
    “The statement about the absence of trends in impacts attributable to natural or anthropogenic climate change holds for tropical and extratropical storms and tornados”
    “The absence of an attributable climate change signal in losses also holds for flood losses”
    And:

    “Some authors suggest that a (natural or anthropogenic) climate change signal can be found in the records of disaster losses (e.g., Mills, 2005; Höppe and Grimm, 2009), but their work is in the nature of reviews and commentary rather than empirical research.”

    You need to have incredibly fine tuned blinders to highlight this instead of helping people understand the implications of what the report is trying to tell us.

    I'm thinking of purchasing a fiddle. I'm expecting that people will scream at me, overly concerned that I should really be playing a lyre.

  6. http://www.agu.org/pubs/crossref/2012/2012GL051000.shtml

    I don't have access to this entire paper, but this appears to blame the Arctic ice loss for the persistence of the jet streams anomalies. Do I have that right?

  7. I'm able to view the entire paper through html viewing:

    http://scholar.googleusercontent.com/scholar?q=cache:8Qh4niPATxsJ:scholar.google.com/&hl=en&as_sdt=0,40

    Conclusions

    In summary, the observational analysis presented in this study provides evidence supporting twohypothesized mechanisms by which Arctic amplification -- enhanced Arctic warming relative tothat in mid-latitudes – may cause more persistent weather patterns in mid-latitudes that can leadto extreme weather. One effect is a reduced poleward gradient in 1000-500 hPa thicknesses,which weakens the zonal upper-level flow. According to Rossby wave theory, a weaker flowslows the eastward wave progression and tends to follow a higher amplitude trajectory, resultingin slower moving circulation systems. More prolonged weather conditions enhance theprobability for extreme weather due to drought, flooding, cold spells, and heat waves. Thesecond effect is a northward elongation of ridge peaks in 500 hPa waves, which amplifies theflow trajectory and further exacerbates the increased probability of slow-moving weatherpatterns. While Arctic amplification during autumn and winter is largely driven by sea-ice lossand the subsequent transfer of additional energy from the ocean into the high-latitudeatmosphere, the increasing tendency for high-amplitude patterns in summer is consistent withenhanced warming over high-latitude land caused by earlier snow melt and drying of the soil.Enhanced 500-hPa ridging observed over the eastern N. Atlantic is consistent with more persistent high surface pressure over western Europe. This effect has been implicated ascontributing to record heat waves in Europe during recent summers (Jaeger and Seneviratne,2011).

    Can the persistent weather conditions associated with recent severe events such as the snowywinters of 2009/2010 and 2010/2011 in the eastern U.S. and Europe, the historic drought andheat-wave in Texas during summer 2011, or record-breaking rains in the northeast U.S. ofsummer 2011 be attributed to enhanced high-latitude warming? Particular causes are difficult toimplicate, but these sorts of occurrences are consistent with the analysis and mechanismpresented in this study. As the Arctic sea-ice cover continues to disappear and the snow covermelts ever earlier over vast regions of Eurasia and North America (Brown et al, 2010), it isexpected that large-scale circulation patterns throughout the northern hemisphere will becomeincreasingly influenced by Arctic Amplification. Gradual warming of the globe may not benoticed by most, but everyone – either directly or indirectly -- will be affected to some degree bychanges in the frequency and intensity of extreme weather events as greenhouse gases continueto accumulate in the atmosphere. Further research will elucidate the types, locations, timing, andcharacter of the weather changes, which will provide valuable guidance to decision-makers invulnerable regions

  8. You have the paper right, although ice loss is not necessary for Arctic amplification. Whether the paper has it right is a different matter.

    However, I am at the least confident that the paper is asking the right sort of question. The jet stream is moving north and getting sticky - that seems observationally clear.

    It's important to discount any answer obtained now, though, as this is post hoc reasoning. Climatology as a whole failed to predict this outcome: IPCC is silent on it. It would be far better to find discussions among the best dynamicists from five years ago.

    I think this is a clear example of uncertainty breaking the wrong way.

    Time will tell if the persistence itself is persistent, but at this point it is striking enough that coincidence (i.e. stochastic variation) is inadequate as an explanation.

  9. Thanks for highlighting this, grypo.

    In the coverage of this, Marty Hoerling continues to be unhelpful. Yesterday's NYT article quoted him and Christy. Balance achieved!

    In other climate science news, I almost missed this. First paper, yes, and no attempt at attribution, but kind of a large-scale effect, no? We live in interesting times, although apparently not interesting enough for this to have gotten any coverage to speak of. (Actually only one outlet of any significance, the Times of India, plus a couple very obscure ones including an outfit called "Global Adventures." I'll say.)

  10. Well, the press won't touch it because the scientists didn't plug it into the AGW horse race for them. Is this good news or bad news?

    This fits in with the informal rule that a complex system acts to reverse any sudden change.

    I'm not sure I should indulge in public speculation about this. Its relationship to the direction of anthropogenic climate change leads me to many wild thoughts, but they might well be wrong.

    Consider this, though. What goes down must come up. Deep water formation must be balanced by upward motion elsewhere, and (last I heard, but I can be out of date) that balancing motion has not been identified. It must be due to some sort of distributed mixing. That mixing must be depressed. Which is cause and which effect is uncertain, but it must be so.

    A consequence would be an unexpected warming in middle-depth ocean temperatures somewhere. Where that is would give us a clue as to where the mixing now isn't...

  11. Section 10.3 of IPCC AR4 WG1 covers a lot of ground on model projections for changes in circulation patterns, etc. - and I think makes it clear there's an awful lot that will be very hard to predict. It doesn't say anything directly about the jet stream that I noticed, but it does talk about changes in the circulation (like the polar vortex which is closely related) including nonlinearities, for example:

    A plausible explanation for the cause of the upward NAM trend simulated by the models is an intensification of the polar vortex resulting from both tropospheric warming and stratospheric cooling mainly due to the increase in greenhouse gases (Shindell et al., 2001; Sigmond et al., 2004; Rind et al., 2005a). The response may not be linear with the magnitude of radiative forcing (Gillett et al., 2002) since the polar vortex response is attributable to an equatorward refraction of planetary waves (Eichelberger and Holton, 2002) rather than radiative forcing itself. Since the long-term variation in the NAO is closely related to SST variations (Rodwell et al., 1999), it is considered essential that the projection of the changes in the tropical SST (Hoerling et al., 2004; Hurrell et al., 2004) and/or meridional gradient of the SST change (Rind et al., 2005b) is reliable.

  12. That's entirely based on a remark of Hansen's that GCMs (presumably at least the GISS one, but it sounded like he meant more than that) need to be tweaked to not start it up for current conditions. I very vaguely recall reading elsewhere that it's thought to have been a feature of mid-Pliocene climate.

    The remark was written and recent (within the last couple of years) but I can't recall where. I'll have a look and see if I can find it.

  13. I remember about a year ago on InIt, we were discussing how to bring these disparate ideas together to make a logical cohesive argument about AGW and weather extremes. I think the elements are there. Are we asking the right questions now? I believe so. Is there a loaded dice problem beginning? I don't know how anyone can deny that. But how do we get away from the "baseball" problem, where we are not just fitting curves? If we are to attribute increased and exacerbated blocking phenomenon to warming, or jet stream anomalies, there needs to be a physical mechanism in place, like the Francis hypothesis on sea ice loss. So we are at least moving in the right direction.

    The statically analysis by R&C 11 says it contradicts the previous (Dole 11) findings that the Russian heat wave was unpredictable. Otto 2012 finds that the results of these two papers are not incompatible. This is not mentioned in Trenberth's paper but it appears to me, to be logically consistent with Trenberth's point.

    About Hoerling, who Steve Bloom mentions, there is a quote from him that is the complete opposite of Francis' work. He says:

    “What’s happening in the Arctic is mostly staying in the Arctic,”

    Francis says :

    So the question is not whether the sea-ice loss is effecting large-scale atmospheric circulation...but how could it not?

    This appears to a large gulf of opinion. Are they really that far off?

  14. OK, can't find the Hansen quote (unsurprising), and AFAICT my memory was wrong about the Pliocene, rather it was the Paleogene that saw deep water formation shift (not sure to what degree) to the North Pacific. The Paleogene being the hottest period of the Phanerozoic, that's a bit different than anything likely to be detectable in the present. Possibly Hansen was referring to an effect of future warming rather than present conditions, but in any case I'm curious enough about this point to want to track it down. More soon, hopefully.

  15. I've seen a website on which the map indicates deep water formation off Kamchatka Pennisula; no other websites do.

    Deep water is formed in the North Atlantic and also around Antarctica. Interestingly, deep water upwelling occurs there as well. Then there is also intermediate water formation as well...

  16. Masters has now picked up on the Arc Amp connection to sticky weather, citing 2 more papers (one with Curry as a co-author), as well as, the Francis paper. He also included a Yale post by Francis that concludes:

    While it’s difficult to point the finger at Arctic amplification in causing any of these weather events, they are the types of phenomena that are expected to occur more frequently as the world continues to warm and the Arctic continues to lose its ice. Further research may find ways to predict which regions will experience which conditions. But in the meantime, it’s increasingly likely that the weather you have today will stick around awhile.


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