Focus 3 - Themes
Although processes that, by definition, involve the whole Earth System are difficult to divide into packets, we consider the science of this focus under four themes:
Variability in the Hydrological Cycle
The global hydrological cycle is of crucial importance for terrestrial ecosystems. Moreover, by transporting large amounts of latent heat, the hydrological cycle is also involved in the large-scale transfer of energy from low to high latitudes. Observations suggest that the hydrological cycle varies on daily- to inter-decadal timescales. Specifically, the detrimental effects associated with perturbations of the hydrological cycle (drought, flooding) are of major concern with respect to future climate change.
Our current knowledge of the hydrological cycle and its sensitivity to changes in climate forcing, as well as its interactions with other components of the Earth System, is still incomplete. As a result, predictions regarding, for example, future changes in precipitation, disagree not only in magnitude but also in the sign of the anticipated change. Accordingly, there is an urgent need to better constrain the feedbacks associated with the hydrological cycle over a range of timescales.
The largest natural variation of the hydrological cycle at a global scale is associated with the monsoon systems. While the temporal evolutions of the regional monsoon systems are generally known, large uncertainties exist with respect to the sequence (phasing) of their response to global forcings. In addition, the potential interaction of interannual modes of climate variability (e.g., AMO, ENSO) and the global monsoon systems remains to be quantified.
Rapid Climate Change
The dominant climate feature of the last glacial period (70-10 ka) (and indeed of earlier ones) is the millennial-scale changes most clearly observed as Dansgaard-Oeschger (DO) events and their counterparts, the Antarctic Isotopic Maxima (AIM). These events are widely believed to be due to changes in ocean heat transport, and they lead to extremely fast (years to decades) changes in atmospheric circulation and regional climate (e.g., Steffensen et al., 2008; Brauer et al., 2008). The question has been raised whether similar changes in ocean circulation could occur under future warming scenarios, and if so what their climatic effects would be.
By studying past occurrences of rapid climatic and oceanographic change we can assess the sensitivity of the Earth’s ocean circulation system and the associated climatic and environmental responses.
> ADOM (Atmospheric circulation dynamics during the last glacial cycle: Observations and modeling)
Interglacial Climate Variability
Climate records of the mid- and late-Quaternary reveal the well-known pattern of glacial-interglacial change. Long glacial periods (with large ice sheets on the North American and Eurasian continents) are interspersed with short interglacials. For the last ~ 1 Myr, these interglacials have recurred on average every 100 kyr, with a dominant 40 kyr periodicity prior to this. The interglacials are particularly compelling periods of interest because we live in such a period. Some earlier interglacials were warmer than the present one, at least in the polar regions. Although the cause of warmth was different from the cause we expect in the next century, they still offer the possibility to discover how the Earth behaved under a range of interglacial conditions. In particular, we can investigate whether increased warmth led to loss of a significant part of either the Greenland or West Antarctic ice sheets, and whether warmer conditions favored certain feedbacks in the climate system.
> PIGS (Past Interglacials)
Paleoperspectives on Ocean Biogeochemistry
Marine biogeochemical dynamics can provide strong feedbacks to climate change. Global-scale variations in marine plankton productivity, ecosystem structures, and nutrient cycling inevitably shift carbon and nitrogen budgets within the Earth System, and therefore are effective in modulating greenhouse gas concentrations in the atmosphere. Other more immediate economic aspects of marine ecosystems relate to their role as a food source, and their importance for biodiversity and protection of coastlines. Future global change is expected to greatly affect the ocean, for example, through the uptake of CO2 and the resulting acidification, through changes in macro- and micro-nutrient inputs from land to ocean, and through global warming-induced water stratification and ocean circulation changes. This theme therefore focuses specifically on past ocean biogeochemistry.
> NICOPP (Nitrogen Cycle in the Ocean, Past and Present)
Brauer, A., Haug, G.H., Dulski, P., Sigman, D.M. and Negendank, J.F.W., 2008: An abrupt wind shift in western Europe at the onset of the Younger Dryas cold period, Nature Geoscience, 1: 520-523.
Steffensen, J.P. et al., 2008: High-Resolution Greenland Ice Core Data Show Abrupt Climate Change Happens in Few Years, Science, 321: 680-684.