First, the facts: it is true that average increase in temperature since 1998 is lower than the previous 20 years or more. But it is also true that the nine warmest years on record have all been in this very 15-year period. The sole exception is 1998 itself, the warmest year on record, itself caused by a historic El Nino event.
But this is not the full story. In this same 15-year period carbon dioxide levels in the atmosphere reached 392.6 parts per million (ppm), the highest in 800,000 years; Arctic sea ice melted to its lowest ever summer extent — about 18% lower than the previous lowest in 2007; global sea levels reached a record high, continuing the average 3.2 mm per year rise; extreme events and more intense water cycles of rain continued. Clearly, all other indicators of climate change are present in abundance. Even global temperatures are rising, but not as rapidly as earlier.
Scientific research has explained this strange event. The four main reasons for a slight slowing down in the warming process are: rise in sea heat, El Nino , aerosols, and what is called the 'solar minimum'. These have not been fully included in the IPCC report.
But before going into that here is another fact for context: it is not the first time that there has been such a 'pause'. There have been two earlier periods when a similar slowdown in warming took place — from 1977 to 1986, and from 1987 to 1996. Both these periods were followed by big jumps in temperatures. Climate change is a complex process and this variability is inherent to it.
According to the US National Oceanic and Atmospheric Administration ( NOAA), "heat content in the upper 2,300 feet of the ocean remained near record high levels in 2012. Overall increases from 2011 to 2012 occurred between depths of 2,300 to 6,600 feet and even in the deep ocean" . About 30% of the heat was dumped into the ocean below the 2,300 feet depth as per research by Kevin Trenberth , a scientist at the University Corporation for Atmospheric Research in the US.
Trenberth also found a link between global warming and El Nino , the warming of water in the South Pacific . "A global temperature increase occurs in the latter stages of an El Nino event, as heat comes out of the ocean and warms the atmosphere," he writes.
It appears that a series of small volcanic explosions in the past decade and a half have also contributed to the warming slowdown by injecting small particles of dust and ash (called aerosols) in the upper reaches of the atmosphere . This layer acts as a shield preventing more of the sun's rays from coming earthwards.
Besides, during this period of slowed warming the heat coming to Earth from the Sun was less because the Sun was passing through its cyclical low, as James Hansen, a leading climate change scientist has pointed out. This solar minimum has lasted from 2005 to 2010 and the sun is now on an upswing. So, the 'pause' is only a passing phase. As Trenberth writes, "global warming has not gone away" .
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