Gifford Miller and Jim White are two of six lead authors on a new synthesis of past Arctic climates and corresponding implications for future climate change. More than 30 scientists contributed to the report, Synthesis and Assessment Product 1.2: Past Climate Variability and Change in the Arctic and at High Latitudes, including John Andrews, Anne Jennings, and INSTAAR affiliates Lesleigh Anderson, Scott Elias, and Dan Muhs. Led by the US Geological Survey, the team found that arctic amplification--the process by which the high northern latitudes respond more intensely to hemispheric or global climate changes--has been a pervasive feature of the Earth's climate system for at least the last four million years, during both warming times and cooling times. Thus, the current enhanced temperature increase in the Arctic is likely to continue into the future with further glacier and ice-sheet melting, sea-level rise, coastal erosion, and loss of summertime sea ice.
The size and speed of the summer sea-ice loss over the last few decades is highly unusual compared to events from previous thousands of years, especially considering that changes in Earth's orbit over this time have made sea-ice melting less, not more, likely. Sustained warming of at least a few degrees above average 20th century values is likely to cause the nearly complete, eventual disappearance of the Greenland ice sheet, which would raise sea level by several meters. The current rate of human-influenced Arctic warming is comparable to peak natural rates documented by reconstructions of past climates. However, some projections of future human-induced change exceed documented natural variability. When thresholds in the climate system were crossed in the past, climate change was often very large and very fast. Similar events cannot be ruled out with human-induced climate change in the future.
The four main chapters of the report are being re-written for scientists and will be published in Fall 2009 by Quaternary Science Reviews.