XII. Heidelberger Graduiertenkurse Physik. Climate Research. Werner Aeschbach-Hertig Pier-Luigi Vidale

Transcription

1 XII. Heidelberger Graduiertenkurse Physik Climate Research Werner Aeschbach-Hertig Pier-Luigi Vidale

2 Part 1: Paleoclimate Session 1 (Tuesday): Introduction and motivation Basics of the Earth's climate system Methods of paleoclimate reconstruction Session 2 (Wednesday): Overview of paleoclimate on different time scales Discussion of important paleoclimate records Driving mechanisms of natural climate changes Recent past and link to climate modeling

3 Paleotemperature on Different Time Scales I Entire Earth history (~ 4.5 Gyr): Alternating warm and cool phases Mostly warmer than today (~ ice free) Exception: Snowball Earth in Late Proterozoic (?)

4 Paleotemperature on Different Time Scales II Last ~ 100 Myr: General cooling Last ~ 1 Myr: 100 kyr glacial cycles Last ~ 150 kyr: Full glacial cycle Last ~ 20 kyr: LGM - Holocene Younger Dryas

5 Paleotemperature Records on Different Time Scales III Last ~ 1000 years: Historical records Climate archives: Tree rings, corals, ice cores, sediments, Recent warming is exceptional! Last ~ 150 years: Direct temperature measurements Warming of ~ 0.6 C since ~1900

6 "Hard Facts"about Paleoclimate Warm, ice free climate for most of the last ~ 250 Myr 100-kyr glacial cycles during the Pleistocene (last ~1 Myr) Relatively stable and mild climate in the Holocene (last 10 kyr) Clear warming trend in the 20th century (last 100 yr) Discussion of Important Paleoclimate Records Ocean sediments (stable isotopes in foraminifera shells): 100 kyr glacial cycles Ice cores (stable isotopes, trapped gas, borehole T ): 100 kyr glacial cycles trace gas history abrupt changes

7 The Oceanic δ 18 O Record Climate Parameter: Ice volume (and temperature) Proxy: δ 18 O in carbonate (CaCO 3 ) shells of foraminifera Archive: Forams in ocean sediments Dating: 14 C, indicators, orbital tuning

8 The Oceanic δ 18 O Record and Marine Isotope Stages warm less ice cold more ice from Broecker, 1995

9 The oceanic δ 18 O record and marine isotope stages

10 Ice sheets of the last glacial maximum (LGM) and today

11 Stable Isotopes in Ice, Ocean and Foraminifera Present day: Ocean: 97 % of water on Earth, δ 18 O = 0 (standard) Mean depth of the ocean: 3800 m Ice: 2 % of water on Earth, δ 18 O -35 Last glacial maximum (LGM): Sea level ~ 120 m lower: 120/ % less water Ice: 5 % of water on Earth, δ 18 O -40 Ocean: ~ 94 % of water, δ 18 O enriched to ~ = 1.2 Benthic (bottom dwelling) foraminifera CaCO 3 shells in isotopic equilibrium with deep sea water mainly record deep water δ 18 O ice volume Complication: T-dependence of eq. isotope fractionation

12 Stable Isotopes in Ocean Sediments δ 18 O record from carbonate shells of benthic foraminifera Temperature effect: T-dependence of water/carbonate fractionation Ice volume effect: Ice caps depleted, glacial ocean enriched in heavy isotopes from Broecker, 1995: The Glacial World According to Wally

13 Example 2: Ice Cores Climate Parameters: Temperature / trace gas concentrations Proxy: stable isotopes (δ 18 O, δ 2 H) in ice / trapped air Archive: Ice sheets, glaciers Dating: Layer counting, flow models

14 Stable isotopes in polar ice cores warm cold Raw data from Vostok (Antarctica), Petit et al., Nature 1999, 399: Dating by layer counting (upper part) and by ice flow model (lower part) (from Broecker, 1995)

15 Calibration of δ 18 O Thermometer in Ice

16 How Cold was the Ice-Age in Greenland? Stable Isotopes δ 18 O ~ 8 Slope: 0.67 / C T ~ 12 C Borehole Temperatures T ~ 23 C Mismatch by factor of 2! T had been underestimated Dahl-Jensen et al., 1998, Science 282:

17 Stable Isotopes and Trace Gases in Polar Ice Cores Strong covariation of temperature (stable isotopes) and greenhouse gases CO 2 and CH 4 Dating problem: ice age gas age

18 Recent CO 2 Increase put into Perspective

19 The Ice Age Gas Age Difference in Ice Cores What is the relative timing of temperature and trace gas variations? Problem: age of ice age of trapped air Air circulates in firn to a depth of ~ 100m Trapped air younger than ice Ice age gas age difference: Time of snow accumulation until closure of bubbles On the order of several 100 yr

20 Relative Timing of Warming and Trace Gas Increase Thermal fractionation of N- and Ar-isotopes in trapped air marks warming Can be correlated to δ 18 O signal in ice Can directly be compared to trace gases Shows that warming leads trace gas increase! Glacial termination not greenhouse driven Severinghaus et al., 1998, Nature, 391:

21 Milankovic: Orbital Parameters Drive Climate

22 What drives Glacial Interglacial Climate Cycles? Milankovic theory: Changes in orbital parameters: Eccentricity Obliquity (tilt) Precession Variation of distribution of solar irradiance on Earth Critical parameter taken as insolation at 65 N

23 Why is Insolation in the North Important? Northern ice sheets are the main factor in glacial radiative forcing! Increased albedo keeps glacial climate cold

24 Relative Timing of Insolation and Temperature Petit et al., 1999, Nature 399:

25 Spectral Analysis of Time Series and Orbital Tuning

26 Discussion of the Milankovic Theory Strengths Physically plausible mechanism Match between orbital and climate record frequencies Explains regularity of climate record Problems Somewhat arbitrary choice of 65 N Power spectra mismatch: Why is 100-kyr dominant in climate? Sometimes mismatch between insolation and warming Despite problems, the Milankovic theory is generally accepted.

27 Abrupt Climate Changes Ice core and sediment record show series of fast (< 100 yr) changes during the last glaciation (occurring in intervals of ~ 1500 yr?) Dansgaard/Oeschger (DO) events: abrupt warming, gradual cooling Heinrich (H) events: layers of ice-rafted debris in N-Atlantic sediments, associated with cooling Younger Dryas (YD): Cool episode during last termination From Rahmstorf, 2002, Nature 419:

28 Heinrich Events

29 What Drives Abrupt Climate Changes? Two major attempts for explanation: Changes in ocean circulation (N-Atlantic deep water formation) Changes in tropical atmosphere-ocean dynamics (~ ENSO)

30 The North Atlantic Climate Switch Two stable modes of ocean circulation: NADW on / off Two climate states: interglacial / glacial (stadial)

31 The North Atlantic Climate Switch Freshwater flux into N-Atlantic is the controlling factor Transition between the two states occurs when perturbation (change of freshwater flux) exceeds certain threshold Transition exhibits hysteresis: System does not switch back when perturbation stops

32 "Exotic" Driving Mechanisms of Climate Change "Svensmark hypothesis": Solar activity, cosmic rays, and clouds 11-yr average T T correlates with - Solar cycle length - Cosmic ray flux - Sunspots -Irradiance (until ~ 1980)

33 Solar Forcing of Climate? Changes of solar irradiance are too weak, but possible feedback by modulation of galactic cosmic ray (GCR) flux by solar activity: High solar activity ( High irradiation) Strong solar wind Strong magn. field Strong shielding Low GCR flux missing link Warmer climate?

34 Missing Link: Cosmic Rays and Clouds? Low GCR flux Less low clouds Lower Albedo Warmer climate! Svensmark and Friis-Christensen, 1997, J. Atm. Sol. Terr. Phys. 59:

35 Svensmark Hypothesis: Does the Correlation Hold? high middle New data show weakening of correlation between GCR flux and (low) clouds. The link between GCR flux and cloud formation remains unclear. Interesting, but controversial hypothesis. low

36 GCR-Climate Link on Long Time Scales? GCR-flux modulated on 100 Myr-timescale by passages of solar system through spiral arms of galaxy, and correlated with Temp? Highly disputed paper (contradicting statement published by several well-known climate researchers) Questionable methods (Rahmstorf et al., EOS, 27. Jan. 2004) Problematic: Speculative theory used to discount greenhouse warming Shaviv and Veizer, 2003, GSA Today.

37 Speleothem Evidence for GCR-Climate Link? Absolute U-Th dating of speleothems allows investigation of relative timing Termination II timing seems inconsistent with Milankovic (65 N insolation) but fits with GCR-flux. Highlights importance of precise dating Further investigation needed Christl et al., 2004, J. Atm. Sol. Terr. Phys. 66:

38 Arguments in favour of the GCR hypothesis Consistent with evidence for sun-climate connections Potential feedback to amplify direct solar forcing Some evidence for GCR-cloud connection May explain timing of some climatic events Arguments against the GCR hypothesis Mainly circumstancial evidence, no clear physical mechanisms Solar activity temperature correlation seems to have ended GCR-cloud correlation may break down Very long-term correlation are not convincing

39 Gas Hydrates and Climate Change Gas/methane hydrates/clathrates are: Crystals consisting of a H 2 O-lattice stabilized by (mostly) methane molecules enclosed in cavities Conditions for methane hydrate formation: adequate supply of water and methane low temperature high pressure Continental margin sediments!

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41 Pictures from the 1996 Geomar cruise to Hydrate Ridge

42 The Carbon Inventory in Methane Hydrates Highly uncertain, best estimate 10'000 Gt = kg of carbon Larger than all other reservoirs of organic carbon combined Twice as much C as in fossil fuels Obviously a potential energy resource

43 Hydrates and climate: The clathrate gun hypothesis Catastrophic CH 4 releases from hydrates drive abrupt climate changes

44 Late Paleocene Thermal Maximum: ~ 55 Myr BP Strong reduction of benthic foraminifera species Large excursions of foraminferal δ 18 O ( T) and δ 13 C (CH 4!?) Kennett and Stott, Nature, 1991 Did similar events happen in the more recent past? Could something like this happen in the (near) future?

45 Hydrate Release Mechanism Warming of intermediate waters destabilises hydrates Submarine slides release large gas amounts Sea level rise stabilises hydrates

46 The Storegga Slump and Tsunami 7000 years ago, off the coast of Norway: Huge submarine slide 5600 km 3 of sediment moved, up to ~10 m high tsunami waves Probably hydrates involved

47 Arguments in favour of the clathrate gun hypothesis Consistent with warming events in the distant past Close association of temperature and CH 4 in climate records Explains rapid warming (involving feedbacks to CH 4 increase) Sufficient CH 4 available: 3000x amount in present atmosphere Arguments against the clathrate gun hypothesis Large part of CH 4 oxidised to CO 2 in the water column No significant impact on greenhouse forcing Alternative explanations for CH 4 increase (wetlands) Killer: CH 4 increase slightly lags behind rapid warming

48 Recent past and link to climate modeling Periods of special importance for model/data comparisons Last Glacial Maximum (LGM): clearly different, relatively well-defined climate state. Last 1000 yr: Relatively well-known Last 150 yr: Best known, direct temperature data Challenge for paleoclimate research: How well do we know it? Major problems: LGM-cooling of the tropics Importance of Medieval Warm Period and Little Ice Age

49 LGM-Holocence T: 1. CLIMAP SSTs LGM-Modern Aug CLIMAP inferred relatively small (< 2 C) (sub)tropical SST This result was often used as prescribed glacial boundary condition in climate models LGM-Modern Feb

50 LGM-Holocence T: 2. Continental Proxies (Pollen, Noble Gases, Speleothems, etc.) Continental proxies imply large (~ 5 C) (sub)tropical T From: Farrera et al., Climate Dynamics 15 (1999):

51 Problems of Model-Data Comparisons From: Crowley, Climate Dynamics 16 (2000):

52 Paleoclimate Modeling Intercomparison (PMIP) Model-data match only if CLIMAP SSTs are NOT prescribed

53 PMIP Simulations with Different Treatment of SSTs

54 Climate of the last 1000 yr Multiproxy reconstructions (but strongly based on tree rings) Most models (including IPCC) rely on curve by Mann et al., 1999: IPCC, taken from: Mann et al., 1999, Geophys. Res. Lett. 26:

55 MWP and LIA Debate centers on two periods: Medieval Warm Period (MWP, ~ ) Little Ice Age (LIA, ~ ) Significant (historical) evidence for these periods in Europe Not prominent in Mann-curve IPCC: MWP and LIA are regional, not global, effects Is this true?

56 Disagreement on Climate of the last 1000 yr Several reconstructions of last 1000 yr have been published Disagreement in the past (MWP and LIA) Agreement that 20th century shows exceptional warming

57 Modeling the Climate of the last 1000 yr Importance: Fine-tuning of response to forcings Good agreement (with Mann) is only achieved if solar forcing and volcanoes are taken into account. In the 20 th century, greenhouse forcing becomes dominant

58 Volcanoes Dust and aerosols from volcanic eruptions significant on short times Prominent example: Tambora (1815) "year without summer" (1816) From Briffa et al., 1998, Nature 393:

59 Summary of Part 2 Lessons from the paleoclimate record: - We live in a mild period of a cold epoch - T, ice sheets, sea level, trace gases have varied in 100 kyr-cycle - Climate has been relatively stable for past 10 kyr (Holocene) - Abrupt changes can occur in the climate system Driving mechanisms of natural climate variations: - Milankovic remains main explanation despite problems - GCR-climate link is interesting, but uncertain - Other drivers (e.g. gas hydrates) are speculative Recent past and modeling: - Improved knowledge of LGM and last 1000 yr needed to better constrain climate models

60 My Personal View of Climate Change Human impact on atmospheric trace gas concentrations is a fact (Enhanced) greenhouse effect is a physically reasonable theory Feedbacks in climate systems (e.g. clouds) are hard to assess Present warming is likely to be partly (mainly?) anthropogenic Further warming in the future is likely (how much?) Climate change is not a threat for life on Earth, but could cause severe disruptions for human economy and society Reasonable measures to reduce fossil fuel burning make sense: reduce air pollution and possible climate effects save energy resources for future generations