A2.3EC3 Perspectives on Environmental Change

1. A2.3EC3Perspectives on Environmental Change LECTURE 6 Mechanisms underlying environmental change

2. SUMMARY • Introduction • Drivers of climatic change • Orbital changes • Changes in the oceanic circulation • Greenhouse gases • Solar output • Volcanic eruptions • Teleconnections within the climate system • Stable modes of the climate system • Perspectives on Environmental Change

3. Introduction

4. Analyses of long climatic proxy records (ice cores, sediment cores) show that the LGH climate has varied over timescales from decades to millennia. • This suggests that several different causes may be involved. • It is also likely that one change may trigger others, perhaps on different timescales.

5. The drivers of LGH climatic variation can be grouped broadly as: • orbital changes • changes in the atmosphere-ocean system • changes in short-term solar radiation received at the ground surface • Within these groups more than one individual driver can often be identified.

6. Orbital changes

7. During the Holocene there has been a half-cycle change in the precession component and a quarter cycle change in the obliquity component. • The former describes the direction of the spin axis to the long and short axes of the orbit. • The latter describes the tilt angle of the spin axis relative to the plane of the orbit.

9. Taken together they have led (in the northern hemisphere) • first to an increase of ~10%+ in received solar radiation between the last glacial maximum and the start of the Holocene • then to a decrease of ~8% from the start of the Holocene to the present day.

10. The effect of the increase was to drive the climate out of glacial mode and into interglacial mode. • The effect of the subsequent decrease was to bring about the transition from the climatic optimum to the neoglacial. • These are quite large changes for a relatively small change in direct insolation. This suggests an amplifying mechanism exists.

11. Changes in oceanic circulation Deep ocean ventilation

12. The North Atlantic is an important area for the transfer of heat from the tropics to the poles: • It has an open connection to the Arctic Ocean • It receives warm surface water from the tropics • It generates a return flow of deep water (NADW) • Overturning of surface water causes mixing with the deeper waters, which later return to the surface at upwelling points. This mixing is termed the ventilation of the deep ocean.

13. There are no other comparable areas on the Earth. • Thus events in the North Atlantic region control the strength of the entire global oceanic circulation. • The rate of overturning is of critical importance. This controls the ventilation timescale - about 500 years at the present day.

15. One of the most significant changes during the LGH involved the position and strength of the ocean currents in the North Atlantic • Of critical importance was the location of the polar front - the area of thermohaline sinking and thus production of NADW.

17. The reorganisation of the surface ocean currents reflects a general shift in the production of deep water during the glacial-interglacial cycle. • In warm periods, the dominant area is the North Atlantic. • By contrast, in cold periods, production shifts to the South Atlantic and Antarctic.

19. During glacial periods North Atlantic surface water freshened significantly. This reduced the production of NADW. • Sea-level fell by around 150m. This reduced the southward overflow across the Wyville-Thompson (Iceland-Scotland) ridge. • Thus the thermohaline circulation was weaker in glacial periods and the ventilation timescale increased to ca. 6000 years.

21. Changes in oceanic circulation Stability of the circulation

22. Variations at millenial scales may also be due to the stability of the thermohaline circulation. • Some changes were abrupt, one-off events due to the final breakup of the North American ice sheet and the associated release of ice and meltwater. • The well-known cold event at 8,200 BP is an example.

24. There also seems to be a 1,500 year cycle in the strength of the North Atlantic deep water outflow from the Arctic. • This is recorded in several deep ocean cores from the Iceland area. • This cycle of this frequency is also found in the Greenland ice core record during the Holocene and appears to be attributable to the atmospheric North Atlantic Oscillation.

28. It is well-known today that the ENSO cycle determines climatic events on a global basis. • There is evidence that the frequency of ENSO events was less in the early Holocene and that there have been periods of greater activity at times in the mid- to late-Holocene.

30. These frequencies appear related to the cold (low frequency) and warm (high frequency) periods known from the pollen record. • It is not known whether the variation in ENSO freqencies is due to external forcing (eg solar output) or internal (dynamical) oscillation.

32. Changes in oceanic circulation Decadal variations

33. The strength of the deep thermohaline circulation (THC) fluctuates on a decadal timescale. • These fluctuations are matched by small variations in the temperature of European winters. • In general, a reduction in THC strength is associated with warmer winters in Europe.

34. (1 Sv [Sverdrup unit] = 1,000,000 cumecs) Thames = 500 cumecs, Rhine = 2,000 cumecs

36. The winter temperature is significant since the THC reflects cooling in the Greenland area, in which depressions originate. • The mechanism may involve less surface cooling in the Greenland area, thus SSTs are higher and more heat enters the atmosphere. • This finding is counter to the effect of a larger shutdown of the THC, which reduces the surface heat flow into the Arctic.

37. Greenhouse gases

38. It is possible to make a direct analysis of the carbon dioxide content of air bubbles trapped in the ice-core layers. • These show how the atmospheric composition has changed over the past few hundreds of thousands of years.

39. Cromerian Hoxnian Ipswichian Holocene

40. Cross-comparison with the isotopic record indicates the relationship of these changes to temperature. • Given the very high resolution of the ice-core layering, it is possible to track atmospheric changes both within the Last Termination and into both the Holocene and modern periods.

43. From the whole ice core, these changes can be seen as part of a longer glacial to interglacial cycle that correlates with the extent of glacierisation. • The fundamental control on atmospheric carbon dioxide is thus the rate of deep ocean ventilation. This governs the rate of release from the deep ocean to the atmosphere.

45. TAKE A SHORT BREAK PLEASE COME BACKIN 10 MINUTES

46. Solar Output

47. Solar activity is known to vary by small amounts on a semi-regular basis. • This activity is associated with the presence of sunspots, which display a number of cycles of varying length. • The best known is the 11-year cycle, which shows a limited relationship to long-term avarage rainfall.

49. Sunspot numbers 1700 - 1990

50. 10-year average precipitation record 1750-1995