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. So why is the NH different and why?
The SH is dominated by oceans and the coverage before 1900 was very poor. The SH is also sparsely populated having less human impact on the environment than the NH. Clearly the NH effect is real and shows that the seasonal variation in temperature between summer and winter has reduced for some reason since the 19th century. This reduction is mostly due to an increase in minimum winter temperatures rather than maximum summer temperatures.
This story is reminiscient of the observed reduction in the Diurnal Temperature Range (DTR). Several papers have shown that most ‘global warming’ has occured through an increase in minimum night-time temperatures rather than any increase in maximum daytime temperatures. Could there be an anthropogenic cause for both effects?
Perhaps an increase in winter clouds reduces winter heat loss, thereby reducing extreme cold temperatures. Could perhaps coal burning for indudstry and domestic heating prior to 1960 be partly responsible?
Is the slow increase in solar activity over the 20th century to blame? An increase in solar activity leads to a higher flux of UV producing more stratospheric ozone. This warms the stratosphere transfering heat to the dark polar winter as the lapse rate collapses. Could a new Maunder like solar minima bring back freezing winters to the NH?
Any other ideas?