July 9, 2011 — andyextance

The full climate impact of China’s massive industrialisation between 1998 and 2008 has yet to be felt, thanks to its reliance on coal, US and Finland-based researchers said this week. Using this fuel for energy generation did release large volumes of the greenhouse gas CO2 that will warm the planet in the long term. However, it also emitted pollutants derived from the element sulphur that oppose this warming effect in the short term, explained Boston University’s Robert Kaufmann. “That let natural variations in that decade really predominate,” Kaufmann told Simple Climate.

These findings help answer a long-standing climate question, which stumped Kaufmann when he was speaking about global warming to the public in New Jersey in 2008. “A member of the audience said that he had heard that global temperatures hadn’t risen for about 10 years,” the researcher explained. “He asked me why not, and I must admit that I was at a loss to explain it.” Climate scientists have conceded that there has technically been no warming in this time, even though the US National Oceanographic and Atmospheric Administration (NOAA) said 2005 was then the warmest year on record. Kaufmann found that in fact there hadn’t yet been any satisfactory explanation why this had happened, partly due to the tools used by climate scientists. Most “general circulation” models (GCMs) used to simulate processes in the atmosphere and on the Earth calculate climate patterns from the laws of physics. While these are good at modelling changes in the long term, they are much less accurate over periods of just a few years, Kaufmann said.

Bridging statistics and physics

Together with economists Heikki Kauppi from the University of Turku, Finland, and Harvard University’s James Stock, Kaufmann had published a method for linking human pollutant emissions to temperature in 2006. This contrasts with GCMs by focusing more closely on connecting numbers, or variables, than the underlying processes. That means some climate scientists find it hard to trust. But the Boston scientist said that even if it doesn’t quite completely capture causes-and-effects, their model includes much more than simple statistical links. “The techniques we used were the basis of the 2003 Nobel prize in economics to Clive Grainger and Robert Engle,” Kaufmann explained. “The statistical techniques are able to detect long-run relationships between variables – in this case, between radiative forcing, which is a measure of how much heat the Earth’s atmosphere traps, and global temperature.”

Before seeing whether this method could explain the slowdown in temperature rise, Kaufmann asked his PhD student Michael Mann to look at the major changes in the amount of gas humans have emitted. “I noticed there was a huge increase in sulphur emissions, due largely to rapid economic growth in China,” Kaufmann explained. China’s electricity is mainly generated by burning coal, which contains large amounts of sulphur, which can counter the warming impact of greenhouse gases. “Sulphur acts like a mirror, it reflects solar radiation back to space,” the Boston scientist said. “It reduces radiative forcing, thereby reducing the amount of energy ultimately absorbed by the Earth’s atmosphere.”

While sulphur’s cooling effect in the atmosphere is well known – it explains worldwide temperature falls after volcanoes erupt – Kaufmann said that researchers just hadn’t spotted how much atmospheric levels had risen. “Climate scientists are generally not following the rapid increases of sulphur emissions,” he commented. “Chinese coal consumption more than doubled in that decade. Once I saw that, I said ‘Michael, go ahead, there could be something here.’”

Replaying the 20th century

In a Proceedings of the National Academy of Sciences of the USA paper published on Monday, Kaufmann’s team reveal that the increases in sulphur and CO2 emissions largely cancelled each other out. That left two other factors to control the Earth’s temperature. One, the energy reaching Earth from the sun, was weakening towards 2008 as part of its usual cycle. The other, the oscillation between warmer El Niño and cooler La Niña weather systems, changed from El Niño in 1998 to La Niña in 2008. Both contributed to 2008 being around 0.2°C cooler on average than 1998.

“This is not the first time sulphur emissions have largely cancelled the warming effects of carbon dioxide,” Kaufmann underlined. “You had rapid increases in CO2 but little or no gain in temperature in the late 40s, the 50s, 60s, through to the early 70s. You had sulphur emissions rising in tandem with carbon emissions and carbon concentrations, so temperature didn’t really rise. Then the US, Europe and Japan adopted environmental legislation aimed at reducing air pollution and acid deposition. Sulphur emissions declined, carbon dioxide emissions continued to rise along with concentrations, and temperature really started to increase rapidly.”

With sulphur emissions responsible for tens of thousands of deaths from respiratory illnesses each year, and acid rain that kills forests and lake inhabitants, limiting them is vital, Kaufmann said. But with sulphur pollutants staying in the atmosphere for a much shorter time than CO2, that will then quickly increase radiative forcing, and therefore temperature. With the NOAA saying that 2010 has now tied with 2005 as the warmest year, Kaufmann believes the effect of renewed efforts to reduce sulphur emissions is already being seen. “You’re almost getting a replay of this post-war economic expansion,” he said. “Now China’s passing environmental legislation it’s quite possible we’ll see a re-run of the kind of rapid temperature increases that characterise the last 25 years of the 20th century again starting soon.”