Introduction to Earth s Climate System Our Geologic Backdrop: Ice Age Cycles MODULE 2.4
2.4 Our Geologic Backdrop: Ice Age Cycles Lesson Goals»» Describe Earth s geologic variability over the past million years.»» Explain evidence that supports the orbital theory of naturally recurring ice ages»» Formulate a hypothesis describing the best orbital configuration to grow (or melt) a continental ice sheet.»» Construct feedback loops that likely amplified climate cycles over the past million years.
More than a million years of warm-cold climate cycles precede today DATA: ZACHOS ET AL., 2001 TRANSFORMED AS IN HANSEN & SATO, 2012 ESTIMATED GLOBAL AVERAGE TEMPERATURE ( C) 15 14 13 12 11 10 9 1000000 800000 600000 400000 YEARS BEFORE PRESENT 200000 0 IMAGE: M. ANTON
Approximately how long does each of the large, prominent climate cycles take? DATA: ZACHOS ET AL., 2001 TRANSFORMED AS IN HANSEN & SATO, 2012 ESTIMATED GLOBAL AVERAGE TEMPERATURE ( C) 15 14 13 12 11 10 9 1000000 800000 600000 400000 YEARS BEFORE PRESENT 200000 0 A. 10,000 years B. 20,000 years C. 50,000 years D. 100,000 years E. 200,000 years
Cyclic changes in Earth s orbit Polaris BIT R O R A EXAGGERATED Circle in space ELL IPT I RBIT LO CA '(" & $% CIR CU L N N Orbital Plane S Tilt (obliquity) S Eccentricity Precession Milankovitch cycles tutorial at: http://www.sciencecourseware.org/eec/globalwarming/tutorials/milankovitch/
Which tilt scenario produces the greatest seasonal contrast? A B C SH DRAWING, NASA IMAGES OF SUN AND EARTH D E
How does precession affect seasonal contrast? Given the orbital configuration shown below, which is approximately today s configuration, which hemisphere has higher seasonal contrast? N N S S A. Northern hemisphere B. Southern hemisphere C. The hemispheres have equal seasonal contrast
Milankovitch Cycles Precession of the equinoxes N. Hem. Winter S. Hem. Summer TODAY N. Hem. Summer S. Hem. Winter South: high seasonal contrast North: low seasonal contrast N. Hem. Summer S. Hem. Winter ~ 11,000 yrs ago N. Hem. Winter S. Hem. Summer South: low seasonal contrast North: high seasonal contrast
To GROW an ice sheet what kind of seasonal contrast would be best? A. High seasonal contrast (hotter summers & colder winters) B. Low seasonal contrast (cool-ish summers & warm-ish winters) C. It wouldn t make any difference
Growing an ice sheet with the ice-albedo feedback Low seasonal contrast in the Northern Hemisphere, so not all the winter snow melts away the following summer Perturbation More area covered with ice & snow Cooler temperatures AMPLIFYING More reflective surface area on Earth More incoming solar energy reflects back to space. Less solar energy absorbed
TOTAL incoming solar radiation changes only slightly over time MODIFIED FROM LASKAR ET AL., 1993) INCOMING SOLAR RADIATION W/m 2 340.6 340.4 340.2 1000000 8000000 6000000 4000000 YEARS BEFORE PRESENT 2000000 0
Which orbital configuration would be best to MELT an ice sheet in the Northern Hemisphere? A. High tilt angle, June 21st farthest from the Sun B. High tilt angle, June 21st closest to the Sun C. Low tilt angle, June 21st farthest from the Sun D. Low tilt angle, June 21st closest to the Sun
Atmospheric CO 2 and global temperature correlate TEMPERATURE DATA: ZACHOS ET AL., 2001 TRANSFORMED AS IN HANSEN & SATO, 2012; CO2 DATA: LUTHI ET AL., 2008 ESTIMATED GLOBAL AVERAGE Temperature ( C) 15 14 13 12 11 10 9 1000000 8000000 6000000 4000000 YEARS BEFORE PRESENT 2000000 0 300 275 250 225 200 175 ATMOSPHERIC CO2 (PPM)
Temperature and CO 2 another amplifying feedback Global temperature gets a little warmer Perturbation Warmer oceans can keep less CO 2 gas in solution Warmer global temperatures AMPLIFYING More outgassing of CO 2 from the oceans Stronger greenhouse effect
No atmospheric CO 2 values like today s, over at least the past 800,000 years ~395 ppm in 2013 TEMPERATURE DATA: ZACHOS ET AL., 2001 TRANSFORMED AS IN HANSEN & SATO, 2012; CO2 DATA: LUTHI ET AL., 2008 ESTIMATED GLOBAL AVERAGE Temperature ( C) 15 14 13 12 11 10 9 1000000 Ice core CO 2 data extends back to about 800,000 years ago 8000000 6000000 4000000 YEARS BEFORE PRESENT 2000000 0 300 275 250 225 200 175 ATMOSPHERIC CO2 (PPM)
Key Points»» Changes in Earth s orbit alter the amount and distribution of incoming solar radiation Earth receives.»» Low seasonal contrast helps grow ice sheets; high seasonal contrast helps melt ice sheets.»» The total incoming energy doesn t vary enough to account for the large observed change in climate, but feedbacks, like the icealbedo feedback and the CO 2 -temperature feedback amplify small perturbations and help produce larger changes in Earth s climate.»» The past million years is our geologic backdrop against which we can compare today s rates of change in climate and absolute values of climate parameters. Today s rates of change, and values like atmospheric CO 2, are higher than observed in the past million years.