The average global temperature of the Earth
has increased by 0.8?C since 1880, according to the Intergovernmental Panel on
Climate Change (IPCC), during this time there have been periods of rapid and
slower growth.  The global warming hiatus
refers 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 for
such a hiatus; missing observational coverage, the Earth’s climates response greenhouse
gas forcings in terms of ocean uptake, the effect of aerosol, and interpretation
of methods and look at whether there has indeed been  a climate hiatus.The climate hiatus began in 1998 with a unusually
strong El Nino, this resulted in global surface temperatures increasing to 0.2?C above the expected temperatures taken from the long-term
warming trend. The following 15 years, to 2012, showed a much smaller increasing
linear 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 Greenhouse
emissions, and was used by many, including the press, to make bold statements suggesting
a halt in global warming; resulting in the need for scientist to provide an
explanation and rebuttal of the hiatus. The first thing to consider is the effect
of the incomplete or missing data on the rate of increase of global surface
temperature. There is incomplete coverage of the globe, particularly in the
polar regions and parts of Africa, due to the way satellite data is collected.
This incomplete data resulted in an underestimation of warming rates in the
polar regions, which was discovered when the hiatus was being investigated. Additional
warming in the polar regions was estimated using interpolation of the limited
observational data. Results showed that warming of a few hundredths of a degrees
Celsius per decade had continued to occur during the hiatus period. This correction
applied to global average temperatures resulted in no statistical decrease in
the rate of warming between the two periods previously considered, 1951 to 2012
and 1998 to 2012, firmly making a case against the hiatus. The slow down
apparent in the non-polar regions will be examined by the following reasons. The Earth’s climates response to aerosol
forcing is one of the reasons for reduction in surface warming. During the
hiatus 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 as
the hiatus suggest. During the hiatus both ocean heat content and sea level
increased, these properties are more representative of the net energy imbalance
of the climate system than surface temperature. Of the excess energy as a
result of greenhouse gas forcing entering the climate system, 90% was stored in
the oceans. Observational estimates of ocean heat show a increase through the
hiatus and over the previous 30 years, with the largest increase seen at the
beginning of the hiatus period. This increased subsurface heat uptake can be
explained by the unprecedented strengthening of the Pacific trade wind,
resulting in increased subduction in the Pacific shallow and increased upwelling
in the central and eastern Pacific. The deep oceans increased uptake of this
energy and heat influenced global surface temperatures and therefore was a
cause of the slow down during the hiatus periodAnother possible reason for the hiatus has
been proposed as being in part caused by the volcanic activity resulting in
increased sulphur emissions. The eruptions of Mt Pinatubo and Mount Hudson in 1991
resulted in 23 million tonnes of SO2entering the stratosphere. SO2
aerosols reflect sunlight therefore reducing the amount of energy reaching the
Earth’s surface. The effect of volcanic activity combined with the bulk of
anthropogenic SO2 emissions which come from human activity,
increased levels of the aerosol in the atmosphere therefore is partly responsible
for the global temperature slow down. Further to the ocean heat uptake and
increased aerosol levels, a major factor to be considered is how different
methods have been used and interpreted. There are currently 5 different observational
data sets for global mean surface temperature, the difference across these datasets
are due to different data sources, calibration of different instruments,
homogenization and interpolation in areas without data. Differences in long
term trend compared to short term trend are dependent on data set and periods
chosen. By selecting a long term period only covering 1950-2000, the 15 year trend
being smaller than the long term trend is only evident on 3 of the
observational data sets, with the difference present not being statistically
significant. This is evident that the global hiatus is directly dependent on
time period chosen and dataset observed, giving further evidence for the claim
of a hiatus not occurring. 

Between 1998 and 2012 is a cherry picked period of time in
which reduced rate of surface warming appeared to occurred. The reduction can
be attributed to the combined effects incomplete coverage and errors, of the
climates response to aerosol forcing, deep ocean heat uptake and the effect of selecting
a time period and data set to give desired results. The global warming hiatus
is a period of time that is useful for close scrutiny of climate models and
observations and research into climate variability and uncertainties. Taking
the 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 and
2016 breaking temperature records, and has not changed overall projections for climate
change in the 21st century. 

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