02 Jun 2011

Forum: Is Extreme Weather Linked to Global Warming?

In the past year, the world has seen a large number of extreme weather events, from the Russian heat wave last summer, to the severe flooding in Pakistan, to the recent tornadoes in the U.S. In a Yale Environment 360 forum, a panel of experts weighs in on whether the wild weather may be tied to increasing global temperatures.

That global air and ocean temperatures are rising, and that human activity is largely to blame, is no longer a subject of debate among the vast majority of the world’s climate scientists. But there is no such consensus when talk turns to another important question: Is climate change already causing more extreme weather events, including worsening downpours and flooding, intensifying heat waves, and more powerful hurricanes?

Yale Environment 360 asked eight leading climate experts whether they think there is growing evidence that human-caused global warming is contributing to an increased incidence of extreme weather — and to cite specific recent examples in their answers. Their responses varied, with some contending that rising temperatures already are creating more tempestuous weather and others saying that more extreme weather may be likely but that not enough data yet exists to discern a trend in that direction. Scientists in both camps said two physical phenomena — warmer air holds more moisture, and higher temperatures exacerbate naturally occurring heat waves — would almost by definition mean more extremes. But some argued that the growing human toll from hurricanes, tornadoes, floods, and heat waves is primarily related to burgeoning human population and the related degradation of the environment.

Kevin Trenberth
Kevin Trenberth, senior scientist at the U.S. National Center for Atmospheric Research’s Climate Analysis Section.
Yes, undoubtedly. The environment in which all storms form has changed owing to human activities. Global warming has increased temperatures and directly related to that is an increase in the water-holding of the atmosphere. Over the ocean, where there are no water limitations, observations confirm that the amount of water vapor in the atmosphere has increased by about 4 percent, consistent with a 1 degree F warming of sea surface temperatures since about the 1970s. The human component does not change much from year to year and affects all storms.

In the absence of water, during a drought, the extra heat goes into raising temperatures and creating a more intense drought and heat waves, no doubt contributing to the 2010 Russian heat wave. However, the most spectacular events over the past year have been extreme heavy rains: flooding in India, China, and Pakistan in July and August, and then Queensland, Australia in December 2010 and January 2011. Further, very heavy rains in the U.S. in April 2011, along with snow melt, have also led to extensive flooding. In all these cases, very high sea surface temperatures have undoubtedly contributed to extra moisture flowing into the storms that produced the heavy rains and likely contributed to the strength of the storms through added energy. While perhaps a major part of these high sea surface temperatures was related to natural variability such as ENSO [El Nino Southern Oscillation], a component is related to global warming. It is when global warming and natural variability come together that records are broken. Our current work is documenting the link between the Asian flooding and the Russian heat wave and why the blocking anticyclone that led to it was so persistent.

Andrew Watson East Anglia
Andrew Watson, professor of environmental sciences at the University of East Anglia.
My answer to this question as posed is no. However, if you were to ask instead whether I expect that human-caused climate change will lead to more extreme weather events, the answer would be yes.

“Human-caused climate change” is something that will develop over decades to a century (the time scale on which greenhouse gas concentrations are changing), and evidence for or against it needs also to be considered over such lengths of time. So if you are thinking about recent events such as Hurricane Katrina, tornadoes in Alabama or Missouri, flooding in Bangladesh, Southeast Asia or on the Mississippi, my answer is that a few such examples of extreme weather cannot, by definition, be used as evidence for, or against, climate change.

Only by long-term statistics can we demonstrate a change that might be attributable to climate. Therefore your request that we give “an example or two of recent extreme weather events that you think either affirm, or refute, the contention that anthropogenic global warming is increasing the frequency and intensity of extreme events” makes no sense to me.

But having said that, I do expect that global warming should lead to more extreme weather events. Extreme weather is powered by large fluxes of energy, much of which comes from latent heat, that is to say from condensation of water vapor. Since a warmer atmosphere will contain more water vapor, I think it makes sense that we should expect more violent weather events in a warmer world. This is a very general prediction however, and certainly not one that any particular extreme event can prove or disprove. Overall I’m prepared to bet that it’s right, but it will take another couple of decades of research and statistics to be sure.

Roger A. Pielke Jr.
Roger A. Pielke Jr., professor of environmental studies at the University of Colorado.
The IPCC [Intergovernmental Panel on Climate Change] defines “climate change” as a change in the statistics of weather occurring over 30 years or longer and persisting for decades. Thus, the detection of a change in climate requires long-term records. To suggest that particular extreme weather events are evidence of climate change is not just wrong, but wrongheaded — every bit as much as the claims made during a particularly cold and snowy winter (or even several in a row) that such events somehow disprove climate change. Weather is not climate and short-term climate variability is not climate change.

The detection of changes in climate requires looking at actual data — and the data on tornadoes, large-scale river floods (in unaltered river basins), and landfalling hurricanes shows no evidence of trends in the direction of more extreme events. This should not be surprising, because even if we assume a strong signal in extreme events from human-caused climate change, the statistics suggest that it would take many decades, and probably longer, before such signals would be detected.

Given this context, claims that particular events can be attributed in a causal fashion to human emissions of greenhouse gases are simply unscientific if not fundamentally incoherent. It is true that overall damage from tornadoes, floods, and hurricanes has been increasing in recent decades. A recent literature review of extreme event impacts around the world found that everywhere that researchers have looked, this increase can be entirely explained by increasing value of property at risk and increasing exposures to these hazards.

Human-caused climate change is real and deserves effective policies in response. The making of claims that are scientifically unsupportable will not further that effort.

Kerry Emanuel MIT
Kerry Emanuel, director of Massachusetts Institute of Technology’s Program in Atmosphere, Oceans and Climate.
There is some evidence that hydrological events are becoming more extreme. This is not so easy to estimate, because rainfall is often quite local, so a good network of observing stations is required. In the U.S., where we have an excellent precipitation network, several published papers present evidence that rainfall is becoming increasingly concentrated into less frequent but more intense events.

Worldwide, satellite-based observations indicate that the most intense tropical cyclones are becoming more frequent even though the far more plentiful weak storms are occurring somewhat less often. In the North Atlantic region, where we have the best observations of such storms, there is a close correlation between their power and the temperature of the tropical Atlantic in summer. This correlation, which was first pointed out in 2005, has grown stronger since. Evidence is mounting that the tropical Atlantic sea surface temperature has been influenced by sulfate aerosol “global dimming” as well as by greenhouse gas increases, while evidence for a “natural cycle” in ocean temperature and storminess is correspondingly less persuasive.

Although this has been an exceptional year for tornadoes in the U.S., strong variations in reporting such events over time prevent us from deducing any long-term trends. Work is under way to examine trends in objective measures of thunderstorm rotation detected by Doppler radar since about 1988, when such radars were first widely deployed. But so far, there has been little theoretical work on how global warming might influence severe local storms, including tornadoes and hail storms.

Judith Curry
Judith Curry, chair of Georgia Institute of Technology’s School of Earth and Atmospheric Sciences.
The substantial interest in attributing extreme weather events to global warming seems rooted in the perceived need for some sort of a disaster to drive public opinion and the political process in the direction of taking action on climate change. However, attempts to attribute individual extreme weather events, or collections of extreme weather events, may be fundamentally ill-posed in the context of the complex climate system, which is characterized by spatiotemporal chaos. There are substantial difficulties and problems associated with attributing changes in the average climate to natural variability versus anthropogenic forcing, which I have argued are oversimplified by the IPCC assessments. Attribution of extreme weather events is further complicated by their dependence on weather regimes and internal multi-decadal oscillations that are simulated poorly by climate models.

I have been completely unconvinced by any of the arguments that I have seen that attributes a single extreme weather event, a cluster of extreme weather events, or statistics of extreme weather events to anthropogenic forcing. Improved analysis of the attribution of extreme weather events requires a substantially improved and longer database of the events. Interpretation of these events in connection with natural climate regimes such as El Nino is needed to increase our understanding of the role of natural climate variability in determining their frequency and intensity. Improved methods of evaluating climate model simulations of distributions of extreme event intensity and frequency in the context of natural variability is needed before any confidence can be placed in inferences about the impact of anthropogenic influences on extreme weather events.

Laurens Bower
Laurens Bouwer, climate scientist at Vrije University, Amsterdam.
The IPCC has established that here is an increase in the frequency of some extreme weather types at the global level, including heavy rainfall events, and extreme temperatures. Part of these changes is also a result of increasing atmospheric greenhouse gas concentrations. It is hard — if not impossible — to link individual local events to human-caused climate change. Still, the likelihood of particular types of extreme events occurring may have shifted, and so-called attribution studies can demonstrate this to a certain extent. At the same time, the occurrence of local weather extremes is influenced to a large extent by natural climatic variations that have a much more significant impact on timescales of tens of years.

Even more important is that other processes determine the impact of extreme weather events — principally the way humans modify their environment and often settle in locations where natural hazards occur. These are therefore the only reason that we have been seeing more frequent and widespread weather disaster over the past decades. For example, severe flooding has occurred in Pakistan before, but its growing population made the impacts of last year's flooding much worse. The 2005 flooding disaster in New Orleans could only occur because the population was not well protected against the surge that the winds of hurricane Katrina caused. At the moment there is no record of weather-related disasters in the scientific literature that can be demonstrated to have been caused by human-made climate change causes. In the near future, we will see more disasters in places where growing populations are not well protected from the weather — but not because of climate change that is happening today and tomorrow.

Gabriele C. Hegerl
Gabriele C. Hegerl, professor of climate system science at the University of Edinburgh.
Not all extreme events are expected to increase or even change. For example, there is quite a bit of evidence that greenhouse gas increases have contributed to recent widespread changes in the frequency of extreme temperatures, but this encompasses both decreases in the number of cold days and nights and increases in the number of warm nights. The widespread recent warming, which for global and continental mean data has been found to be likely due largely to human influences, leads to a changing probability of extreme temperatures. However, there are also cases where the tail of the temperature distribution is changing differently from the mean. Warming can lead to more severe drought in regions and seasons where precipitation decreases or remains largely unchanged, while evaporation increases due to warmer temperatures. Furthermore, the warming atmosphere has been shown to become moister. This probably explains the statistically significant shift towards more extreme precipitation events worldwide, which cannot be explained by climate variability and is best explained by human influences.

Individual weather extremes can generally neither confirm nor dispute the role of humans in climate change. The only meaningful approach is to estimate changes in the probability of events of the kind observed, and then see if human influence has changed this probability. For example, there is a very convincing study that shows that the probability of a heat wave of the magnitude of the 2003 European heat wave has very likely substantially increased due to global warming. Recently, another similar study showed that the fall 2000 floods in the UK were more likely in a warming world than they would have been without human influence on climate.

William Hooke American Meteorological Society
William Hooke, director of the American Meteorological Society’s Policy Program.
We live on a planet that does much if not most of its business through extreme events. What we call "climate" reflects the summing-up of these extremes to find averages. And what we call climate "variability" or "change" will therefore be reflected in variability or change in the locations, timing, patterns, intensity, and duration of these extremes.

That said, the statistics of the extremes are inherently noisy. Teasing out long-term changes in the relationships linking the extremes and the averages merits concerted and sustained scientific attention, but will remain a multi-year aspiration. Scientists — let alone non-experts — will be debating any findings for some time to come. (The most likely place to be convincing earliest may prove to be the statistics for heat waves and cold snaps.)

In the meantime, society has to deal with extremes. The truth is that we're not proving very adept at coping with the extremes and hazards we're facing today. Furthermore, the biggest change in our experience with hazards in


Science Targets Human Role
In Extreme Weather Events

Science Targets Human Role in Extreme Weather Events
Employing increasingly sophisticated methods of studying weather extremes, climate scientists say they are closer to determining whether human-induced climate change is leading to more heat waves, floods, and other extreme events.
coming decades is most likely going to prove to be the result of social change and technology advance: population growth, urbanization, and movements to hazard-prone areas such as coasts; dependence on critical infrastructure; the tendency toward zero-margin societies, in both the developed and developing world; and so on. Land-use and building codes; no-adverse-impact policies for levees and other hazard measures; learning from experience; and public-private collaborations for building community resilience are going to be urgently needed in parallel with the natural science.


SHARE: Tweet | Digg | | Reddit | Mixx | Facebook | Stumble Upon


The post very interesting, and it seems that most of the scientists believe there is some impact (of climate change), even if it's just to make an event that would've happened anyway, worse. I think a lot of them are just saying we need more data over a longer period of time to say yes or no, but really... it starts sometime, and it starts being noticeable at some point... yeah, the data 30 years from now will prove it either way but we have to live it now :)

Posted by Krista on 02 Jun 2011

Stunningly absent is any mention of stalled, persistent, stationary, or stagnant weather systems/patterns that are already resulting as a consequence of global warming. Moreover, the jet stream, instead of flowing predominantly west to east as usual, meanders more north and south probably due to deicing of the Arctic ocean....

Posted by Stanley Scharf on 02 Jun 2011

We have heard for a long time that it is not possible to attribute any single climate event to human-induced warming. Quite so. But how many times can that claim be made? If we experience a couple of heat waves, a drought, some floods and several extreme hurricanes over couple of years, the claim that each cannot be attributable to human-induced warming starts to look like the inverse of the gambler's delusion (nine straight tails from tossing a coin increases the chances that the next toss will produce a head).

Posted by Clive Hamilton on 02 Jun 2011

How can anyone take Mr Trenberth seriously when he refers to "global warming" without any quantifiable qualification of the term, and gives a 100% "yes" with no supporting evidence? He also gets rather mixed up by talking about increased temperatures causing droughts, then excessive rains and floods, again without any qualification or reference. Also, what snow melts? What very high sea surface temperatures? These again sound like alarmist scaremongering, especially when the term "likely" is used, which generally means "I can't prove it".

From what I've seen and as has been referenced by the other writers, is the main human influence that we see is an increased population exposed to weather events and not protected. Had the levees in New Orleans been high and strong enough, there would have been no human disaster. We also have very short term memories (about as long as political election cycles). If we look back, the Australian floods were quite predictable, as there is a history of them.

This raises the fundamental question. What are we measuring here? Climate event severity, or the human cost of said events? You cannot mix these two and get any rational answer. One notable ting I've noticed in the UK when reporters report floods, is that they often refer to the location as a flood plain, then totally disconnected from that, say what a disaster it is. If only we could connect the two halves, the answer lies before us - if we build in flood plains (without adequate defences, as is normal), we're going to get flooded, and there will be a large human cost (lives and property value).

The natural variability/forcings have also been given far less weight than they should have. For example, the sun's activity level and wobble cycles are now being recognised as being far more significant than has previously been given credit, so making any possible human influence much smaller.

What no-one has mentioned though is the fact that more of these events are now reported than used to be, far sooner, and also now with a lower threshold of severity. This is called statistics, and can be made to demonstrate anything. Where's the objectivity?

So, sorry Mr Trenberth, your rather feeble, so self-assured, but fictitious assertions just don't wash with the quickly increasing majority who just don't accept the AGW lies any more.

How about we go away, observationally measure everything (not models) at a higher resolution in all dimensions across the globe, land and sea, valleys, mountains and sky, north and south, east and west, for 100+ years, then come back and ask the question again.

Final observation - No physicists invited to answer the question.

Posted by SCS on 02 Jun 2011

Hard to let last comment go unchallenged. Climate models, and too bad scs engaged in an ad Joni en attack on Dr. Trenberth without giving his own name.

The predictions of climate change do predict both droughts in some areas as the temperature rises and wind patterns change, as well as increased rainfall in other areas, leading to more floods, as warmer air can hold more water. There is no contradiction there.

SCS then disputes measured surface sea temperatures, and measured ice volumes, without
giving any supportive data. He says that solar radiance and the earths wobble can account for the changes we are seeing in the climate. At least, therefore, he must believe they there are changes.

Posted by Nick snow on 02 Jun 2011

The question was whether climate change is contributing to extreme weather events, and some of them turned it around, saying that extreme weather events don't prove climate change, which wasn't what they were asked.

Extreme weather events don't prove climate change, but many other lines of evidence do. Given that evidence (and people like Curry and Pielke accept that), it is reasonable to think that it contributes to some types of extreme weather, and there is evidence for that. Trenberth was spot on when he said that climate change combined with natural variability is what is causing some of these extreme events, particularly floods.

Posted by Dean Myerson on 02 Jun 2011

This disregard for risk fundamentals in this debate undermines its credibility and service. Frequency of events aside, the differential vulnerability of urbanized and coastal societies is not academic and can't wait for multi-decadal oscillation data sets. Actuarial statisticians, reinsurance companies, and financial institutions are quickly working with real-time data and intuitive models that are drawing circles around risks and driving coverage and pricing. Climate science needs to come off the mountain, forget about the IPCC, and bring human risk back to the table.

Posted by Dave G on 02 Jun 2011

Insurance companies have raised rates to meet higher loss rates from severe thunderstorms over the past 3 years. Despite the lack of economic growth and recent building activity, something has happened with weather patterns that is contributing to increasingly damaging events.

A quick review of the Climate Data Center's web site shows that the Mississippi Valley has increasing amounts of precipitation over the past 30 years. A quick review of Atlantic ocean sea surface temperatures shows that the western Atlantic and Gulf of Mexico have warmed to record or near record temperatures since 1980.

So, while, by definition, weather is not climate, there is strong evidence supporting Dr Trenberth's position that extreme precipitation events and severe thunderstorms are more frequent and that heat stored in the oceans underlies the increase in extreme events.

The denier's position that solar activity is causing warming is absurd given that the sun went through the longest and quietest solar minimum in 100 years. The reduction in solar luminosity should have caused a slight cooling. Solar luminosity has declined slightly over the past 30 years.

Dr. Roger Pielke is not a climate scientist. He should not be treated as one by this blog, in my opinion.

Note that Dr. Kerry Emanuel is politically conservative. Also, he does not depend on climate science funding for his research. He's an expert on the physics of atmospheric convection. He literally wrote the book on the physics of tropical storms. He has no political or economic dog in this fight. His opinion is based on sound science, not politics.

Posted by FishOutofWater on 02 Jun 2011

Dr Kevin Trenberth is a climate scientist and thinks that the 2011 floods in QLD Australia are something out of the ordinary? Maybe he should study some weather history before he opens his mouth. Floods of the severity that recently hit Brisbane (these were NOT as severe as some previous floods) occur every few decades (last major flood was in 1973). For the Brisbane basin severe floods have actually decreased in regularity and severity compared to the late 1800's- there were 5 major floods in the 15yrs between 1885 and 1900. Here is a link from the Australian bureau of meteorology.

Its such lack of historical knowledge and incorrect information by leaders in the climate science field that erode confidence in their opinions.

Posted by brett on 02 Jun 2011

I think SCS is spot on with his comment. We all need to look at the historical record (say for the last 200 years) to put things in perspective so as to avoid any knee-jerk reactions to the 'weather events' of the last few months and attempts to link them to 'climate'. Our modern communications networks (tv, internet, etc) allow for much more coverage of these events and distorted perceptions of frequency.

Kudos to the above writers who (mostly) showed restraint in blaming any one weather event on Climate.

Posted by Hank on 02 Jun 2011

Unfortunately, Dr. Trenberth left the field of science several years ago to pursue a career in sales.

All of us continue to wait for the "missing heat" to be located but that has not prevented Dr. Trenberth from assuring us that it is out there somewhere and that — notwithstanding the missing evidence— the hypothesis of catastrophic anthropogenic global warming is as solid as a mortgage-backed security.

Posted by John Garrett on 03 Jun 2011

Just one point regarding the impossibility of demonstrating a correlationat this time: Taken as a whole, world weather is a closed system and is governed by well-known physical laws. If the total heat energy present is increasing, then on average, local energy thresholds for extreme weather events must logically be reached more frequently. Perhaps it's not much, perhaps patterns are changing and therefore difficult to recognize. But some effects should be apparent.

Posted by James on 03 Jun 2011

The comments are interesting such as higher temperatures world wide means more evaporation from the seas and more precip I'll go with the majority for a 30 year gathering of data and waite to see for the findings then. I'm 92 years old.

Posted by george d whiyney on 03 Jun 2011

The oceans are heating up. There is no "missing heat". The personal attacks on Trenberth are standard right wing political hits.

Please stick to the science.

A careful review of Australian flood and La Nina data not only shows a correlation between La Nina and Australian floods, it also shows a correlation with Mississippi River floods the spring following January Australian floods. The past year there have been many floods on many continents all correlated with La Nina.

However, close inspection of NOAA identified ocean heat anomalies shows that the floods also correlate with areas of exceptionally high heat content.

Strong La Nina winds have carried large amounts of water vapor evaporated from areas of high heat content towards adjacent land areas causing massive flooding from south America to south Asia.

Trenberth is correct. Natural variability and natural cycles combine with the increasing precipitable water due to anthropogenic climate change to produce years like this with exceptional numbers of extreme weather events.

Posted by FishOutofWater on 03 Jun 2011

SCS et al.,

Trenberth's assessment is systems-based. It's theoretical. It's backed by our understanding of the climate system and its changing variables. His position is scientifically tenable with current empirical data. His position is probably a good starting point. If we dump more energy into a system, what do we expect?

Of course, the system is complex. It's unlikely everything will become more extreme. Recent research has indicated a decrease in extreme wind and overall atmospheric circulation. Other research, as Emanuel noted, indicates that less powerful hurricanes are becoming less frequent. But these observations should give us some confidence in our systems-based assessments: they were predicted by models.

It's completely true that we generally can't attribute specific events to anthropogenic warming-- though there have been numerous attempts published in high-impact journals. But that doesn't mean we can't use our understanding of the system our empirical data to attribute global weather trends to global temperature trends. Sure, the attribution has error bars, but we have a pretty good idea of how the system should respond, at least in a hazy, general sense, and we have a pretty good understanding of how the system has changed over the past couple decades, at least, again, in a hazy, general sense.

We expect the hydrological cycle to intensify. We expect this due to our systems-based understanding of the climate, climate model output, and our understanding of paleoprecipitation and paleoclimate. So far, data seem to indicate this is happening.

We expect drought in some places to become more severe. We expect this to our systems-based understanding of the climate and climate model output, which both indicate an expansion of the Hadley Cell. So far, data seem to indicate Hadley Cell expansion and expected weather impacts.

We expect heat waves to become more severe and frequent (at least, if defined against previous means). We expect this due to our systems-based understand of the climate and our model output. So far, data indicate that record temperatures are becoming more frequent.

We expect an increase in the frequency of the most severe hurricanes. We expect this due to our systems-based understanding of hurricane formation (which is certainly imperfect). So far, data indicate the most severe hurricanes are becoming more frequent. We are just starting to learn how to assess how hurricanes may have responded to paleoclimate.

These basic predictions are based on a systems-understanding of our climate. They are, so far, scientifically tenable. Empirical data indicate they are accurate predictions. But we don't have a great deal of data, so it's perfectly reasonable to remind ourselves that we aren't yet in a position to distinguish anthropogenic influence from natural variation.

From a risk management standpoint, however, we should be more concerned that we may already see correlations with so little warming! If even one of these trends continues with further warming, consequences could be greater than anticipated, especially if emissions continue unabated.

For those who fear historical perspective isn't being taken into account when assessing recent weather for potential trends, read the actual scientific literature. Research treats the subject very conservatively. Data are tested for statistical significance. Careful thought and controls are used to mitigate bias. Conclusions are understated, and unknowns are always front and center. Many seem to think this is a political issue. It's not. It's scientific, and the proof is in the science. It's in the journals. It's in the conferences.

And yes, Roger Pielke is not a climate scientist. Notice he also indicates that data do not indicate an increase in the frequency of the most extreme hurricanes. Emanuel is a hurricane/climate expert. He may be considered THE hurricane/climate expert. Emanuel indicates the most extreme hurricanes are becoming more frequent. He can explain why. And he's demonstrated it in the scientific literature. It's one thing to argue that recent trends reflect natural variability, and that Emanuel's theoretical understanding of hurricanes is inaccurate, but suggesting the trend doesn't exist is just wrong. Pielke is overstating his position.

Weather is created in climate. We're altering climate. We're increasing climatic energy. It's not that great a stretch to anticipate increases in extreme events. We may not yet know the details, but we know our fingerprint is now in the climate, so we know it's in weather. And that's Trenberth's point. And it's a fair point. The real questions are, how big is our fingerprint, how much larger will it become, and how will it influence future weather?

Posted by Anthony No on 03 Jun 2011

To me the exact obverse seems to be true. Extreme weather events are more likely to be linked to global cooling, which is currently just beginning its progress. North-western USA temperatures have been low this year with massive snowfalls etc. These lower temperatures and warmer air form the Gulf of Mexico generate increased differentials and thus give rise to the more extreme weather events that we have seen recently. As Global Cooling proceeds the differential - Poles to the Equator grows and one can expect more weather extremes not less.

A warmer climate is likely to be more benign and a colder climate is truly deadly.

From Brian H | June 1, 2011 at 6:22 am | 
At a rough guess, the odds of warming being benign are about 80%, and of cooling being benign about 0.01%. The odds of warming occurring are about 10%, and of cooling occurring about 60%. The ratio of the riskiness is thus [(1-.8)(.1)]/[(.6)(1-.9999)] = .02/.00006 = 333. So it makes 333X more sense to prepare for cooling disaster than
for warming.

Posted by Edmh on 04 Jun 2011

I rather hurriedly skimmed through the 8 assessments, so I may be mistkaen, but in my oinion ALL of these experts, plus the people who formulated the question, seem to have missed some basic considerations.

First, humanity is engaged in multiple major modifications of the biosphere, only one of which and likely not the most significant, is global warming. The appropriate question is theefore not whether extreme conditions are increased by global warming but whether they are increased by human intervention, much of which ALSO causes global warming. So there may be correlations that are not cause-and-effect but the result of there being related or identical human causes for global warming and extreme events.

Second, I have long viewed ecosystems as being if not precisely then very similar to Laplace transforms of the past history of energy and other resource inputs. Another wayof saying approximately the same ting is to obsrve that each species has a characteristic time constant describing the rapidity with which it adapts its total biomass to varying conditions. It will tend to take advantage of comparable time constants in the change of inputs related to the seasons, longer-term solar events, etc. One sort of generalized niche inan ecosystem in which a species takes advantage of a gradient in space or time of some measure like temperature, elevation, etc., extracting energy by lessening the gradient, in accordance with the second law of thermosynamics. Which is to say that an inherent characteistic of an ecosystem is that it feeds off variations in conditions by extracting energy by deceasing those variations. So it is inherent in biodiversity and the existence of a mature ecosystem that it will stabilize local conditions of temperature, wate supply, you name it. So without having to know etails, we can be certain that decreasing biodiversity is going to increase extreme conditions. I am virtally certain that the concept can be made quantitative, because it is to large degree the "purpose" of ecosystems and biopdivesity to moderate extremes and by doing so extrct energy for the survival of the species constituting the ecosystem. Forests are obvious cases in point, since they moderate changes in temperature and water supply.

Posted by Nicholas Arguimbau on 04 Jun 2011

The more the IPCC promotes the global warming hysteria the more irrelevant it becomes. To suggest that AGW is rampant is just basically a well proven lie. Too many studies clearly demonstrates that to be a fact but it shpuld not stop Global Warming Supporters from standing back and taking another good look at what must be circumstantial evidence rather than fact. Too much needs to be learned before anyone can claim that the weather is involved in climate change and vice a versa. Meanwhile we wait for the hysterics to die down and wait for some more facts rather fiction which scientists appear to want to expose without political intervention. The public is still waiting.

Posted by Christian J. on 04 Jun 2011

Those commentators who presume to know more than Dr. Trenberth and who are disparaging him for his comments made above, might offer a more reasoned and scientific opinion had they actually read his many scientific papers and the scientific literature which support his statements.

We have altered the earth's climate system by, in very short time, increasing CO2 levels to their highest levels in 800 000 years, possibly even millions of years-- it is naive and myopic to assume that doing so will have had, and will continue to have, no significant ramifications.

There is in fact abundant evidence in the literature to support Trenberth's statements. As Dr. Emanuel noted, there is quite a bit of evidence suggesting that heavy and extreme rainfall events are on the increase, including droughts. There is also evidence that the hydrological cycle may be accelerating, and that the frequency of heat waves is increasing.

The much more worrisome events though include the worldwide loss of terrestrial land ice, and the dramatic loss of Arctic sea ice (an extreme event in its own right).

Seemingly omniscient readers here ridiculing Dr. Trenberth might wish to show some respect and address the science and substantiate their misguided assertions with some facts, rather than making ad hominem attacks.

Posted by Albatross on 04 Jun 2011

The extreme rainfall events which recently afflicted Australia came at the end of a national drought which at the time was said by our CSIRO to represent a new dry reality which we would have to become attuned to. They've since updated their position to "uncertain, but more rain and drought likely depending on where you live". Our just released Climate Commission Report has attributed increased bush fire damage to climate change though they preferenced a non-normalized study over those which did normalize for urbanisation. The latter don't find a connection. With regard to this topic I have no doubt that some authorities are simply seeing what they want to see.

Posted by David Anderson on 05 Jun 2011

brett on 02 Jun 2011 says: For the Brisbane basin severe floods. . .
Its such lack of historical knowledge and incorrect information by leaders in the climate science field that erode confidence in their opinions. “
~ ~ ~

B., Don’t know what you’re trying to prove with that. It means nothing without including consideration of the flood control structures that have been getting erected during those years.

Also lets not forget Trenberth isn’t only looking at Brisbane floods, he was looking at the whole atmospheric event and others.
Selectivity of data helps no one.
~ ~ ~ ~ ~ ~ ~

JG., the missing heat has been found in the deep ocean. The Earth Observation data is out there, just need to be willing to face it.

Here’s a start on that missing heat story:
~ ~ ~ ~ ~ ~ ~

Great point James, one worth repeating:

“Taken as a whole, world weather is a closed system and is governed by well-known physical laws. If the total heat energy present is increasing, then on average, local energy thresholds for extreme weather events must logically be reached more frequently.”

And as my above post links to, the evidence of these actual physical changes is out there.

Posted by citizenschallenge on 05 Jun 2011

Interesting responses, both by the experts and those who posted comments. Thank you all for your thoughts on the matter.

One thing I found unclear in Dr. Curry's response: How are we to get the data for natural vs. human-influenced variability? If we are using the data from our own times, we must hypothesize a baseline level of human influence, as it would be a needed assumption in the discussion. If we are using data from recent times - as far back as the start of the Industrial Revolution - the assumption of some degree of human influence would still be needed.

I have seen several pieces regarding differences in human influences over time, based on differences in the optical properties and concentrations of our industrial outputs - some aerosols seem to reflect a lot of insolation, for instance.

If we assume that nearly all of our direct data from current and historical sources is tainted by human influences on the system (the urban heat island effect is a favorite among many arguing against the validity of the science) then we must go back to times before human influence had much chance of being measurable, and we must rely on proxy data sources -which are often disputed in the public discourse.

Does anybody have any ideas on how to separate the signal from natural cycles from the signals from human influences, including any disruptions to existing natural feedback loops?

There were three separate attacks on Dr. Trenberth's response in the comments, so far. SCS seems to make a number of unwarranted assumptions, insisting on definitions for "global warming" (a worldwide, long-term increase in average temperature) and "likely" (established in IPCC reports as a confidence of 90% or greater). SCS wants numbers and citations in this non-peer-reviewed venue (an inappropriate demand) then denies that statistics can be used with enough reliability to be trusted. Dr. Trenberth's response to the question posed is clear enough, even if he generalizes for this audience. He has given well-supported numbers in other writings, and should not be expected to do the same here in limited space.

The second attack on Dr. Trenberth, by Brett, is over the floods in Queensland. I, for one, expect to find gaps in the knowledge of any expert. We are all human. The best of us are always learning new things. Even after a long life spent in diligent effort, gaps will remain.

The third attack on Dr. Trenberth, by John Garrett, is simply wrong. The so-called "missing heat" has been known to be in the oceans (and heat of fusion for melting ice) for over a decade, complete with accurate measurements and reproduction of results. If Mr. Garrett and "all of us" are not aware of this, they need to read more on the subject.

Finally, the question of whether risks from human population distributions are what we are really seeing in the recent data is a red herring. Certainly this tends to dominate coverage in the popular press. Certainly this tends to dominate the actuarial processes. These tragedies may impact our views, but professional scientists work hard to keep such biases out of their work. Human risk is not the question, here. Localized extremes and long-term trends are.

Posted by Daniel Gilsdorf on 05 Jun 2011

Both sides have arguments based on the measured evidence, but Trenberth's basic approach to the question is different from everyone else's:

- Everyone else is arguing from the measured statistics of extreme-weather events. They are saying that the statistics are not good enough (and will not be good enough for quite a while) to come to a statistically valid statement on the question.
- Trenberth is arguing from measured modifications in the factors that cause extreme weather. He is saying that we can see increases in the factors that must naturally lead to extreme-weather events.

The point is that Trenberth has enough faith in his understanding of how the climate works that he believes he can see the cause/effect in-progress; whereas the others are saying they need to see a statistically valid trend established for more time.

Under the circumstances, it will take a longer time for consensus to arise on this particular point; probably on the order of a decade or more. If we look back in 20 years and find that it has been a period of extreme-weather events, we will say that Trenberth was a visionary, who dared to say that he saw the signal amidst the noise. If instead we find that the period has not been particularly extreme, we will say that Trenberth was over-interpreting the data.

Posted by Neal J. King on 05 Jun 2011

Excellent post. Yes. There is definite evidence that weather extremes and global warming and climate change are related.

Dr.A.Jagadeesh Nellore(AP),India

Posted by Dr.A.Jagadeesh on 05 Jun 2011

My immediate reaction after reading the statements of the "experts":

"While Nero fiddles, Rome burns!"

Trenberth is the only expert who seems to grasp the urgency of now.

Posted by John Hartz on 05 Jun 2011

Were scientists who specialize in attribution contacted for this forum? For example, Zwiers, Stott and Santer? Instead, we have people like Curry glibly dismissing the body of science on
attribution and extreme precipitation events around the globe.

Munich Re and other insurance groups have documented a rapid increase in damages from
meteorological events, far more so than for other natural disasters like earthquakes and volcanoes.

Posted by Albatross on 06 Jun 2011

> ...eight leading climate experts ...

That is not true. E360 has 'balanced' the opinion of several experts with 'celebrity contrarians'.

Pielke is not a climate scientist and is notoriously unreliable and frequently *wrong*. Curry has been excoriated for her anti-science ranting and is now more accurately described as a political blogger than climate scientist.

It's clearly not a coincidence these two were picked from thousands of possible experts - someone knew they could be relied on to 'spice up' the story with contrarian views.

This is just more of the never-ending false balance from a media that is more concerned with drawing traffic than informing the public with good science.

Posted by BlueRock on 07 Jun 2011

If Clive Douglas wants to be taken seriously, he should stop running away from physics .

Increasing the heat content , flux, and contrast of any heat engine, an atmosphere in dynamic equilibrium with a hydrosphere included, will tend to accelerate the motion of its working fluid, and accelerated fluid motion and heat flux conduces to higher rates of evaporation.

So when he dislikes the weather he sees, he would do his predictable line of argument better service by reflecting on the magnitude of its anthropogenic component instead of denying its existence. Thermodynamics doesn't need an invitation to anybody's tea party.

Posted by Russell Seitz on 07 Jun 2011

I see this over and over again. Journalists keep forwarding the same wrong question from the public, which perpetuates the public misunderstanding around climate change as it relates to these blip-in-time phenomena like Hurricane Katrina.

It's clear that the scientists understand this and are trying to work around that, but the journalists and the public don't quite get it, as suggested by Professor Pielke. Regardless of whether he's right (or peer-reviewed, or whatever), the lay person doesn't understand the nuance here, which is why the wrong question keeps getting repeated. Scientists should not be trying to answer the question as asked, but rather they should be re-educating the public by redirecting the question into one that can actually be answered by the data: "Well, unfortunately, you're asking the wrong question, because climate is measured over much longer time scales, so it may be another decade or so before we know if these extreme events like Katrina were tied to global climate change."

As a science communicator, I know that the public wants a more useful answer than that--something to help them make policy decisions, perhaps? What good is the science (to them) if it can't advise the decision-making process? I would be disappointed if I didn't have some kind of practical follow-up answer like: "But many of the current models *do* predict an increase in extreme weather events associated with climate change, and if we continue to see more of these events, the evidence for our models becomes stronger. Scientists prefer not to speculate, but if the models are consistent, we probably will look back 10 years from now and say with more certainty that yes, in fact, these events were tied to global warming or global climate change."

Posted by Stevon on 09 Jun 2011

Why do you put forward discredited denialists like Pielke and Curry into this debate? They have been shown to present biased and distorted views and simply muddy the waters.

Posted by IA on 25 Jul 2011

"Extreme weather is linked to Global Warming on steroids."

Posted by Stanley Scharf on 11 Nov 2011

First: Pielke and Curry are pseudoscientists and bloggers. They offer opinions.

Second: Scientific consensus is reached by a preponderance of evidence at a given time, obeying known physical, chemical and biological laws, that can be reproduced. The results are published in peer-reviewed journals. Hypotheses are accepted and rejected over time. (If we knew everything now our understanding would be complete and research would end.)

Third: We live in a stochastic world constrained by boundaries, not a deterministic one in which are actions are linearly connected. This leads to various levels of uncertainty and models need to be reinitialized periodically. Human nature favors certainty, whether in the behavior of markets or the natural world. Expressions of uncertainty leads to fear, disbelief, and superstition. They also are amplified by the media. Unfortunately, simple statistical theory and probability are not taught in most educational systems.

Fourth: Obervations and data from the past that are leading to what we call 'global climate change' do not disobey and known natural laws and in many cases are predicted by them. Exponential increases in phenomena are characterized by a long initial period in which the multiplying can hardly be discerned. Thus an unusual or unprecedented rate of increase in a property requires attention. This seems to be the case for increasing global atmospheric temperature.

Finally, some factoids: NOAA oceanographers in the 1960's predicted increasing frequency and severity of weather events without benefit of models. Melting of permafrost is releasing methane, a major greenhouse gas, from rotting, previously frozen vegetation. A large percentage of atmospheric CO2 is entering the ocean, causing acidification which is affecting shellfish, zooplankton and corals. Had this CO2 stayed in the atmosphere its heat content would have increased more than at present.

Posted by Herb Curl on 26 Dec 2011

Hottest weather since 1936. Proof that Global Warming is real. Right?

Uh....wait a minute. Why was it so damned hot in 1936?

Posted by Scott on 07 Jul 2012

I would like to see each individuals credentials. I am curious if the actually have a degree in
climate science.

It is one thing to be head of an environmental department, for example, or an actual climate
scientist. many scientists refer to themselves as a climate scientist,

A geologist could be the head of a climate organization without being an actual climate

It is obvious they are well versed in climate science, but I prefer not to take this kind of info
on face value.

Posted by Micheal Smith on 09 Jul 2012



Abrupt Sea Level Rise Looms As Increasingly Realistic Threat
Ninety-nine percent of the planet's freshwater ice is locked up in the Antarctic and Greenland ice caps. Now, a growing number of studies are raising the possibility that as those ice sheets melt, sea levels could rise by six feet this century, and far higher in the next, flooding many of the world's populated coastal areas.

From Mass Coral Bleaching, A Scientist Looks for Lessons
For climate scientist Kim Cobb, this year’s massive bleaching of coral reefs is providing sobering insights into the impacts of global warming. Yale Environment 360 talked with Cobb about the bleaching events and the push to make reefs more resilient to rising temperatures.

Climate Change Adds Urgency To Push to Save World’s Seeds
In the face of rising temperatures and worsening drought, the world’s repositories of agricultural seeds may hold the key to growing food under increasingly harsh conditions. But keeping these gene banks safe and viable is a complicated and expensive challenge.

As World Warms, How Do We Decide When a Plant is Native?
The fate of a tree planted at poet Emily Dickinson's home raises questions about whether gardeners can — or should — play a role in helping plant species migrate in the face of rising temperatures and swiftly changing botanical zones.

Can We Reduce CO2 Emissions And Grow the Global Economy?
Surprising new statistics show that the world economy is expanding while global carbon emissions remain at the same level. Is it possible that the elusive “decoupling” of emissions and economic growth could be happening?


Donate to Yale Environment 360