Global temperature change observed over the last hundred years or so is well within the natural variability of the last 8,000 years, according to a new paper by a former Intergovernmental Panel On Climate Change (IPCC) lead author.
Dr. Philip Lloyd, a South Africa-based physicist and climate researcher, examined ice core-based temperature data going back 8,000 years to gain perspective on the magnitude of global temperature changes over the 20th Century.
What Lloyd found was that the standard deviation of the temperature over the last 8,000 years was about 0.98 degrees Celsius– higher than the 0.85 degrees climate scientists say the world has warmed over the last century.
“This suggests that while some portion of the temperature change observed in the 20th century was probably caused by greenhouse gases, there is a strong likelihood that the major portion was due to natural variations,” Lloyd wrote in his study.
The United Nations’ IPCC claims there’s been 0.85 degrees Celsius of warming since the late 1800s, and concludes that most of this warming is due to human activities– mainly, the burning of fossil fuels and changes in land use. The IPCC says that “more than half of the observed increase in global average surface temperature from 1951 to 2010” have been caused by human activity.
If Lloyd’s results hold, the IPCC may have to revise how much warming it attributes to mankind. In any case, the IPCC’s estimate of man-made and natural warming (0.85 degrees) is still below the standard deviation for the last 8,000, according to Lloyd’s results. This means that warming is not very significant within the context of the Earth’s recent climate history.
Lloyd arrived at his conclusion after the “differences in temperatures between all records which are approximately a century apart were determined, after any trends in the data had been removed.” Lloyd noted the “differences were close to normally distributed.”
But Lloyd’s study hits at a larger debate within climate science: how much warming is attributable to mankind or nature. Clearly, Lloyd and the IPCC he once contributed to now represent different ends of the spectrum.
“The key challenge in understanding climate change is to assess the natural climate variability,” Dr. Judith Curry, a climate scientist at Georgia Tech, told The Daily Caller News Foundation in April.
At the time, Ronald Bailey, a science write for Reason magazine, wrote that there has still not been enough observed warming to meet the IPCC’s standard of “enhanced warming” — that is, warming above natural levels.
In his article, Bailey noted that there has not been enough temperature rise since the IPCC set its benchmark for “enhanced warming” in 1990. Curry noted that there was a big jump in temperature between 1993 and 1998, but that was basically because of the latter year’s El Niño.
“The magnitude of natural climate variability over the past 1000 years and even the past 100 years is hotly debated,” Curry added. “Personally, I think the role of natural climate variability has been substantially underestimated in our interpretation of recent climate change.”
But not all scientists agree with Bailey’s article, and some argue that signs of human influence on the Earth’s climate were evident in the 1970s. Indeed, by 1995 the IPCC stated that the “balance of evidence suggests a discernible human influence on global climate.” The international body has only made stronger statement on man’s climatic influence ever since.
“I would not pin anything on what was said by IPCC in 1990,” Dr. Kevin Trenberth, a climate scientist with the National Center for Atmospheric Research, told TheDCNF in April. “In the reports since then there have been thorough evaluations of past IPCC projections and whether they were out of line.”
Human influence on the climate may have been observable in the 1970s, but scientists have had trouble explaining why satellite data shows that average global temperatures have been virtually flat for more than 18 years. Satellites measure the troposphere — the lowest few miles of the atmosphere — in contrast, to surface temperature measurements, which most climate bodies rely on for estimates of global average temperature average.
But even surface temperature data showed a hiatus in warming for about 15 years or so. Scientists have offered up dozens of explanations for why global temperatures have been flat since the late 1990s. The most prominent explanation is that oceans have been absorbing most of the “heat” from increased greenhouse gas emissions, meaning surface temperatures show less warming than they otherwise would.
“What is evident now is that the signal of global warming emerged from the noise of natural variability about the mid 1970s,” Trenberth added. “There are fluctuations in global mean temperatures: from year to year with El Niños, etc., and from decade to decade, so that trends reflecting global warming need to be taken over at least 20 years.”