Von Sebastian Lüning und Fritz Vahrenholt
As Portugal came out of its second unusually wet winter in a row, some people already fear these could be the first signs of global climate change. Can the seemingly endless rainy period be blamed on ourselves because we are driving our cars to work, heating and air-conditioning our homes, and flying on holidays or on business to Brazil? Undoubtedly the atmospheric carbon dioxide (CO2) concentration has been steadily increasing over the past 150 years. In its latest report released last September the United Nations Intergovernmental Panel on Climate Change (IPCC) warned of dire consequences should CO2-emissions not be drastically curtailed in the near future.
Among the contributors to the IPCC report were also two Portuguese academics, Dr Pedro Viterbo, Director at the Portuguese Instituto de Meteorologia, and Professor Filipe Duarte Santos of the Lisbon University’s Faculty of Sciences, both serving as review editors for the IPCC. In conjunction with the report’s launch Santos warned that Portugal would be among the European countries most vulnerable to climate change. He suggested that the country in the future will suffer from more extreme weather events like heat-waves and droughts, which in turn will lead to more forest fires and reduced agricultural output. Santos prognosis sees an overall drop in rainfall but with the threat of short bursts of torrential rains that will raise the risk of flooding. Moreover, the Lisbon-based professor expects sea level to rise by more than half a metre before the end of this century, which would put two thirds of Portugal’s coastline at risk for a loss of terrain.
But is it really so? Checking the facts
Are these scary IPCC future scenarios really justified? This is an important question, especially as the European Union has decided to spend at least 20 percent of its entire 2014-2020 European Union budget on climate-related projects and policies – money that is already lacking in other fields. It is clear that the global temperature has risen by nearly one degree since 1850. A similar amount of warming has occurred also in Portugal, as evidenced by historic temperature measurements and geological investigations in the Tejo delta area.
What is interesting, however, is that there has been no warming in Portugal over the last 19 years. This corresponds with the global situation, which has not warmed in the past 16 years, a situation that all of the IPCC’s highly praised climate models failed to foresee. Scientists have been taken by surprise and are now nervously discussing what might have gone wrong in their models. A first explanation emerging is that important 60-year natural ocean cycles apparently had been overlooked. Historical temperature data recorded by weather stations in Lisbon and Coimbra during the last 140 years confirm these stunning cycles. Hardly known today is the fact that around 1950 temperatures in Portugal were as warm over a ten-year period as they are today. And 60 years before that, during the late 19th century, another warm peak had occurred in Portugal, though temperatures were not quite as high as modern levels. Strangely many high temperatures recorded at many places around the world during the 1940s and 1950s have been “corrected” downwards recently by official climate agencies such as NASA’s Goddard Institute for Space Studies (GISS). Whether these data alterations are justified is today the subject of heated debate among some climate scientists.
How stable was the climate before CO2 levels rose?
To better understand the context of global warming since the industrial period started in 1850, it is also important to study pre-industrial temperature development when atmospheric CO2 concentrations were low and fairly stable at about 280 ppm. Obviously any change in climate during those pre-industrial times would essentially have to be owing to natural factors. Geological studies from all over the world have documented the occurrence of significant climate fluctuations during the last 10,000 years. Warm phases and cold phases alternated about every thousand years, often in sync with changes in solar activity. For example 1000 years ago, during the Islamic Period in Portugal, average global temperatures were at or above the present-day level. This period is referred to as the Medieval Warm Period and coincides with a phase of high solar activity.
A research team led by Fatima Abrantes from the Laboratório Nacional de Energia e Geologia (LNEG) investigated this period and examined a sediment core they had drilled out of the Tagus delta. In their report published in 2005 in the Quaternary Science Reviews journal, the scientists document temperatures that were on average more than half a degree warmer than today for the Lisbon area. The water discharge of the Tagus during this time was less.
Figure: Temperature development of the Tagus delta near Lisbon over the past 2000 years. From Abrantes et al. 2005.
A few years later Fatima Abrantes and her team expanded their studies to the Porto area. Not surprisingly the data confirmed the existence of the Medieval Warm Period also at this location. In their study the researchers recorded persistently elevated temperatures between AD 960-1300 and documented climatic conditions for northern Portugal that were at times more than one degree warmer than today. Studies in Spain show similar results.