The average global temperature of the Earthhas increased by 0.
8?C since 1880, according to the Intergovernmental Panel onClimate Change (IPCC), during this time there have been periods of rapid andslower growth. The global warming hiatusrefers to a period of relatively little change in global surface temperatures,one such period appeared to occur between 1998 and 2012. Here I will discuss the proposed reasons forsuch a hiatus; missing observational coverage, the Earth’s climates response greenhousegas forcings in terms of ocean uptake, the effect of aerosol, and interpretationof methods and look at whether there has indeed been a climate hiatus.The climate hiatus began in 1998 with a unusuallystrong El Nino, this resulted in global surface temperatures increasing to 0.2?C above the expected temperatures taken from the long-termwarming trend. The following 15 years, to 2012, showed a much smaller increasinglinear trend, of 0.05?C, than over the 60 years from 1951 to 2012, of 0.
12?C. This time also saw a rapid increase in levels of Greenhouseemissions, and was used by many, including the press, to make bold statements suggestinga halt in global warming; resulting in the need for scientist to provide anexplanation and rebuttal of the hiatus. The first thing to consider is the effectof the incomplete or missing data on the rate of increase of global surfacetemperature. There is incomplete coverage of the globe, particularly in thepolar regions and parts of Africa, due to the way satellite data is collected.This incomplete data resulted in an underestimation of warming rates in thepolar regions, which was discovered when the hiatus was being investigated.
Additionalwarming in the polar regions was estimated using interpolation of the limitedobservational data. Results showed that warming of a few hundredths of a degreesCelsius per decade had continued to occur during the hiatus period. This correctionapplied to global average temperatures resulted in no statistical decrease inthe rate of warming between the two periods previously considered, 1951 to 2012and 1998 to 2012, firmly making a case against the hiatus. The slow downapparent in the non-polar regions will be examined by the following reasons. The Earth’s climates response to aerosolforcing is one of the reasons for reduction in surface warming. During thehiatus more heat entered than left the climate system at the top of the atmosphere.This implies that the trend of the climate system as a whole did not plateau asthe hiatus suggest.
During the hiatus both ocean heat content and sea levelincreased, these properties are more representative of the net energy imbalanceof the climate system than surface temperature. Of the excess energy as aresult of greenhouse gas forcing entering the climate system, 90% was stored inthe oceans. Observational estimates of ocean heat show a increase through thehiatus and over the previous 30 years, with the largest increase seen at thebeginning of the hiatus period.
This increased subsurface heat uptake can beexplained by the unprecedented strengthening of the Pacific trade wind,resulting in increased subduction in the Pacific shallow and increased upwellingin the central and eastern Pacific. The deep oceans increased uptake of thisenergy and heat influenced global surface temperatures and therefore was acause of the slow down during the hiatus periodAnother possible reason for the hiatus hasbeen proposed as being in part caused by the volcanic activity resulting inincreased sulphur emissions. The eruptions of Mt Pinatubo and Mount Hudson in 1991resulted in 23 million tonnes of SO2entering the stratosphere. SO2aerosols reflect sunlight therefore reducing the amount of energy reaching theEarth’s surface. The effect of volcanic activity combined with the bulk ofanthropogenic SO2 emissions which come from human activity,increased levels of the aerosol in the atmosphere therefore is partly responsiblefor the global temperature slow down. Further to the ocean heat uptake andincreased aerosol levels, a major factor to be considered is how differentmethods have been used and interpreted. There are currently 5 different observationaldata sets for global mean surface temperature, the difference across these datasetsare due to different data sources, calibration of different instruments,homogenization and interpolation in areas without data.
Differences in longterm trend compared to short term trend are dependent on data set and periodschosen. By selecting a long term period only covering 1950-2000, the 15 year trendbeing smaller than the long term trend is only evident on 3 of theobservational data sets, with the difference present not being statisticallysignificant. This is evident that the global hiatus is directly dependent ontime period chosen and dataset observed, giving further evidence for the claimof a hiatus not occurring. Between 1998 and 2012 is a cherry picked period of time inwhich reduced rate of surface warming appeared to occurred.
The reduction canbe attributed to the combined effects incomplete coverage and errors, of theclimates response to aerosol forcing, deep ocean heat uptake and the effect of selectinga time period and data set to give desired results. The global warming hiatusis a period of time that is useful for close scrutiny of climate models andobservations and research into climate variability and uncertainties. Takingthe global warming hiatus in the context of the evidence for its occurrence,does not suffice to show a true slowdown in the surface temperatures,particularly when taken in context of the years that followed, with 2015 and2016 breaking temperature records, and has not changed overall projections for climatechange in the 21st century.