Natural and Human Influences on Climate Change Natural climate influence only Natural and Human Influences on Climate Change Human climate influence only All Climate Influences

Sources of Climate Variability Orbital forcing Solar output Volcanic eruptions Earth system feedbacks GHGs Land surface, cryosphere, etc. Oceans Humans

Orbital Forcing

Orbital forcing: eccentricity Currently, difference in distance between aphelion (farthest from sun) and perihelion (closest to sun) is only 3%; ~ 6% variation in W*m^-2. When orbit is most eccentric, ~ 25% variation between aphelion and perihelion

Orbital forcing: obliquity Less tilt = solar radiation more evenly distributed between winter and summer. However, less tilt = increased difference in radiation between equator and poles Currently, middle of range (23 degrees)

Orbital forcing: precession Wobble of the spin of the earth around its axis causes solstices to vary in their coincidence with apehelion and perihelion. Winter solstice occurred when the earth was furthest from the sun ~11,000 ybp. + greater seasonal variability

Theory vs. Observations?

Imbrie and Imbrie (chillin’ for 23k) vs Imbrie and Imbrie (chillin’ for 23k) vs. Berger and Loutre (cookin’ for 50k) vs. Archer and (!) Ganopolski (CO2, 500kyr) The amount of solar radiation in the northern hemisphere at 65°N seems to be related to occurrence ice ages. Astronomical calculations show that 65°N summer insolation should increase gradually over the next 25,000 years. No declines in 65°N summer insolation sufficient to cause an ice age are expected in the next 50,000 - 100,000 years. Archer: CO2 may be sufficient to dampen orbital variation for 500 kyr.

Solar Output Swedish Royal Academy November 2002

Sunspot Numbers (C14 and Be10 proxy

Note 11 year cycles (peak to trough = -1.3 Wm-2 Royal Observatory of Belgium record: observations from around the world since 1749; other data spotty. Note 11 year cycles (peak to trough = -1.3 Wm-2 Note variability: lower numbers associated with lower solar luminosity Maunder and Dalton minima associated with obvious northern hemisphere climatic variability 1900-1950 increase associated with warming in early 20th century (16-36% of warming<1950, none since 1970) Solanki, S.K., I.G. Usoskin, B. Kromer, M. Schussler and J. Beer (2004). Unusual activity of the Sun during recent decades compared to the previous 11,000 years, Nature, 431(7012): 1084-1087.

Observations of Climate Variability Instrumental observations (~1700s - present): Decadal climate variability Centennial climate variability Proxy records: Multi-centennial, millennial, multi-millennial

Instrumental Observations Human records (both measurements and other observations documented the LIA). Other observations only for the Medieval warm period. A Scene on the Ice (Hendrick Avercamp)

LIA spatially, temporally variable Any of several dates ranging over 400 years may indicate the beginning of the Little Ice Age: 1250 for when Atlantic pack ice began to grow 1300 for when warm summers stopped being dependable in Northern Europe 1315 for the rains and Great famine of 1315-17 1550 for theorized beginning of worldwide glacial expansion 1650 for the first climatic minimum

1. dark blue 1000-1991): P. D. Jones, K. R. Briffa, T. P 1. dark blue 1000-1991): P.D. Jones, K.R. Briffa, T.P. Barnett, and S.F.B. Tett (1998). "High-resolution Palaeoclimatic Records for the last Millennium: Interpretation, Integration and Comparison with General Circulation Model Control-run Temperatures". The Holocene 8: 455-471. DOI:10.1191/095968398667194956 2. (blue 1000-1980): M.E. Mann, R.S. Bradley, and M.K. Hughes (1999). "Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations". Geophysical Research Letters 26 (6): 759-762. DOI:10.1029/1999GL900070 (pre-print) 3. (light blue 1000-1965): Crowley and Lowery (2000). "Northern Hemisphere Temperature Reconstruction". Ambio 29: 51-54. Modified as published in Crowley (2000). "Causes of Climate Change Over the Past 1000 Years". Science 289: 270-277. DOI:10.1126/science.289.5477.270 (data available from NCDC : [2]) 4. (lightest blue 1402-1960): K.R. Briffa, T.J. Osborn, F.H. Schweingruber, I.C. Harris, P.D. Jones, S.G. Shiyatov, S.G. and E.A. Vaganov (2001). "Low-frequency temperature variations from a northern tree-ring density network". J. Geophys. Res. 106: 2929-2941.DOI:10.1029/2000JD900617 5. (light green 831-1992): J. Esper, E.R. Cook, and F.H. Schweingruber (2002). "Low-Frequency Signals in Long Tree-Ring Chronologies for Reconstructing Past Temperature Variability". Science 295 (5563): 2250-2253. DOI:10.1126/science.1066208 6. (yellow 200-1980): M.E. Mann and P.D. Jones (2003). "Global Surface Temperatures over the Past Two Millennia". Geophysical Research Letters 30 (15): 1820. DOI:10.1029/2003GL017814. 7. (orange 200-1995): P.D. Jones and M.E. Mann (2004). "Climate Over Past Millennia". Reviews of Geophysics 42: RG2002. DOI:10.1029/2003RG000143 8. (red-orange 1500-1980): S. Huang (2004). "Merging Information from Different Resources for New Insights into Climate Change in the Past and Future". Geophys. Res Lett. 31: L13205. DOI:10.1029/2004GL019781 9. (red 1-1979): A. Moberg, D.M. Sonechkin, K. Holmgren, N.M. Datsenko and W. Karlén (2005). "Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data". Nature 443: 613-617. DOI:10.1038/nature03265 10. (dark red 1600-1990): J.H. Oerlemans (2005). "Extracting a Climate Signal from 169 Glacier Records". Science 308: 675-677. DOI:10.1126/science.1107046