Understanding and attributing climate variations: The role of energy

Kevin E TrenberthNCAR

NCAR: attribution

• A way to organize a lot of research• Makes it relevant to societal concerns• Has some implications on transferring

research developments and data analysis methods into operations: a legacy

• Helps to sell the research• Some examples in session 3• A broad framework in session 4.3

Data from Climate Monitoring and Diagnostics Lab., NOAA. Data prior to 1974 from C. Keeling, Scripps Inst. Oceanogr.

Changing atmospheric composition: CO2

Mauna Loa, Hawaii

Rate increasing

2000-2004 (CERES Period)

Where does energy go?

•Heat storage in the ocean (sea level)•Melting land ice (sea level)•Melting sea ice and warming melted water•Can we track it?

Snow cover and Arctic sea ice are decreasing

Arctic sea ice area decreased by 2.7% per decade(Summer: -7.4%/decade)up to:2007: 22% (106 km2) lower than 20052008, second lowest

IPCC

•To melt 106 km2 ice 1 m thick (2007) to 10°C = 0.8x1020 J•Globally per year this is 0.005 W m-2.

Global temperatures and carbon dioxide through 2008

0

10

20

30

40

50

60

deg Cx100

1997 1999 2001 2003 2005 2007year

Global temperatures

T

Trend 1998 to 2008 is slightly positive but not significant. Hadley Centre and CRU

Solar irradiance

Solar irradiance from composite of several satellite-measured time series based on Frohlich & Lean (1998; http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant)

Solar irradiance

03 04 05 06 07 08 09

1360.9

1360.8

Drop of about 0.5 W m-2 or 0.09 W m-2 for radiative forcing

Where does energy go?• An imbalance at TOA of 1 W m-2 is 3.2x107 J/yr m-2 = 1.6x1022 J/yr globally• To melt 106 km2 ice 1 m thick (2007) to 10 C = 0.8x1020 J• To produce 1 mm rise in sea level requires melting 360 Gt ice or 1.2x1020 J

Plus 12.5% to warm melted waters to ambient 1.35x1020 J• To produce 1 mm rise in sea level by warming the ocean (thermosteric)

depends greatly on where energy is placed Fresh water has a maximum in density at 4 C, but not so for sea water.

• Coefficient of expansion varies with temperature and pressure by factor of 6 from 0 C to 20 C

• For warming over top 700 m to give 1 mm can take from 50 to 75x1020 J, or below 700 m 110x1020 J

• Hence melting ice vs warming ocean is a factor of about 40 to 70more effective in raising sea level (if in top 700m) or 90 (if below 700 m)

• 1 W m-2 gives sea level rise of 93 mm (melting ice) vs 3 to 1.5 mm (thermal expansion)

• Need to distinguish eustatic vs thermosteric sea level rise wrt energy

Where does energy go?1 W m-2 globally:• This would be a warming of 1.0°C/yr over a

30 m thick layer for 20°N to 20°S = 4 W m-2

– Or 0.75°C/year over a 10 m thick layer globally.– Or 0.1°C/yr over 75 m layer. – Or 0.01°C/yr over 750 m layer.

• Locally we can detect changes in SST at perhaps 0.3 °C anomalies (random)

• Can we detect changes of these magnitudes?

Estimates of upper 700 m ocean heat content and SST.Domingues et al Nature 2008

Ocean heat contentcorrected for XBT drop rate:Old versions inred and blue vs new

upper 700 mupper 100 mSST

Sea level contributions”Upper 700mDeep oceanIce sheetsGlaciers/ice capsLand storage

Sum of above vsObserved SL and altimeter

Domingues et al 2008

Sea level is rising:from ocean expansion and melting glaciers

Since 1992 Global sea level has risen 48 mm (1.9 inches)

• 60% from expansion as ocean temperatures rise, • 40% from melting glaciers

Courtesy Steve Nerem

Sea levelAnomaliesAltimeter

ARGOOcean heat content =Thermosteric

GRACEOcean mass sea level

Willis, Chambers, Nerem JGR 2008

Sean Swenson

Sea level 2003-2008Sea level (altimetry) 2.5 0.4

Ocean mass (GRACE) 1.9 0.1

Ice sheets (GRACE) 1 0.15

Glaciers and ice caps (Meier et al., 2007) 1.1 0.24

Terrestrial waters 0.17 0.1

Sum of ice and waters 2.2 0.28

Sea level (altimetry minus GRACE) 0.31 0.15

Steric sea level (Argo; 04–08) 0.37 0.1

mm/yr Cazenave et al 2008 GPC.

Sea level 2003-2008

Cazenave et al 2008 GPC.

Ocean mass Greenland Sea level

Ice sheet Antarctica Ocean steric sea levelGIA corrected GRACE altimetry, ice, GRACE

ARGO

Residuals (i) Altimetry and GRACE ocean(ii) Altimetry plus land ice

GreenlandFeb 2003 to Jan 2008Equiv water height change

Mass Changes Over the Entire Greenland Ice SheetYear A-M-J A-S-O Winter Summer Net Balance2003 525 260 235 -265 -302004 355 149 95 -206 -1112005 199 80 50 -279 -2292006 26 214 54 -188 -1342007 146 484 68 -338 -270Average values of the total mass with respect to the (2003– 2008) mean are given for April-May-June (A-M-J) and August-September-October (AS-O), together with winter gain, summer loss and net balance. Units are Gt.

Mean SL rise0.5 mm/yr

Wouters et alGRL 08

Commentary• Cazenave et al claim to be able to reconcile recent

changes in sea level with land ice melt, both from direct estimates and GRACE, plus ocean expansion (heat content from ARGO)

• Depends a lot (uncomfortably so) on Glacial Isostatic Adjustment in GRACE

• Implication is that since 2003, main source of sea level rise is melting of Greenland and Antarctica, and glaciers.

• These require about a factor of 50 less heat to produce same sea level rise as expansion

• If correct, implies a slow down in ocean heat uptake and TOA energy imbalance in past 4 years.

• Does NOT solve energy imbalance problem.

Need to know energy balance

• CERES data on TOA radiation???• Cloud data (ISCCP, HIRS etc)???

– ISCCP into 2007, but not homogeneous• Some stuff available: Flashflux:

CERES plus MODIS cloudshttp://eosweb.larc.nasa.gov/PRODOCS/flashflux/table_flashflux.html

CERES Flashflux data

OLR: part of energy, and indicator of clouds in tropics

SST

Note diff base periods(bad)

HIRS cloud amount trends

Wylie et al 2005

Need to know energy balance

• A 1% increase in clouds is about 0.8 W m-2

• Need clouds and radiation data in closer to real time.