Mind the gap – distant climates and immediate budgets

Time for some updates from the world of climate science.

First up, the December issue of Nature Reports Climate Change is definitely worth checking out. (This spin-off internet magazine, produced by the Nature Publishing House, is always worth reading, and you can download a full-colour PDF if you prefer this format — good for printing).

Three articles, in particular, grabbed my attention this issue. The first revisited the premise of carbon budgets proposed by Allen et al. 2009 — a concept I covered in a BNC post back in May 2009. The conclusion was that to have a half-decent (50%) chance of keeping global temperature rise to <2°C below pre-industrial levels, given a climate sensitivity in the range of 2 — 4.5°C, humanity’s cumulative carbon budget between now and ‘forever’ (the next 100,000 years or so), is 1 trillion tonnes of carbon. We’ve burned 500 billion tonnes of fossil carbon and forests already, and on our current trajectory, we’ll break the global carbon bank within the next two to three decades.

In this latest paper, the authors suggest that in order  to better focus our attention to the immediate rather than perpetual task, we need a supplementary short-term budget for the period 2010 — 2030. They calculate that to avoid a rate of change of +0.2 per decade, the carbon ‘expense’ for the next 20 years must stay within 190 billion tonnes, or about 9.5 billion tonnes per year (for context, in 2008 global emissions were 9.8 billion tonnes). If we met this goal, we would then have a further 300 billion tonnes to spend for the period 2030 — 100,2030 AD (or thereabouts). Given the seeming inevitability of emissions growth for at least the next 5 — 10 years, we’ll have to have a serious turn around and decline in emissions in the period 2020 — 2030. Sobering thought. Massive deployment of nuclear and renewable power, anyone?

The second article worth reading is called “Mind the Gap”. Here, the question of novel and disappearing climates is considered (this problem has previously been addressed in the technical literature, here). Take a look at this grim figure:

This is a 4°C warming scenario — what’s expected by mid-century under a business-as-usual approach to fossil fuel use, or the likely climate state by 2100 under a mid-range mitigation scenario (BAU gives up to 7°C by 2100, but let’s  not go there). The grey areas are climates that currently exist nowhere on Earth. The orange-red are areas where the equivalent climate is 6,000 to 10,000 km away. Look again at that map. The grey and orange-red areas are predominant in the tropics. The tropical biomes — mostly humid and wet-dry tropical rain forest, and coral reefs – support over 60% of the world’s biodiversity. Under this scenario, they’re cooked. They’re stuffed. They’ve got nowhere to go. Sure, the tropics are naturally hot, but they’re also stable, with little temperature variation compared to the huge seasonal swings that most of us experience in the temperate realms. This makes well-adapted tropical species acutely vulnerable to rapid change. Crank up the warming ratchet in tropical areas during this century, and the only place these species have to go is back to the Palaeocene. Time machine anyone? I thought not.

The third paper of interest in this issue is an interview with my friend and colleague, Jim Hansen of NASA’s Goddard Institute of Space Studies. He gives is frank and clear assessment of what to expect in Copenhagen, why cap-and-trade is a disaster, and a preview of his new (and first!) popular book, Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe and Our Last Chance to Save Humanity (I was fortunate to read an advanced copy that Jim sent me, and I can thoroughly recommend it — he writes extremely well, and his science is superb).

What else in the world of climate change?  For those readers who  might be interested to hear me on a couple of radio interviews I did this week. In this one, from 5aa Adelaide, I give a frank assessment of Copenhagen. The morning show interviewers, usually a jocular bunch, seemed a bit down after our little chat. I guess I have a depressing effect on people when I’m talking climate change impacts rather than nuclear power prospects! Then, there is a ‘he said… no, he said‘ type of exchange on ABC 891, with an interview with Ian Plimer from Copenhagen, followed by one from me. Suffice to say that, in getting sick of this 50:50 denier vs science cr@p, I let rip.

The World Meteorological Organisation released its annual climate statement for 2009, stating that the decade 2000 — 2009 was the hottest 10-year period on record. This beat the period 1990 — 1999 (yes, 1998 was part of that average), which in turn beat 1980 — 1989. No surprises there, at least to anyone with half a brain. They also report on various unusual ‘extreme events’, including the unprecedented heat waves in Australia, the ongoing melt in the polar regions, intense storm activity, severe droughts, and record sea surface temperatures. Overall, 2009 looks to be heading for the 5th hottest year on record, despite the strong ameliorating effects earlier in the year of La Nina, and the sun bottoming out in a deep and persistent solar minimum. With a strengthening El Niño now coming into play, there is a pretty reasonable chance that 2010 will be the new hottest year on record. We’ll see, but I reckon it’s a better than 50:50 proposition, despite the lack of solar forcing at present.

Finally, if you want to know the reality behind the ‘Climategate’ nonsense (the hacked emails from the Climatic Research Unit of the University of East Anglia), then you must listen to this ABC World Today radio interview with former CRU Director, Prof Tom Wigley (full disclosure — Tom is a close friend and colleague of mine, and we are currently writing a number of papers together — Tom is also an extremely strong supporter of IFR Nuclear power). Despite he and others receiving a number of abusive emails, including quite frightening death threats from social psychopaths, he is keeping his chin up, and explains the situation with candour and dignity. I’ll end this post with a quote from Tom:

ELEANOR HALL: Of course climate change sceptic Andrew Bolt named you as a sort of hypothetical whistle blower on this whole affair. He says that if you weren’t then you should’ve been. How do you respond to that?

TOM WIGLEY: Well there are two things, I mean using the word “whistleblower” is really just another ploy on the part of Andrew Bolt and others to attempt to make it look as though the person who hacked these emails was a good guy and that they had a motive of trying to expose nefarious activities within the Climatic Research Unit.

Well of course there was no such nefarious activities and that’s the reason why what Andrew Bolt has said is just totally ludicrous. He says, “when did Tom Wigley finally choke on all that deceit and if he didn’t why the hell not”?

Well you know, I didn’t choke on the deceit because there was no deceit. All I did was ask a number of pointy questions and I received perfectly adequate answers. It would be really nice if someone like Andrew Bolt used the same approach and tried to get both sides of the picture and then he might learn to understand some of these issues better.

There is also a new ‘Climate Crock of the Week’ video on this issue, called “Smacking the hack attack“. Definitely worth a watch.

Australia’s weird winter

Guest post by Blair Trewin. Blair is a senior climatologist at the Bureau of Meteorology’s National Climate Centre. He recently took the lead in writing the Special Climate Statement, Exceptional winter heat over large parts of Australia, issued 26th August 2009, updated 1st September 2009.

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Australia has just experienced an exceptionally warm August. Almost the entire country experienced above-average temperatures during the month, but the warmth was most extraordinary in the subtropics. Over most of the southern Northern Territory and the southern half of Queensland (away from the coast), maximum temperatures for August were more than 5°C above the long-term average. Maximum temperatures were the highest on record for August over 49% of Australia.

Averaged over Australia as a whole, maximum temperatures were 3.20°C above the long-term August average, and daily mean temperatures (day and night combined) were 2.47°C above average. Both values are the highest on record for August by close to a degree. In terms of how far the month was above normal, the maximum temperatures in August 2009 are also the highest on record for any month, breaking the record of +3.11°C set in April 2005; the daily mean temperatures rank second behind April 2005.

The month was marked by some individual days which were exceptionally hot for August, especially in northern NSW and Queensland. State records were set for August in both states (37.8°C at Mungindi and 38.5°C at Bedourie respectively). Perhaps more exceptional were the margins by which some records were broken, and the number of days on which previous records were exceeded. Collarenebri broke its pre-2009 August record by 5.4°C, and numerous other locations, including Murwillumbah, Moree, Gatton, Miles and Taroom, broke August records by 4°C or more. Such margins are not unheard of at exposed coastal sites – where everything has to go right to achieve an extreme high temperature (not only having a very hot air mass, but having the wind in the right direction to prevent conditions being moderated by sea breezes) – but are virtually unknown at inland locations.

Many locations exceeded pre-2009 August records on five or more days. An especially striking example was Windorah in western Queensland, which prior to 2009 had never reached 35°C in August. In 2009 it happened seven times, and their August record was lifted six times, eventually peaking at 38.0°C on the 29th.

The extremely warm August combined with generally above-average (but not record-breaking) temperatures in June and July to give record or near-record winter temperatures in many areas. Australian daily mean temperatures for winter (1.33°C above average) fell just 0.01°C short of the 1996 record, and maximum temperatures surpassed the record set in 2002. NSW, Victoria and South Australia all had their warmest winters on record, which may come as a surprise to residents of the latter two states, in a season which was distinguished more by an almost complete absence of significant cold than by any major warm extremes.

In terms of weather systems, the month was marked by a persistent high-pressure ridge over the subtropics, preventing cooler air from penetrating from the south into central and northern Australia (until the last two days of the month, by which time it was too late to make much difference). Pressures were also well below normal south of Australia, resulting in very strong and persistent westerlies south of Australia (which made it an extremely wet month in Tasmania). An interesting comparison exists with October 1988, which had very similar pressure patterns, and was also dry over the mainland and very wet in Tasmania. In October 1988 Australian mean temperatures were 2.16°C above average, which was a record at the time (it now ranks fourth). The difference of 0.31°C between the two months is close to the size of the warming trend over Australia in that 21-year period, and suggests that the long-term background warming trend is playing a role in increasing the frequency of high temperature extremes of the type seen in August 2009.

Heatwave update and open letter to the PM

Maximum temperature anomalies (differences from the 1971-2000 average) for 7 February 2009

The Australian Bureau of Meteorology (BOM) has released a detailed analysis of the 2009 southern Australian heatwave. Some of the figures presented are staggering, with numerous temperature records smashed. Indeed, a colleague at BOM pointed out just how exceptional this event was:

“Given that this was the hottest day on record on top of the driest start to a year on record on top of the longest driest drought on record on top of the hottest drought on record the implications are clear...

It is clear to me that climate change is now becoming such a strong contributor to these hitherto unimaginable events that the language starts to change from one of “climate change increased the chances of an event” to “without climate change this event could not have occured”.

I couldn’t have said it better. With the shifting climate we are rapidly moving into uncharted territory with unknown return times (but surely already well above what the long-term records might lead us to expect).

My sincere condolences also go out to the people whose family members or friends were killed the shockingly severe bushfires that followed these unprecedented ‘tinderbox’ conditions. I note that BOM will be releasing further updates in due course on the fire weather aspects of the event.

Some particularly interesting snippets from the BOM report, entitled “SPECIAL CLIMATE STATEMENT 17: The exceptional January-February 2009 heatwave in south-eastern Australia“:

On the morning of 29 January, an exceptional event also occurred in the northern suburbs of Adelaide around 3 a.m. when strong north-westerly winds mixed hot air aloft to the surface. At RAAF Edinburgh, the temperature rose to 41.7°C at 3.04 a.m. Such an event appears to be without known precedent in southern Australia…

The January-February 2009 event has now been responsible for seven of the eight highest temperatures on record in Tasmania; a total of eight sites reached 40°C, a mark which had only been reached on 16 previous occasions in the state’s recorded history…

On 7 February (Figure 2), the focus of the most extreme heat, which was accompanied by high winds and very low humidity, was in Victoria. An all-time state record was set at Hopetoun, in the state’s north-west, when the temperature reached 48.8°C, exceeding the old record of 47.2°C, set at Mildura in January 19395 by a considerable margin. Seven other sites, in the Wimmera and in the area immediately west of Melbourne, also exceeded the old record, including Avalon Airport (47.9°C), Horsham (47.6°C), Longerenong (47.6°C) and Laverton (47.5°C). The Hopetoun temperature is also believed to be the highest ever recorded in the world so far south. A total of 14 sites exceeded the previous Victorian February record of 46.7°C…

Many all-time site records were also set in Victoria on 7 February, including Melbourne (154 years of record), where the temperature reached 46.4°C, far exceeding it’s previous all-time record of 45.6°C set on Black Friday (13 January) 1939. It was also a full 3.2°C above the previous February record, set in 1983. Three of Melbourne’s five hottest days have now occurred during this event. Geelong (47.4) and Wilsons Promontory (42.0) were among long-term sites which broke all-time records which had been set only the previous week. In total, of the 31 currently open sites in Victoria with 30 years or more of data which reported on 7 February, 21 set all-time records, five set February records, and only five failed to set records at all. 7 Record high temperatures for February were set over 87% of Victoria…

Both Adelaide and Melbourne set records for the most consecutive days above 43°C. Adelaide’s temperatures were at this level on each of the four days 27-30 January, and Melbourne’s for three days from 28-30 January, breaking the previous records of two at both locations… Adelaide ultimately had nine consecutive days above 35°C; after never having experienced more than eight consecutive days above 35°C before March 2008, it has now happened twice within twelve months…

Melbourne had no measurable rain from 4 January to 7 February, the equal second-longest dry spell on record for the city (35 days). This approaches the record of 40 days set in 1954-55. Melbourne (0.8 mm) had its second-driest January on record, and with only 2.2 mm to 8 February has now experienced its driest start to a year on record…

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Second, Dr Andrew Glikson has written an open letter to the Prime Minister, once again reiterating the urgency of the climate emergency. I have to ask (rhetorically I suppose), what will it take for the politicians to switch modes, from the awfully clichéd ”Climate change is happening, but let’s move slowing in doing anything meaningful about it so as to protect X, Y and Z [insert your favourite short-term political issue]” to the realistic “This is an emergency!”. I dunno. Let’s just hope it doesn’t take too many more disasters, such as this heatwave and its manifold consequences, or the crossing of Earth system tipping points such as the Arctic sea ice loss, to trigger the full-scale ‘war footing’ that is now so desperately required.

OPEN LETTER TO THE PRIME MINISTER OF AUSTRALIA

Dear Hon Kevin Rudd, MP, Prime Minister of Australia

A WARNING FROM THE PAST CLIMATE HISTORY OF EARTH

In his letter to you of 27 March, 2008, Professor James Hansen, leading US climate scientist and chief scientist of NASA’s Goddard Institute of Space Research (http://www.aussmc.org.au/Hansen_letter_to_Rudd.php), wrote, among other:

“Global climate is near critical tipping points that could lead to loss of all summer sea ice in the Arctic with detrimental effects on wildlife, initiation of ice sheet disintegration in West Antarctica and Greenland with progressive, unstoppable global sea level rise, shifting of climatic zones with extermination of many animal and plant species, reduction of freshwater supplies for hundreds of millions of people, and a more intense hydrologic cycle with stronger droughts and forest fires, but also heavier rains and floods, and stronger storms driven by latent heat, including tropical storms, tornados and thunderstorms”

Since this letter was written new research demonstrates the Earth’s atmosphere is more vulnerable to the rise in trace greenhouse gases, which regulate its temperatures, than we wish to believe, and that such rises in the past resulted in extreme shifts in the state of the atmosphere, oceans and biosphere, triggering mass extinction of species. Examples of some of these papers:

http://www.nature.com/nature/journal/v451/n7176/full/nature06588.html

http://www.agu.org/pubs/crossref/2008/2008EO490001.shtml

http://www.sciencemag.org/cgi/content/abstract/1157707v1

http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf

The new findings indicate that targets considered in the Garnaut Review, namely 450 ppm or 550 ppm CO2, can not be sustained. This is because carbon cycle feedbacks, including looming methane emissions, and the dynamics of ice/warming melt water interactions, threaten runaway warming leading toward tipping points, as occurred repeatedly in the past.

Current atmospheric CO2 levels (387 ppm) are already in the danger zone, while carbon gas emissions proceed at high rates (2.2 ppm in 2007; 1.8 ppm in 2008). It emerges that, unless simultaneous efforts are made to sharply cut carbon emissions and develop the technology for down-draw of atmospheric CO2, the future of our young and future generations looks grim.

The Greenland and Antarctic ice sheets formed under atmospheric conditions at, or below, 450 parts per million, which continued emissions and feedbacks will reach within a couple of decades, leading to temperature increases above 2 degrees C, advanced ice melt and metres-scale sea level rise.

Large mammals can hardly exist on land on an ice-free Earth, nor can human civilization survive such conditions.

In the wake of your election commitment to evidence-based policies (http://www.onlineopinion.com.au/view.asp?article=8049) you were given a historic opportunity to lead the world by example in relation to what you have correctly described as the great moral challenge of our generation (http://www.alp.org.au/labortv/uKTHPU1yia), through conversion of a coal-intensive highest per-capita carbon-emitting economy into an alternative energy-based system.

This could tilt the scales in an increasingly desperate global effort to avert what has been recently described by John Holdren, Obama’s new chief science advisor, as the global climate disruption (http://www.climatesciencewatch.org/index.php/csw/details/holdren_global_climate_disruption/)

Less than one year elapsed since Hansen’s letter was sent, and while isolated weather events are not necessarily related to climate change, a dangerous trend has developed consistent with projections of atmospheric science, relegating southern Australia to droughts and fire and the north to intense cyclones and floods.

Given the gravity of the matter, I suggest you consider to urgently convene a climate summit, where your government can listen to reports of severe climate disruption around the globe and in Australia, and to what the science says regarding future generations your government was entrusted to protect.

Honorable Prime Minister, as communicated by James Hansen, your leadership is required (http://www.aussmc.org.au/ Hansen_letter_to_ Rudd.php). I hope this will happen in the spirit of Dietrich Bonhoeffer (http://www.abc.net.au/rn/ religionreport/stories/ 2006/1755084.htm).

Yours faithfully

(Dr) Andrew Glikson

Earth and paleo-climate scientist

Australian National University

9 February, 2009

How hot should it have really been over the last 5 years?

Fig. 1. Seasonal average global temperature anomaly (TA), normalised sign-inverted southern oscillation index (SOI) and normalised total solar irradiance (TSI), from 1979 to 2008.

Last year, 2008, was about the 9th hottest year in the instrumental record (range: 7th to 10th). It certainly wasn’t the hottest year. That record goes to either 1998 or 2005, depending on which temperature record you look at. NASA’s GISTEMP gives it (slightly) to 2005, whilst the Hadley Centre and the two satellite measures (UAH and RSS) give it to 1998. Note that only GISTEMP averages over the northern polar regions, an area which has warmed more than the planet as a whole due to the retreat of snow and ice (which makes that part of the world duller, and so able to absorb more sunlight).

A question that is often asked by the naive or disingenuous is: “If the buildup of greenhouse gases is causing global warming, then why isn’t each year hotter than the previous?” This is a simplification the more common meme: “Global warming has stopped since 1998” (I note that it ceased being “Earth hasn’t warmed in the last 10 years” when 1998 passed more than a decade into history). The latter question is addressed explicitly in this earlier post — it has to do with the amount of energy transferred from the oceans to the atmosphere, given that most (>95%, on average) of the extra energy being trapped by increased greenhouse gases in going into slowly heating the oceans and melting ice across the planet.

Climate scientists obviously recognise that the Earth’s temperate is influenced by a whole range of forcings and feedbacks. Greenhouse gases, predominantly CO2, is one forcing causing gradual, inexorable warming. Transfer of heat from the oceans to the atmosphere is another and can cause temporary warming or cooling — perhaps the most important being characterised by the El Niño / La Niña oscillation. The solar cycle, lasting an average of 11 years, is yet another. A fourth powerful but occasional influence is large volcanic eruptions which cause temporary dimming (cooling). Global climate models include all of these factors when attempting to reconstruct past temperatures, and can be used to make future predictions of climate if these forcings are specified for future scenarios.

But we don’t need a global climate model to get a rough appreciation of how these forcings affect year-to-year temperature. We can approximate them with some basic correlative analysis that are uncomplicated and straightforward to understand.

Fig. 1 shows three relevant time series. In green is the seasonal average global temperature anomaly (TA), based on the composite measure provided at WoodforTrees. This is useful as it side-steps the ‘debate’ over which of the four major temperature measures is ‘best’ – it uses all of them and corrects for different baselines. Seasons are Dec-Jan-Feb, Mar-Apr-May, Jun-Jul-Aug and Sep-Oct-Nov. In blue is the southern oscillation index (SOI), in this case inverted (so that a higher SOI generally equates with a positive forcing) and normalised to scale between 0 – 1. In red is the satellite measure of total solar irradiance (TSI; similar to sunspot number), originally reported in watts per metre squared but here also normalised to a 0-1 scale so that it can be plotted alongside temperature and the SOI. The SOI and TSI have been offset by 3 months compared to Temperature, because we would expect some lag between the forcing and the response. The period 1979 to 2009 is chosen for two reasons — it is the period covered by all four temperature measures, and it also represents a span of time sufficient to represent ‘climate’ rather than weather — 30 years.

Fig. 2. Actual temperature (green), predicted temperature (blue, based on greenhouse gases, SOI, TSI and volcanic forcing) and temperature trend (grey dashed) with SOI, TSI and volcanic forcing held at its 30-year average.

Visually, it is clear that there is quite a good relationship between the up and down fluctuations of temperature and both SOI and TSI, although the gradual upwards trend in temperature is also apparent. We can quantify this relationship statistically. I fitted a simple linear ANCOVA model with TA as the dependent variable and Time (each season since DJF of 1978-1979), SOI and TSI as continuous predictors and volcanic forcing as a categorical predictor (VOL; a value of 1 is assigned for the few seasons following the 1982 eruption of El Chichon and the 1991 eruption of Pinatubo). I’ve ignored other complicating matters such as tropospheric aerosols. Still, the model is structurally not too bad a fit (for those familiar with the method, the residual model diagnostics confirm a reasonable conformation with assumptions, and 67% of the deviance is explained by the four predictors).

These results are useful. They show a strong positive trend in temperature (+0.0054C per season [or about 0.216C per decade]), a forcing of about +0.4C for a strong El Niño, +0.24C for the peak of solar forcing, and -0.27C for a volcanic event. With this model-based information, we can make two visual additions to the temperature plot. They are show in Fig. 2. First, the original temperature data is plotted in green. In blue is the season temperature as predicted by the ANCOVA model, based on historical forcings. The dashed trendline is the warming trend expected if all other forcings (SOI, TSI and VOL) are held at their 30-year average values.

Of relevance to the title of this post, these correlations can be used to work out (crudely!) what temperatures we might have expected over the last five years based on three different scenarios. First, if SOI, TSI and VOL were all held at their 30-year average, what would the global temperatures have been in 2004, 2005, 2006, 2007 and 2007? (AvF) Also, what if they all been at their minimum (MinF) or maximum (MaxF) 30-year values? These scenario results, along with the composite observed temperature anomalies, is shown in the table below. The last three columns represent deviations from the observed values (so, for example, in 2006 the observed anomaly was +0.69C but if all forcings had been at their 30-year average [AvF] it would have been +0.03C hotter than this, or +0.72C).

These numbers indicate that 2004 and 2007 were right on the average forcing expectation . 2005 was slightly hotter than we’d have expected (it was an moderate El Niño year and near the top of the solar cycle) and almost spot on the MaxF prediction. Conversely 2008 was quite a bit cooler and close to the MinF expectation. Why? It was a La Nina year and the sun had bottomed out in its irradiance/sunspot cycle. If 2008 had been an average year, we should have expected it to be 0.25C hotter, at around +0.75C anomaly.

To cap of this little venture into what-if land, I’ll have a bit of fun and predict what we might expect for 2009. My guess is that the SOI will be neutral (neither El Niño or La Niña), the solar cycle 24 will be at about 20% of its expected 2013 peak), and there will be no large volcanic eruptions. On this basis, 2009 should be about +0.75, or between the 3rd and 5th hottest on record. Should we get a moderate El Niño (not probable, based on current SOI) it could be as high as +0.85C and could then become the hottest on record. I think that’s less likely.

By 2013, however, we’ll be at the top of the solar cycle again, and have added about another +0.1C worth of greenhouse gas temperature forcing and +0.24 of solar forcing compared to 2008. So even if 2013 is a La Niña year, it might still be +0.85C, making it hotter than any year we’ve yet experienced. If it’s a strong El Niño in 2013, it could be +1.2C, putting it way out ahead of 1998 on any metric. Such is the difference between the short-term effect of non-trending forcings (SOI and TSI) and that inexorable warming push the climate system is getting from ongoing accumulation of heat-trapping greenhouse gases.

Is there a link between Adelaide’s heatwave and global warming?

Schematic illustrating the disproportionate effect on extreme and record temperatures when the mean temperature increases, for a normal temperature distribution.

Adelaide is the hot place to be right now. We’re in the middle of an extreme, enduring heatwave, and the city’s residents are suffering. Indeed, we’ve had rolling blackouts as the power system fails to meet peaking loads, and more people are suspected to have died from heat stress over the last week than were killed in the infamous Ash Wednesday bushfires of 1983.

Now, make no mistake, the month of January in Adelaide is renowned for its hot, dry weather. For instance, back in 1908 the city felt the brunt of a run of 6 days above 40C, and in 1939 the temperature soared to its historical single-day record (still unbeaten) of 46.1C. So given this context, an obvious question is whether the current heatwave is anything remarkable, and can a climate change signal be detected in this event? The standard scientific answer I would usually give is something like this: ‘Extreme weather has occurred in the past, and it is not possible to definitively attribute any one unusual event to climate change. That said, a higher frequency of intense heatwaves like this is consistent with the expectations of a rising global temperature‘ (see figure showing ‘more record hot weather’). But in the case of Adelaide’s 2009 heatwave, a bit of deeper investigation does indeed suggest that a climate change link is very likely.

Starting on January 26, we’ve had daily maxima of 36.6C, 43.2C, 45.7C [3rd hottest day ever recorded], 43.4C, 43.1C, 41.1C, 40.6C and 38.8C. This also included the hottest night ever recorded in South Australia, when around midnight on 29th Jan, it dropped to a minimum of 33.9C. For the last 5 days the temperature has not dropped below 25.9C at night. The run of 6 days above 40C equals the record from 101 years ago. The current 5-day forecast is for 38C [update: actual = 36.3C], 38C [33.0C], 37C [35.6C], 40C [43.9C] and 34C [41.5C]; if this holds, we’ll have had a string of 12 days above 35C, or perhaps 13 if Saturday nudges up a degree or so over the forecast. The heatwave is hitting more than Adelaide by the way — Melbourne got its 2nd hottest day on record and Tasmania its hottest ever. The town of Kyancutta on the Eyre Peninsula sizzled at 48.2C.

Historical records from the Bureau of Meteorology show that there have been 6 previous ‘heatwave events’ (here defined as >35C) that lasted 8 days, many more of 7 days, more still of 6, and so on. This is useful information for analysis, because it turns out that the return time of any given string of hot days is logarithmically related to it’s length (see below for how I know this). From Bureau records, we can infer that if the current heatwave does last for 12 days, such an extreme outlier should only occur, by chance (i.e. if the climate is not trending), once every 400 years or so. If it goes for 13 days, then that’s roughly a 1 in 1000 year event — such is the nature of a logarithmic relationship! Statistically speaking, there is always a danger in extrapolating beyond the bounds of your data, but in the case of rarely (or never) observed events, there is little other empirical recourse (a mechanistic simulation such as a general circulation model would also give useful inference on this matter).

So why might I be as bold as to suggest a climate change link with this current heatwave? Well, the hottest night ever recorded is certainly notable, and is consistent with the expectations of a greenhouse gas (GHG) induced warming. You see the basic hypothesis goes that if solar forcing is causing the surface warming, we’d expect there to be relatively more hot days (when the sun is shining) compared to hot nights (when the sun is hitting the other side of the planet and the warmth is maintained by the insulating effect of GHGs and high altitude clouds). If it’s a build-up of GHG that is driving the warming, you’d conversely expect relatively more hot nights, because the ‘atmospheric blanket’ has thickened. Another way to think about this commonsense prediction is to consider deserts, which have few high clouds and so tend to get very cold at night. Or in the extreme, to look to the Moon, which has no GHG at all and so has daily temperatures soaring above 100C and night times plummeting to below -100C! But let’s not read too much into one extremely hot night in Adelaide — it could still just be unusual weather, after all. Better in this case to reflect on large meteorological data sets which quite clearly confirm the GHG prediction, showing that relatively more warming has occurred at night compared to the day time. Another nail in the solar warming idea (but that’s another story).

Adelaide’s record-smashing 15 day March 2008 heatwave

Okay, so if it’s not the hottest night on record that seals the deal in my mind, what is it? Well, it’s another recent heatwave actually — one that occurred only 10 months ago in Adelaide. The record-smashing March 2008 heatwave persisted for a whopping 15 straight days and surpassed the previous longest stretch of above 35C degree temperatures recorded in any Australian capital city (formerly held by Perth in 1988). Poor old Kyancutta took a hot hit then too, recording 13 days straight of over 40C temperatures. The hottest night ever in March in Adelaide, of 30.2C, was also recorded during this event. Overall, last year’s heatwave was not as consistently hot as this year’s event (see chart), but it will still likely hold the record for duration. A frequency analysis on this monster implies that it was a 1 in 3,000 year event in a stationary climate! Read the media release by SARDI to show how Dr Warwick Grace calculated its return time. (Put another way, that’s about as likely as tossing a coin 12 times and getting all heads [or tails] — try it, I dare you…)

Now global warming is quite clearly expected to both increase the frequency of heatwaves (i.e., greater number of events per unit time) and cause those heatwaves that do occur to be hotter and to last longer (on average). This is a fairly simple expectation one derives from a change in the average temperature, as the top figure in this post indicates. The Intergovernmental Panel on Climate Change Fourth Assessment Report (2007) has this to say about it:

“Since 1950, the number of heat waves has increased and widespread increases have occurred in the numbers of warm nights… In several regions of the world, indications of changes in various types of extreme climate events have been found. The extremes are commonly considered to be the values exceeded 1, 5 and 10% of the time (at one extreme) or 90, 95 and 99% of the time (at the other extreme). The warm nights or hot days are those exceeding the 90th percentile of temperature, while cold nights or days are those falling below the 10th percentile… In the last 50 years for the land areas sampled, there has been a significant decrease in the annual occurrence of cold nights and a significant increase in the annual occurrence of warm nights. Decreases in the occurrence of cold days and increases in hot days, while widespread, are generally less marked. The distributions of minimum and maximum temperatures have not only shifted to higher values, consistent with overall warming, but the cold extremes have warmed more than the warm extremes over the last 50 years. More warm extremes imply an increased frequency of heat waves.“

So, in Adelaide we have two freakishly rare extreme events happening with a 10 month period. How likely is that? Well, if the events are totally independent, we’d expect the joint likelihood of two such heatwaves (of 0.25% probability per year [the 2009 event] and 0.033% per year [2008 event], respectively), occurring within the same 12 month period, to happen about once every 1,200,000 years. Is that unlikely enough for you? But if there is ‘autocorrelation’ (dependencies between the two events due to a linked cause — such as climate change), this calculated probability is not valid.

What exactly do I mean by this? Well, the heatwave that struck Europe is 2003 provides a good way to illustrate my final point, thanks to a neat analysis published in Nature in 2004. The authors of this study estimate that warming to date has at least doubled the probability of such an extreme heatwave occurring. Moreover, under ongoing heating, climate models suggest that by 2040, this extraordinarily hot summer (in historical terms) will be just a run-of-the-mill average summer. By 2060, it will be among the coolest of summers the future residents of Europe will thereafter ever experience.

The figure below really says it all. Such is the nature of coping with a ‘gradually warming’ climate…

The European heatwave of 2003 in historical and future context. The black line shows recorded summer temperatures in Europe, with the cross showing the extremely hot summer of 2003. Also shown in various colours are some climate model simulations which match historical records and project ongoing warming under a scenario which includes only a gradual reduction in human-caused greenhouse gases. Climate variability is superimposed on the warming trend, but the impact on extremes is also clear. Following the red dashed line it is clear that by 2040, the anomalously hot summer of 2003 with be merely an average summer. By 2060, it will be positively chilly.