|Anomaly map of HadCRUT4, showing the gaps in data coverage (in white)|
Cowtan and Way (2013) converted UAH satellite temperature data, which measures air temperature 1,000 meters above the Earth's surface, to surface temperature data (measured 2 meters above the surface) to fill in the blanks in the HadCRUT4 coverage map. The way they did this was to take the difference between UAH data and existing surface temperature stations and interpolate the differences in surface data gaps via a method called "kriging." Kriging is a well-established statistical technique and is used by NASA GISS as well as the Berkley Earth team for their global temperature data sets, although those teams interpolate directly from existing surface stations to cover the gaps. Cowtan and Way's method should be more precise, as it uses satellite temperature data that covers the gaps to calculate the surface temperature in the gaps.
Covering the gaps in HadCRUT4 data has a large impact on temperature trends, especially the trends since 1998. The trend in HadCRUT4 data since 1998 is +0.01976 ± 0.05555ºC per decade (trend ± 1σ standard error). The trend since 1998 in the coverage-corrected version (I'll call it the "Cowtan-Way" data set) is +0.10508 ± 0.05458ºC per decade, 5.3x faster. Once changes in ENSO, aerosols, and solar output are factored out of the Cowtan-Way data, the rate of rise since 1998 increases to +0.1880 ± 0.02765ºC per decade, 9.5x faster than the trend in HadCRUT4.
What does this mean for the "pause"? Quite simply, there really is no pause. The apparent "pause" is really just an artifact—the artificial effect of poor coverage of polar regions, combined with cooling from a shift in ENSO, an increase in aerosols, and a decrease in solar output. While the rate of increase in the has been slower since 1998, an increase of +0.10508 ± 0.05458ºC per decade still an increase, not a pause. Once the data is adjusted for ENSO, aerosols, and solar output, the rate of rise due to greenhouse gases for the Jan. 1998-Dec. 2012 is +0.1880 ± 0.02765ºC per decade—which is slightly faster than the overall rate of rise due to greenhouse gases since 1979 (+0.1841 ± 0.010282ºC per decade). The other thing this shows is the over-sized effect small changes can have on short-term trends, as small changes in temperature can produce very different trends. This effect is the consequence (and danger) of using short time periods. Trends become far more stable over as the length of the time period increases.
The next time someone talks about a "pause" in global temperatures, the proper response is really "What pause?" Global warming hasn't paused. If anything, the rate of warming due to greenhouse gas concentrations has increased slightly since 1998.