Global warming and Northern Hemisphere sea ice extent

Surface and satellite-based observations show a decrease in Northern Hemisp here sea ice extent during the past 45 years. A comparison of these trends to control and transient integrations (forced by observed greenhouse gases and tropospheric sulfate aerosols) from the Geophysical Fluid Dynamics Labo ratory and Hadley Centre climate models reveals that the observed decrease in Northern Hemisphere sea ice extent agrees with the transient simulations , and both trends are much Larger than would be expected from natural clima te variations. From Long-term control runs of climate models, it was found that the probability of the observed trends resulting from natural climate variability, assuming that the models' natural variability is similar to th at found in nature, is Less than 2 percent for the 1978-98 sea ice trends a nd Less than 0.1 percent for the 1953-98 sea ice trends. Both models used h ere project continued decreases in sea ice thickness and extent throughout the next century.