Where climate is made in a greenhouse world

New scientific results for the Late Cretaceous greenhouse indicate radically different climatic mechanisms operating about 75-90 million years ago compared to the ones that control today’s climate. The study, published on 29 May 2006 in “Palaeogeography, Palaeoclimatology, Palaeoecology” as part of a special issue on “Causes and Consequence of Marine Organic Carbon Burial Through Time” by Sascha Floegel from the IFM-GEOMAR in Kiel/Germany and Thomas Wagner from the University of Newcastle upon Tyne/UK aims to identify the main ‘climate kitchen’ in a world with about 5-9°C warmer global temperatures than today.

The researchers focus their interest on the causal relationships and feedbacks between the tropics and higher latitudes. Using marine geological records and data from global paleoclimate simulations they identify a previously unrecognized link between higher latitude climate dynamics and tropical African climate, the latter leading to exceptionally high burial of organic carbon in the deep tropical Atlantic. Marine geological record show that enhanced burial of organic carbon in the deep sea was confined to short time envelops of about 5 thousand years that reoccurred over millions of years at a regular pattern (see Beckmann and co-workers, published 8 September 2005 in Nature 437).

Climate modelling is one key technique to identify and understand the larger-scale mechanisms that result in geological evidence. By varying one of Earth’s orbital parameters, the precession of the equinoxes, the modelling setup used in this study provides new insights to the dynamics of global climate during past greenhouse conditions. Accordingly, changes in the amount of energy approaching the top of the atmosphere, called “insolation”, finally triggered cyclic variations of the tropical water cycle in tropical Africa. Periods of enhanced precipitation and freshwater runoff then resulted in massive burial of organic carbon at the sea floor suggesting that processes in the atmosphere drive changes in the ocean. The remaining, fundamental question on the source area(s) where cyclic fluctuations in tropical water cycling and marine carbon burial were triggered was addressed using global climate simulation.

Applying four different orbital configurations of one complete precession cycle the model identifies cross-latitudinal variations of atmospheric pressure systems, fluctuations in the magnitude and direction of surface winds, and associated precipitation and runoff patterns. Previously unrecognized, the model identifies the strongest variations in atmospheric pressure above the South Atlantic at mid-southern latitudes between 25–55°S. Establishment of an atmospheric teleconnection between this area and tropical Africa, however, is limited to one specific orbital configuration, which lasted for about 5 thousand years and caused strongest climate contrasts in a seasonal cycle.

These new results challenge current notions on role of the tropics as main driver of Cretaceous climate. They rather support the conclusion that tropical climate in a greenhouse world is ultimately triggered by climate change at mid-southern latitudes, with precipitation and river discharge being the transport mechanisms.

Today the tropics control a big fraction of Earth’s climate. The new findings reported here suggest that the mid-latitudes will have a much stronger impact on low latitude climate system at predicted future levels of atmospheric CO2. This conclusion has severe consequences for the future low latitude water cycle and associated nutrient and carbon fluxes to coastal areas. The latter fluxes from the continent strongly influence surface ocean productivity, O2 consumption in the water column and thus marine ecosystems, and many other processes affecting the global carbon balance. The broader implications support substantial interaction between the water cycle and atmospheric circulation on regional and hemispheric scales during times of global warmth. As evident from this study we probably still do not realise all the relevant processes that drive future global warming. Knowing them, however, is critical to get prepared and mitigate the effects for society and ecosystems.