Baroness Verma (Whip, House of Lords; Conservative)

The assessment that there has been a long-term upward trend in global average near-surface temperatures since the late 19th century is based upon three global temperature records, compiled from observations, by groups in the US and UK. The rate of global temperature rise on different timescales is summarised in table 1 below. The underlying trend over the period from 1880 to 2011 is 0.062 celsius per decade, giving a total change of 0.81 celsius. Such a rate of change has been judged by major scientific assessments to be large and rapid when compared with temperature changes on millennial timescales.

Over this period some parts of the world have warmed at a much faster rate. The land surface average temperature has risen by about 1.1°C and Arctic temperatures have increased by almost twice the global average rate. The consequences of this warming are already seen across the globe. For example, northern hemisphere sea-ice and snow cover have decreased markedly, most glaciers have retreated and the risks of certain extreme weather events occurring have increased.

Statistical (linear trend) analysis of the HadCRUT4 global near surface temperature dataset compiled by the Met Office and Climatic Research Unit (table 1) shows that the temperature rise since about 1880 is statistically significant.

Time period	Linear trend (°C/decade)	Absolute change in temperature described by linear trend (°C)
1880-2011	0.062±0.009	0.81±0.13
1900-2011	0.074±0.011	0.82±0.13
1950-2011	0.106±0.025	0.66±0.16
1970-2011	0.166 ± 0.038	0.70 ± 0.16

Table 1. Trends fitted to monthly global temperature anomalies for HadCRUT4, with uncertainties describing 95% confidence interval bounds for the combination of measurement, sampling and bias uncertainty and uncertainty in the linear trend fitted to the data. The statistical model used allows for persistence in departures using an autoregressive process (ie that an individual value is not independent of the previous one).

Statistical analyses and modelling of the global temperature record have shown that, because of natural variability in the climate system, a steady warming should not be expected to follow the relatively smooth rise in greenhouse gas concentrations. Over periods of a decade or more, large variations from the average trend are evident in the temperature record and so there is no hard and fast rule as to what minimum period would be appropriate for determining a long-term trend.