>This post shows, based on GISS LOTI data, there is nothing unusual about the Polar Amplification taking place during the current warming period and also shows Polar Amplification exaggerates the cooling during periods when global temperatures decline.
This post is a follow-up to my post Can Most Of The Rise In The Satellite-Era Surface Temperatures Be Explained Without Anthropogenic Greenhouse Gases?
I used GISS Land-Ocean Temperature Index (LOTI) data for the latitudes of 60S-60N in my post Can Most Of The Rise In The Satellite-Era Surface Temperatures Be Explained Without Anthropogenic Greenhouse Gases? Even though I explained why I had excluded the polar data, the fact that I had deleted it was not well received by some around the blogosphere. Why did I exclude the polar data?
First, GISS Deletes Arctic And Southern Ocean Sea Surface Temperature Data. GISS then extends land surface data out over the Arctic and Southern Oceans. Since land surface temperatures warm more than sea surface temperatures during warming periods (and cool more during cooling periods), replacing sea surface temperature data with land-based data as GISS does creates a bias during seasons of sea ice melt.
Second: As I wrote in that post, Keep in mind that the Arctic is amplifying the effects of the rise in temperature at lower latitudes. This is the basis of the concept of polar amplification. If the vast majority of the change in temperature at the lower latitudes is natural, the same would hold true for the Arctic.
My simple explanation of Polar Amplification received complaints as well, but it does not differ significantly from other definitions.
For example, the Wikipedia definition of Polar amplification : “Polar amplification is defined by International Arctic Science Committee on page 23 of the Arctic Climate Impact Assessment ‘Polar amplification (greater temperature increases in the Arctic compared to the earth as a whole) is a result of the collective effect of these feedbacks and other processes.’ It does not apply to the Antarctic, because the Southern Ocean acts as a heat sink. It is common to see it stated that ‘Climate models generally predict amplified warming in polar regions’, e.g. Doran et al. However, climate models predict amplified warming for the Arctic but only modest warming for Antarctica.”
So let’s take a look at the GISS data and see if my description of Polar Amplification is confirmed or contradicted by it.
THE GISS LOTI DATA INDICATES POLAR AMPLIFICATION EXAGGERATES TEMPERATURE INCREASES AND DECREASES
Figure 1 shows Annual (January to December) GISS LOTI data (1880 to 2010) that has been detrended. (Source: Global-mean monthly, seasonal, and annual means webpage.) Figure 1 serves as a reference for the multidecadal periods of warming and cooling used in this post. Based on the detrended annual GISS LOTI data, the recent warming period began when global temperature anomalies rose from their minimum in 1976. The cooling era runs from its maximum in 1944 to the 1976 minimum. And the early 20th Century warming period starts at the minimum in 1917 and ends in 1944. (Later, I explain why I used detrended data in Figure 1.)
The GISS website allows users to create two different types of maps at their Global Maps webpage:(1) global temperature anomalies, and (2) “trends,” which are the changes in surface temperature over user-defined periods (based on local linear trends).
GISS also creates “Zone Mean” data in their Surface Temperature Analysis output page. If you were to scroll down to the plot below the GISS map, you’d find a graph with “Latitude” as the x-axis and “Zonal Mean” as the y-axis. When “trend” maps are created using the GISS map-making feature, GISS calculates the average surface temperature change for every 2-degree latitude band from pole to pole, where the temperature change data is based on the local linear trends for the period selected. The data for those graphs is also available toward the bottom of the page.
Figure 2 shows the average Change in Annual Surface Temperature Anomalies per latitude band for the periods of 1944 to 1976 (the 20th Century cooling period) and 1976 to 2010 (the current warming period), according to the GISS data. Arctic temperatures dropped more than the rest of the hemisphere or globe during the cooling period. The reverse holds true during the current warming period. Figure 2 illustrates that, since the early 1940s, Polar Amplification exaggerates the multidecadal change in global temperature when temperatures rise and when they cool.
THE DATA FOR THE EARLIER WARMING PERIOD REVEALS THE CURRENT ARCTIC AMPIFICATION IS NOT UNUSUAL
Referring to Figure 1 again, Global Temperatures warmed from 1917 to 2010. In Figure 3, I’ve added the Zonal Mean data for the period of 1917 to 1944 to the graph. The change in Arctic surface temperature anomalies during the early warming period is comparable to the current warming period. One could conclude that the Polar Amplification during the current period is not unusual. It’s a natural response to warming.
ON THE USE OF DETRENDED GISS LOTI DATA IN FIGURE 1
Figure 4 shows the GISS LOTI data without the detrending. I used the detrended GISS LOTI data because it was difficult to determine when the early warming period started in the “raw” data, Figure 4. Was it 1907 or 1917? But based on the detrended data, the minimum occurred in 1917.
If we were to use 1907 as the start year for the early 20th Century warming period, it would make a significant difference in the Arctic warming. The GISS Zonal Mean data with that start year indicates the Arctic warmed much more during the earlier warming period than the current warming period. Refer to Figure 5. (Wouldn’t want someone to accuse me of cherry-picking the start year, would I? So I used the 1917 start date).
This post illustrated that Polar Amplification not only causes the Arctic to exaggerate a rise in global surface temperature, it also causes the Arctic to amplify a decrease in global surface temperature. It additionally showed the current warming of the Arctic is not unusual.
Someone might try to argue the Arctic data is very sparse during the early warming period, and the lack of data prevents a true comparison of the two warming periods. Realistically, one needs to consider the fact that most of the Arctic data presented by GISS in all periods is make-believe data. During seasons with sea ice, GISS uses 1200km radius smoothing to infill the vast majority of the Arctic data. That part of the GISS 1200km radius smoothing process for the Arctic is logical.
BUT (big but) sea ice melts every year, some years more than others. During seasons of sea ice melt, the fact that GISS deletes the SST data makes a significant difference, because SST varies less and has a lower trend than land surface data. This biases the Arctic (and Southern Ocean) data. GISS also deletes the SST data so they can extend the land data over open waters; that is, if they were to include the sea surface temperature data when it’s available, GISS could not extend the land data over the open ocean to reach the pole.
The Arctic amplifies the lower latitude trends when the globe is warming and when it is cooling. The Arctic should amplify any multidecadal warming or cooling signal regardless of source. If a major portion of the current warming period is due to natural variability, as suggested in the post Can Most Of The Rise In The Satellite-Era Surface Temperatures Be Explained Without Anthropogenic Greenhouse Gases?, then I believe my statement in that post was correct. And that statement was, If the vast majority of the change in temperature at the lower latitudes is natural, the same would hold true for the Arctic.