Understanding and attributing climate variations: The role of energy. Kevin E Trenberth NCAR

Transcription

1 Understanding and attributing climate variations: The role of energy Kevin E Trenberth NCAR

2 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

3 Changing atmospheric composition: CO 2 Mauna Loa, Hawaii Rate increasing Data from Climate Monitoring and Diagnostics Lab., NOAA. Data prior to 1974 from C. Keeling, Scripps Inst. Oceanogr.

4 (CERES Period)

5 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?

6 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% (10 6 km 2 ) lower than , second lowest To melt 10 6 km 2 ice 1 m thick (2007) to 10 C = 0.8x10 20 J Globally per year this is W m -2. IPCC

7 Global temperatures and carbon dioxide through 2008

8 Global temperatures deg Cx year T Trend 1998 to 2008 is slightly positive but not significant. Hadley Centre and CRU

9 Solar irradiance

10 Solar irradiance from composite of several satellite-measured time series based on Frohlich & Lean (1998;

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

12 Where does energy go? An imbalance at TOA of 1 W m -2 is 3.2x10 7 J/yr m -2 = 1.6x10 22 J/yr globally To melt 10 6 km 2 ice 1 m thick (2007) to 10 C = 0.8x10 20 J To produce 1 mm rise in sea level requires melting 360 Gt ice or 1.2x10 20 J Plus 12.5% to warm melted waters to ambient 1.35x10 20 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 75x10 20 J, or below 700 m 110x10 20 J Hence melting ice vs warming ocean is a factor of about 40 to 70 more 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

13 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?

14 Estimates of upper 700 m ocean heat content and SST. Domingues et al Nature 2008 Ocean heat content corrected for XBT drop rate: Old versions in red and blue vs new upper 700 m upper 100 m SST

15 Sea level contributions Upper 700m Deep ocean Ice sheets Glaciers/ice caps Land storage Sum of above vs Observed SL and altimeter Domingues et al 2008

16 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

17 Sea level Anomalies Altimeter ARGO Ocean heat content = Thermosteric GRACE Ocean mass sea level Willis, Chambers, Nerem JGR 2008

18 Sean Swenson

19 Sea level Sea level (altimetry) Ocean mass (GRACE) Ice sheets (GRACE) Glaciers and ice caps (Meier et al., 2007) Terrestrial waters Sum of ice and waters Sea level (altimetry minus GRACE) Steric sea level (Argo; 04 08) mm/yr Cazenave et al 2008 GPC.

20 Sea level Ocean mass Greenland Sea level GIA corrected GRACE altimetry, ice, GRACE Ice sheet Antarctica Ocean steric sea level ARGO Cazenave et al 2008 GPC. Residuals (i) Altimetry and GRACE ocean (ii) Altimetry plus land ice

21 Greenland Feb 2003 to Jan 2008 Equiv water height change Mass Changes Over the Entire Greenland Ice Sheet Year A-M-J A-S-O Winter Summer Net Balance Average values of the total mass with respect to the ( ) 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 rise 0.5 mm/yr Wouters et al GRL 08

22 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.

23 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 clouds

24 CERES Flashflux data

25 OLR: part of energy, and indicator of clouds in tropics SST Note diff base periods (bad)

26 HIRS cloud amount trends Wylie et al 2005

27 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.