Research supports global warming theory
A study of global temperatures over the past 2,000 years has lent fresh weight to the so-called hockey stick graph which suggests that humans caused global warming. The graph, first published in the late 1990s by US palaeoclimatologist Professor Michael Mann and colleagues, shows temperatures stayed roughly flat for about 900 years, like the handle of the hockey stick laid down, before rising sharply upwards in the 20th century, like the blade, after the industrial revolution prompted a rise in fossil fuel emissions.
The image has become a potent symbol of global warming, especially after the Intergovernmental Panel on Climate Change, a UN body that delivers the most comprehensive scientific assessments of climate change, included it in a major report in 2001.
But the chart’s validity has been attacked by climate sceptics. Some say the graph played down, or even omitted, data showing that the climate changed without human intervention in the past.
Doubters often cite events such as the so-called “medieval warm period” from about AD900 to 1250, when some say vineyards flourished in northern England and other now chilly regions. They say this shows natural variability causes warming, not greenhouse gas emissions produced when humans burn coal or oil for energy.
A number of studies have challenged such theories. Now a paper by 78 researchers from 24 countries, published on Sunday, seems to offer more confirmation of human impact on global climate, based on what its authors said was the most comprehensive reconstruction of past temperature changes at the continental scale.
It shows an overall cooling trend across nearly all continents over the past 1,000-2,000 years that was reversed by what the authors described as “distinct warming” at the end of the 19th century.
“This pre-industrial cooling trend was likely caused by natural factors that continued to operate through the 20th century, making the 20th century warming more difficult to explain without the likely impact of increased greenhouse gases,” the authors said.
“The temperature averaged across the seven continental-scale regions indicates that 1971-2000 was warmer than any time in nearly 1,400 years.”
The only continent that bucked the warming trend was Antarctica. Earlier events such as the medieval warm period do stand out but not in a uniformly global pattern, said co-author Professor Heinz Wanner of the University of Bern.
In other words, the hockey stick “handle” was getting “some bumps”, he said, “but the main form of the hockey stick will remain”.
Professor Mann, co-author of the hockey stick graph, said the study added to growing body scientific evidence that the most recent warming is probably unprecedented for a period even further back in time than the 1,000 years covered by his contentious diagram.
“While the study doesn’t attribute causality to the warming, there is an extensive body of other research that shows that we can only explain the anomalous recent warming with human impacts – ie the burning of fossil fuels and resulting increase in greenhouse gas concentrations,” he said.
Another co-author of the new study, Professor Darrell Kaufman of Northern Arizona University, said the research, nearly seven years in the making, did not set out to look at whether or not humans were changing the climate.
Rather, it used data from 511 locations across seven continent-sized regions to try to enhance understanding of how global changes were translated in different parts of the world. The paper’s authors hope the data they have assembled will be used by other scientists to improve knowledge of phenomena such as the monsoon season.
They warn that their findings contain uncertainties because, like all studies of temperatures going back centuries, they could not rely on readings from thermometers or other instruments. Instead, they had to reconstruct temperatures from evidence in tree rings, corals, ice cores and other indicators of past climate, a common practice for such research.
Fonte: ft.com
