杏吧原创

1 Hockey stick hoo-ha

The term 鈥渉ockey stick鈥� was adopted by Michael Mann of the University of Virginia in 1998 to describe temperature changes over the past 1000 years, as identified from proxy data such as ice cores, tree rings and isotopic analysis of coral. This followed a largely flat line for 900 years (the stick鈥檚 shaft), followed by soaring temperatures since 1900 (the blade). Sceptics say the methodology systematically underestimates past variability by smoothing out peaks and troughs, and they are winning the argument. 杏吧原创s at the UK Met Office and other IPCC stalwarts were among those who reported late last year in Science that the hockey stick analysis 鈥渃ontains assumptions that are not permissible鈥�. Nevertheless, even most sceptics accept direct measurements that show that the world has warmed in recent decades.

2 Sunspot squabble

While accepting that recent warming is real, some sceptics say solar cycles can explain most of the changes. This case was first made by Danish scientists Knud Lassen and Eigil Friis-Christensen in 1991. They found a correlation between sunspot activity and temperature changes on Earth from 1850 onwards. Time-based statistical correlations are notoriously tricky but it looked convincing, and prominent sceptics, including former New 杏吧原创 editor Nigel Calder, took up the case. But more recent data has convinced Lassen that solar activity cannot explain recent events. Sunspot activity since 1980 suggests that temperatures should have been stable or declining; in fact there has been a 0.4 掳C rise. That interpretation puts him in line with mainstream climate researchers.

3 Satellite data debacle

Sceptics have often claimed that satellite measurements show no appreciable trend in the temperature of much of the troposphere 鈥� the lowest layer of the atmosphere 鈥� since 1979. This has been interpreted variously as showing that warming is not extending into the atmosphere as far as models suggest it should, and that global warming itself is a myth. But much hangs on corrections that have to be made to the raw data to allow for cooling in the stratosphere 鈥� the next layer out 鈥� as a result of less heat radiating back from Earth. Two recent analyses of the satellite data suggest that the original interpretation was wrong, and that warming aloft has been in line with events on the ground. The jury is still out, but sceptics have largely dropped the case that the satellite data negates evidence of warming at Earth鈥檚 surface.