Global Warming : Choose Your Baseline

Just recently you may have overheard conversations like “the Earth has heated up by around 0.7 or 0.75 or 0.8 degrees Celsius since the start of the Industrial Revolution”. “No it hasn’t, it’s only warmed by 0.2 or 0.4 degrees Celsius !”

So who’s right ? And how can you tell ? It all depends on where you choose your baseline, the year you count from. And it also depends on the reach or spread of the data in the data set you are looking at.

If you believe that the 1800s were colder than normal, then you will think that the Earth has not warmed up so much, because you will discount everything before about 1940 because it was, to you colder than “average”.

Some of the data available on global temperatures “interpolates” for areas where measurements may be missing in certain geographical regions of the world. This can give the appearance that the data sets are different, when actually, they’re not.

If you are of the correct opinion that Global Warming has contributed just under a degree Celsius or Centigrade of warming in the last 200 years or so, I offer you here a very crude analysis to help you stand your ground. First of all let’s take a look at two of the best-loved line graphs of Global Warming.

The Meteorological Office Hadley Centre Temperatures :-

“Global average temperature 1850 – 2008” it reads until the end of the year 2009.

Now compare and contrast to the NASA GISS data :-

What’s the first thing you see ? They look somewhat different. Note how the zero line is on top of the smoothed annual average for 1940 on the HadCRUT3 series from the Met Office, but under the curve for the NASA GISS data.

And look a the more recent data : the HadCRUT3 data series seems to show a peak in temperatures of around 0.4 degrees C, and the NASA GISS data 0.55 degrees C. What’s going on ?

Well, it’s explained here :-

https://hadobs.metoffice.com/indicators/index.html

“The GISS analysis uses an interpolated sea-surface temperature analysis, which fills in some of the gaps in the sea-surface temperature data. The land station data is also interpolated over data free regions (including over the oceans) to a maximum distance of 1200km. This has a particularly large effect over the Arctic and Antarctic where there are few data points and temperature variability is large. One important thing to note is that the difference between the GISS and HadCRUT3 analyses are smaller than the calculated uncertainties on the HadCRUT3 data set – the data sets are not inconsistent. The largest component of the uncertainty arises from the fact that temperatures over large areas of the Earth’s surface remain unobserved. There are very few observations in the Arctic and Antarctic. GISS attempts to estimate temperatures in these areas, HadCRUT3 does not. This is the major source of difference between the analyses, which can be seen if, instead of a global average, one takes the average temperature anomaly between 60S and 60N. Over this slightly smaller area, the GISS and HadCRUT3 analyses give very similar results.”

If you look at the rather feint smoothed upper “uncertainty” curve in the HadCRUT3 graph, it reaches 0.5 degrees C. The HadCRUT3 “anomaly” in global temperature is in the same ballpark as the NASA GISS, then.

Also, take a look at 1880 on both graphs, which is generally the year used as the “baseline” for the Science : -0.2 degrees C on both graphs. By common agreement, this is the year and baseline chosen by pretty much everyone.

But why ? Why is 1880 chosen as being -0.2 degrees C ? For that, you need to take a broader look at Earth temperatures. Here’s the general synopsis for the last 2,000 years :-

(Bear in mind that the zero point on this graph is different than the other two you’ve already seen). As you can see, the temperatures were roughly level with strong variability between the year 0 AD and 600 AD, then rose to about the year 1,000 AD, and then were slowly sliding down to the year 1,700 AD or so. After that they started to rise quite sharply.

Now a lot of explanation has gone into explaining these changes, but if the long-term average for global temperatures reflects over the last few thousands of years looks like that between 0 AD and 600 AD, then 1880 AD was around -0.2 degrees C below that long term average.

So let’s go back to the top two graphs. Look at the temperature rise between 1910 and 1940 : HadCRUT3 shows 0.5 degrees C rise; NASA GISS shows 0.45 degrees C rise. For the period 1950 to 2008, NASA GISS data shows 0.6 degrees C rise, HadCRUT3 shows 0.6 degrees C rise. In total, between 1880 and 2008, the overall temperature rise is 0.75 degrees C for the NASA GISS data set, and 0.6 degrees C rise for the HadCRUT3 dataset. Pretty strong coherence within the parameters of treatment of the data.

Climate Change denier-sceptics have a habit of knocking the -0.2 degrees off the 1880 temperature anomaly, claiming that Global Warming has only caused a 0.4 degrees C (2008) or 0.2 degrees C (1995) rise.

There is however, cause to believe that the 1880 temperature was indicative of a generally decreasing natural trend which began 8,000 years ago :-

This is generally accepted as being the case. Which means that Global Warming is implicated in the totality of the temperature rise since 1880.