A natural cooling fluctuation between 1998 and 2013 largely masked the warming effects of a continued increase in man-made emissions of carbon dioxide and other greenhouse gases, reveals a new study.
According to research by McGill University physics professor Shaun Lovejoy, has ruled out with more than 99 percent certainty, the possibility that global warming in the industrial era is just a natural fluctuation in the earth's climate.
The researchers found that deceleration in rising temperatures during this 15-year period is sometimes referred to as a "pause" or "hiatus" in global warming, and has raised questions about why the rate of surface warming on Earth has been markedly slower than in previous decades. Since levels of greenhouse gases have continued to rise throughout the period, some skeptics have argued that the recent pattern undercuts the theory that global warming in the industrial era has been caused largely by man-made emissions from the burning of fossil fuels.
The study concludes that there has been a natural cooling fluctuation of about 0.28 to 0.37 degrees Celsius since 1998 -- a pattern that is in line with variations that occur historically every 20 to 50 years, according to the analysis.
The researchers said that they found many examples of these variations in pre-industrial temperature reconstructions based on proxies such as tree rings, ice cores, and lake sediment and being based on climate records, this approach avoids any biases that might affect the sophisticated computer models that are commonly used for understanding global warming.
The study was published in Geophysical Research Letters.