Christoph Kull

Modeling past atmospheric CO2: Results of a challenge

The models and concepts used to predict future climate are based on physical laws and information obtained from observations of the past. New paleoclimate records are crucial for a test of our current understanding. The Vostok ice core record [Petit et al., 1999] showed that over the past 420 kyr (1 kyr = 1000 years), Antarctic climate and concentrations of the greenhouse gases carbon dioxide (CO2) and methane (CH4) were tightly coupled. In particular, CO2 seemed to be confined between bounds of about 180 ppmv (parts per million by volume) in glacial periods and 280 ppmv in interglacials; both gases rose and fell with climate as the Earth passed through four glacial/interglacial cycles.

The EPICA challenge to the Earth system modeling community

One of our major aims as Earth systems scientists is to predict how the Earth will behave in the future, particularly in the face of changes imposed upon it as a result of human activities. These predictions are made using models and concepts that are in part derived from observation of how the system has behaved in the past. However, these observations, which come from paleo-records, are also one important tool for validating the models. The imminent appearance of a new ice core data set presents a unique opportunity for a test of our understanding, particularly of the climate/carbon system. Members of the European Project for Ice Coring in Antarctica (EPICA) and others here present a challenge to the modeling communities and other interested parties. The Vostok ice core record has become an iconic data set. It presents the climate of the last 420 kyr, showing the rise and fall of Antarctic temperature through four complete glacial/interglacial cycles. The most striking finding is that CO2 and CH4, the two most significant greenhouse gases (after water vapor), also rise and fall, in a remarkably similar fashion. When Antarctic temperature is calculated including a correction for the climate of the water vapor source region, the correlation between CO2 and Antarctic temperature over the last 150 kyr has an r2 of 0.89!

Reconstruction of past Mediterranean climate

Mediterranean Climate Variability and Predictability (MEDCLIVAR; http://www.medclivar.eu) is a program that coordinates and promotes research on different aspects of Mediterranean climate. The main MEDCLIVAR goals include the reconstruction of past climate, describing patterns and mechanisms characterizing climate space-time variability, extremes at different time and space scales, coupled climate model/empirical reconstruction comparisons, seasonal forecasting, and the identification of the forcings responsible for the observed changes. The program has been endorsed by CLIVAR (Climate Variability and Predictability project) and is funded by the European Science Foundation. The main purpose of this first MEDCLIVAR workshop was to identify sources of early instrumental data and natural and documentary climate proxies that had not been previously explored and/or identified and that could be relevant for the reconstruction of the Mediterranean climate or weather extremes covering the past millennia. A key focus was on weather and climate information with high temporal (annual or higher) and spatial resolution as well as the potential to resolve past climate variability based on low-resolution proxies covering the past tens of thousands to hundreds of thousands of years.

Past millennia climate variability

Human influences on climate operate against a background of long-term natural climate variability. Our ability to characterize this long-term variability and to distinguish it from climate change due to human activities is limited by the relative shortness of the instrumental record. Thus, investigators turn to a combination of indirect paleoclimate proxy evidence and theoretical climate models to ascertain the nature and causes of climate changes on centennial and longer timescales. Particularly relevant in this context is the time frame of the last few millennia, which is termed the ‘Late Holocene.’ During this period, the fundamental external boundary conditions on the climate, such as the configuration of the continents, the size and locations of the major ice sheets, and the mean radiative forcing due to changes in Earth-orbital geometry, are similar to those today. Study of this interval thus allows insights into the natural variability that might be expected today in the absence of human influences.

Preface: Climates of Past Interglacials — a PAGES Perspective

Publisher Summary The cultural evolution of humans has accelerated considerably during the Holocene interglacial. This explosion of civilization has probably only been possible under the mild and relatively stable climatic conditions that have prevailed for the last 11,000 years. However, these conditions cannot be taken for granted. This is one of the rather simple but unequivocal and important lessons learned from the palaeoclimate record. All other interglacials terminated after a few thousands to a few tens of thousands of years. The interglacial states similar to those of today, with little land ice and largely elevated temperatures at mid-high latitudes, prevailed during no more than 15% of the last half million years. These simple empirics provide clear evidence that interglacials are rather unstable on a 10,000-year timescale. Also on shorter timescales of millennia to decades, late Holocene climate fluctuations, such as the Little Ice Age and those associated with the Maunder Minimum prove that interglacial climate is not entirely stable on a regional scale but responds to even subtle changes in radiative forcing.

Reconstruction of past Mediterranean climate: Unexplored sources of high resolution data in historic time

R. GARCÍA-HERRERA, J. LUTERBACHER, P. LIONELLO, F. GONZÁLEZ-ROUCO, P. RIBERA, X. RODÓ , C. KULL AND C. ZEREFOS Facultad de CC Físicas, Universidad Complutense, Ciudad Universitaria, Madrid, Spain; rgarciah@fi s.ucm.es NCCR Climate, University of Bern, Switzerland Department of Material Sciences, University of Lecce, Italy Universidad Pablo de Olavide, Sevilla, Spain Climate Research laboratory LRCPCB-UB, Barcelona, Catalunya (Spain) PAGES IPO, Sulgeneckstrasse 38, 3007 Bern, Switzerland 7 University of Athens, Panepistimiopolis, Zografou, 157 84, Greece