Northern Hemisphere Ice

Throughout Earth's history there have been significant changes in the size of the ice caps. Given the recent trend of global warming, is there evidence of a melting of the ice sheets?   Box 14.2 discussed iceberg calving in Antarctica. There is no indication that the Antarctic ice is rapidly melting.   But what about the ice amount in the Northern Hemisphere?

Using NASA's airborne laser altimeter, scientists have identified a pronounced thinning of Greenland's ice cap. The laser altimeter is an active remote sensing instrument that measures precise distances by measuring the amount of time necessary for a pulse of light to leave an emitter, hit a target, and return (Chapter 5). In this application, researchers determine changes in ice thickness through these distance measurements.   By comparing changes in ice thickness measured in 1999 to observations made earlier in the decade, NASA scientists concluded that the Greenland ice cap is melting nearly 50 gigatons of water. The thinning is most severe along the coasts, while the interior ice mass appears to thicken slightly (See figure).

Records of the annual freezing and thawing of lakes provides additional evidence of changing ice conditions in the Northern Hemisphere. For more than a century, people in various regions of the world have been observing when local lakes freeze over and when they thaw.   Over the 1846-1995 period, the lake freezing occurred an average of 8.7 days later and the ice break-up date occurred about 9.8 days earlier. So, on average, the lakes are now open approximately an additional one-half a month. Of the 39 sites analyzed, 38 showed a decrease in the amount of days the lake was frozen over.

Climate change includes combinations of many interrelated processes and feedbacks. This challenges researchers who try to distinguish human activities that affect changes from natural processes. The exact cause of the recent global warming trend is not known, but it is accompanied by changes in the Greenland ice cap and the freeze over dates of many Northern Hemisphere lakes.