After has led to the consensus that there had


After the 10-15 year period following
1998, it was reported in the Intergovernmental Panel on Climate Change’s Fifth
Assessment Report (IPCC AR5) that there had been a ‘hiatus’ in the rate of
global warming when referring to global mean surface temperature (GMST). This
announcement led to a significant public backlash prompting a renewed
investigation into the period by the scientific community. There is still
debate as to why the increase GMST appeared to slow down. It can be argued that
there was no actual hiatus in real terms, but the way in which data was
collected and analysed was at fault, resulting in a perceived slow-down. Or,
there was reduction in the rate of GMST (whilst taking into account adjustments
to errors in data collection), but the reasons for which have yet to be truly


After the announcement in
IPCC AR5, climate data from the end of the 19th century up until the
present day was investigated. One of the main issues that was found with the
data was that there was simply a lack of it. For a large portion of the 20th
century, the majority of stations observing the surface temperature had been on
the land and sea, with little data obtained from the Arctic. This has led to
the consensus that there had been an underestimate of warming rates.

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It has now also been
found that there were inconsistencies with the ways in which surface
temperature data collected from the oceans was interpreted. Ocean surface data
can be collected from buoys, ship engine intake thermometers or bucket water
temperatures. Each of these data collection methods requires different corrections
to reach the values of the actual surface temperature which is homogenous with
the rest of the global data set. For example, it was found that temperatures
from ships were systematically warmer than that from buoys. This led to a
correction of -0.12°C being added to buoy data. It
had also been assumed that the practice of taking surface temperatures by bucket
measurements had ceased. As this was not the case, a new correction had to be
applied to these data, leading to 0.030°C decade-1
of the 0.064°C decade-1 trend
difference being accounted for.

Another aspect of
the findings surrounding the hiatus that came under scrutiny was the 10-15 year
period in which it took place. There is no scientific basis for the start and
end points of this time period, they were chosen arbitrarily. This has also
been shown by the fact that the results found during this period are
statistically insignificant. In IPCC AR5, the hiatus was said to have lasted
from 1998-2012. However, if the period from 1998-2014 had been observed
instead, the decadal temperature increase would have been 0.020°C higher. The occurrence
of an El Niño in 1998 also
meant that the 10-15 period started warmer then would have been typical.


Despite these findings, it
is argued that the global warming hiatus did occur in real terms. The issues
surrounding data collection and analysis, although valid, may not be as
significant as thought. Satellites provided near-global time invariant temperature
data of the lower troposphere which showed a slow-down in global warming. These
satellites provide independent data when compared to a vast array of data collection
techniques that constantly require recalibration and correction.

                  The hiatus might be able to be explained internal decadal
climate variability. It could be the case that during the period 1998-2012, ocean
currents and winds led to heat energy being taken far below the surface by
processes that have yet to be understood fully. As it stands, most of the data
collected is taken from the surface, but the effect of global warming on the
deep oceans is also significant. For example, the top 3.5m layer of the sea holds
the same amount of energy as the whole atmosphere. Not enough time has passed
or data collected in order to draw accurate conclusions with respect to how the
deep oceans respond to large amounts of greenhouse gases and particulates being
released to the atmosphere. If this concept is to be applied, then studies that
have said that the period in which the hiatus has occurred is statistically insignificant
are invalid as they have only referenced periods in which greenhouse gas
emissions have been much lower. In order to obtain truly independent findings
of GMST increase rates, start and end dates of periods analysed should be
chosen with respect to a physical understanding of the forcings and processes
involved in global warming and climate change.


Since the IPCC report in
2013 a significant amount of research into the different aspects climate change
has taken place. Not only in the ways that data is collected and analysed, but
also in the fundamental understanding of the processes that dictate the climate
of the planet. In conclusion, I believe that the extent to which the hiatus occurred
was far less significant than was originally reported. The arguments
surrounding the insignificance of reanalysed GMST ocean data could also be seen
to be invalid. This is due to the fact that the lower troposphere temperature
data obtained from satellites comes with large uncertainties, so can only be included
in studies when compared to data from other sources. Also, since IPCC AR5, a
significant amount of research and data collection has been undertaken in the
Arctic specifically. This new data was run through retrospective models (this
time including these Arctic regions) and it was indicated that no hiatus had occurred.

Despite this, I believe that far more research has to be done in the ways that
the deep oceans are affected by global warming and climate change so that
possible future changes in warming rates can be predicted and accounted for. 


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