1. Atmospheric Impact of the 1783-1784 Laki volcanic eruption
David Stevenson (University of Edinburgh)
Thanks to:
Ellie Highwood (Univ. Reading),
Colin Johnson, Bill Collins, Dick Derwent (Met Office)
Funding:

2. Talk Structure Motivations: volcanoes and the atmosphere Introduce:
The Laki eruption
Atmospheric chemistry model
Tropospheric sulphur cycle
Model experiments & results
Radiative forcing & climate impact
Conclusions / problems

3. June 1991

4. June 1991

5. June 1991

6. Solar
radiation
Aerosol reflects
Solar radiation – net cooling
SO2 oxidises to H2SO4 aerosol:
Residence time ~year (mid-stratosphere)
weeks-months (UT/LS)
days-weeks (troposphere)
Ash falls out
quickly (hours-days)

7. Pinatubo aerosol from the Space Shuttle
Aerosol layer ~20-25 km
Tropopause ~15 km
SO2 (gas) + OH → H2SO4 (aerosol)

8. Volcanoes and Climate Natural climate variability
How much of this is due to volcanoes?
IPCC
2001

9. Volcanoes and Climate Test climate model sensitivity/response
Hansen et al IPCC, 2001
GISS GCM

10. Volcanoes and Climate
Focus to date on large explosive eruptions, or those that leave a record in the Greenland or Antarctic ice.
What about large effusive eruptions?

11. ‘Fires of the Earth – The Laki Eruption 1783-1784’
Eyewitness account of the eruption by the local vicar (Rev. Jon Steingrimsson)
Recently reprinted by the University of Iceland, and translated into English
1/5 of Iceland’s population
died (10,000 people)
Much better than that cack by Tolkien

13. 27 km long fissure
580 km2 of lava

14. 1783-84 Laki eruption, Iceland
8 June 1783: 27 km long fissure opens
15 km3 of basalt erupted in 8 months
60 Tg(S) released
60% in first 6 weeks
Fire-fountaining up to ~ m
Eruption columns up to ~ km
Tropopause at ~10 km
‘Dry fog’ or ‘blue haze’ recorded over Europe, Asia, N. Atlantic, Arctic, N. America
This appears to have been a sulphuric acid aerosol layer in the troposphere and/or lower stratosphere

15. ~200m Fire-fountaining at Etna, 2002
Photos from Tom Pfeiffer’s web-site:

17. Environmental impacts
20% of the Icelandic population die
Acid deposition destroys crops & grazing
Similar impacts across Europe
Cooling of NH (regionally extreme, e.g. Alaska)
Cooling for several years (Franklin, 1785)
Famine

18. Questions
Using our best estimate of the Laki SO2 emissions, what is the modelled impact on the global atmospheric composition?
Does it agree with observations?
Can it generate a climate impact?

19. Atmospheric model: STOCHEM
Global 3-D chemistry-transport model
Meteorology: HadAM3
GCM grid: 3.75° x 2.5° x 58 levels
CTM: 50,000 air parcels, 1 hour timestep
CTM output: 5° x 5° x 22 levels
Detailed tropospheric chemistry
CH4-CO-NOx-hydrocarbons
detailed oxidant chemistry
sulphur chemistry
Normally used for air quality/climate studies
This version has high resolution tropopause

20. STOCHEM framework Air parcel centres Eulerian grid from GCM provides
meteorology
Interpolate met. data for each
air parcel

21. For each air parcel Advection step
Interpolated winds, 4th order Runge-Kutta
Plus small random walk component (=diffusion)
Calculate emission and deposition fluxes
Prescribe gridded emissions for NOx, CO, SO2, etc.
Integrate chemistry
Photochemistry (sunlight, clouds, etc.)
Gas-phase chemistry (T, P, humidity, etc.)
Aqueous-phase chemistry (cloud water, solubility, etc.)
Mixing
With surrounding parcels
Convective mixing (using GCM convective clouds)
Boundary layer mixing

22. Sulphur chemistry SO2 SO4
Oxidants normally determined by background photochemistry – but very high SO2 levels will deplete them
SO2
gas
emissions
SO4
aerosol
+OH
+H2O2(aq)
(in clouds)
+O3(aq)
Oxidation and
deposition rates
determine the
SO2 lifetime
dry(wet) deposition
wet(dry) deposition
Only
deposition rates
determine the
SO4 lifetime

23. SO2 OH SO4 H2O2 O3 DMS MSA Present-day tropospheric sulphur cycle
0.29
Burden
Tg(S)
1.1
Lifetime
Days
Fluxes in Tg(S)/yr OH
SO4
6.3
0.81
5.3
Dry
Wet
Deposition
9.2 30 32 17
H2O2 O3
Wet
Deposition
7.1 49
Dry
Biomass
burning
1.4
Anthro-
pogenic 71
Volcanic 9 12
Oceanic
DMS 15 4
MSA
Soil 1

24. Laki sulphur emissions
Analysis of the S-content of undegassed magma suggests ~60 Tg(S) released by Laki (Thordarson et al., 1996)
~1990 global annual anthropogenic input
Compare to Pinatubo: ~20 Tg(S)
What was the vertical profile of emissions?

25. 1990 Anthropogenic SO2 emissions (annual total)
Laki
value 61
Total 72
Peak
value ~2
0.1 1 10
100
Tg(S)/yr/5x5

26. Model experiments 1990 atmosphere
Background ‘pre-industrial’ atmosphere
Two Laki emissions cases
‘lo’: emissions evenly distributed 0-9 km
‘hi’: 75% emissions at 8-12 km, 25% at 0-3 km
All runs had fixed (‘ ’) meteorology
No attempt made to simulate 1783 weather
Run for one year following start of eruption
Generate aerosol distributions
No feedback between aerosols  climate
Calculate radiative forcings and climate effects later

27. Zonal mean JJA SO2 & sulphate
Pre-industrial background
SO2
Laki
Tropopause
SO4

28. July SO2 (ppbv) Laki hi Surface 0.5 km 550 hPa 5 km 350 hPa 8 km
0.1
0.2
0.5 1 2 5 10 20 50
100
0.1
0.2
0.5 1 2 5 10 20 50
100
350 hPa
8 km
200 hPa
12 km
0.1
0.2
0.5 1 2 5 10 20 50
100
0.1
0.2
0.5 1 2 5 10 20 50
100

29. July SO4 (pptv) Laki hi Surface 0.5 km 550 hPa 5 km 350 hPa 8 km
100
200
500
1000
2000
5000 50
100
200
500
1000
2000
5000
350 hPa
8 km
200 hPa
12 km 50
100
200
500
1000
2000
5000 50
100
200
500
1000
2000
5000

30. Laki SO4 evolution Surface Upper Trop Lower Strat lo hi 90°N Eq 90°S
June 1783
May 1784 10 20 50
100
200
500
1000
2000
5000 10 20 50
100
200
500
1000
2000
5000 10 20 50
100
200
500
1000
2000
5000
SO4 / pptv
90°N hi Eq
90°S

31. Laki sulphur budget SO2 SO4 Hi case 22 Tg(S) or 89 Tg (H2SO4.2H2O) gas
aerosol
16%
Emissions
61 Tg(S)
16%
+H2O2(aq)
+O3(aq) 4%
Dry
dep
28%
Wet
dep
37%
Dry
dep
10%
Wet
dep
90%

32. Impact on oxidants (JJA)
H2O2 OH O3

33. Laki Sulphate budget (JJA)LS
tSO4 = 67 days
(no transport: >7 yrs) UT
tSO4 = 10 days
(no transport: 32 days) LT
tSO4 = 5.3 days

34. Acid deposition to Greenland

35. Greenland Ice-core data
H2SO4 deposition rates mg(S)/m2/yr
Modelled
Observed
Background
5.0
Laki
63-65
18-107

36. Total atmospheric aerosol mass

37. Aerosol yield/peak loading
Total yield
Peak load
This work
51-66
Clausen & Hammer(1988)
280 -
Zielinski (1995)
~40
Stothers (1996)
150
4.5
Clausen et al (1997)
Thordarson and Self (2002)
200
(~150?)
Tg(H2SO4)

38. Radiative forcing & Climate impact Ellie Highwood (Reading Univ)
Aerosol fields inserted into Reading IGCM
3 experiments:
1. Hi/long decay (10 month e-fold)
2. Hi/short decay (3.6 month e-fold)
3. Lo
Each has a 10 member ensemble of 3 yr runs
Compare to control run with no forcing
Only direct aerosol effect

39. Radiative forcings

40. Climate Impact lo hi

41. Climate impact
Hi runs have NH cooling of –0.21K, in good agreement with observations (-0.14 to –0.27K)
Lo runs show no significant cooling
BUT: runs neglect indirect aerosol effects
Hi runs also have cooling persisting for 3 years, due to feedbacks (ice/snow albedo)

42. Conclusions(1)
1st attempt at chemistry-climate modelling of the Laki eruption
Simulated a sulphate aerosol cloud across much of the NH during the 8-month eruption
Deposition to Greenland similar to ice-core record
60-70% of emitted SO2 is deposited before forming aerosol (previous studies assumed it all formed aerosol)
Mean lifetime ~week
Atmospheric loading less than previous estimates

43. Conclusions(2) Oxidants H2O2 & OH strongly depleted
lengthens the SO2 lifetime
more likely to be deposited as SO2
Climate modelling suggests Hi scenario gives ~0.2K cooling, and persists for >2 years; this matches observations
But: many processes missing
For more info: 2 papers in ACP:

44. Problems(1) Volcanology: Emissions uncertain
Magnitude
Vertical profile
Temporal distribution – episodic
Plume processes, e.g. scavenging of SO2/SO4 by ash in the eruption column

45. Problems(2) Chemistry modelling Not fully coupled
No aerosol microphysics
No coupling of aerosol to photolysis rates
Only 1 heterogeneous reaction (N2O5 loss)

46. Problems(3) Climate model
Simplified radiation scheme – more bands suggest forcing is smaller by factor 0.6
Humidity assumptions – 80% RH increases forcing by factor 2.6
No aerosol indirect effects
Climate sensitivity low compared to other models: suggests duration of forcing may be underestimated

47. 2 Papers in Atmospheric Physics & Chemistry: