This 1980-2008 discrepancy between GISS and UAH is important, as it is nearly equal to the claimed warming trend since 1980.
Taking this one step further, I made a graph of the difference between the GISS and UAH monthly anomalies since 1980.
As you can see below, the discrepancy has increased over time. Using Google’s linest() function, the divergence between GISS and UAH is increasing at a rate of 0.32C/century. (GISS uses a different baseline than UAH, but the slope of the difference should be zero, if the data sets correlated properly.) The slope is not zero, which indicates an inconsistency between the data sets.
Some readers will undoubtedly again point out that the GISS baseline (“normal”) temperature is lower than the UAH baseline. This is true, but as I said above does not affect the slope calculation. The difference between the GISS and UAH monthly baselines is a constant, which affects the relative position along the y-axis – but it does not affect the slope. Subtracting a monthly constant from each point in a graph does not alter the slope over a large set of years. It only alters the y-offset.
The equation of a line is y = mx + b, where m is the slope and b is the y-offset. m and b are completely independent. The different baselines affect only b, not m. If the UAH and GISS data were closely tracking each other, the slope (m) would be close to zero. The fact that GISS shows 2008 temperatures much higher than 1980, and UAH shows 2008 temperatures lower than 1980, is also a clear indicator that the two data sets are divergent.
Steve McIntyre has coincidentally just done a similar comparison of NOAA USA yearly data vs. GISS USA yearly data, and came to the conclusion that the NOAA slope is even steeper than GISS, diverging from UAH by 0.39C/century.
This would imply that NOAA is diverging from UAH by an even larger amount than GISS is diverging from UAH.