3 May 2007 GIST May 2007 1 Professor John Harries, Professor John Harries, Space and Atmospheric...

3 May 20073 May 2007 GIST May 2007GIST May 2007 11

Professor John Harries,Professor John Harries,

Space and Atmospheric Physics group,Space and Atmospheric Physics group,Blackett Laboratory, Blackett Laboratory,

Imperial College,Imperial College,London,London,

UKUK

A natural experiment:The Mt. Pinatubo eruption; net flux;

time constants; and the ERB


o Work reported in Harries and Futyan GRL, 33, L23814, 2006

o Acknowledge provision of data by Prof. Brian Soden, U.Miami, USA]


Some Background

Climate may be described in terms of ‘Forcing’ and ‘feedback’ processes.

Forcing processes (External processes which impose a change of climate balance):

•greenhouse gas changes; •solar variations;•volcanic eruptions.

Feedback processes (Internal processes which respond to a forced change):

•water vapour feedback;•cloud feedback;•land surface feedback;•ice feedback;•ocean feedback.


Terrestrial Energy Budget (per unit surface area) Terrestrial Energy Budget (per unit surface area) and greenhouse forcingand greenhouse forcing

Input SW Power Pin = ITS (1 – A) / 4 = S (1 – A)

Output LW Power Pout = TE4 = (1 – g)TS

4

Power deposited/lost = p

G = greenhouse radiative forcing (in Wm-2) = (TS4- TE4)g = normalised greenhouse effect, g = G / (TS4 ) G / 390 0.40

A = planetary albedo 0.31 = Stefan-Boltzmann constant = 5.6696 10-8 Wm-2K-4

TE = effective temperature of Earth / atmosphere 254K

TS = mean surface temperature of Earth 288K

Pin = Pout + p

1 Wm-2

(Hansen et al., Science, 2005)

235 Wm-2 (ITS 1366 Wm-2)

How big is p = FN = Pin – Pout? Does it vary with time?Can volcanic eruption help?

Pin - Pout = p = FN


Terrestrial Energy Budget: feedbacks and Terrestrial Energy Budget: feedbacks and volcanic forcingvolcanic forcing

S (1 – A) = (1 – g) TS4 + p1 + p2 + …

hydrological cycle, circulation patterns,cloud cover & type

greenhouseforcing

delayedresponses

Delay due to feedback processes: eg. deep ocean warming

SW LW

Volcanic eruption forces a direct effect on A, and a response in g on scale of days to years

In this experiment, we use Pinatubo to “ping” the system, and we watch how the system responds

Direct decrease in A (and smaller increase in g) due to volcano


Pinatubo: A natural perturbation to the systemPinatubo (Phillipines, June 12 1991) was powerful (20 Mt), and directed vertically: so, a large mass of injecta quickly reached the stratosphere.

Tropospheric material was quickly washed out.

Stratospheric zonal circulation is strong, and particles quickly circulated equatorial zone, spreading N and S more slowly. Decay from stratosphere slow.

http://en.wikipedia.org/wiki/Mount_Pinatubo


Pinatubo: A natural perturbation to the systemPinatubo (Phillipines, June 12 1991) was powerful (20 Mt), and directed vertically: so, a large mass of injecta quickly reached the stratosphere.

Tropospheric material was quickly washed out.

Stratospheric zonal circulation is strong, and particles quickly circulated equatorial zone, spreading N and S more slowly. Decay from stratosphere slow.


Some Context:

Recent work in USA has attempted to make measurements of stability of TOA radiation balance, and of evidence for “stored energy”, p, by measuring net flux anomaly at TOA…..

p = FN

20N -20S: Wielicki et al, (2001): revision in press

Mt. Pinatubo,

June 1991

Net Flux, FN


Combination of ERBE and CERES data (Wong et al 2005; Loeb et al, 2006)


…and to model it (Hansen, 2005)

Pinatubo

FN

(stored energy)

(lost energy)


• Following work on Pinatubo by Soden et al. [2002], we have used the perturbation caused by Pinatubo to study some of the process time constants in the system;

• We have analysed the time series of the parameters shown in next Figure, and measured the (assumed exponential) rise and decay of the perturbation in each parameter;

• Results produce characteristic time constants for certain processes, which ought to be captured by models.


Figure 1.

The time series of the anomalies of the following parameters [adapted from Soden et al., 2002]:(top to bottom)

•observed longwave and shortwave TOAfluxes for latitudes 60N–60S and

for 1991–1996**;

•Observed net flux formed from the difference between absorbed SW and emitted LW fluxes;

•Observed total column water vapour and lower tropospheric temperature for 90N–90S; (NVAP project; Randel et al., 1996).

•Observed 6.7 mm brightness temperature for 90N–90S (TOVS Radiances Pathfinder project: Bates et al., 1996).

SW and LW flux anomalies

Net flux anomalies(“stored energy”)

T6.7

Water vapour column and T


Concluding remarks:• Pinatubo offers a natural perturbation to the climate system;

• FN –ve for volcanic eruption: +ve for stored energy;

• Processes which can respond immediately to the “instantaneous” insertion of aerosol from the volcano show very short time constants (few months), driven by the time taken for aerosols to become distributed;

• Processes which involve slower dynamical processes, eg moving water vapour around, take much longer (1-2 years);

• Rise and decay process time constants differ;

• Models ought to reproduce these relaxation times as validation.


End


Some of the evidence for climate change,

and the uncertainties


The temperature signal at the surface and the coincident changes in CO2 , CH4 , sulphates, etc…


CERES (polar orbiter) monthly averages :

LW SW


Do we have evidence of “climate forcing” by increasing greenhouse gases?

Harries et al., Nature, March 15 2001

Yes!


There are, of course, uncertainties in many forcing processes…..

IPCC


But the major uncertainties are in feedbacks, not the forcings: Should we believe that we understand “climate change” well enough to predict our future?

No!

The feedback processes, especially clouds, water vapour, oceans, cause large uncertainty

Climate change runs by different models for same conditions


Terrestrial Energy BudgetTerrestrial Energy Budget

Shortwave Longwave

Albedo 1/3


Variability and complexity in climate


Studies of the Physics of the Earth’s Climate Balance, and the new

Geostationary Earth Radiation Budget experiment (GERB)

Professor John Harries

Head, Space and Atmospheric Physics

• Climate is highly variable:+ Many processes are non-linear;+ Some processes are chaotic;+ Natural variability in climate components;+ Feedback processes cause variability.

•Climate is very complex:+ Many greenhouse absorbers (CO2, CH4, H2O,

FCC, O3, clouds..);+ Many SW scatterers (clouds, aerosols, dust);+ Both Forcing and Feedback processes;+ Wide range of time and space scales are significant;

• Variability is in spectral, spatial and temporal space.

3 May 2007 GIST May 2007 1 Professor John Harries, Professor John Harries, Space and Atmospheric...

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