Over the 4.6 billion years of its evolution, the Earth has seen a large variety of climate states, which in their extremes have ranged from either the snowball Earth or hot house. Extended periods of an intermediate climate with only moderate changes have also been deduced from proxy data. However, climate variability is the rule and not the exception and the evolution of life on Earth was closely linked to climate and its change (Kasting and Catling 2003).
For about 250 years, mankind has interfered stronger with the climate system in the sense that, in addition to land use changes, radiative active gases and particles have been released in substantial amounts into the atmosphere. Over this period especially, the atmospheric concentrations of the greenhouse gases CO2 and methane have risen significantly and account for an additional radiative forcing of more than 2 W m-2 (IPCC 2007a). A global mean temperature increase can be deduced from instrumental observations for the same time span. The recent warming recorded in the instrumental observations over the last decade exceeds the temperatures (including the uncertainty ranges) of at least the past 1,600 years, as reconstructed for the Northern hemisphere from an expanded set of proxy data (Mann et al. 2008). Climate model projections driven by greenhouse gas and particle emissions according to a set of socio-economic scenarios emphasise a further increase in global temperature for the present century and beyond (IPCC 2007a). These projections encouraged some scientists to speak of a dangerous interference with the climate system, which could place the Earth perilously close to dramatic climate change that could run out of our control, resulting in great dangers for humans, other creatures and ecosystems (Hansen et al. 2007a,b). Based on the likelihood of strong El Niños in the future, Hansen et al. 2006 argue that a global warming of more than about 1°C, relative to the year 2000, would constitute a "dangerous" climate change for some regions, as judged from the likely effects on sea level and extermination of species. Although the term "dangerous interference" holds some subjective notion, there does appear to be a widely acknowledged awareness that, besides emission mitigation actions, adaptation measures have to be implemented.
Even if there have been hotter times in the deep history of our planet, there are two aspects which enhance the relevance of the current change in temperature and other climate parameters. Firstly, there are many more people than ever which live in regions of the world which are vulnerable to climate impacts. Secondly, the pace of the temperature increase in relation to the amplitude is almost unprecedented in the analysed climate records and will lead to a geographical shift in climate conditions, which may make it difficult for some ecosystems to adapt to or for some species to migrate in time (IPCC 2007b).
There are a few climate elements which may be sensitive to sudden, abrupt changes when the boundary of a set of conditioning parameters is overstepped or when certain thresholds are passed. Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past (Alley et al. 2003). An often cited example of an abrupt change is that of a possible collapse of the thermohaline circulation originating in the North Atlantic.
The discussion on abrupt changes has, generally, only considered the implications from an economic and ecological points of view; however, Hulme (2003) also discusses the implications from a more social and behavioural science perspective. Although abrupt changes might be of enhanced relevance for the global society, a detailed knowledge of the geophysical processes involved is not complete and a scientific consensus regarding the overall relevance and expected timing has not yet been fully reached. Therefore, extending our research into the area of abrupt climate change is urgently needed. Lenton et al. (2008) define tipping points in the climate system and report transition times of between 10 and 100 years. Among the discussed tipping elements are those of the thermohaline circulation, the Arctic sea-ice, the Greenland and West Antarctic ice sheets, the Indian summer monsoon, the Sahara/Sahel, West African monsoon and the Amazon rainforest (not the complete list). Lenton et al. (2008) also note that society may be lulled into a false sense of security by smooth projections of global change. According to their study, a synthesis of our present knowledge suggests that a variety of tipping elements could reach their critical point within this century. In their more general analysis, Steffen et al (2007) conclude with the statement: "Whatever unfolds, the next few decades will surely be a tipping point in the evolution of the Anthropocene".
Acknowledgement The author thanks Dr. Jan Habel and Professor Thorsten Assmann for having given the opportunity to contribute to the symposium on relict species in Luxembourg. Prof. Raymond Pierrehumbert, University of Chicago, USA, Prof. Leo Donner, Princeton University, USA, and Dr. Victor Brovkin, MPI for Meteorology, Hamburg, Germany, provided yet unpublished material, thank you all for the kindness. Valuable comments on the manuscript provided by Dr. Desmond Murphy are gratefully acknowledged. A part of the work is based on lectures on environmental meteorology and climate physics by the author at the University of Lüneburg, Germany.
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