Thursday, August 2, 2018

The ongoing flash drought is (probably) the most intense experienced at Phoenix Park since records began

Simon Noone, Conor Murphy and Peter Thorne

Drought has begun to fade from the national conversation following a week of rain, but observations show that Dublin remains in the grip of the one of the most intense flash droughts, probably the most, since at least the middle of the nineteenth Century.

Defining drought

There are numerous definitions of drought. None are perfect. In Ireland we have tended to use an index called the Standardised Precipitation Index (SPI) (McKee et al. 1993). This makes sense in that most of the time it is a lack of rainfall that leads to drought in our relatively moderate climate. But there are other indexes such as the Standardised Precipitation and Evapotranspiration Index (SPEI) (Vicente-Serrano et al.2010) which, in addition, account for removal of water via evaporation and plant transpiration. This index is arguably better for characterising flash droughts that tend to occur in the warm season. It uses a combination of monthly rainfall and temperature records.

The long-term context

The Noone et al. (2015) assessment of variability and change in precipitation over the period 1850–2010 indicates positive trends in winter and negative trends in summer precipitation. Noone et al. (2017) produced a 250 year drought catalogue for the Island of Ireland by applying SPI-12 ( a 12 month drought index) to identify long hydrological drought rich periods. The results show that Ireland is drought prone but recent decades are unrepresentative of the longer-term drought climatology. Ireland has experienced seven long drought rich periods over the period 1850-2015 impacting the whole of the island of Ireland; (1854–1860, 1884–1896,1904–1912, 1921–1923, 1932–1935, 1952–1954 and 1969–1977). But it is also possible to have ‘flash’ droughts, such as that in 2018.

Phoenix Park

Phoenix Park is one of the longest continuous meteorological stations in Ireland. Records started in the early 1800s and continue through present. This long record is thanks to the significant efforts of Met Eireann, Ordnance Survey Ireland and others. Although instrumentation has changed over time, the site has always stood in the grounds of Ordnance Survey Ireland and the regional environment has remained largely unchanged. The precipitation series was homogenised by ICARUS as part of Noone et al. (2015). The temperature record has not been homogenised to date.* Met Eireann make monthly and daily summaries available. To perform the present assessment we have appended the July daily values aggregated to monthly averages** to the long monthly series and calculated SPEI-3 (a 3 month drought index) for the entire series.

Phoenix Park site photo sourced from Met Eireann53°21‘50” N, 06°20’00’’ W, 48 m above mean sea level

SPEI-3 at or very close to record levels

The provisional value of SPEI-3 for May to July is -2.70 which is the most extreme value on record, beating October 1995 by a whisker***. The next most extreme was August 1995. Before that the index values are sufficiently different to conclude that either 2018 or 1995 represent the worst flash drought since modern meteorological records began in Ireland, with 2018 marginally more likely. 

SPEI-3 values calculated at Phoenix Park January 1850 to July 2018. Red lines show 3 month accumulative deficits and blue lines show 3 month accumulative surplus.

One important thing to note is that the two most extreme prior months occurred in the same year. This shows that drought can appear to diminish only to quickly return. We may well have had a week of rain (although even that in Phoenix Park amounted to only 47% of the long term monthly average rainfall for July), but that does not even begin to undo months of deficit. 

Table of the ten most extreme values of SPEI-3 in the Phoenix Park record. Negative values denote deficits. The more negative the index the more severe the drought conditions.

Perhaps more worrying still is the seasonal forecasts for the coming two months which show a real possibility of the meteorological set-up that led to the current drought returning and persisting. Don’t be fooled by a week of recent rains. We are potentially far from done yet.

Current seasonal forecast output from the US National Oceanic and Atmospheric Administration CFSv2 system. Daily deficits of >-0.2mm/day are predicted. Note large uncertainties exist in seasonal prediction systems which should only be used as indicative guidance.

*A reason to say probably rather than definitively
** Another reason to say probably rather than definitively in that monthly summaries have quality control applied by Met Eireann which may lead to a mismatch between the aggregated to monthly daily reports and eventual monthly summary of record.
*** Yet another reason to say probably rather than definitively.


McKee TB, Doesken NJ, Kleist J. (1993). The relationship of drought frequency and duration of time scales. In Eighth Conference on Applied Climatology. Am. Meteorol. Soc. January 17–23, 1993, Anaheim CA, 179–186

Noone S, Murphy C, Coll J, Matthews T, Mullan D, Wilby RL, Walsh S. (2015). Homogenization and analysis of an expanded long-term monthly rainfall network for the Island of Ireland (1850–2010). Int. J. Climatol. 36: 2837–2853, doi: 10.1002/joc.4522.

Noone S, Broderick C, Duffy C, Matthews T, Wilby R. and Murphy C. (2017), A 250‐year drought catalogue for the island of Ireland (1765–2015). Int. J. Climatol, 37: 239-254. doi:10.1002/joc.4999

Wednesday, April 18, 2018

Reflections on the Citizens Assembly consideration of climate change

The final report from the deliberations of the Citizens' Assembly on How the State can make Ireland a leader in tackling climate change is now available and delivered to the Oireachtas (and to our non-Irish readers, yes, whenever the political institutions of Ireland are discussed there is indeed a hint of Lord of the Rings). That means I am now allowed, apparently, to discuss this as a member of the Expert Advisory Group (other members were: Anna Davies, Margaret Desmond, John Garry, Aine Ryall, and Diarmund Torney).

The Citizens' Assembly is an exercise in deliberative democracy. There are people far more qualified than I to provide a definitive outline of what that is. But, at its most basic, it is picking randomly from those eligible to vote and bringing them together to hear evidence, discuss and then vote on issues within their charge. The Citizens' Assembly has considered 5 topics of which climate change was the third.

The assembly members were given two weekends to discuss the issues and then vote on a series of resolutions. The role of the Expert Advisory Group was to aid the Chair and secretariat to shape these deliberations. I got to know how to get to Government buildings rather well.

What were the key issues?

Finding the angle: The scope given was very broad and could have been taken in any number of directions. At the same time there were existing national activities around the national mitigation and adaptation planning and several existant bodies.  Ultimately, the citizens wanted to know what would change in their lives were Ireland to become a climate leader. So, the final structure was to spend the first day as a scientific primer and scene setting and then dive into the three major sectors of energy, transport and agriculture. Each sector should show what the status quo is, what 30 years hence may look like with leadership and take examples of how we are currently doing (ideally drawn from Irish exemplars).

Finding the right mix of speakers: Citizens needed to hear from speakers who could speak to the range of issues objectively. This was a challenging and rewarding puzzle to help solve. Particularly important was to hear inspirational examples of actual leadership in the here and now.

Helping find questions that answer the charge: The charge was how the State can make Ireland a leader so questions needed to be cast in the context of legal frameworks, incentives, policy and taxation. Questions needed to be realistic and recognise the need for trade-offs (no questions enabling having one's cake and eating it), and most importantly of all actionable.

The weekends themselves were fascinating. I'd urge people to look at the video presentations to see what was presented to the citizens. The citizens really engaged thoughtfully with the issue and asked challenging questions.

The final voting was very strongly (>80% in all cases) for a range of policy and fiscal measures which, if enacted in full would, indeed, make Ireland a leader in tackling climate change.

What happens next? Well, that, ultimately is up to members of the Oireachtas to decide. The report contains a wealth of information for their consideration.

It was a tremendous honour to serve the Assembly on this topic. It took me somewhat outside my comfort zone quite often but it was a real privelige to see the process in action and work with experts I wouldn't ordinarily interact with. The Chair and secretariat did sterling effort, but most of all the citizens impressed me with how much they engaged.

It is just a pity that this level of civic engagement is so much the exception rather than the norm in our modern democracies. When our governments do not care because we do not care or don't wish to become informed we quite frankly get the governments we deserve. Exercises such as the Citizens' Assembly offer a tantalising glimpse of an alternative.

Wednesday, January 10, 2018

The Atlantic Overturning Circulation and Ireland


Palm trees in Sligo

Figure 1:  Palm trees grow in Mullaghmore in Sligo (left), which is as far north as glaciated Grytviken, South Georgia (right) is south, where glaciers grow.[image modified from:Wikipedia]

Sometimes it’s easy to forget how far north Ireland is. Palm trees grow in Sligo on the northwest Atlantic coast whereas glaciers---though in retreat ---overshadow Grytviken, South Georgia (Fig. 1). Grytviken is as far south as Sligo is north, so why the big difference? 

Frequently people will cite the Gulf Stream as the reason for Ireland’s mild climate. This is an idea dating back to Matthew Fontaine Maury who coined the idea that the Gulf Stream was responsible for Europe’s mild climate in 1855. But the Gulf Stream itself is not inherently unique. It is an ocean current, called a ‘Western Boundary Current’ due to its location in the western North Atlantic, analogues of which exist in every major ocean basin. In the North Pacific, the Kuroshio is a western boundary current just as vigorous as the Gulf Stream. But when we look at surface temperatures relative to relative to the average for that latitude (Fig. 2), the Atlantic does stand out as anomalously warm.

Figure 2: Deviations from zonally averaged temperatures based on NCEP/NCAR reanalysis (annual mean) in degrees Celsius.[image: Joel Hirschi, NOC]

Atlantic Exceptionalism

The ocean process that drives the anomalous warmth in the Atlantic is the Atlantic Overturning Circulation. This is a system of currents that carries warm water northwards, through the Gulf Stream and its extended current, the North Atlantic Current. This water is warm and salty. As it loses heat, through a variety of processes, it forms deep, cold water that returns southwards. This exchange of warm and cold water leads to the largest movement of heat by any ocean. This heat is then released from the Atlantic and is carried by the prevailing winds over northwest Europe. This combination of ocean and wind is the reason for the exceptionally mild climate of northwest Europe.

Figure 3: Dublin is around 9C warmer than St Johns in Newfoundland and 4C warmer than the similar maritime climate of Seattle on the Pacific Northwest.[image from McCarthy et al., 2015, Weather]

The combination of winds and ocean is important and leads to a subtlety in discussing this Atlantic Exceptionalism. It might be tempting to look across from the cliffs of Moher in winter to the frigid Labrador coast of Canada and ascribe the almost 10 degrees C difference in temperature to the Atlantic Overturning. However, much of this temperature difference is due to this coast being downwind of the frigid Canadian landmass whereas Ireland is downwind of the ocean. A better comparison to reveal the impact of the Atlantic overturning on climate is to compare Ireland to similar maritime climates of the Pacific northwest. Dublin is 4C warmer than Seattle in winter and this difference can be mainly ascribed to the influence of the Atlantic Overturning Circulation. While not as dramatic a comparison as with the coast of Labrador, a mean difference of 4C is approximately the difference in temperature between the climate of Ireland and the climate of Northern Portugal.

Threats to the Overturning Circulation

The Atlantic Overturning has changed in the past, with large fluctuations in climate in the last glacial period associated with collapsing and recovering of the Overturning circulation, and it is predicted to decline in the future---the Intergovernmental Panel for Climate Change ranking as 'very likely' that the Overturning Circulation will weaken by 30% in the coming century.  If the overturning declines, this will have a serious impact on Irish climate. Evidence for a declining overturning is already mounting. Temperatures globally continue to rise due to anthropogenic global warming but, in the North Atlantic, a cold spot appears where a small temperature fall is occurring. 

Here at ICARUS, work is ongoing to understand the ongoing changes in the Atlantic, including understanding the drivers and the impacts that has on Ireland, northwestern Europe and beyond.

Figure 4: Global surface temperatures show a general increase due to anthropogenic climate change. A blue area in the North Atlantic shows where temperatures have not risen and could be indicative of a slowdown in the Atlantic Overturning Circulation.

Leagan Gaeilge

Crainn phailme i Sligeach

Is fusa dearmad a dhéanamh ar cé chomh fada thuaidh is atá Éire. Fásann crainn phailme i Sligeach ar an gcósta thiar ó thuaidh. Ar an leithead céanna, i nGrytviken, sa tSeoirsia Theas (Fig. 1), feictear oighearshruthanna, cé go bhfuil siad ag cúlú de bharr athrú aeráide. Cad í an chúis don difríocht seo?

Go minic, deirtear gurb é an teas a thugann Sruth naMurascaille leis i dtreo na hEorpa cúis na h-aeráide chineálta in Éirinn. Matthew Fontaine Maury a mhol an smaoineamh sin i 1855. Ach ní hé Sruth na Murascaille amháin gur cúis le seo. Is sruth aigéin é Sruth na Murascaile ar a glaotar ‘Sruth Teorann Iartharach’ de bharr a shuíomh in iarthar an Atlantaigh Thuaidh. Agus tá a leithéid de na sruthanna seo i ngach aigéan. San Aigéan Ciúin, is sruth teorann iartharach é an Kuroshio atá chomh láidir le Sruth na Murascaille. Ach, nuair a fhéachaimid ar na teochtaí dromchla i gcomparáid le meán-teocht an leithid (Fig. 2), seasann teochtaí arda aimhrialta an Atlantaigh amach. 

Eisceachtúlachas Atlantaigh

Glaotar Iompu Imsruthu an Aigéin ar an gcóras aigéin a bhogann an teas seo i dtreo iarthuaisceart na hEorpa. Is córas sruthanna aigéin é Iompú Imshruthú na Mara a bhogann uisce te ó thuaidh, i Sruth na Murascaille mar shampla. Tá an t-uisce seo te agus goirt. Agus an t-uisce seo ag cailliúint teasa de bharr próiseas éagsúla, cruthaíonn sé uisce fuar, domhain a fhilleann ó theas. Cruthaíonn an malartú den uisce fuar agus uisce te seo an iompar teasa is mó san aigéan. Scaoiltear an teas seo ón Atlantach agus iompaítear é i dtreo na hEorpa ag na príomhghaotha. ‘Sé meascán an aigéan agus an ghaoth cúis don aeráid chineálta eisceachtúil atá in iarthuaisceart na hEorpa.

Is tábhachtach é an meascán de ghaotha agus aigéan sa chomhrá faoi Eisceachtúlacht Atlantaigh. Ba éasca é féachaint ó Aillte an Mhóthair i rith an gheimhridh go dtí cósta fuaránta Labrador igCeanada  agus an difríocht 10 gcéim Celsius a chur i leith iompú an Atlantaigh. Ach is í cúis an difríocht teochta seo ná go bhfuil Labrador le cóir na gaoithe ó mhórchríoch fuaránta Cheanada, agus tá Éire le cóir na gaoithe ón aigéan. Is fearr an comparáid chun tionchar iompú an Atlantaigh ar an aeráid a léiriú, ná an comparáid idir Éire agus aeráid mhara den chineál céanna in iarthuaisceart an Aigéin Chiúin. Tá Baile Átha Cliatha 4C níos teo ná Seattle sa gheimhreadh agus is de bhuíoch iompú an Atlantaigh an difríocht seo. Cé nach bhfuil an comparáid seo chomh géar leis an gcósta Labrador, is ionann é difríocht 4C idir aeráid na hÉireann agus aeráid Portaingéil thuaidh.

Bagairtí i leith an Sruthú Iompaithe

Tá Iompú an Atlantaigh athruithe, le luaineacht mór san aeráid san oighearthréimhse deireanach ceangailte le titim agus athbheochan an sruthú iompaithe. Táthar ag tuar go dtiocfaidh meath ar an sruthú sa todhchaí – deir an Painéal Idir Rialtais don Athrú Aeráide go bhfuil an-seans go laghdóidh an Sruthú Iompaithe de 30% sa chéad atá amach romhainn. Má thagann an meath seo ar an sruthú, beidh tionchar ollmhór ar aeráid na hÉireann. Tá fianaise de seo ag méadú cheana féin. Tá teochtaí domhanda ag ardú ar bhonn leanúnach de bharr téamh domhanda antrapaigineach ach, san Atlantach Thuaidh, feictear spota fuar áit a bhfuil teocht íseal ag tarlú.

Ag ICARUS, táimid i mbun oibre chun tuiscint a fháil ar athruithe leanúnacha an Atlantaigh, chomh maith le tuiscint a fháil ar an tionchar atá ag sin ar Éirinn, ar iarthuaisceart na hEorpa agus ar áiteanna níos faide ó bhaile.