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If you examine the detailed temperature records that form the basis for our understanding of global warming, there are some strange anomalies (at least from a naive point of view where warming affects everywhere equally).

For example, by far the largest temperature increases are confined to winter months in the northern hemisphere. As one commentator has it:

Winter months in the Northern Hemisphere are responsible for most of the observed land warming in CRUTEM4. The summer months (June, July, August, September) essentially show no warming at all from 1850 to 1990. Overall, NH winters have warmed about twice as much as NH summers.

...The same exercise for the southern hemisphere shows very little difference where of course the seasonal months are inverted.

Is there a sensible explanation of this effect? Is it reproduced, for example, in climate models?

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Energy balance in the winter is highly dependent on the total amount of area covered by snow or ice. Snow increases the albedo of the Earth which directs the short-wave radiation from the Sun back to space, stopping it from being absorbed by the Earth. Winters with minimal snow/ice cover effectively allow more solar radiation to be absorbed by the Earth. Depending on the winter, the amount of snow cover can be quite different from year to year, which can cause great variability in temperature. In contrast, summer months generally have more stable temperatures because snow cover is less dynamic during those months, and there is roughly the same total solar radiation absorbed from year to year.

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  • $\begingroup$ Is this effect modelled by any global climate models? $\endgroup$ – matt_black Sep 21 '15 at 9:39
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The rate of heat transfer increases with the difference in temperature between two objects. The difference in temperature between the equatorial and other regions is greatest in winter (because of the geometry between the earth and the sun). So it would be surprising only if the increase in local temperatures weren't biggest during local winter.

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  • $\begingroup$ So why is this a northern hemisphere only effect? And does your hypothesis imply that cold places should warm faster than warm ones when the whole world is warming up from increased greenhouse effects? Is that what climate models predict? $\endgroup$ – matt_black Sep 21 '15 at 9:39

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