Abstracts of Special Invited Lectures

President Invited Lecture

Speaker Lennart Bengtsson,  Max Planck Institute for Meteorology, Hamburg, Germany and Environmental Systems Science Centre, University of Reading, UK, "Is the Earth's climate changing?".

Abstract: The climate of the Earth has over the last 500.000 years undergone major climate changes. So far at least 4 major glaciation cycles have been identified from ice core measurements at Vostok, Antarctica. The glaciation periods dominate some 80% of the time, interrupted by short interglacials lasting some 20.000 years or so. Mankind has presently spent some 10.000 years in such an interglacial. During the glacial periods the amount of land ice were some 25 milj. km3 more than now covering significant areas of
the higher latitudes.
On much shorter timescales of several hundred years, the climate in certain regions at least have been reported to undergo variations. The warm episode during the early medieval time and the cold periods during 1600-1850 called the Little Ice Age in Europe, are examples of such events.
During the 20th century we can identify two periods of warming, one during the first part of the century and another during the last 25 years. A period of colder climate/weather occurred in the 1960s and 1970s. A reliable observing system making it possible to analyse the global atmosphere accurately with high time and space resolution as a matter of fact only exist for the last 25 years and for the Northern Hemisphere extra tropics for some 50 years. Surface observations are available since the late 19th century covering some 50% of the Earth and in some limited regions 50-100 further back in time. Consequently our knowledge of climate and climate change is strongly dependent on indirect information and the analysis of incomplete and partly less accurate meteorological records. There is a broad scientific agreement that the long term climate changes of
the ice ages are caused by variations in the Earth´s orbit around the sun, while the changes on centennial time scales are open to debate. Our theoretical knowledge of climate is nowadays mainly coming from the study of numerical models of the Earth climate system. Such models have undergone an enormous development in recent decades partly as a consequence of computer performance development. The models are further systematically used in weather and short term climate prediction such as for El Niño.
Model improvements and more accurate observations from satellite based systems have led to significant improvements in predictive skill in recent years, thus increasing the confidence in climate models. Based on advanced model studies the indications are that climate variations during the last thousand years or so are dominated by internal chaotic
processes in the climate system. Such internal processes can create significant climate anomalies over several decades. Possible variation in solar irradiation cannot be ruled out, but so far only indirect data exist. Major volcanic eruptions do also contribute but hardly beyond a few years. There are strong indications that the warming in the first part of the last century, which was geographically limited, was caused by natural internal
processes in the climate system. However, this appears not to be the case for the present warming, which has a different and more widespread distribution. Here the indications are that this warming is driven by the increasing concentration of greenhouse gases.
In my lecture I intend to give a broad review of our present understanding of climate, discuss results from climate models, analyse their interpretation and examine conditions for climate change during the 21st century.