Using 15 year trends to replicate GISTEMP average surface temperature anomalies

At Jo Nova’s Unthreaded on 22/06/14, Philip Shehan posted some GISTEMP temperature trend figures that caused a good deal of controversy.

In 15 year steps they are

1924/39 Trend: 0.142 ±0.148 °C/decade (2σ)

1939/54 Trend: -0.088 ±0.144 °C/decade (2σ)

1954/69 Trend: 0.024 ±0.151 °C/decade (2σ)

1969/84 Trend: 0.165 ±0.162 °C/decade (2σ)

1984/99 Trend: 0.234 ±0.167 °C/decade (2σ)

1999- Trend: 0.099 ±0.138 °C/decade (2σ)

Two issues with the trends are

1. They do not really capture the trends in the data.

2. They slope of the 15 year OLS lines is sensitive to shifting the period by one year – for instance replacing 1999-2013 with 1998-2012.

I replicated Philip Shehan’s data (or a least tried to – he does not use J-D years) on a graph, along with shifting the periods a year backwards. Compare these slopes to the 5 year centred moving average curve in light blue.