- What was the last interglacial? The last interglacial was the last time Earth's climate was generally comparable to our preindustrial climate (around 1750 CE): CO2 was about 280 ppm and there were no large ice sheets covering North America. The last interglacial lasted from about 129,000 years ago, when the Earth came out of the second-to-last ice age (called the penultimate glacial maximum or Marine Isotope Stage 6), until about 116,000 years ago, when the Earth began to descend into the most recent ice age (culminating in what is called the last glacial maximum).
- Why is the last interglacial important to me? Many lines of evidence point to global sea level being at least ~20 feet higher than present during the last interglacial (Dutton et al., 2015), but global mean temperatures appear to be about the same as today (McKay et al., 2011). This relationship suggests that Earth's climate experienced a series self-amplifying feedbacks to generate a higher global mean sea level. Can these feedbacks contribute to higher sea levels than we've predicted for future climate change? Tens of millions of people live within ~20 feet of sea level today, so figuring this out is an important contribution to our knowledge of past and future climate changes.
- So what did I do? Using a global database of climate proxies (or natural thermometers left behind by the Earth for us to figure out), I reevaluated global and regional patterns of sea surface temperature during the last interglacial. According to my (yet to be published) results, the global ocean during the last interglacial reached temperatures about 1°C warmer than the preindustrial ocean. This result is significant because human-caused climate change has already warmed the ocean about 0.85°C since 1880 (IPCC, 2013). Basically, we're approaching ocean temperatures not seen by Earth for almost 130,000 years! If the ocean played a role in generating the last interglacial sea-level high stand as is hypothesized by some scientists (see the Dutton et al. paper cited above), we might be committing ourselves to around ~20 feet of sea-level rise in the next century. This risk should play a role in planning for adaptation strategies for coastal communities and cities.
Hoffman, Jeremy S., *Brown, Z.E.I., Clark, P.U., Mix, A.C., Haley, B.A., *de Jesus, E., Andrews, J.T., and He, F. (in prep), Deglacial subsurface warming at intermediate depths of the North Atlantic and its relation to Heinrich event 1.
*denotes mentored undergraduate research assistants
- What are Heinrich events? Few discoveries have galvanized the paleoclimate research community more so than Heinrich events. Heinrich (1988) related rapidly deposited, quasi-periodic or repeated layers of coarse sands found in ocean sediment cores from the North Atlantic ocean to iceberg discharge events from the major ice sheets during the last ice age. Broecker et al. (1992) coined the term "Heinrich event," and interpreted the layers as being created by "armadas of icebergs" originating from the Laurentide Ice Sheet, a now extinct mountain of ice that sat on top of North America and extended as far south as Des Moines, IA, during the last ice age. Heinrich events thus represent a striking example of a spectacular Earth process where ice sheets that connect to the ocean rapidly disintegrate and produce icebergs at an almost unbelievable scale. What remains to be determined about Heinrich events, however, is exactly how and why they occur.
- Why are Heinrich events important to me? Figuring out the cause of Heinrich events can play a role in how we predict the rates of future sea-level rise, which will redefine the Earth's coastlines. One leading hypothesis for the origin of Heinrich events involves how ocean temperatures interact with and change the stability of the ice itself - and has an analogy with the present-day West Antarctic Ice Sheet. When glaciers flow into the ocean, part of them stays frozen to the seafloor, and the rest extends out over the surface of the ocean like a giant floating ice tongue. These are called ice shelves. If ocean temperatures get warm enough at the place where the ice is still frozen to the seafloor, it sets off a chain reaction that causes the ice shelf to break up catastrophically and any ice behind it to surge into the ocean. Some modern-day ice shelf breakup events in West Antarctica (for example, see the Larsen B ice shelf collapse) can be partially attributed to this process. If Heinrich events are attributable to rising ocean temperatures and their destabilizing effect on ice shelves, how will future warming of the ocean affect similar ice shelves in West Antarctica?
- So what did I do? With the help of two undergraduate research assistants, I explored how a climate proxy of ocean temperature changed over time in two sediment cores from nearby the Hudson Strait outlet, right in the pathway that icebergs would have taken during a Heinrich event. Climate models also predict that these ocean sediment core locations would capture any ocean temperature warming that would affect the frozen base of the ice shelf that formed prior to Heinrich events. Preliminary results suggest that indeed, ocean temperatures increased prior to Heinrich events. These results suggest that continued ocean warming around the base of West Antarctic ice shelves could lead to more catastrophic disintegration events. The scientific community should continue efforts to monitor the ocean temperatures around these floating ice shelves in order to help refine predictions of future sea-level rise.
Hoffman, Jeremy S., Andrews, J.T., Clark, P.U. (2016) Characterizing Late Quaternary Labrador Sea sediments using qXRD and principal component analysis (in prep).
Hoffman, Jeremy S., Carlson, A.E., Winsor, K.,Klinkhammer, G.P., LeGrande, A.N., Andrews, J.T., Strasser, J.C. (2012), Linking the 8.2 ka event with its freshwater forcing in the Labrador Sea, Geophysical Research Letters 39, L18703, doi:10.1029/2012GL053047.
- What was the 8.2 ka event? The 8.2 ka event (8.2 ka = 8,200 years ago) is the most significant large-scale climate change event recorded in Greenland ice cores during the otherwise relatively stable Holocene (the last ~11,000 years). The effects of the climate cooling during the 8.2 event were varied around the Northern hemisphere, including reductions in the Asian, African, and Indian monsoon cycles, loss of precipitation in northern Brazil, and advances of Norwegian glaciers (Rohling and Palike, 2005). These effects are all consistent with an abrupt shift to colder temperatures in the Northern hemisphere. The leading hypothesis for the cause of the 8.2 ka event suggest that the sudden drainage of glacial Lake Agassiz into the Labrador Sea and North Atlantic ocean caused a slowdown in the world's oceanic heat regulator, the Atlantic Meridional Overturning Circulation (AMOC). Some estimates (Li et al., 2012) suggest that this lake drainage contributed as much as ~2 meters (almost 7 feet) of sea-level rise. However, evidence from climate proxies was inconclusive about whether or not the cold & fresh waters of Lake Agassiz actually entered the relatively warmer & salty Labrador Sea - until now.
- Why is the 8.2 ka event important to me? Rapid shifts in climate (on the order of years to decades), termed "abrupt climate change" (National Research Council, 2002) would greatly outpace society's ability to effectively adapt to them. Knowing how such Earth processes work can provide us with "worst-case scenario" estimates of future climate change. Is it likely that in the future a lake the size of Lake Agassiz will form and drain rapidly into the North Atlantic ocean, causing a similar cooling to the 8.2 ka event? Not very. Is there a chance that the cold & fresh water flowing off of the melting Greenland Ice Sheet could disrupt the AMOC in the future? Yes, but it depends on how quickly and how much our global temperature rises.
- So what did I do? Using a climate proxy for surface ocean temperature in the shells of plankton (foraminifera), we show that the fingerprint of the Lake Agassiz drainage was obscured in previous studies by the competing effects of temperature and salinity on the isotopes (or, "flavors") of oxygen in these plankton shells. When plankton form their shells they lock in the chemistry of the ocean in which they are living, including the oxygen isotope ratio of the water. Cold water usually has more of the heavy oxygen isotope, oxygen-18, in it. Melting glacial ice has more of the light oxygen isotope, oxygen-16, in it. Thus, the cold & fresh water in the Lake Agassiz drainage would have been "stealth" chemistry for the plankton shells. By isolating one of the variables in that equation (temperature) we were able to show that surface ocean cooling of around 3°C masked the fingerprint of the glacial Lake Agassiz water in the ocean.