Researchers from ETH Zurich, which is a University in Switzerland, have been looking into the possible causes of the recent (past 10-15 years) slowdown in global temperature rise.
NASA global temperature anomaly analysis, which also shows major volcanic eruptions and ENSO changes bottom (red El Nino, blue La Nina)
The scientific team, led by Reto Knutti, a professor of Climate Physics, has for the first time systematically examined all current hypotheses in regards to the leveling off of the global temperature rise.
El Nino and La Nina
El Nino years typically lead to warmer global temperatures, while the opposite is true for La Nina. Why 1998 was so warm had a lot to do with the super El Nino of 1997/98. Recently, we have experienced more La Nina's compared to El Nino and these natural variations would certainly explain a large part of the reduced warming rate. However, even though climate models do take El Nino/La Nina into account it is almost impossible to predict what year in the long term that there will be an El Nino or a La Nina.
Solar cycles and volcanic eruptions
The team found that the solar irradiance has been weaker than predicted over the past few years and that the last period of weak solar irradiance has lasted 13 years compared to the normal 11-year cycle. The combination of reduced irradiance and the increase in volcanic eruptions such as the one in Iceland back in 2010, which have increased the amount of light reflecting aerosols in the atmosphere is likely the second leading cause of the warming slowdown.
Measured data and model data
The researchers found that measured temperature data over the past 16 years is likely to be too low, since global average temperature is only estimated using values obtained from weather stations on the ground, and that these do not exist everywhere on the planet.
Much of the Arctic has become much warmer over recent years, but since there are hardly any stations in that region the specified global average temperature is too low, according to the ETH Zurich report.
Some researchers propose using satellite data to estimate temperatures where there are no stations. If the model data is corrected downwards as proposed by the ETH researchers and the measured data is corrected upwards, then the model and actual observations are very similar, according to the report.
Climate still will become much warmer in the long term
Knutti believes that despite the short-term climate fluctuations, which can be easily explained, the more rapid global warming rate will resume once again as soon as solar activity, aerosol concentration and El Nino activity return to the values of previous decades.
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