What we covered in class on September 10, 2009 Richard Alley rba6@psu

1. What we covered in class on September 10, 2009 Richard Alley rba6@psu.edu

3. First, the really big picture… •  In round numbers, estimated use by humans (together with corn, cows, cats, etc.) is approaching half of everything on the planet • With population growth and rising expectations, a mere doubling of demand is an optimistic view • Our energy use is something like 85% fossil-fuel unsustainable, but surely is important in maintaining us and biodiversity (whales, trees, …) • Yet science is indicating that a wise path would be to reduce fossil-fuel use

4. Henry David Thoreau, Cape Cod, 1865 (visited 1849-1857), ch. 7: “The old houses also are built of the timber of the Cape; but instead of the forests in the midst of which they originally stood, barren heaths, with poverty-grass for heather, now stretch away on every side. The modern houses are built of what is called “dimension timber,” imported from Maine, all ready to be set up, so that commonly they do not touch it again with an axe. Almost all the wood used for fuel is imported by vessels or currents, and of course all the coal. I was told that probably a quarter of the fuel and a considerable part of the lumber used in North Truro was drift-wood. Many get all their fuel from the beach.” He further noted that much of what the Cape residents of the middle 1800s called “woods” was only 4-5 feet high, with the belts of taller trees so restricted that “for the most part, we could see the horizon through them” Ch. 2: “There were almost no trees at all in this part of Dennis, nor could I learn that they talked of setting out any. It is true, there was a meeting-house, set round with Lombardy poplars, in a hollow square, the rows fully as straight as the studs of a building, and the corners as square; but, if I do not mistake, every one of them was dead.”

5. First, the really big picture… •  Estimated recurring cost of a carbon-neutral economy is ~1% of world economy per year (order-of-magnitude in true sense of word); • To avoid damages growing to be a few times bigger in a few decades; • But requires remaking a few percent (or more) of world economy; • All economic “remakes” involve loss of real jobs that are now held by real people, offset (more-or-less) by gain of jobs that don’t now exist and nobody now is doing, so there is a strong asymmetry in political pressure.

6. First, the really big picture… •  Not surprisingly, those real people demand that the science behind decisions be exceptionally good • Our political bodies routinely make decisions on other issues with much weaker scientific justification than is now available on climate • But we are expected to do better • (Doing better is necessary but not sufficient grounds for decision-making)

7. As paleoclimatologists: •  If it happened, it was possible, and might still be • If it changed, it might still change and should be in the models; • A weather-forecasting student can test daily-forecast skill a thousand times in a student career; climate-model testing under diverse conditions and in time to inform interpretation of projections requires that we reconstruct the past; • The last IPCC had a paleoclimatology chapter, and a lot of demand from paleoclimatology from other chapters, for good reasons!

8. Focus on ice-age cycling: •  Very large changes, so test many aspects of our understanding of the climate system • Strong involvement of greenhouse gases, so highly relevant in assessing climate sensitivity, etc. • Recent enough that we can get high-time-resolution, spatially resolved records • Recent interglacials include slightly warmer conditions and so do address warm climates to some extent (note that in a burn-it-all world, we may pass the Cretaceous…)

9. Focus on ice-age cycling: •  Very large changes, so test many aspects of our understanding of the climate system • Strong involvement of greenhouse gases, so highly relevant in assessing climate sensitivity, etc. • Recent enough that we can get high-time-resolution, spatially resolved records • Recent interglacials include slightly warmer conditions and so do address warm climates to some extent (note that in a burn-it-all world, we may pass the Cretaceous…)

10. A bit of ancient history: •  Highly likely that people living near Alpine glaciers knew for a long time that the moraine ridges, scratched and polished rocks, etc. that were observed in front of glaciers were evidence that glaciers had been bigger in the past • European academics (once such existed, and prior to mid 1800s) were apt to disagree • Glacial deposits are called “drift” because they were believed to have drifted into place in icebergs during Noah’s flood

11. A bit of ancient history: •  A few geologists in late 1700s and early 1800s, including de Saussure and Schimper, figured out that there were glacier tracks beyond the glaciers • Agassiz picked up on this, did good science, and published (1838 and immediately thereafter) that ice ages had extended far beyond the Alps • With Buckland, extended idea to Scotland • (Later, Agassiz sort of went wonky and found evidence of ice almost everywhere, including where there isn’t any such evidence, such as in the Amazonian rain forest.)

31. A bit of ancient history: •  Orbital forcing—Adhemar (1842) precession • Croll (1875) added obliquity and eccentricity • Milankovitch (1920-1941) calculated history of top-of-atmosphere sunshine by latitude band, also focused on summer rather than winter sunshine • Lots of improvements since, but Milankovitch usually gets most of the credit

32. Swiped from Lowell Stott

33. Precession: • From sun, etc. tugging on equatorial bulge • 19-23 ka cyclicity in insolation • Controls season (N summer/S winter or N winter/S summer) when Earth closest to sun • Little effect on total yearly energy at a latitude • Much change in midsummer peak energy, and thus in summer-winter difference at a latitude • High midsummer peak in far north when low midsummer peak in far south

34. Obliquity: • From sun, etc. tugging on equatorial bulge • 41 ka cyclicity in insolation • Is inclination of Earth’s spin axis from normal to orbital plane • Notable effect on total received energy at a latitude over a year • Also affects peak summer sunshine • High midsummer peak in far north when high midsummer peak in far south

35. Eccentricity: • From Jupiter, etc. tugging on Earth as we pass it in orbit • 100 ka, 400 ka cyclicity • Is deviation of Earth’s orbit from circular • Slight effect on total sunshine received by planet (as eccentricity rises, planet spends more time far from sun and so gets less total sunshine) • Mostly modulates precession (for a perfectly circular orbit, precession would not matter)

36. From Glacial World According to Wally

37. From Glacial World According to Wally

38. From Peter Huybers

39. 100 41 23 19ky Insolation at 65 degrees N, Peter Huybers

40. From Peter Huybers

41. 100 41 23 19ky Insolation at 90 degrees S, Peter Huybers

42. From Peter Huybers

43. 100 41 23 19ky Mean Annual Insolation, Peter Huybers

44. From Glacial World According to Wally

45. From Glacial World According to Wally

46. From Glacial World According to Wally

47. From Glacial World According to Wally