In 2009 a widely used dataset indicated that the average temperature of the Earth’s surface may have stopped warming, or that it was warming at a lower rate than the long term average. Now in 2017 it is clear that global temperatures have continued to rise at about the same rate prior to 2000. A recent study has looked back to determine whether a global warming “hiatus” really occurred or whether it was an artifact of incomplete data or changes in how global temperatures are measured. The new analysis shows that the trend for 1998–2012 is indistinguishable from the best estimate of the long trend for 1951–2012.
The Fifth Assessment Report of the Intergovernmental Panel (AR5) on Climate Change (IPCC) stated that the Earth’s global surface temperature “has shown a much smaller increasing linear trend over the past 15 years (1998-2012) than over the past 30 to 60 years.” The apparent slowdown, which has been referred to as a “hiatus” in global warming, was taken by some outside the scientific community as evidence that global warming may have halted even though greenhouse gas concentrations in the atmosphere continue to rise.
Calculating global surface temperature
Because global surface temperature is a derived quantity, computed by averaging temperatures recorded by tens of thousands of weather stations and ships and buoys at sea, it can be affected by a number of factors relating to sampling locations and how temperatures are measured. Technologies for recording temperatures change over time and this can affect the computation. There are different ways of choosing which sites (weather stations, ships and buoys) to include in the calculation. Different methodologies used to interpolate over regions such as high latitudes, uninhabited regions in central Asia and some oceanic areas which have few weather stations or are not on frequently traveled sea lanes can affect to calculated surface temperature.
In AR5 important changes in temperature measurement technologies and methodologies were not taken into account. The most important changes are an increasing amount of ocean data from an expanding network of buoys; temperature data from ships which comes from engine intake thermometers as well as bucket sea-water temperatures; and a large increase in land-station data especially in regions that have had limited weather coverage in the past.
The new analysis which takes these factors into account indicates that short- and long-term warming rates are far more similar than previously estimated in the AR5. There are five or six recognized time series of computed global surface temperatures that use different data sets and methologies – BEST, GISTEMP, HadCRUT4, HadCRUT3, NOAA, and Cowtan and Way. The global surface temperatures computed with these data sets and methodologies vary over time as new data is added, older data revised, and methodologies improved. In 2009 the trend in global surface temperatures from HadCRUT3 showed a slight cooling of 0.01 C per decade since 1998. With the corrections that have been applied since then none of the current datasets (BEST, GISTEMP, HadCRUT4, NOAA, and Cowtan and Way) show a cooling trend starting in 1998 that lasted for more than five years. Only HadCRUT4 exhibited a cooling trend beginning in 2001 and lasting 12 years.
Breaks in the predominant warming trend can be defined AS periods of a decade or more where the trend in global surface temperatures is less than a long term average. Based on this definition of a hiatus in the predominant warming trend, HadCRUT4, GISTEMP, and NOAA have exhibited 15-year periods with trends less than the long term 1951 to 2000 trend, but the differences between the trends for the 15-year periods and the long term trend are not statistically significant, meaning that what looks like a slower trend may be an artifact in the observed data.
In conclusion with the new corrections the surface temperature trend for 1998–2012 is indistinguishable from the best estimate of the long trend for 1951–2012.
Ocean heat content
A factor that is more important than errors induced by natural variability, incomplete coverage, and methods of measuring temperatures is that 93% of the excess energy (the difference between the energy captured from solar radiation impinging on the Earth and the energy radiated back into space) is accumulated in the oceans. Thus the ocean heat content is a more representative measure of the energy uptake by the Earth than the surface temperature. Data from the recently deployed, extensive oceanic buoy system reveals warming at all levels during the period 1998 to 2012, primarily at the surface above 700 meters where most of the Earth’s excess energy has accumulated.