Heat wave events, once rare, are becoming more frequent due to global warming, says NASA scientist James Hansen. Such summertime heat events covered only 1% of Earth's surface for most of the time since humanity has been studying the climate. Now, when they occur, they cover closer to 10% of the global land area.
"We conclude that extreme heat waves, such as that in Texas and Oklahoma in 2011 and Moscow in 2010, were 'caused' by global warming, because their likelihood was negligible prior to the recent rapid global warming," Hansen writes in a new paper, "Perceptions of Climate Change: The New Climate Dice," to be published in the Proceedings of the National Academy of Sciences.
This summer saw another major and anomalous climate related event; over the course four days, from July 8 to July 12, the amount of melting occurring over the surface of Greenland went from a seasonally normal 40% to an abnormal 97%, according to NASA scientists working with satellite data.
Massive melting was even observed at the Summit Central station located at the highest and coldest point of the Greenland ice sheet, nearly two miles above the sea level. Though extremely unusual, the melting phenomenon is not without precedent. A corollary event occurred in 1889, and ice-core samples suggest that Greenland melting seems to occur every 150 years.
However, scientists say that this year's event was likely caused by extremely unusual high pressure systems, or heat ridges, which have been moving over Greenland since May. "Each successive ridge has been stronger than the previous one," says University of Georgia climatologist Thomas Mote. This latest heat ridge made its way over the central part of Greenland on July 8 and stayed there until about July 16. Scientists believe that the ice will regrow, but the new ice, at least at first, will not be as thick as the ice that melted.
Scientists believe that the Arctic is heating twice to four times as fast as the rest of the globe. The bigger heat ridges in the Arctic may be linked to changes in the jet stream, as a result of manmade global warming.
Sources: NASA; an early draft of the paper by James Hansen et al. is available from Columbia University,Download PDF