Global pattern of moraine-dammed Gacial Lake Outburst Floods

Stephan Harrison

John Reynolds (Aberystwyth) Christian Hugel (Zurich)

Jeff Kargel (Arizona) Andy Wiltshire (UKMO) Richard Betts (UKMO)

Liam Reinhardt (Exeter)

Leverhulme Trust Helix

Rationale and structure

• AR5 stress on adaptation to climate change

• New focus on Detection and Attribution as important for extreme events (includes litigation as well as development)

• Important debates on Equilibrium Climate Sensitivity, Transient Climate Response and policy implications

• Geomorphological sensitivity and D&A for GLOFs

• Stress global scale approach needed

Climate Change: Detection

Bottom: estimate of uncertainty for one dataset (black). Anomalies are relative to the mean of 1961-1990.

Energy accumulation (1021 J on Earth from 1971-2010. Upper ocean is 700m. Estimates below 2000m started in 1992.

The Radiative Forcing of WMGHG in AR5 is 2.83 ± 0.28 Wm–2

Milankovitch cycles around 1.5 Wm–2 The range of climate sensitivities in climate models used for IPCC AR4 is a warming of 0.6–1.2K per Wm–2 of forcing.

Attribution: Global average radiative forcing estimates and ranges

We can show a response to this:

• Glacier recession

• Paraglaciation

• GLOFs – Important as responsible for thousands of deaths per

year – Huge infrastructure costs – Implications for economic and social development in

remote mountain regions

Global glacier recession

From 169 glacier length records (Oerlemans 2006)

West Rongbuk Glacier in 1921 and 2008. (RGS & David Breashears)

Imja Glacier, central Himalaya in 1955 and 2009 (Fritz Müller and Alton Byers).

Does paraglaciation become the dominant mechanism of sediment movement in glaciated mountain systems? Implications for changes in the magnitude/frequency of natural hazards?

Paraglacial.... landsystems and processes driven by deglaciation

Knight and Harrison, 2013,2014

GLOFs seen as paraglacial phenomena and as a non-linear response to warming and glacier recession

No global assessment Major problem in parameterising RCMs……so development of predictive tools difficult

ICIMOD

11 Nare Glacier GLOF, Khumbu Himal, Nepal ASTER image, Bhutan & Tibet

Questions: Can we detect a climate signal in GLOF development? Will Climate change affect the

magnitude/frequency of GLOFs?

Three issues have to be addressed

• Climate sensitivity

• Geomorphological sensitivity

• Attribution and glacial systems

How much will the temperature warm? i.e. what is climate sensitivity?

AR5 definitions:

Equilibrium climate sensitivity (ECS): “defined as the equilibrium change in global surface temperature following a doubling of the atmospheric equivalent CO2 concentration”*

Transient climate response (TCR): “the change in global surface temperature in a global coupled climate model in a 1%yr-1 CO2 increase experiment at the time of CO2 doubling”*

*Note: Units are in degrees Celsius (°C).

PDFs, distribution and ranges for ECS (IPCC AR5)

• no central figure given • lower limit may be reduced (problem of using recent data sets to constrain this?) • long tail…..although figures greater than 6°C unlikely (and palaeo record doesn’t suggest this) • TCR lower than this….but may be more relevant for adaptation policy

What is ‘geomorphological sensitivity’? GLOFs become valuable metric with which to test ‘geomorphological sensitivity’ and to discuss uncertainties 1. Is there a warming

trend in mountain regions?

2. Can we detect trends in magnitude and frequency of GLOFs?

ICIMOD

However, Attribution problem....How can we Attribute Extreme Events to Human Forcing

• Difficult problem to solve because:

– Relates to a set of small scale events

– These may be stochastic and/or display chaos

– Understanding them depends on assessing nature of initial conditions as well as forcing (i.e. the precise conditions of the system before the event began as well as whatever might be acting on the system)

– To model events we need probabilistic framework...lots of model runs (ensembles)….and with a suitable model!

Event based attribution

P0: Probability of exceeding a threshold in a “natural world” (no anthropogenic forcings).

P1: Probability of exceeding a threshold in the real world.

FAR = 1 – (P0 / P1)

P0: Probability of exceeding a threshold in a “natural world” (no anthropogenic forcings).

P1: Probability of exceeding a threshold in the real world.

FAR = 1 – (P0 / P1)

Approach:

Find out how many times a specific event of interest occurs in a model with predindustrial GHG forcing (‘natural world’). Find out how many times a specific event of interest occurs in a model with current levels of GHG forcing (‘actual world’). Assess the difference (Fraction of Attributable Risk).

Requires:

a model capable of simulating the event(s) of interest many runs of the model

FAR = 1 – P0 / P1

Problem of running meaningful model ensembles which can help develop attribution

for GLOFs

Very difficult in mountains:

• Large changes in albedo, water vapour fluxes, ice temperature, supraglacial debris fluxes, landcover etc over small spatial scales

• Topography difficult to model and model resolution still much too coarse

• How to model paraglacial triggers?

• How to model initial conditions?

Tasman Glacier, New Zealand Charles Warren

Attribution and debris covered glaciers Debris obscures the glacier-climate signal...and difficult to model and parameterise

Baltoro Glacier

The GLOF record

• We need a Global record of GLOFs and a more complete data set than hitherto.

In Zailiskiy Alatau GLOFs concentrated in 1960s-90s…few in recent years

Passmore, Harrison et al 2008. AAAR

Developing a record for D&A was the motivation for the GLOF inventory

• 567 glacier hazard events had to be cross-checked • Problem is that many events named after the glacial lake in some datasets; in others the same event named after the glacier or mountain region • Eventually these whittled down to 191 GLOFs associated with moraine dam failure…best candidates for Atrribution

We can show that world’s mountains are warming….but data resolution is 5°

Black crosses are glaciers from WGI-XF

Harrison et al. in submission

• For Attribution….we need integration between glacier and climate models (RCMs) at scales where debris supply, lake initiation, trigger mechanisms can be modelled.

• And we need an uncensored data base of GLOF magnitude and frequency in the past.

• Pattern so far shows rising temperatures in the world’s mountains but the number of GLOFs has fallen and there is no trend in magnitude.

Conclusions

• Attributing GLOFs to AGW too early because attribution studies have not been done (and are problematic).

• Modelling at scale required and in mountain regions also difficult. Parameterisation of crucial processes needed.

• No global GLOF inventory and data sets are incomplete (so Detection difficult)

• However, an important task….for adaptation policy, litigation issues (Climate Justice Programme) and assessing impact of climate on extreme geomorphological events.