Is the Earths climate approaching dangerous thresholds that could cause melting of the Greenland Ice Sheet or sudden changes in the ocean circulation («the gulfstream»), that impacts Norways climate? Using marine sediment cores recovered from the deep ocean through participation in the International Ocean Discovery Program, together with advanced computer models, the THRESHOLDS project investigated past changes in climate and ocean circulation to find past climate thresholds. The results reveal clearly that during past warm climate periods, each just a little bit warmer than today, large reductions in ocean circulation and the Greenland Ice Sheet occurred.
Previous studies of ocean circulation focused on changes occurring over thousands of years leading scientists to concluded that ocean circulation was strong and stable during periods of the recent geological past that were as warm or even warmer than today. However, almost no one had looked at changes in ocean circulation that occur on shorter timescales, such as centuries or even less, to see if ocean circulation could vary more rapidly. This is critical, since changes that occur rapidly could have large impacts on society due to their influence on climate, sea level, food production, and ocean carbon uptake.
To answer this question THRESHOLDS investigated unique archives from the seafloor near Greenland where mud accumulated many times faster than in other locations. The sediment deposits on the seabed act as a tape recorder that has stored millions of years of climate information. Analyzing thousands of sediment samples from these locations we found that large changes in ocean bottom water occurred during every climate period in the last half million years that was warmer than today. These changes indicate that rapid reductions in ocean circulation occurred much more commonly than had been predicted and consistently occurred when temperatures in the North Atlantic rose above current levels. Using state-of-the-art modeling tools we confirm that changes in ocean circulation during the last interglacial alter the deep water biogeochemical properties in the North Atlantic similar to the signal recorded in sediments. Our model suggests that sea-ice extent in the Nordic Seas is a key pacemaker of the Atlantic circulation and therefore climate variability. Future model studies would benefit from even more observational coverage of changes in the deep ocean, including from a range of different past climate conditions. Globally, past climate-induced overturning circulation changes significantly affect the ocean capacity to take up carbon. Our findings therefore highlight the need to consider circulation-carbon feedbacks (beyond the IPCC timescale of 2100) when determining future oceanic carbon uptake.
The North Atlantic temperature reconstructions also revealed that we are rapidly approaching the climate threshold for past Greenland Ice Sheet demise. We showed that only minor amounts of warming from today?s climate was enough to melt much of the ice sheet during past warm climates. By using these real world observations we show that the actual threshold for ice sheet decay may be on the lower end of that previously determined by models, and that decay may soon become an unavoidable consequence of our human activities.
We identified a new and previously unrecognized frequency of large magnitude deep water variability (centennial scale) with potentially high impact climatic consequences. The persistence of this variability in the past argues that we must adjust our models and concepts of deep circulation to include the possibility ocean circulation could exhibit significant variability under certain climate conditions; including those of the near future. Given that the overturning is a high impact tipping point in the climate system this suggests further work should be done to constrain the magnitude of circulation changes and their climate consequences. Any such future changes would incur serious climate consequences in the form of rapid changes in sea level, climate, drought, and the oceans ability to take up CO2. These events provide real world case examples of past abrupt change against which we can validate models in order to better estimate future risk .
Thermohaline circulation (THC) variability impacts regional climate and its potential predictability. While THC is thought to be relatively vigorous and stable during interglacial periods models forecast significant changes by 2100 A.D.. In addition, the latest highly-resolved proxy reconstructions suggest that large centennial-scale variability could have occurred in the past when conditions were warmer than present; challenging the notion of interglacial THC stability inferred from lower resolution records. We will generate the first high-resolution proxy reconstructions of ocean state variables for characterizing the (sub)centennial scale behavior of deep ocean circulation during late Pleistocene interglacials. These records spanning the most recent examples of warm(er) conditions will constrain the climate-ocean system behavior under a range of boundary conditions including many found in simulations of future climate, such as warmer/fresher surface ocean conditions, increased regional radiative forcing, reduced sea ice, and rapid Greenland Ice Sheet retreat.
Utilizing expanded sediment sequences recovered by IODP coring we will reconstruct past variations in the transport and properties of the lower branches of the THC. We focus on two key locations in order to portray centennial-scale variability in both branches of the Nordic Seas overflows (DSOW and ISOW) contributing to North Atlantic Deep Water. We will use the same sediments to reconstruct hydrographic (SST and SSS) and cryospheric (IRD) changes in the subpolar gyre and North Atlantic and determine their phasing relative to deep ocean variability. Together, these constraints will provide a new empirical benchmark allowing us to elucidate the relationship between climate and THC and to identify hydrographic thresholds or triggers for past THC variability. Finally we will use model (ESM) experiments to explore the dynamics of past interglacial ocean-climate variability and its relevance for our future.