Abstract
In this paper evidence of anthropogenic influence over the warming of the 20th century is presented and the debate regarding the time-series properties of global temperatures is addressed in depth. The 20th century global temperature simulations produced for the Intergovernmental Panel on Climate Change’s Fourth Assessment Report and a set of the radiative forcing series used to drive them are analyzed using modern econometric techniques. Results show that both temperatures and radiative forcing series share similar time-series properties and a common nonlinear secular movement. This long-term co-movement is characterized by the existence of time-ordered breaks in the slope of their trend functions. The evidence presented in this paper suggests that while natural forcing factors may help explain the warming of the first part of the century, anthropogenic forcing has been its main driver since the 1970’s. In terms of Article 2 of the United Nations Framework Convention on Climate Change, significant anthropogenic interference with the climate system has already occurred and the current climate models are capable of accurately simulating the response of the climate system, even if it consists in a rapid or abrupt change, to changes in external forcing factors. This paper presents a new methodological approach for conducting time-series based attribution studies.
Highlights
For more than two decades a debate regarding the time-series properties of global and hemispheric temperatures has taken place in the climate change literature (e.g., [1,2,3,4,5]), and it has hardly been settled at the present time [6,7,8,9,10]
This paper analyzes the time-series properties of several General Circulation Models (GCM) runs of the 20th Century Climate Experiment (20c3m) conducted for the Intergovernmental Panel on Climate Change’s (IPCC) Fourth Assessment Report (AR4) and a set of the radiative forcing series used to drive the 20c3m simulations to investigate four main issues: 1) Can the nonstationarities in global temperatures be tracked to the anthropogenic radiative forcing? Analyzing the timeseries properties of climate models simulations offers the advantage of knowing the experimental design from which they were generated, facilitating the detection and attribution of the nonstationarities present in temperature data
Similar findings have been reported for observed global and hemispheric temperatures as well as for radiative forcing series using these tests (e.g., [4,5,6]). From these results it could be erroneously concluded that both global temperatures and radiative forcing are integrated processes and that cointegration techniques would be adequate for investigating their long-run relationships
Summary
For more than two decades a debate regarding the time-series properties of global and hemispheric temperatures has taken place in the climate change literature (e.g., [1,2,3,4,5]), and it has hardly been settled at the present time [6,7,8,9,10]. Analyzing the timeseries properties of climate models simulations offers the advantage of knowing the experimental design from which they were generated, facilitating the detection and attribution of the nonstationarities present in temperature data. One approach for testing whether or not a unit root representation is a valid assumption for global temperatures in terms of the climate physics is to analyze the time-series properties of GCM simulations. This could be considered as another characteristic to evaluate GCM performance for reproducing current climate and their ability for representing the ‘‘climate change forcing signal’’. To answer these questions, we present a new methodological approach based on recent advances in econometric methods that provides an alternative to the cointegration approach commonly used for attribution studies. The proposed methodological approach is broad enough to have wide applicability in the analysis of trending variables and their long-term relationships in climate research
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