Global climate change: Did we pass a tipping point?

Abstract

Systems that have an internal feedback mechanism can principally behave non-linear. The climate on earth is such a complex system with internal feedback and a non-linear characteristic should therefore be expected. Nevertheless it is common practice to publish linear trends from many global data sets without considering their relevance. The classical example is the often cited (IPCC) global warming trend of 0.05°C/decade for air temperature during the last 150 years. The inadequacy of this linear trend model over such a long period is obvious when looking at the difference of the observed 2009 mean temperature to that estimated from the calculated linear trend. The trend calculation gives an underestimation of about 48% (~0.36°C to low, compared to a total change of 0.75°C). A similar inspection of several global atmospheric and oceanographic data time series provides reasonable doubt concerning the validity of the linear model for century long time series. Using a test for breakpoints we can show that most of the analyzed global climate time series contain statistical significant structural changes (breaks in the mean or in the slope of a linear regression). We will reveal the existence of breakpoints for most investigated parameters at the end of the 70ties of the last century. Breakpoints detected at a comparable time in many different regional and global climate variables are a strong indication for a corresponding regime shift in the state of the global climate. The enormous difference between trends calculated with and without consideration of breakpoints invalidates predictions based only on overall linear trends. Despite these statistical indications for the existence of a tipping point we cannot infer the underlying process dynamics from the statistics alone. The final confirmation has to come from an accurate and complete geophysical description of the climate system, a difficult and complex task still to be done.