Sunday, July 29, 2012

Climate results turn sceptic: 'let the evidence change our minds'

Leo Hickman   
The Age, July 30, 2012

THE Earth's land has warmed by 1.5 degrees Celsius in the past 250 years and ''humans are almost entirely the cause'', according to a scientific study set up to address climate sceptic concerns about whether human-induced global warming is occurring.

Richard Muller, a climate sceptic physicist who founded the Berkeley Earth Surface Temperature (BEST) project, said he was ''surprised'' by the findings. ''We were not expecting this, but as scientists, it is our duty to let the evidence change our minds.''

He said he considered himself a ''converted sceptic'' and his views had received a ''total turnaround'' in a short space of time.

''Our results show that the average temperature of the Earth's land has risen by 2½ degrees Fahrenheit over the past 250 years, including an increase of 1½ degrees over the most recent 50 years. Moreover, it appears likely that essentially all of this increase results from the human emission of greenhouse gases,'' Professor Muller wrote in an opinion piece for The New York Times.

The team of scientists based at the University of California, Berkeley, gathered and merged 14.4 million land temperature observations from 44,455 sites across the world dating back to 1753. Previous datasets created by NASA, the US National Oceanic and Atmospheric Administration, Britain's Meteorological Office and the University of East Anglia's Climate Research Unit had gone back only to the mid-1800s and used five times fewer weather station records.

The funding for the project included $US150,000 from the Charles G. Koch Charitable Foundation, set up by the billionaire US coal magnate who is a key backer of the climate sceptic Heartland Institute think tank. The research also received $US100,000 from the Fund for Innovative climate and Energy Research, created by Bill Gates.

Unlike previous efforts, the temperature data from various sources was not ''homogenised'' by hand - a key criticism by climate sceptics - but, instead was ''completely automated to reduce human bias''. The BEST team's findings, despite their deeper analysis, closely matched the previous temperature reconstructions, ''but with reduced uncertainty''.

Last October, the BEST team published results that showed the average global land temperature has risen by about one degree Celsius since the mid-1950s. But the team did not look for possible ''fingerprints'' to explain this warming. The latest data analysis reached much further back in time but, crucially, also searched for the most likely cause for this rise in land temperature by plotting the upward temperature curve against suspected ''forcings''. It analysed the warming impact of solar activity - a popular theory among climate sceptics - but found that, over the past 250 years, the contribution of the sun is ''consistent with zero''.

Volcanic eruptions were found to have caused ''short dips'' in the temperature rise in the period from 1750 to 1850, but ''only weak analogs'' in the 20th century.

''Much to my surprise, by far the best match came to the record of atmospheric carbon dioxide, measured from atmospheric samples and air trapped in polar ice,'' Professor Muller said. ''While this doesn't prove that global warming is caused by human greenhouse gases, it is currently the best explanation we have found, and sets the bar for alternative explanations.'' Professor Muller said his team's findings went further and were ''stronger'' than the latest report published by the Intergovernmental Panel on Climate Change.

In an unconventional move aimed at appeasing climate sceptics by allowing ''full transparency'', the results have been released before being peer-reviewed by the Journal of Geophysical Research. All the data and analysis may be freely scrutinised at the BEST website. This follows the pattern of previous BEST results.

Guardian News & Media

Wednesday, July 25, 2012

97% of Greenland ice sheet melting

The Age, July 25, 2012
Ben Cubby, Environment Editor   

The Greenland ice sheet is melting at an "unprecedented" rate, according to NASA satellite data that shows 97 per cent of the vast mass is undergoing some form of melting.

"This was so extraordinary that at first I questioned the result: was this real or was it due to a data error?" a NASA researcher, Son Nghiem, said.

About half the ice sheet usually shows signs of melting in a northern hemisphere summer, but the satellite data shows that between July 8 and July 12 the melt extended to cover almost all of Greenland.

It follows the breaking off of a giant chunk of ice, about twice the size of Manhattan, from Greenland's Petermann Glacier, this month.

"For several days this month, Greenland's surface ice cover melted over a larger area than at any time in more than 30 years of satellite observations," NASA researchers said in a statement.

"Nearly the entire ice cover of Greenland, from its thin, low-lying coastal edges to its two-mile [3.2-kilometre] thick centre, experienced some degree of melting at its surface, according to measurements from three independent satellites analysed by NASA and university scientists."

They described the event as being without precedent because such a massive loss of ice has not been observed by humans before, although estimates derived from studying old, compressed ice suggest that melts on this scale happen about once every 150 years.

"Researchers have not yet determined whether this extensive melt event will affect the overall volume of ice loss this summer and contribute to sea-level rise," NASA said.
About one-fifth of the annual sea-level rise experienced globally is attributed to the melting of the ice sheet.

The manager of Australia's climate monitoring section at the Bureau of Meteorology, Karl Braganza, said the observation was a disturbing development.

"In terms of just one event taken in isolation, you can't tell much from it. We had a similar event back in the 1800s so it does happen from time to time," Dr Braganza said.

"But clearly there is a trend going on in the Arctic this century. We have warmer ocean temperatures, now what looks like particularly large reductions in sea ice, and large chunks of glaciers breaking off."

The Arctic appears to be locked in a vicious cycle, where rising concentrations of greenhouse gases meant higher temperatures, and more melting ice, which meant that, in turn, less of the sun's incoming heat was reflected away from the Earth.

"What's alarming to scientists is that we know the Arctic ice is a key feedback, and the warming in the Arctic has been slightly faster than was predicted 10 or 20 years ago," Dr Braganza said.

"This year, we measured CO2 emissions in the Arctic at above 400 parts per million for the first time. That's the first time it's been at that level in 3 million years. Back then during the Pliocene period, the Greenland ice sheet wasn't a feature. Now we've taken the atmospheric chemistry back to that territory."

The NASA statement said the huge melt had been driven by an unusual "heat dome" of relatively warm air that travelled across Greenland this month. The warm patch spiked just before July 18, and has now dissipated, they said.

Wednesday, July 11, 2012

How global warming is driving our weather wild

by Stephen Battersby 
New Scientist 09 July 2012 

Not only is global weather becoming much more extreme, it is becoming even more extreme than anyone expected

Editorial: "As freak weather becomes the norm, we need to adapt"

ITS NICKNAME is the icebox of the nation. The village of Pellston in Michigan often sees arctic winters, with a thermometer-shattering record low of -47 °C in 1933. Even by late March, it is usually a very chilly place. But not this year. On 22 March the Pellston weather station registered a temperature above 29 °C, vaporising the previous record for that date by more than 17 degrees.

This was just one of thousands of weather records smashed by the "summer in March", a 10-day event that affected much of North America. Many people enjoyed the unseasonal warmth, but most of the other extraordinary weather events of the past decade have been far less welcome. In 2003, the summer in Europe was so hot it killed tens of thousands. Russia in 2010 suffered even more staggering heat. Rainstorms brought unprecedented floods to Pakistan in 2010 and again in 2011. Tropical cyclone Gonu in 2007 was the most powerful ever recorded in the Arabian Sea.

Climate scientists have long warned that global warming will lead to more heatwaves, droughts and floods. Yet some of these recent extremes, such as the summer in March, are way beyond the predictions of our climate models. And there have been extremes of cold as well as heat. In Rome, ancient monuments are crumbling because of the big freeze that hit Europe this February. And on the northern edge of the Sahara desert, the streets of Libya's capital Tripoli were blanketed with snow.

It seems that our weather is getting wilder - more variable as well as steadily hotter. The big question is why. Is this just a blip, or are we in for even more freakish weather as global warming accelerates over the coming decades?

See more: "Worst and wildest: Five off-the-charts weather events"

Even in an unchanging climate, our weather varies a lot. Each summer will be different. Take the average summer temperature each year, and you will get a series of numbers scattered about a long-term mean, distributed in a pattern more or less like a bell curve. Wait long enough, and you will sweat though a few very hot summers and grumble through a few very cool summers.

Over the past century, the surface temperature of the planet has increased by 0.8 °C on average, which has shifted the familiar range of weather into warmer territory. Cooler summers have become less likely and warmer summers more likely. Contrary to what you might think, this kind of shift increases the odds of extremely hot summers by more than it increases the odds of slightly warmer summers (see "Shifting weather").

The rising temperature is leading to other kinds of extreme weather, too. Warmer air can hold more moisture - in fact, its capacity increases exponentially as the temperatures rises. This means that when rain falls it can become a deluge, increasing the chance of catastrophic floods (New Scientist, 19 March 2011, p 44).

Damper downpours

Floods are not the only result. When water vapour condenses to form clouds, it releases latent heat, and this heat is what powers most kinds of storms, from thunderstorms to hurricanes. With a wetter atmosphere there may not necessarily be more storms, but those that do occur will tend to be more powerful because there is more heat to power them. The damage done by storms rises rapidly as wind speeds increase.

So simple physics tells us that global warming should make extreme weather more extreme, from stronger storms to hotter heatwaves, drier droughts and damper downpours. This is indeed what has been happening around the world - except that in recent years, the magnitude of some of the record breakers has been jaw-dropping.

In 2003, temperatures in Europe were much higher than any summer for at least 500 years. Stefan Rahmstorf of the Potsdam Institute in Germany points out that in Switzerland the average summer temperature broke the previous record by 2.4 °C. It is not unusual for the records for particular days to be broken by fairly wide margins but for the average of an entire season to be so much warmer is extraordinary. Then there was the Russian heatwave of 2010. Even averaged over Europe as a whole, this heatwave was more extreme than the one in 2003.

Most recently, there was the summer in March. Because it was so early in the year, it was a disaster only for fruit growers - trees blossomed too early and then got hit by frost, wiping out over 90 per cent of crops in some places - but it could have been much worse. "If such unusual conditions had occurred during July or August, the impact would have been enormous," says Dim Coumou, a colleague of Rahmstorf.

More and more people are being affected by all this extreme weather. In a recent poll in the US, 82 per cent of people reported that they had personally experienced extreme weather or a natural disaster in the past year, and 35 per cent said they were personally harmed either a great deal or a moderate amount by one or more of these events.

There is little doubt that things are going to get even worse. What is especially worrying, though, is that the rise in extremes can't be accounted for solely by the 0.8 °C warming so far. Events like the 2003 and 2010 heatwaves were projected to occur only after much greater warming, towards the end of this century. And while one or two freak events might be dismissed as simple bad luck, there have been suspiciously many of them in the past decade.

James Hansen at NASA's Goddard Institute for Space Studies in New York has analysed records of local temperatures across the globe, in each case totting up June, July and August to get an overall temperature for this period. The results show that an increasing area of the planet's surface is experiencing highly anomalous heat extremes each year, relative to the period 1951 to 1980 (see charts).

To a large extent, this is just what is expected in a warming world. However, Hansen's analyses show there is more to it than that. The weather is not only getting warmer, but more variable. Between 1951 and 1980, the average range in local summer temperatures across the entire globe was 0.55 °C; from 1981 to 2010, it had gone up to 0.58 °C. Over land the variability is greater, and its increase is faster. Some locations, especially those far from the stabilising influence of the ocean, see much more variability and more increase. Project that into the future, and we already have more cause for concern than we had with the mere rise in mean temperature.

Implausibly hellish

But even in the context of this somewhat more jittery climate, the mega-heatwaves of the last decade stand out as implausibly hellish. Is something else happening to make temperatures soar like this?

Quite possibly, says climate modeller Pier Luigi Vidale at the University of Reading, UK. He thinks plants and soils might explain some of the unprecedented heatwaves. Where land is covered by vegetation, much of the sun's heat is absorbed by plants. They stay cool - and keep the land cool - by sucking up water and letting it evaporate from their leaves. But when the soil dries out, plants close their pores and stop transpiring. "It is the same as if you don't drink any water and stop sweating," says Vidale. When the sun's heat is no longer channelled into evaporating water, it all goes into the land and the air above it. The result is a jump in temperature.

This has been happening for as long as there have been plants on land, but it is becoming more common and affecting greater areas because winter precipitation has become more erratic, sometimes failing to soak the soil thoroughly. At the same time, the growing season has lengthened and become warmer, so plants are sucking up more water.

In 2004, while he was part of a team led by Christoph Schär at the Swiss Federal Institute of Technology in Zurich, Vidale studied this process in a regional climate model. Although previous models had included drying soils, the representation was too simple, Vidale says, as plants shut down transpiration too suddenly. With the improved model, some of the simulations looked like the summer of 2003, and other models have since produced similar results. They all suggest that over land where soils can dry out, summer temperatures do not follow an exact bell curve. Instead, there are more mega-heatwaves.

However, a lot of uncertainty remains. Part of the problem is accurately modelling the role of plants and soil. Even details such as the species of plant are important, because plants with deep roots will keep transpiring long after those with shallower roots. "The biophysical models are not yet accurate enough," says Robert Vautard at the University of Versailles in Saint-Quentin-en-Yvelines, France.

Measuring the actual moisture content of soils could improve things, but it is not easy. "You can make a measurement here, but a few metres away it's not valid any more," says Vidale. "In Europe we only have a few monitoring stations for soil moisture. Many of us have been arguing for more." Satellites can give an indication of surface soil moisture over a wide area, but not how much is available to deep roots.

While drying soils could be partly to blame for some recent off-the-chart heat extremes, they are not the whole answer. Vidale's model may have reproduced the scorcher of 2003 - but it also predicted that such hot summers would be unlikely before the end of this century. And of course this phenomenon cannot account for all the weird weather we are experiencing. You don't need to be a climate expert to conclude that a heatwave did not cause snow in Tripoli. But some researchers think they know what might be to blame for that - a lazy jet stream.

Jet streams are high-speed winds that carve a snaking path through the upper atmosphere. The two polar jet streams, one in each hemisphere, are driven by the difference in temperature between warm tropics and cold poles. In the tropics, the atmosphere is puffed up by higher temperatures: "It's like there is a hill from the tropics tilted down towards the poles," says Jennifer Francis of Rutgers University in New Jersey.

Gravity pulls some of this air down towards the poles. Because of Earth's spin, the air gets deflected off to one side, which is what drives the polar jet streams from west to the east.

The positions of the jet streams aren't fixed. They move around, shifting south or north and also developing big meanders, or waves. "You can get such a big wave, it breaks off as an eddy that gets left behind, just sitting and stewing in its own juice," says Francis. "When this happens, the weather near the eddy stays the same for days or even weeks."

Humanity is now messing with this vital component of the atmosphere. The Arctic is warming far faster than the rest of the planet, in part because its sunlight-reflecting snow and ice is melting to expose dark, sunlight-absorbing land and sea. This is reducing the temperature difference between the tropics and the Arctic. In work published in 2009, Francis showed that in summers with less sea ice in the Arctic - meaning more heat being absorbed by the ocean - the atmospheric hill had a more gentle slope (Geophysical Research Letters, vol 36, p L07503). The upshot is that the engine driving the northern polar jet stream is weakening.

As the jet stream slows down, it takes a more mazy path, with meanders that move around more slowly. That is crucial, because the jet stream pushes weather systems around. So when the stream's position changes more slowly or stays in one place for weeks - what meteorologists call a blocking pattern - the weather is more likely to become extreme. If the jet stream shepherds one low pressure system after another towards you, then you will soak - as happened to the UK this April, producing record rainfall. If the sluggish stream holds a high pressure system in place, you will roast.

"It's not news when you have one or two hot or cold days. If it goes on for a week or two, then people are freaking out because their harbour is freezing over," say Francis. Blocking patterns have played a part in much of the extreme weather around the northern hemisphere in recent years, including some of the freezing winter weather and record snowfalls, and the summer in March.

Other researchers have confirmed that the jet stream has been weakening, and shown that this leads to more blocking events. Now Francis has found another effect of the warming Arctic. "I got thinking - if you are warming the north more than the south, that will stretch the northern peaks of high-pressure ridges farther northward." Working with Stephen Vavrus of the University of Wisconsin-Madison, she used highly detailed weather models to recreate past events and trace the contours of atmospheric pressure. And indeed the high-pressure ridges have tended to stretch further north in recent years. That makes the meanders of the jet stream more extreme, bringing warm air further north, and cold air further south - to places such as Rome and Tripoli.

So it appears the northern hemisphere is in for more weather chaos as the planet warms. In some years, the jet stream mechanism could cancel out the drying-soils mechanism but in others it could amplify it, because a lazy jet stream will occasionally produce exceptionally dry winters and springs as well as hot spells in the summer. The polar jet stream in the southern hemisphere is unlikely to be affected in the foreseeable future, though, because Antarctica is warming more slowly than other parts of the world.

Provoking the elements

There could well be other, as-yet-unidentified mechanisms contributing to the wildness of our weather now, or which might kick in as the world warms further. For example, having roused the air and the earth against us, we may also be provoking another element. The ocean joins with the atmosphere in a roughly periodic pattern called the El Niño Southern Oscillation, or ENSO, in which warm water sloshes back and forth across the surface of the Pacific Ocean partly in response to changes in the trade winds.

ENSO's changing moods already cause all sorts of mayhem and as the water sloshing around gets even warmer the mayhem is likely to increase. The Australia and Pakistan floods of 2010 and 2011 were due to unusually warm surface waters loading the air with moisture, probably caused by a combination of ENSO and climate change.

A big question is whether things could get even worse. What if ENSO and other climatic oscillations don't just continue as before in an ever hotter world, but become even greater in magnitude? Are we pushing these pendulums in a way that makes them swing more wildly? "So far there is no clear evidence either way," says Rahmstorf, "but we are changing the whole energy balance of the climate system, so in a way it would be surprising if these patterns of variation did not change."

Part of the problem with studying these phenomena is that our climate models are still relatively coarse, although they are improving. A new international collaboration called RESOLVE, which includes Vidale's group in Reading, will run global climate models with a spatial resolution of 2 kilometres, compared with a more typical resolution of tens of kilometres. Vidale hopes this will be able to show how processes are connected across the globe, and allow researchers to tease apart the influence of soils and atmospheric circulation on weather extremes.

Such models might give a better idea of just how extreme future extremes could become, although there is of course no way to predict one-off weather events years in advance. Then again, as earlier models missed the changes in the jet stream, these new ones could still be blind to some unanticipated mechanisms.

In the meantime, there are some things we can do to prepare. European health services now have better contingency plans for a mega-heatwave than they had in 2003. We can design buildings to cope with extreme heat, and planners might consider avoiding putting vital infrastructure in areas at risk of flooding.

But adaptation can be very costly, and the very nature of more variable weather poses problems. Farmers could learn to cope if it was consistently drier or hotter, for instance, but if the weather continues to become more variable and there is no way to know whether to expect frost or floods, hail or heatwaves, then each season will become an ever greater gamble. "It is difficult to adapt to unprecedented extremes, as they always involve some element of surprise," says Rahmstorf.

While no one can say exactly what's going to happen to our weather, all the signs are that we're in for a bumpy ride. "We are seeing these extremes after only 0.8 degrees of global warming," says Rahmstorf. "If we do nothing, and let the climate warm by 5 or 6 degrees, then we will see a very different planet."

Stephen Battersby is a freelance writer based in London

Tuesday, July 10, 2012

Climate change boosts odds of extreme weather: report

Posted July 11, 2012
Severe droughts, floods and heat waves rocked the world last year as greenhouse gas levels climbed, boosting the odds of some extreme weather events, international scientists say.
The details are contained in the annual State of the Climate in 2011 report, compiled by nearly 400 scientists from 48 countries and published in the peer-reviewed Bulletin of the American Meteorological Society.
The report itself remains "consciously conservative" when it comes to attributing the causes of certain weather events to climate change, and instead refers only to widely understood phenomena such as La Nina.
However, it is accompanied for the first time by a separate analysis that explains how climate change may have influenced certain key events, from droughts in the US and Africa to extreme cold and warm spells in Britain.
"2011 was notable for many extreme weather and climate events. La Nina played a key role in many, but certainly not all of them," said Tom Karl, director of the National Oceanic and Atmospheric Administration (NOAA)'s National Climatic Centre.
Last year was among the 15 warmest since records began in the late 1800s, and the Arctic warmed at about twice the rate of lower latitudes with sea ice at below average levels, according to the report.
Greenhouse gases from human pollution sources like coal and gas reached a new high, with carbon dioxide emissions exceeding 390 parts per million - up 2.10 parts per million from 2010 - for the first time since modern records began.
Despite the natural cooling trend brought by back-to-back La Nina effects, which chill waters in the eastern equatorial Pacific Ocean, 2011 was among the 12 highest years on record for global sea surface temperatures.
The double La Nina punch influenced many of the world's significant weather events, like historic droughts in East Africa, the southern US and northern Mexico, the report said.
La Nina trends also were associated with the wettest two years on record in Australia.
An accompanying analysis in the same journal, titled "Explaining Extreme Events," examined the links between human-driven climate change and six selected weather crises in 2011, including the Texas drought that lasted half the year.
The authors found that "such a heat wave is now around 20 times more likely during a La Nina year than it was during the 1960s," said Peter Stott, climate monitoring and attribution team leader at the UK Met Office.
"We have shown that climate change has indeed altered the odds of some of the events that have occurred.
"What we are saying here is we can actually quantify those changing odds."
Looking at Britain's unusually warm November 2011 and the cold snap of December 2010, scientists found that frigid Decembers are half as likely to occur now compared to 50 years ago, and hot Novembers are 62 times more likely.
However, a close look at the floods along the Chao Phraya River that swamped Thailand last year showed that climate change was not to blame, but rather human activities that increased construction along the flood plain.
The damage caused by the floods was unprecedented, but the amount of rain that actually fell "was not very unusual," said the analysis by experts from NOAA and Britain's Met Office along with international colleagues.
While it remains hard to link single events to human-caused climate change, "scientific thinking has moved on and now it is widely accepted that attribution statements about individual weather or climate events are possible," the report added.
The key is analysing to what extent climate change may be boosting the odds of extreme weather, said the report, likening the phenomenon to a baseball player who takes steroids and then starts getting 20 per cent more hits than before.
Scientists can consider steroids as the likely cause for the increase in hits, but must still take care to account for natural variability in the player's swing.

Friday, July 6, 2012

Is it now possible to blame extreme weather on global warming?

Tuesday 3 July

Wildfires, heatwaves and storms witnessed in the US are 'what global warming looks like', say climate scientists

Whenever an episode of extreme weather – heatwave, flood, drought, etc – hits the headlines, someone somewhere is sure to point the finger of blame at human-induced climate change.

Such claims are normally slapped down with the much-aired mantra: "You cannot blame a single episode of bad weather on global warming." But with the on-going record high temperatures affecting large parts of the US, there seems to be a noticeable reduction in such caveats and notes of caution.

This week, scientists have been queuing up, it seems, to explain how thewildfires in Colorado, the heatwave across the eastern seaboard, and the "super derecho" are all indicative of "what global warming looks like". Most pulled back, though, from directly blaming global warming for such weather events.

"In the future you would expect larger, longer more intense heat waves and we've seen that in the last few summers," Derek Arndt of NOAA Climate Monitoring told the Associated Press." The same report added: "At least 15 climate scientists told the Associated Press that this long hot US summer is consistent with what is to be expected in global warming."

So, can we now say, or not, that specific extreme weather events are caused, or at least exacerbated, by global warming? Has anything changed in climate scientists' understanding of the attribution - or "anthropogenic fingerprint" - of such events? Are they now more confident about making such links?

I put this question to a number of climate scientists...

Kerry Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology:

In my view, the only responsible statement scientists can make about this regards the probabilities of such events with and without climate change. We should be able to say something like "the annual probability of a heat wave of magnitude A and duration B before the advent of climate change was x but as a result of climate change has increased to y and is expected to further increase to between z1 and z2". It would take some work to actually fill in the numbers x,y, z1, and z2, but there are studies along these lines for events such as the 2003 European heat wave.
In my view, any statement that goes appreciably beyond statements like this one probably involves spin of one kind or another. In addition, once could talk about the particular routes by which climate change affects particular events. For example, the fires in the Rockies have apparently been affected by the ill health of many trees, owing to a population explosion among pine beetles, which is in turn partly owing to climate change.

Dr Peter Stott, head of climate monitoring and attribution, at the Met Office Hadley Centre:

Unusual or extreme weather events are of great public concern and interest, yet here are often conflicting messages from scientists about whether such events can be linked to climate change. While it is clear that across the globe there has been an increase in the frequency of extreme heatwaves and of episodes of heavy rainfall, this does not mean that human-induced climate change is to blame for every instance of such damaging weather. However climate change could be changing the odds and it is becoming increasingly clear that it is doing so in such a way as to increase the chances of extremely warm temperatures and reduce the chances of extremely cold temperatures in many places.
At the Met Office, in collaboration with international colleagues, we have formed the Attribution of Climate-related Events initiative (or ACE for short) to develop the tools needed to quantify the changed risks of extreme weather. Extreme rainfall and flooding is a particular challenge. The globally warmer atmosphere now carries 4% more moisture over the oceans than in the 1970s and in many places this extra moisture would be expected to lead to increased rainfall when storms form over land. But in some regions, weather patterns as a whole could change due to natural as well as human causes. For example, if there were a systematic shift in the jet stream, the fast flowing ribbon of air high in the atmosphere that steers storm systems, this could reduce the risk of extreme rainfall in some places. The next meeting of the ACE group, taking place in Oxford in October, will discuss the development of authoritative assessments of extreme weather risk that can be produced shortly after the extreme weather events in question when interest is at its height.

Professor Michael Mann, director of the Earth System Science Center at the Penn State Department of Meteorology:

I like to use the analogy of loaded dice. Here in the US, we've seen a doubling in the frequency of record-breaking heat, relative to what we would expect from chance alone. So far this year, we're seeing those records broken at nearly 10 times the rate we would expect without global warming. So there is no question in my mind that the "signal" of climate change has now emerged in our day-to-day weather. We are seeing the loading of the random weather dice toward more "sixes". We are seeing and feeling climate change in the more extreme heat we are witnessing this summer, the outbreak of massive forest fires like the one engulfing Colorado over the past week, and more extreme weather events like the Derecho that knocked out power for millions in the eastern US during a record-breaking heat spell.

Dr Clare Goodess, senior researcher at the University of East Anglia'sClimatic Research Unit:

Over the last five years or so, a growing number of peer-reviewed studies have provided convincing evidence of a detectable human influence on the increases in high temperature extremes which have been observed over the last few decades over the globe as a whole and over large-scale regions such as Europe. Attribution of observed trends to human influences (anthropogenic climate change) requires a clear signal of change, which is stronger than the inherent natural variability of climate (so-called 'noise'), and where the different driving mechanisms of change can be separated out. Thus it is a more challenging task for rainfall and other weather variables than for temperature, and for areas smaller than continents. An anthropogenic influence has, however, recently been detected over Northern Hemisphere land areas in the largest daily rainfall events experienced each year.
These approaches to what climate scientists refer to as detection and attribution do not, however, address the questions that are uppermost in people's minds when they are personally affected by an extreme weather event. Unfortunately, I do not believe that it will ever be possible to look at a single event and say definitively if it would or would not have occurred in the absence of human influence. However, what can be, and has been, done is to estimate the extent to which human activity has increased the risk of such events occurring. It has, for example, beendemonstrated that human influence has more than doubled the risk of a hot European summer like that of 2003 occurring, and substantially increased the risk of floodingwhich occurred in England and Wales in autumn 2000.

Dr Doug Smith, who leads decadal climate prediction research and development at the Met Office Hadley Centre:

I think it is inevitable that climate change will affect the frequency and intensity of extreme events. Weather can be characterised in terms of a mean value (the climate) and variability around the mean. Climate change will shift the mean value (by definition), and hence change the probability of extremes unless the variability also changes to compensate exactly (and there is no reason to expect this). The difficulty is in calculating the contribution of climate change to an individual extreme event. This is currently an active research area, known as operational attribution, in which many climate model simulations are made with and without forcing due to climate change in order to compute differences in the probabilities of particular events.

Michael Oppenheimer, professor of geosciences and international affairs at Princeton University's Woodrow Wilson School and Department of Geosciences:

The link between extreme events which have occurred recently and the build-up of the greenhouse gases is best represented by the "loading the dice" analogy – as the world warms, the likelihood of occurrence (frequency), intensity, and/or geographic extent of many types of extreme events is increasing. The events are individual data points in a broader pattern, akin to pixels on a computer screen. You can't say much from any one pixel, but a picture emerges when you step back and look at the pattern. That said, for a few types of extreme events, particularly heat waves, it is sometimes possible to connect the pixel to the bigger picture more directly. The best case is the European heat wave of 2003. According to computer simulations of climate, the likelihood that such an event would occur was about doubled by the buildup of the greenhouse gases. A few other events have been examined using similar techniques, including the 2010 heat wave in Russia.
As for the willingness of scientists to make such statements: as the climate signal due to the ever-increasing greenhouse effect strengthens and emerges more and more from the noise in the system, and as statistical techniques for doing such "fingerprinting" studies as I mention above improves, scientists have become more confident in making such claims, which is to be expected.

Harold Brooks, head of the mesoscale applications group at Noaa'sNational Severe Storms Laboratory:

Attribution of extremes is challenging. We're faced with separate, but related, questions. First, how much did the warming of the planet contribute directly to the extreme event? Second, how much more likely was the event because of a warmer planet? For things that are closely
related to temperature (e.g., heat waves. fires), the first question can be addressed in a relatively straightforward manner and, typically, the answers are conservative. Even with a degree or two of global warming, the direct contribution to extreme heat, such as in the southern Plains of the US in 2011 and in much of the US in 2012, is small.
The second question is more challenging to address. There are two issues that need to be considered. First is a statistical problem about how the likelihood of low probability events changes as the average condition changes. For example, if you flip a fair coin 100 times, on average you get 50 heads and 95% of the time, you'll get between 40 and 60 heads and 2 or 3 times, you'll get 65 heads. If you get a weighted coin that is 55% likely to be heads, it will 10 times as likely that you'll get 65 heads. The small change in the average chance means the chance of an extreme becomes much more likely. The same thing happens for temperature extremes, but there's another issue. Did the change in the average temperature make it more likely that the flow in the atmosphere was even more likely to occur than just by chance? For instance, when it doesn't rain much over a large area, the ground dries out and heats up. The atmosphere responds to this by flowing around the area of hot air in a way that makes rain even less likely in the hot area, leading to more heating of the ground, reinforcing the flow around that area.
For things that aren't temperature, we have to work to understand the relationship between the global temperature and the phenomenon in question. For instance, we understand that warming the planet will likely lead to a more intense water cycle, with heavier rain when it rains and longer periods without rain in between. On the other hand, our understanding of how global scale atmospheric changes affect things like tornadoes and severe thunderstorms is that global warming will make some of the ingredients for them more likely and others less likely. As a result, it appears that long-term trends in tornado occurrence or intensity are unlikely to be large. Even without the planet warming, we would expect to see some years with many tornadoes and others with few tornadoes.

Michael F. Wehner, staff scientist at the Lawrence Berkeley National Laboratory:

Whenever an extreme weather event occurs, it is natural for the public to ask, "Is this event a result of global warming?" This is not the quite the correct question to ask, as to date, all individual weather events observed could have happened prior to the human intervention in the climate system, however unlikely that may have been. The more relevant question to ask is "How has the risk of this event changed because of climate change?" 
This risk of extreme weather, particularly very severe heat waves, has already changed significantly due to human induced global warming. For instance, the chances of the 2003 European summer heat wave, responsible for as many as 70000 additional deaths, at least doubled and likely increased by a factor of 4 to 10. The chances of the 2010 Russian and 2011 Texas events also undoubtedly increased. While these events could have occurred without the human changes to the climate, it is important to know that the amount of climate change that we have experienced so far is very small to what is projected to occur by the middle and end of this century. By 2100, today's most extreme weather events will seem relatively normal.