To add to Avery's excellent answer, I'd like to note that the absolute surface temperature of Earth isn't actually used to measure changes in climate, and that estimates of it changing over a few decades wouldn't indicate that scientists aren't sure of the changes in the Earth's temperature.
NASA has a FAQ discussing how their GISTEMP measure of temperature is calculated:
The GISTEMP analysis concerns only temperature anomalies, not absolute temperature. Temperature anomalies are computed relative to the base period 1951-1980. The reason to work with anomalies, rather than absolute temperature is that absolute temperature varies markedly in short distances, while monthly or annual temperature anomalies are representative of a much larger region.
They calculate temperature by, for each climate station, subtracting the average for that climate station from 1951 to 1980, so each value is now a difference from the average temperature for that station from 1951 to 1980. This is a 'temperature anomaly' value. Then, those anomalies are averaged, across all stations. This works because temperature anomalies are correlated over large distances - if it's unusually cold in Chicago it's probably unusually cold in the entirety of Illinois. Obviously if it's 15c in Chicago it's probably not 15c in the entirety of Illinois.
This allows NASA and the other groups providing global surface temperature records, to provide a much more accurate measure of change in surface temperature, as discussed in this post on Realclimate:
That means you need fewer data points to make a good estimate of the global value. The 2σ uncertainty in the global mean anomaly on a yearly basis are (with the current network of stations) is around 0.1ºC in contrast that to the estimated uncertainty in the absolute temperature of about 0.5ºC (Jones et al, 1999).
This is because you can average more stations together, essentially.
When absolute surface temperatures are reported in the press, like the example quoted in your article:
This year the Earth's average temperature was 14.64C, compared with the long-term average of 14C, said James Hansen, of NASA's Goddard Institute for Space Studies, who analyses the data collected from thousands of weather stations around the world.
This value is calculated by adding the anomaly value for the current year to an estimated preindustrial absolute surface temperature, not by taking the absolute temperature today and subtracting the preindustrial absolute temperature. From the NASA FAQ:
Q. What do I do if I need absolute SATs, not anomalies?
A. In 99.9% of the cases you'll find that anomalies are exactly what you need, not absolute temperatures. In the remaining cases, you have to pick one of the available climatologies and add the anomalies (with respect to the proper base period) to it. For the global mean, the most trusted models produce a value of roughly 14°C, i.e. 57.2°F, but it may easily be anywhere between 56 and 58°F and regionally, let alone locally, the situation is even worse.
The implication in the American Thinker blog post that scientists cannot be sure of global warming because they are not confident of absolute surface temperature is very much misunderstanding how temperatures are measured. To make a strained metaphor, if you were measuring a child's growth by getting them to stand up against a wall and drawing a line over their head, you wouldn't say you were unsure how much taller they were getting because your estimated height above sea level isn't precise to a metre.
Some more discussion of this aspect of climate science can be found in this discussion from NOAA, and this Realclimate blog post.