A global warming slowdown was reported in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report. It reported that 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, has been ascribed to a variety of mechanisms, for example, oceanic heat uptake.

In this study new land and ocean data with enhanced coverage of the arctic and older ocean data with new corrections has been analyzed to recompute the global surface temperature for 1950 to 2014 and compare the rate of warming for the latter half of the 20th Century with the first 15 years of the 21st Century.

Observations and corrections


The data is primarily surface air temperature observations from thousands of weather observing stations over land sea surface temperature (SST) from thousands of commercial ships and drifting surface buoys.

However, over time there have been important changes in the measurement technologies that were not taken into account in IPCC AR5. The most important changes are an increasing amount of ocean data from buoys; an increasing amount of ship data from engine intake thermometers instead of bucket sea-water temperatures; and a large increase in land-station data especially in regions that have had limited weather coverage in the past.

Several studies have found that ship data are systematically warmer than buoy data. This is important because because much of the ocean is now sampled by both types of measurement. Also the proportion of the Earth's ocean that is covered by buoys has increased significantly since AR5. Ship observations have come increasingly from engine intake thermometers. However, some ships continue to report temperatures taken from buckets. Therefore, a correction for these different types of ship-board measurements has been continued up to the present time. The impact of these changes in oceanic input measurements has been to increase the rate of rising temperature by 0.064°C per decade for the period 2000-2014. The can be broken down and ascribed to different changes in the input data.

CorrectionContribution(°C per decade)
Ship correction0.030°C
Buoy correction0.014°C
Greater weighting of buoys0.012°C


There have also been advances in the calculation of land surface air temperatures (LSTs). The most important is that land surface temperatures are now calculated using the International Surface Temperature Initiative (ISTI) databank. The ISTI databank integrates new data sources, which more than doubles the number of temperature reporting stations and improves spatial coverage over many areas such as the Arctic where temperatures have increased rapidly in recent decades. New analyses since AR5 have revealed that incomplete coverage of the Arctic led to an underestimate of warming since 1997 in the data used in AR5.

Analysis


Comparison of global surface temperature trends for several periods
Comparison of global surface temperature trends for several periods
Temperature trends from the “new” analysis (squares), “old” analysis (circles) and new corrections with the polar interpolation method (triangles). 90% confidence intervals from the new analysis are shown by a horizontal dash. (A) base period (1951–2012), (B) “hiatus” period used in IPCC, (C) alternate base period—the second half of the 20th century, (D) The 21st century through 2014, (E) 1998 through the 21st century. Click to enlarge

The new analysis suggests that short- and long-term warming rates are far more similar than previously estimated in IPCC. The new corrections show that the 90% confidence interval for 1998–2012 encompasses the best estimate of the trend for 1951–2012.

Conclusion


To summarize the newly corrected and updated global surface temperature data do not support a global warming “hiatus". The new analysis shows the trend over the period 1950-1999 is indistinguishable from the trend over the period 2000-2014.

PeriodRate of increase (°C per decade)
1950-19990.113°C
2000-20140.116°C


Source


Possible artifacts of data biases in the recent global surface warming hiatus, Thomas R. Karl, Anthony Arguez, Boyin Huang, Jay H. Lawrimore, James R. McMahon, Matthew J. Menne, Thomas C. Peterson, Russell S. Vose, and Huai-Min Zhang, Science 04 Jun 2015 DOI: 10.1126/science.aaa5632


Slowdown in global surface temperature rise attributed to oceanic heat uptake

IPCC Fifth Assessment Report Technical Summary