Sections

Services

Information

UK United Kingdom

The blame for rain is mainly done in vain

As a climate scientist, it seems for every extreme event - be it the recent hottest 12 months on record for Australia or the floods and heavy rains of 2011 and 2012 - one question is inevitably asked…

Attributing heavy precipitation to climate change isn’t that easy. LordKhan/Flickr

As a climate scientist, it seems for every extreme event - be it the recent hottest 12 months on record for Australia or the floods and heavy rains of 2011 and 2012 - one question is inevitably asked: is human-made climate change to blame?

That is the wrong question.

It is impossible to say whether any single weather event was caused by man-made climate change. What we can do is examine whether the human influence on the climate has increased (or decreased) the odds of certain types of severe weather events happening.

The branch of climate science that looks at this is called attribution studies. Attribution research follows similar methods to those used in epidemiology.

Epidemiologists look at the patterns of disease in communities over a period of time to see where unusual outbreaks may have occurred and to determine what changes in the community might have led to these outbreaks. They may not be able to identify a single case of disease as being caused by the change but they can show that the change has caused the incidences of certain diseases to increase – and even define by how much.

Climate scientists take a similar approach, looking at the patterns and trends of climatic observations over a period of time to see why unusual or persistent weather patterns have changed and to determine what may have caused these changes.

Like epidemiologists, climate scientists cannot identify a single incident as being caused by climate change but they can define within a margin of error whether or not climate change has contributed to the likelihood of these events occurring.

Attribution, it’s just not cricket

To get a sense of how these kinds of attribution studies are applied we can take Australia’s national pastime, cricket, as an example.

Consider a cricketer who starts taking performance-enhancing drugs. Suddenly he hits 50% more boundaries in this season than he did in the last one.

For any single four or six he belts back over the bowler’s head, it would be impossible to say with certainty that one particular shot was made because of the drugs he was taking.

However, we might be able to say that the probability of this cricketer hitting a boundary has increased by 50%, supposing nothing else has changed in his performance.

This is exactly the approach we take to weather events. And certainly, it is useful when it comes to detecting the influence of global warming on large-scale periods of intense heat, as we have experienced over the past 12 months.

However, attributing precipitation events, such as rain and snow, to climate change is a much harder task.

The challenge of drying clothes

The relatively frequent experience of hanging your clothes outside to dry can help explain why.

Many of us may recall the feeling of coming home worried about leaving our washing outside when it’s been pouring with rain close to where we work.

Sometimes we get lucky (although not always!) and, despite passing through showers and storms on the trip back home, our clothes are still dry because it hasn’t rained where we live.

This is completely different to temperature extremes - if it is hot at work, it’s more than likely hot at home.

In short, extreme temperatures generally occur more uniformly over a wide area when compared to intense, localised rainfall events. For this reason, climate models fail to capture extreme precipitation events as well as they capture extreme temperatures.

Climate change and our long wet summers

This is also the reason that two studies into south-east Australia’s wet summers in 2010-2011 and 2011-2012 drew slightly different conclusions.

Figure 1: Summer 2011-2012 was exceptionally wet over parts of Australia.

In the study that my colleagues and I used to investigate these summers, we applied the same kind of methods described above to analyse these extreme weather events.

We looked at the wet summers of 2010-11 and 2011-12 over south-east Australia and tried to assess whether the probability of this kind of extreme rainfall event occurring had altered due to climate change.

It is important to note, these exceptionally wet periods coincided with two consecutive La Niña phases of the El Niño-Southern Oscillation (ENSO). La Niña events are typically associated with cool, wet summers in eastern Australia and therefore may by themselves be likely to be “blamed” for the heavy rains rather than additional human influences.

We used several state-of-the-art climate models to perform this analysis, so we could examine multiple possible realisations of the past century and a half.

All of these models had greater greenhouse gas concentrations later on in the model runs than they did at the beginning to coincide with observed changes over the past century. This meant we could compare extreme rainfall events near the start and end of the model runs to see if heavy rainfall events in south-east Australia had become more (or less) likely.

In the end, we couldn’t find much evidence that the human influence on the climate had played a significant role in these kinds of extreme rainfall events occurring over south-east Australia. In fact natural climate variability related to ENSO was shown to have a far greater influence than the effect humans have had on the climate.

Figure 2: Man-made influences on the climate (left) have less impact on extreme rainfall than ENSO variability (right).

Our study has been featured in the latest issue of the Bulletin of the American Meteorological Society (BAMS), which is devoted to similar analyses looking at other recent extreme weather events, such as the droughts in the central US and the record Arctic sea ice minimum of last year.

By contrast, another study of similar events published in this issue of BAMS found a small influence from man-made climate change on above-average rainfall that fell in March 2012 in eastern Australia (increasing the probability of a wet March by 5-15%).

On the face of it, our study and this one may appear to have reached different conclusions. However, this is not the case.

The two investigations of Australian extreme rainfall events used different methodologies and applied to slightly different regions and time periods.

They both agreed that natural climate variability was the main driver of the heavy rainfalls. While our study found a small and non-significant human influence, the latter study also found that any human influence was likely to be relatively small.

So, while some recent studies have clearly detected substantial human influences on extreme temperatures across Australia, including the record summer temperature of 2012-13 and the record temperature for the last 12 months, the ability to detect the human-induced effects on extreme precipitation events over our continent remains elusive.

Join the conversation

21 Comments sorted by

  1. John Newlands

    tree changer

    My question would be how bad does it have to get before we take nontrivial action? This summer my 30 kL water tank nearly ran dry, flames got to the bottom of the hill in on which I live in January but it was also -8C in July. I suggest a couple of other metrics such as the ability of the frail elderly to afford aircon (including night time so PV is no help) and whether all fires can be extinguished within say 3 days.

    Clearly we're not too worried when coal ports are being expanded and seemingly our modest carbon pricing efforts are to be retracted. Since we're in denial mode I guess when more coal ships run aground on the GBR it will a case of accidents will happen. When a swathe of vulnerable people succumb to heat, cold, fire or flood it will be character building for the survivors. We could still be a decade away from decisive climate action.

    report
    1. John Whelan

      logged in via Facebook

      In reply to John Newlands

      We don't need non-trivial action on climate. Leave things exactly as they are and stop wasting money on uneconomic nonsense. The planet needs more CO2 to make it as lush and green as it once was. It is currently heading in the right direction. We also need our planet to warm up by at least 3 degrees Centigrade on average to make it a more hospitable place to live. As for vulnerable people succumbing to heat, the Left-leaning Labor party with its Carbon Tax and push for Green energy has given…

      Read more
    2. John Whelan

      logged in via Facebook

      In reply to John Whelan

      Further to the above, does anyone else find it an amazing coincidence that Karoly publishes two papers right in the middle of the Election campaign? Is the Left-leaning professor that keen to retain Labor and their cohorts the Greens? Oh dear, its too late for that. With the Coalition vowing to cancel Carbon Tax, perhaps the game is over for the 'Karolys' of the world and they now have an uncertain future as nobody is listening any more. You can cry Wolf for only so long and people turn off.

      report
    3. David Arthur

      resistance gnome

      In reply to John Whelan

      It's time, Mr Whelan, to ask some searching questions of those who persist in Denying that anthropogenic perturbation of atmosphere are a cause for concern.

      The fact is irrefutable, and well-established by 19th century physicist John Tyndall - and yet there are those who insist that climate change is a scare campaign waged by Commie/Greenie/Hippie/Druggie/Poofter Atheists (excuse my language, but I'm trying to reflect the mindset of such people as Alan Jones and his ilk).

      So my question to you, Mr Whelan, is: given this certainty, why are you so determined that your children, your grandchildren and this nation generally will suffer so grievously through your obstinacy?

      We need to ask the question, Mr Whelan: why are you betraying your nation?

      report
    4. ian cheong

      logged in via email @acm.org

      In reply to David Arthur

      eeek. tyndall did a great job for science especially spectroscopy. he did not measure nor demonstrate thermalization of infrared by CO2. Nor has anybody else. if anyone knows of such a paper, please tell.

      any number of physics experiments could demonstrate this effect, but they havent been done either. no as far as i can find.

      report
    5. David Arthur

      resistance gnome

      In reply to ian cheong

      Not sure what you're talking about here: thermalisation of IR?

      Actually, I've not heard the term thermalisation before, but I interpret its Wikipedia definition as referring to particles reaching thermal equilibrium through interaction. IR photons don't interact with each other, only with molecules of the medium through which they travel; it's the molecules of the medium that thermalise.

      report
    6. ian cheong

      logged in via email @acm.org

      In reply to David Arthur

      thermalsation = conversion of IR waves to heat by absoption, which presumably is not instantaneously remitted as IR. absorption + reemission results in scattering without necessarily causing a rise in temperature but easily detectable by spectroscopy.

      report
    7. David Arthur

      resistance gnome

      In reply to ian cheong

      Ian, I read your description of thermalistion as the process by which emitted radiation approaches the black body distribution.

      Has anyone measured the black body distribution? I assume so, since it was well-known before Planck derived its distribution by assuming energy quantisation.

      report
    8. ian cheong

      logged in via email @acm.org

      In reply to David Arthur

      thermalisation = heating

      conversion of IR to heat by absorption

      report
    9. David Arthur

      resistance gnome

      In reply to ian cheong

      Ian, thermalisation is NOT heating. Thermalisation is a process of absorbtion AND RE-EMISSION of energy by matter in which the input energy of any distribution (anything from radio waves to gamma, it doesn't have to be IR) is ABSORPED by matter and re-emitted as black body radiation (ie distributed in accordance with the Planck distribution, which is only dependent on the temperature of the emitting matter).

      Before just replying to this email by repeating the half-arsed definition you think you understand, please go look it up.

      report
  2. David Arthur

    resistance gnome

    Good article, makes a useful distinction between localised (time and space) events and general trends.

    report
  3. Wade Macdonald

    Technician

    A great article. In Adelaide we have had a wet , warm winter. Typically drought years are colder because of the night minimum under clear skies.

    http://www.bom.gov.au/climate/current/season/sa/adelaide.shtml

    As for the western half of the countrlies rainfall decreasing in comparison the eastern half increasing, Adelaide's geographical location appears to be giving us a bit of both sides of this pattern?

    report
  4. Mark Pollock

    Analyst

    Professor, of course it's impossible to attribute a single weather event to climate change. Could you please tell,your fellow boosters to stop drivelling on aboutnhurricanes Sandy and Katrina?

    report
  5. Mark Pollock

    Analyst

    I am very reassured that the professor is using "state of the art" climate models" to conclude that our recent heavy rain was not caused by excessive SUV use. I am heartened to read that he thinks that regular old natural variation might be causing some of the "extreme weather" that so many zealots blather on about.

    I hope Senator Milne is reading too.

    report
    1. David Arthur

      resistance gnome

      In reply to Mark Pollock

      On the other hand, Mr Pollock, what's less reassuring is that of 19 analyses of a dozen major 2012 extreme weather events, (ref "Explaining Extreme Events of 2012 from a Climate Perspective" - Special Supplement to the Bulletin of the American Meteorological Society Vol. 94, No. 9, September 2013, http://www.ametsoc.org/2012extremeeventsclimate.pdf) about half the analyses concluded that human-caused climate change contributed to the extreme event in question.

      If that's too long for you, there's…

      Read more
  6. Gerard Dean

    Managing Director

    Sorry, but do I see these two things in the same article?

    The name 'Professor Karoly' and statement, 'the ability to detect the human-induced effects on extreme precipitation events over our continent remains elusive.'

    Amazing, and gratifying that finally some sense is being written about 'climate change'. For too long pseudo scientists and for that matter, real scientists, have been too eager to make short term predictions that have proven totally wrong. We only need remember Professor Flannery's prediction our dams would never fill again or the many scientists who told us that the arctic would be ice free this year - all wrong.

    Reasoned articles like the one above will gain more than the crash-and-burn headline in the poor old dying Age newspaper.

    To show my approval, I will not mention JetA1 fossil fuel at all.

    Thanks

    Gerard Dean

    report
    1. Gerard Dean

      Managing Director

      In reply to Gerard Dean

      Sorry, I did mention JetA1 fuel, but only to say that I would not mention it, otherwise you may not be aware that I did not mention it.

      Makes sense to me - I think.

      report