Discussions of the Arctic often include references to “Polar Amplification,” defined thusly by climate scientists (wikipedia):
“Polar amplification refers to the observation that any change in the net radiation balance (for example greenhouse intensification) tends to produce a larger change in temperature near the poles than the planetary average.”
NSIDC adds in the notion of positive feedbacks and concern over “tipping points.”
“Scientists have already seen evidence that positive feedbacks are occurring in the Arctic. They call this Arctic amplification. Predicting the Arctic climate is difficult. Some of the changes in the Arctic could also have negative feedback effects, or effects that reduce the amount of warming. For example, if warm temperatures make the Arctic growing season longer, more plants can survive and take up more carbon from the air. However, most evidence suggests that the positive feedback effects outweigh the negative effects. A recent report by NOAA concluded that Arctic climate is unlikely to return to previous conditions.”
No doubt there is an amplification effect seen in Arctic air temperatures. This graph by Bob Tisdale presents the evidence:
So what is going on?
Basics of Air Parcels in the Arctic
To begin with, let’s consider the characteristics of the air parcels presenting this effect.
The central region of the Arctic is very dry. Why? Firstly because the water is frozen and releases very little water vapour into the atmosphere. And secondly because (according to the laws of physics) cold air can retain very little moisture.
Greenland has the only veritable polar ice cap in the Arctic, meaning that the climate is even harsher (10°C colder) than at the North Pole, except along the coast and in the southern part of the landmass where the Atlantic has a warming effect. The marked stability of Greenland’s climate is due to a layer of very cold air just above ground level, air that is always heavier than the upper layers of the troposphere. The result of this is a strong, gravity-driven air flow down the slopes (i.e. catabatic winds), generating gusts that can reach 200 kph at ground level.
The heating characteristics of an air parcel
Now consider that raising the temperature of dry air requires 1kJ/kg per degree C. But moist air requires 3 times as much energy, depending upon the air temperature and the saturation amount.
From the Engineering Toolbox:
Enthalpy is the measure of the total thermal energy in air (often called specific heat capacity.)
Energy content is expressed as energy per unit weight of air (Btu/lbair, J/kgair).
Air with the same amount of energy may either be drier hotter air (higher sensible heat) or cooler moister air (higher latent heat).
Moist air is a mixture of dry air and water vapor. In atmospheric air water vapor content varies from 0 to 3% by mass. The enthalpy of moist and humid air includes:
The enthalpy of the dry air – the sensible heat – and
The enthalpy of the evaporated water – the latent heat
Specific Enthalpy of Dry Air – Sensible Heat
Assuming constant pressure conditions the specific enthalpy of dry air can be expressed as:
ha = cpa where
cpa = specific heat capacity of air at constant pressure (kJ/kgC, kWs/kgK, Btu/lbF)
t = air temperature (C)
For air temperature between -100C and 100C the specific heat capacity can be set to
cpa = 1.006 (kJ/kgC)
Enthalpy of Moist Air
The enthalpy of humid air at 25C with specific moisture content x = 0.0203 kg/kg (saturation), can be calculated as:
h = (1.006 kJ/kgC) (25C) + (0.0203 kg/kg) [(1.84 kJ/kgC) (25C) + (2501 kJ/kg)]
= (25.15 kJ/kg) + [(0.93 kJ/kg) + (50.77 kJ/kg)]
= 76.9 (kJ/kg)
The same calculation for moist air at 20C gives a heat capacity of 58.2, so the 5C increase requires 18.7 kj/kg for moist air vs. 5.0 kj/kg for dry air, or a ratio of 1:3.7. Similar ratios apply at all air temperatures above 0C. Subzero air, like that in the Arctic most of the year, shows little difference in heat content between dry or saturated, since cold air doesn’t hold much water vapor.
So there is a physical explanation for why the Arctic air temperatures should warm more than the Northern Hemisphere generally. The same amount of thermal energy applied to the cold, dry air in the Arctic has 3 times the effect on temperatures than when applied to warm, moist tropical air. Not surprisingly, the observed amplified Arctic warming has been about twice the average rate of warming estimated over all latitudes of the hemisphere.
And also, when the trend is cooling the Arctic also amplifies the drop in temperatures. This further demonstrates that polar amplification is a feature of the air itself, and operates independently of CO2 radiative qualities or possible feedbacks from ice or snow extent.
This discussion of enthalpy blends into a point that Roger Pielke Sr. was making years ago. Moist and dry air at the same temperature has considerably different heat content, as demonstrated above. And since temperature is being used as a proxy for changes in heat content of the air, to average them while ignoring humidity gives misleading results, especially in places like the Arctic desert. Dr. Norman page contends that sea surface temperatures are a much better indicator of changes in heat in the global climate, since they are directly reporting enthalpy.