Various processes, including albedo change (the change in reflectivity when ice and snow are replaced by open land or ocean), amplify the warming which is observed in the Arctic. Yet the situation is complicated. Cloud cover can change, which also affects reflectivity and can reduce or increase Arctic warming. Atmospheric patterns can likewise change, as can the state of the atmosphere in general. Overall, although we know that the Arctic is warming faster than the planet as a whole, how great this amplification will be in the future remains uncertain.
The question also arose, how will Arctic warming patterns differ at different times of year? I decided to take a look at some data, to see what differences can be identified in what’s happened to Arctic temperature recently. Hence I determined the Arctic warming rate in two satellite data sets of temperature in the lower troposphere (from RSS and UAH), and one of surface temperature (from the NCAR/NCEP reanalysis). To make the results more comparable, I used surface data (from NCAR/NCEP) only since 1980. This is about the same period covered in the satellite lower-troposphere data (since 1979).
It’s important to bear in mind that these data sets measure different things. The lower troposphere is the lower layer of earth’s atmosphere, roughly the bottom 5 km or so. Its temperature will not be the same as that of the surface, and likewise its temperature changes will not be the same. Nonetheless, examining the rates of change will give us some insight into how Arctic temperature is changing.
For each data set, I isolated the data for each month of the year and estimated the warming rate by linear regression. For the RSS data, that gives this:
There are hints, but not really any significant evidence, of slower warming during February and faster during April. Overall, however, the warming rate at different times of year is surprisingly consistent.
The UAH data are also for the lower troposphere, giving this:
Again there’s a hint of faster warming during April, and for these data of slower warming during summer months. But again, the most surprising thing (to me) is that warming is so consistent throughout the year.
It’s also clear from the satellite data that the Arctic really has warmed faster than the globe as a whole — a lot faster.
The surface temperature estimates from NCAR/NCEP give this:
There are significant differences between the trends in different months. However, it’s not what I was expecting (although perhaps I should have). The fastest warming is during autumn, peaking in October. This is probably due to the loss of sea ice — during October (when there’s much less sea ice than in the recent past) the open ocean allows heat to be transferred from ocean to atmosphere, warming the surface air considerably. It also surprised me that the warming during October is so fast, with temperature increasing at a whopping 1.5 deg.C per decade.
Comparing the results from all three data sets gives this:
All results confirm the rapid warming of the Arctic. The lower-troposphere data show little difference in warming rates throughout the year. The surface data not only show considerably faster warming than the lower-troposphere data except during summer, they also show a marked, and very strong, annual pattern, with autumn especially heating up at an alarming rate.