Michael Schulz Prof. Michael Schulz Research Center Ocean Margins University of Bremen Germany If you would like to interview this scientist, please contact Leah Christen. > biographical information > abstract > paper

Biographical Information:

My current research focuses on understanding processes responsible for natural climate change. Specifically, I am interested in the origin of millennial-to-centennial scale climate variations during the late Quaternary as well as Milankovitch climate variations in icehouse and greenhouse worlds. My main research tools are numerical climate models, covering the full range of complexity from conceptual to comprehensive climate models. Since 2002, I have been conducting research and teaching at the Department of Geosciences of the University of Bremen, Germany.

Abstract:

Past Climate Variability at Centennial-to-Millennial Timescales: Does it Matter for Predicting Future Climate Change?

Climate change at millennial timescales is best known from the last glacial period, when so-called Heinrich events and Dansgaard-Oeschger cycles perturbed the climate system. These climate fluctuations were associated with rapid (i.e. happening in a decade or even less) and large-amplitude transitions in Atlantic Ocean’s thermohaline circulation and impacted climate at least at a hemispheric scale, more likely even globally. While the origin of the Heinrich events was traced to internal instabilities of continental ice caps, the cause for the Dansgaard-Oeschger cycles remains elusive. Recently, Dansgaard-Oeschger events have been tentatively linked to external forcing mechanisms, mainly because of their regular pacing (“1500-year cycle”), which seems difficult to reconcile with an Earth-bound origin.

Based on experiments with an ocean-atmosphere-sea ice model of intermediate complexity, it is argued that Dansgaard-Oeschger cycles are generated within the Earth climate systems and do not necessarily call for an external origin. These model experiments reveal that the coupled climate system can exhibit unforced climate variability at centennial-to-millennial timescales. The modeled oscillations are associated with on- and off-states of deep convection in the Labrador Sea, while convection in the Norwegian-Greenland Seas remains active during all phases of the oscillations. The oscillations represent a three-dimensional phenomenon, linked to the intricate interaction between two deep-water formation sites in the North Atlantic.

A widely held view is that the rate of North Atlantic deepwater production may weaken (or even cease) during the next century due to anthropogenic climate forcing. Based on our modeling results, we add to this complexity by suggesting that future climate may be associated with the renewed onset of oscillations of the Atlantic Ocean’s thermohaline circulation.

Paper:

The Earth’s Variable Climate—Lessons for the Future from Paleoclimatic Research

The Earth’s climate is not steady. It exhibits natural variations on timescales of a few years to tens of thousands of years. Paleoclimate research over the past decade has revealed that within this range climate shows a preference to vary on multi-century to millennial timescales.

Evidence from glacial climate variations
Climate during the last glacial period (~10,000 to 100,000 years ago) was characterized by repeated transitions between warm and cold phases. These temperature variations have been unequivocally detected in ice core, marine and terrestrial paleoclimatic archives. The transitions from cold-to-warm states host a remarkable and, as yet, unexplained feature: their timing appears to be spaced in multiples of approximately 1500 years. That is, a transition from cold to warm occurs 1500, 3000, 4500, etc. years after the previous transition.

Insights from interglacial climate variations
Climate varies significantly less during warm interglacials (such as the Holocene; the last 10,000 years) than during glacials. Nevertheless, a growing body of evidence points to discernable climate variability at multi-century timescales, namely around 400-500 and 1000 years. Whether or not these interglacial variations are linked to the glacial 1500-year “cycle” is a matter of ongoing debate.

Why care about such long-term climate fluctuations?
The concerns of society about future climate change rarely exceed a timeframe of, say, 50 years (i.e., two generations). So what is the societal relevance of the multi-century to millennial-scale climate variations detected in the paleoclimate record? There are two reasons to be concerned: Firstly, any anthropogenically forced climate change will occur on top of natural long-term climate variations. Due to this superposition, natural long-term climate fluctuations may significantly amplify or attenuate man-made climate perturbations. Secondly, one cannot rule out the possibility that anthropogenic climate perturbations result in a strengthening of natural long-term climate variability. Our current understanding of the processes that are responsible for natural multi-century to millennial-scale climate change is too limited to dependably address these two issues.

Potential mechanisms for natural long-term climate variations
Results from climate computer-modeling studies imply a link between the multi-century scale variations during interglacials and the millennial-scale fluctuations during glacials. Specifically, these experiments suggest that oscillations in the strength of deep-water production in the North-Atlantic Ocean may be responsible for the reconstructed climate variations at multi-century to millennial timescales. Deep-water formation is intimately linked to the transport of heat from low to high latitudes. Accordingly, a change in deep-water formation is an effective mechanism for amplifying climate change.

Lessons for the future
An often-cited scenario for future climate change is the cessation of deep-water production in the North-Atlantic Ocean, due to humanly induced excess freshwater input into polar oceans (Fig. 1, left). Such a cessation would, for example, lead to a dramatic temperature drop by up to approx. 10°C in the North-Atlantic region, with severe socio-economic consequences. However, our model experiments suggest the existence of an alternative route for future climate change: We may be heading into a world with century-scale transitions between warm and cold climates in large parts of the northern hemisphere (Fig. 1, right).



Figure 1. Future fate of deep-water formation in the North-Atlantic Ocean. (left) Cessation scenario, in which deep-water formation becomes extremely weak or even stops completely. (right) Oscillation scenario, in which deep-water formation flips every few hundred years between strong and weak states. In both scenarios, the transition in the mode of deep-water formation is triggered by increasing freshwater input into polar oceans (indicated be the arrow at the top). Changes in deep-water production cause temperature variations in large parts of the northern hemisphere. Strong deep-water production goes along with warm phases (red), whereas weak deep-water production leads to cold conditions (blue).