Where is the "Hiatus" in Global Warming?
- Friday, 26th June 2015
In the past several years, climate deniers have been bleating about the slowdown of global warming since 1998, or the so-called "hiatus". Against the background of increasing atmospheric carbon dioxide concentration in recent decades, some deniers even claim that carbon dioxide does not cause global warming and hence there is no need to curb emissions. The Fifth Assessment Report of the Intergovernmental Panel on Climate Change has explained that short-term trends, such as the trend over 1998-2012, would be very sensitive to the start and end dates selected and would not in general reflect the long-term climate trend (e.g. the trend over 1951-2012). Short-term temperature fluctuations of the globe can be strongly affected by natural variations of the climate system, such as the El Niño that we are experiencing now, as well as by volcanic and solar activities. In our earlier blog "Has global warming stopped?", we showed how climate deniers used cherry-picking tactics to magnify selected short-term trends in support of their claims while ignoring the big picture.
Estimating the Earth's temperature has always been a challenge to climate scientists because of differences in evolving observing practices, changes in instrumentation, methods and locations of measurement, as well as the uneven and insufficient coverage of temperature measurement, especially over the oceans. Some studies have suggested that natural variations, such as the increase in heat uptake by the oceans, has an important role to play in determining the global temperature in the past couple of decades. Recent developments however have enabled scientists to come up with a better estimate of global average temperature: over land, a much larger surface temperature dataset (viz. the International Surface Temperature Initiative databank ) has been released and the number of stations available for analysis has doubled; while over the oceans, surface temperature observations on buoys have increased and at the same time a new method has been developed to improve the correction of the systematic bias of ship data.
Figure 1 shows the old and new data analyses of the US National Oceanic and Atmospheric Administration (NOAA)  that depict a global warming trend since the 1950s and, based on the analyses, Figure 2 compares the old and new warming rates over different periods of time. While the long-term overall warming rates in NOAA's old and new analyses are rather similar (0.117 and 0.129 oC/decade respectively) during the period of 1951-2012, the warming rate for the period of 1998-2012 has more than doubled from 0.039 oC/decade in the old analysis to 0.086 oC/decade in the new analysis. Even more intriguing is the fact that if the latest data of 2013 and 2014 are included in the new analysis, the warming rate over 1998-2014 would become 0.106 oC/decade, bringing it even closer to the long-term overall warming rate. As such, the fragility of the so-called "hiatus" claim of significant warming slowdown is evident for all to see as it has obviously failed to withstand the test of time!
Figure 1 NOAA's old and new analyses of global temperature anomaly (data source: NOAA)
Figure 2 NOAA's analyses of global warming rate over different periods of time (data source: NOAA)
S M Lee
 Release of the International Surface Temperature Initiative's Global Land Surface Databank
 Science publishes new NOAA analysis: Data show no recent slowdown in global warming