Climate change: warming effect of greenhouse gases 'has been overestimated', ice core study suggests
Antarctic ice has revealed that pre-industrial air pollution was worse than thought, suggesting climate models have overstated the warming from . Key to modelling the climate of the future is understanding how the rate at which surface temperatures are likely to rise in response to greenhouse gas levels. To do this, modern climate models start by looking at how temperatures responded to known changes in the past and then extrapolating from there. The problem is that while past greenhouse gas levels are well documented, they are not the only things in the atmosphere that can affect surface temperatures. Aerosols, as released by volcanoes and fires, have but, unlike greenhouse gases, their levels before the Industrial era are poorly understood. A team led from Harvard University analysed cores of Antarctic ice, trapped in which were soot particles from Africa, Australia and South America dating back to 1750. Analysing this suggested that in the southern hemisphere at least pre-industrial times were more fiery than was previously anticipated, with four times the soot. This means that, before 1780, the atmosphere was being cooled by soot more than expected with implications for atmospheric models based on this assumption. Specifically, in order to account for the observed increase in surface temperatures since then, models may have overestimated the warming from greenhouse gases. While the world is 'clearly' warming, as the team put it, the new findings suggest that it might not be heating up at quite the rate that was previously feared. 'Up till now, the magnitude of past fire activity, and thus the amount of smoke in the preindustrial atmosphere, has not been well characterized,' explained paper author and atmospheric chemist Pengfei Liu of Massachussets' Harvard University. 'These results have importance for understanding the evolution of climate change from the 1750s until today, and for predicting future climate.' In total, Dr Liu and colleagues analysed the content of 14 different ice cores, each bored up from a different location across the southernmost continent. 'Soot deposited in glacier ice directly reflects past atmospheric concentrations so well-dated ice cores provide the most reliable long-term records,' explained hydrologist Joseph McConnell of the Desert Research Institute in Nevada. The researchers were surprised to find that the pre-industrial (here defined as 17501780) soot levels were considerably higher than was long thought. 'While most studies have assumed less fire took place in the preindustrial era, the ice cores suggested a much fierier past, at least in the Southern Hemisphere,' said atmospheric chemist Loretta Mickley, also of Harvard University. To account for these surprising levels of soot, the team ran computer simulations which explored both the impact of wildfires and the burning practices of indigenous peoples in the southern hemisphere. 'The computer simulations of fire show that the atmosphere of the Southern Hemisphere could have been very smoky in the century before the Industrial Revolution,' said earth scientist Jed Kaplan of the University of Hong Kong. 'Soot concentrations in the atmosphere were up to four times greater than previous studies suggested,' he added. 'Most of this was caused by widespread and regular burning practiced by indigenous peoples in the pre-colonial period.' Indeed the smelting of copper is known to have taken place in South America from as early as 1400 BC, with the Incas smelting silver ore later that century and air pollution increasing when the conquistadors invaded the century after. Both the ice core data and the models conclude that soot levels were abundant before the industrial era and remained relatively constant across the 20th century. As land use changed and fire activity decreased emissions from industry increased instead, the models suggest. 'Climate scientists have known that the most recent generation of climate models have been over-estimating surface temperature sensitivity to greenhouse gasses, but we haven't known why or by how much,' explained Dr Liu. 'This research offers a possible explanation', he noted. The new finding, added Dr Mickley, 'allows us to refine our predictions moving forward.' 'Clearly the world is warming but the key question is how fast will it warm as greenhouse gas emissions continue to rise, she added. The full findings of the study were published in the journal .