RESEARCH GROUP: PALEOCLIMATE DYNAMICS We study the mechanisms that caused the last glacial termination, millennial-scale variability, the Paleocene Eocene Thermal Maximum and past changes of ENSO. The climate of the past may hold important clues to understand its future evolution. Recent discoveries on past ice-sheet instabilities have prompted a surge of activities to monitor, understand and predict the response of the Greenland and Antarctic ice-sheets to greenhouse warming. A partial rapid meltdown of these ice-sheets would lead to a global sea-level rise of several meters with catastrophic effects for low-lying countries and islands in the Pacific. Moreover, meltwater from the disintegrating Greenland ice-sheet could trigger a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC), with climate-effects that would be felt worldwide: relative cooling of the northern hemisphere, weakened Indian summer monsoon, increased El Nino variability and reduced upwelling and marine productivity in the major southeastern basin upwelling regions. Assessing the sensitivity of the major ice-sheets and the oceans thermohaline and wind-driven circulation to perturbations, such as an increase in CO2 concentrations, requires an understanding of their past behavior. Paleo-data have provided a unique means to decipher important aspects of abrupt climate change. With the discovery of Heinrich and Dansgaard-Oeschger events in the late 80s and early 90s evidence emerged that under glacial conditions the climate system is capable of generating spontaneous rapid transitions from one state to another. What caused such abrupt transitions still remains elusive. Possible threshold behavior has been suggested for the AMOC, ice-sheets and the carbon cycle. Moreover, on millennial to orbital timescales these climate components seem to interact with each other vigorously. The nature of these interactions has not been explored satisfactorily. The main goals of the paleo-climate research group at the IPRC are to assess the stability of the major ice-sheets using paleo-climate data and coupled ice-sheet climate models to identify the climate and biogeochemical impacts of reorganizations of the ocean circulation to elucidate the mechanisms that drive glacial cycles and millennial-scale glacial climate variability to develop a better understanding of climate-carbon cycle interactions under past and future climate conditions to quantify ENSO’s sensitivity to past and future climate change using paleo-proxy archives from the Pacific and state-of-the art forced climate models One key lesson that can be learned from the paleo-climate history is that longterm climate variability, while often generated in particular regions, has far-reaching effects on climate elsewhere. Understanding fundamental dynamics of past climate variability hence requires a global perspective that encapsulates oceanic and atmospheric teleconnections. 1. Transient Modeling of past climates Ratios of heavy-to-light oxygen and hydrogen isotopes stored in the ice matrix of Antarctic ice cores reflect the local temperature evolution over the last 8 glacial cycles. With the exception of oxygen isotopes recorded in the Taylor Dome ice core, temperature proxies from different Antarctic sites, such as EPICA Dome C , Vostok, Dome Fuji and Siple Dome, as well as sea surface temperature (SST) proxies from the Southern Ocean consistently show that the major rise in local temperatures during the last glacial termination began between 20 and 17.5 ka B.P. and ended around 9-10 ka B.P. Within the same time period, atmospheric CO2 concentrations measured in air-bubbles trapped in ice cores rose from glacial to interglacial concentrations by about 80 ppmv. Furthermore, sea ice proxies, derived from sea-salt fluxes in Antarctic ice cores and marine sediment cores indicate that Antarctic sea-ice started to retreat around 20-17ka ago. It is still unclear what processes controlled the timing of the last glacial termination in the Southern Hemisphere and the corresponding CO2 increase. Covarying spring time insolation and sea-salt sodium flux (considered to be a spring sea-ice indicator) from EPICA Drauning Maud Land ice core. Using coupled climate-biogeochemical models configured to simulate the climate and carbon-cycle evolution for the Late Pleistocene, we address the following questions: What triggered glacial terminations? How did ice-sheets respond to the occasional shutdowns of the AMOC? How do ice-sheets respond to orbital forcing? How stable is the Antarctic ice-sheet with respect to future greenhouse warming? What processes triggered meltwater pulses during the last glacial termination? What effects did changes in ice-sheet orography have on North Pacific climate? A web-interface of our 21,000-year long gacial-interglacial simulation can be accessed here. For more information click here. 2. Climate carbon-cycle modeling Ice cores from Antarctica reveal that atmospheric greenhouse gas concentrations co-varied with temperatures throughout the last 700,000 years. While a substantial amount of the recorded glacial-interglacial climate variability can be attributed to greenhouse gas forcing, the origin of glacial cycles in CO2 and methane still remains elusive. Dozens of hypotheses have been proposed during the last decade to explain the origin of glacial-interglacial CO2 changes. None of them alone seems to explain the full magnitude of these variations. Earth system models of intermediate complexity will be used to elucidate the fundamental driving mechanisms of global carbon cycle variability on millennial to orbital timescales. About 55 mio years ago a massive release of isotopically light carbon to the atmosphere led to climate conditions that were comparable to those predicted by greenhouse warming simulations for the next 100-200 years. This so-called Paleocene Eocene Thermal Maximum (PETM) has often been interpreted as an analogue of what is in store for us if future greenhouse gas emissions will not be reduced significantly. During the PETM the Arctic Ocean was ice-free, breadfruit trees grew in Siberia and tropical sea surface temperatures may have exceeded 40C in some areas. With an open Panama Isthmus a northern North Atlantic just opening up and the Tethys Seaway connecting the Atlantic and Indian Ocean, the oceanic circulation was very different from todays. Understanding the Paleocene Ocean circulation and its interaction with a massive partly submarine volcanic eruption in the northern North Atlantic may help to identify potential trigger mechanism for the massive carbon release and its subsequent influence on the climate system. Ocean general circulation model studies will be conducted to elucidate the general circulation patterns under PETM conditions, the sensitivity of the ocean circulation to large geothermal heat fluxes and the stability of methane hydrates. The following questions shall be addressed: What mechanisms are responsible for glacial-interglacial cycles in CO2? How does the carbon cycle respond to orbital forcing? What ocean/climate conditions are favorable for releasing massive amounts of carbon? What led to the 14C drop of about 190 permil during the Mystery Interval (~ 17ka B.P.)? How stable was the lysocline during the late Pleistocene? What role did the North Pacific play in the deglacial atmospheric CO2 increase? What triggered the carbon release during the PETM? How did the PETM carbon release affect climate and the ocean circulation? Were PETM climate conditions in the Pacific and Indian Ocean favorable for generating hypercanes? Atmospheric CO2 and C reservoir response to a meltwater pulse that leads to a shutdown of the AMOC 3.Coral Bleaching and Global Warming   Future climate change may put coral reefs at high risk. Their survival will depend on their skill to adapt to future warming, ocean acidification and other anthropogenic forcings. If the rate of adaption can not keep pace with these stress factors, massive coral mortality is likely to occur. Recent studies have focused on the effect of projected thermal stress on corals using state-of-the art climate model simulations for the 21st century and empirically-derived bleaching thresholds. While an overall warming will lead to widespread threshold exceedances in tropical regions by the year 2050, the regional patterns of coral vulnerability in a warming world have not been studied in great detail yet. Projected annual mean Degree Heating Weeks for the decade 2090-2010 using a multi-model downscaling technique based on 5 IPCC SRES-A1B emission scenario simulations. To obtain annually accumulated DHWs, these numbers have to be multiplied with 52. Note, the IPCC climate models do not simulate reduced warming in the western tropical Pacific as conjectured by Kleypas, GRL (2006). There is neither observational nor modeling evidence for an “ocean thermostat” that decelerates future warming in this region. 4. Paleo-ENSO Current generation global coupled general circulation models however, show little consensus with regard to the projected future behavior of ENSO . While some state-of-the-art coupled general circulation models simulate an intensification of ENSO variability under CO2 doubling conditions, other models show no significant change or even a weakening of ENSO activity. Partly, this large uncertainty can be attributed to the simulated differences in the background states, partly to the different regimes in which ENSO operates in the climate control simulations. One possibility to constrain the sensitivity of ENSO to climate change is to study the past history of ENSO using proxy-based climate reconstructions as well as numerical models. Paleo proxies, documentary research and instrumental data, all capture variations in ENSO behavior over the past centuries and throughout the Holocene. Much of this variability appears to be internal to the earth’s climate system, but there is evidence from intermediate coupled models and coupled general circulation models that orbital variations have been responsible for systematic changes of ENSO statistics throughout the Holocene. Such changes can occur quite abruptly, on timescales that are much shorter than the orbital forcing timescale. Recently, also volcanic aerosol forcing as well as changes in the solar irradiance have been suggested as potential drivers for low-frequency changes of ENSO. Separating externally forced signals in tropical Pacific climate reconstructions and model simulations from the ones that are generated by internal instabilities is a fundamental problem that will be addressed. The IPRC paleo group will further elucidate the physical mechanisms responsible for the sensitivity of tropical Pacific climate under paleo and future greenhouse warming conditions. The following questions will be addressed by conducting a suite of climate sensitivity experiments using state-of-the art CGCMs as well as intermediate coupled models What is the range of internally generated ENSO variability on decadal and centennial timescales in comparison with the externally-induced low-frequency modulation of ENSO? What are the mechanisms of internally generated and externally-induced long-term changes of ENSO? How does ENSO respond to the radiative forcing induced by strong volcanic eruptions? Can changes of solar irradiance modulate ENSO activity? In what ways does orbital forcing influence ENSO variability? What is the effect of orbital forcing on tropical instraseasonal variability? How stable were ENSO teleconnections during the Late Pleistocene? What are optimal paleo-proxy locations to reconstruct ENSO variability? What is the degree of consistency between different paleo-ENSO reconstructions during the last millennium? Wavelet spectrum of Nino3 SST from 5 glacial climate simulation conducted with NCAR CCSM3, During years 25-150 the Atlantic Meridional Overturning circulation collapses due to the applied northern North Atlantic freshwater forcing. The annual cycle in the eastern equatorial Pacific weakens, whereas ENSO variabiity is strongly amplified. This illustrates the effect of the AMOC on ENSO and eastern tropical Pacific climate. Paleo-records from the Hawaiian Islands climate reconstructions from old Mamamne trees from the slopes of Mauna Kea (with Ed Cook, Lowell Stott, Niklas Schneider, Patrick Hart) climate reconstructions from Ka’au Crater, Oahu (with Dave Beilman, Oliver Timm, Niklas Schneider) climate reconstructions from sediment cores of Lake Kauhako (with Niklas Schneider) climate reconstructions from Lake Waiau (planned) climate reconstruction from Ko’olau swamp (planned)

Axel Timmermann

Oliver Timm

Malte Heinemann,

Tobias Friedrich

Fabian Schloesser

Megumi Chikamoto

Former Members

Flavio Justino

Ben Marzeion

Uta Krebs

Audine Laurian

Close Collaborators

Lowell Stott,

USC, USA

Ayako Abe-Ouchi

CCSR, Japan

Naomi Harada

RIGC, Japan

Yusuke Okazaki

RIGC, Japan

Anne Mouchet

Universitee de Liege, Belgium

RESEARCH GROUP:

PALEOCLIMATE DYNAMICS

We study the mechanisms that caused the last glacial termination, millennial-scale variability, the Paleocene Eocene Thermal Maximum and past changes of ENSO.

The climate of the past may hold important clues to understand its future evolution. Recent discoveries on past ice-sheet instabilities have prompted a surge of activities to monitor, understand and predict the response of the Greenland and Antarctic ice-sheets to greenhouse warming. A partial rapid meltdown of these ice-sheets would lead to a global sea-level rise of several meters with catastrophic effects for low-lying countries and islands in the Pacific. Moreover, meltwater from the disintegrating Greenland ice-sheet could trigger a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC), with climate-effects that would be felt worldwide: relative cooling of the northern hemisphere, weakened Indian summer monsoon, increased El Nino variability and reduced upwelling and marine productivity in the major southeastern basin upwelling regions.

Assessing the sensitivity of the major ice-sheets and the oceans thermohaline and wind-driven circulation to perturbations, such as an increase in CO2 concentrations, requires an understanding of their past behavior. Paleo-data have provided a unique means to decipher important aspects of abrupt climate change. With the discovery of Heinrich and Dansgaard-Oeschger events in the late 80s and early 90s evidence emerged that under glacial conditions the climate system is capable of generating spontaneous rapid transitions from one state to another. What caused such abrupt transitions still remains elusive. Possible threshold behavior has been suggested for the AMOC, ice-sheets and the carbon cycle. Moreover, on millennial to orbital timescales these climate components seem to interact with each other vigorously. The nature of these interactions has not been explored satisfactorily.

The main goals of the paleo-climate research group at the IPRC are

to assess the stability of the major ice-sheets using paleo-climate data and coupled ice-sheet climate models

to identify the climate and biogeochemical impacts of reorganizations of the ocean circulation

to elucidate the mechanisms that drive glacial cycles and millennial-scale glacial climate variability

to develop a better understanding of climate-carbon cycle interactions under past and future climate conditions

to quantify ENSO’s sensitivity to past and future climate change using paleo-proxy archives from the Pacific and state-of-the art forced climate models

One key lesson that can be learned from the paleo-climate history is that longterm climate variability, while often generated in particular regions, has far-reaching effects on climate elsewhere. Understanding fundamental dynamics of past climate variability hence requires a global perspective that encapsulates oceanic and atmospheric teleconnections.

1. Transient Modeling of past climates

Ratios of heavy-to-light oxygen and hydrogen isotopes stored in the ice matrix of Antarctic ice cores reflect the local temperature evolution over the last 8 glacial cycles. With the exception of oxygen isotopes recorded in the Taylor Dome ice core, temperature proxies from different Antarctic sites, such as EPICA Dome C , Vostok, Dome Fuji and Siple Dome, as well as sea surface temperature (SST) proxies from the Southern Ocean consistently show that the major rise in local temperatures during the last glacial termination began between 20 and 17.5 ka B.P. and ended around 9-10 ka B.P. Within the same time period, atmospheric CO2 concentrations measured in air-bubbles trapped in ice cores rose from glacial to interglacial concentrations by about 80 ppmv. Furthermore, sea ice proxies, derived from sea-salt fluxes in Antarctic ice cores and marine sediment cores indicate that Antarctic sea-ice started to retreat around 20-17ka ago. It is still unclear what processes controlled the timing of the last glacial termination in the Southern Hemisphere and the corresponding CO2 increase.

Covarying spring time insolation and sea-salt sodium flux (considered to be a spring sea-ice indicator) from EPICA Drauning Maud Land ice core.

Using coupled climate-biogeochemical models configured to simulate the climate and carbon-cycle evolution for the Late Pleistocene, we address the following questions:

What triggered glacial terminations?

How did ice-sheets respond to the occasional shutdowns of the AMOC?

How do ice-sheets respond to orbital forcing?

How stable is the Antarctic ice-sheet with respect to future greenhouse warming?

What processes triggered meltwater pulses during the last glacial termination?

What effects did changes in ice-sheet orography have on North Pacific climate?

A web-interface of our 21,000-year long gacial-interglacial simulation can be accessed here. For more information click here.

2. Climate carbon-cycle modeling

Ice cores from Antarctica reveal that atmospheric greenhouse gas concentrations co-varied with temperatures throughout the last 700,000 years. While a substantial amount of the recorded glacial-interglacial climate variability can be attributed to greenhouse gas forcing, the origin of glacial cycles in CO2 and methane still remains elusive. Dozens of hypotheses have been proposed during the last decade to explain the origin of glacial-interglacial CO2 changes. None of them alone seems to explain the full magnitude of these variations. Earth system models of intermediate complexity will be used to elucidate the fundamental driving mechanisms of global carbon cycle variability on millennial to orbital timescales.

About 55 mio years ago a massive release of isotopically light carbon to the atmosphere led to climate conditions that were comparable to those predicted by greenhouse warming simulations for the next 100-200 years. This so-called Paleocene Eocene Thermal Maximum (PETM) has often been interpreted as an analogue of what is in store for us if future greenhouse gas emissions will not be reduced significantly. During the PETM the Arctic Ocean was ice-free, breadfruit trees grew in Siberia and tropical sea surface temperatures may have exceeded 40C in some areas. With an open Panama Isthmus a northern North Atlantic just opening up and the Tethys Seaway connecting the Atlantic and Indian Ocean, the oceanic circulation was very different from todays. Understanding the Paleocene Ocean circulation and its interaction with a massive partly submarine volcanic eruption in the northern North Atlantic may help to identify potential trigger mechanism for the massive carbon release and its subsequent influence on the climate system. Ocean general circulation model studies will be conducted to elucidate the general circulation patterns under PETM conditions, the sensitivity of the ocean circulation to large geothermal heat fluxes and the stability of methane hydrates.

The following questions shall be addressed:

What mechanisms are responsible for glacial-interglacial cycles in CO2?

How does the carbon cycle respond to orbital forcing?

What ocean/climate conditions are favorable for releasing massive amounts of carbon?

What led to the 14C drop of about 190 permil during the Mystery Interval (~ 17ka B.P.)?

How stable was the lysocline during the late Pleistocene?

What role did the North Pacific play in the deglacial atmospheric CO2 increase?

What triggered the carbon release during the PETM?

How did the PETM carbon release affect climate and the ocean circulation?

Were PETM climate conditions in the Pacific and Indian Ocean favorable for generating hypercanes?

Atmospheric CO2 and C reservoir response to a meltwater pulse that leads to a shutdown of the AMOC

3.Coral Bleaching and Global Warming

Future climate change may put coral reefs at high risk. Their survival will depend on their skill to adapt to future warming, ocean acidification and other anthropogenic forcings. If the rate of adaption can not keep pace with these stress factors, massive coral mortality is likely to occur. Recent studies have focused on the effect of projected thermal stress on corals using state-of-the art climate model simulations for the 21st century and empirically-derived bleaching thresholds. While an overall warming will lead to widespread threshold exceedances in tropical regions by the year 2050, the regional patterns of coral vulnerability in a warming world have not been studied in great detail yet.

Projected annual mean Degree Heating Weeks for the decade 2090-2010 using a multi-model downscaling technique based on 5 IPCC SRES-A1B emission scenario simulations. To obtain annually accumulated DHWs, these numbers have to be multiplied with 52. Note, the IPCC climate models do not simulate reduced warming in the western tropical Pacific as conjectured by Kleypas, GRL (2006). There is neither observational nor modeling evidence for an “ocean thermostat” that decelerates future warming in this region.

4. Paleo-ENSO

Current generation global coupled general circulation models however, show little consensus with regard to the projected future behavior of ENSO . While some state-of-the-art coupled general circulation models simulate an intensification of ENSO variability under CO2 doubling conditions, other models show no significant change or even a weakening of ENSO activity. Partly, this large uncertainty can be attributed to the simulated differences in the background states, partly to the different regimes in which ENSO operates in the climate control simulations. One possibility to constrain the sensitivity of ENSO to climate change is to study the past history of ENSO using proxy-based climate reconstructions as well as numerical models.

Paleo proxies, documentary research and instrumental data, all capture variations in ENSO behavior over the past centuries and throughout the Holocene. Much of this variability appears to be internal to the earth’s climate system, but there is evidence from intermediate coupled models and coupled general circulation models that orbital variations have been responsible for systematic changes of ENSO statistics throughout the Holocene. Such changes can occur quite abruptly, on timescales that are much shorter than the orbital forcing timescale. Recently, also volcanic aerosol forcing as well as changes in the solar irradiance have been suggested as potential drivers for low-frequency changes of ENSO. Separating externally forced signals in tropical Pacific climate reconstructions and model simulations from the ones that are generated by internal instabilities is a fundamental problem that will be addressed. The IPRC paleo group will further elucidate the physical mechanisms responsible for the sensitivity of tropical Pacific climate under paleo and future greenhouse warming conditions.

The following questions will be addressed by conducting a suite of climate sensitivity experiments using state-of-the art CGCMs as well as intermediate coupled models

What is the range of internally generated ENSO variability on decadal and centennial timescales in comparison with the externally-induced low-frequency modulation of ENSO?

What are the mechanisms of internally generated and externally-induced long-term changes of ENSO?

How does ENSO respond to the radiative forcing induced by strong volcanic eruptions?

Can changes of solar irradiance modulate ENSO activity?

In what ways does orbital forcing influence ENSO variability?

What is the effect of orbital forcing on tropical instraseasonal variability?

How stable were ENSO teleconnections during the Late Pleistocene?

What are optimal paleo-proxy locations to reconstruct ENSO variability?

What is the degree of consistency between different paleo-ENSO reconstructions during the last millennium?

Wavelet spectrum of Nino3 SST from 5 glacial climate simulation conducted with NCAR CCSM3, During years 25-150 the Atlantic Meridional Overturning circulation collapses due to the applied northern North Atlantic freshwater forcing. The annual cycle in the eastern equatorial Pacific weakens, whereas ENSO variabiity is strongly amplified. This illustrates the effect of the AMOC on ENSO and eastern tropical Pacific climate.