Greenhouse effect VS global warming

By Genevieve Wanucha

There’s a tricky chemical trade-off at work in our skies. As greenhouse gases provide their famous warming effect to Earth’s surface, aerosol pollution in the atmosphere actually partly counteracts it. Aerosols are tiny particles suspended in the air, both natural and industrial, including sea-salt, mineral dust, ash, soot, sulphates, nitrates, and black carbon. They hang around in the air for around 10 days, scattering and absorbing radiation from the sun. Aerosols also provide nuclei for water droplets, boosting cloud formation, thus decreasing the amount of energy reaching the ground and providing a net cooling force. In short, greenhouse gases warm the surface; aerosols cool the surface.

This trade-off applies to the real world, considering that China and India’s coal burning creates an aerosol problem far worse than the US or Europe ever had before the Clean Air Act in 1970 and the collapse of the Eastern Block’s dirty economy reduced aerosol emissions dramatically. In principle, if China and India were to begin fixing their aerosol problem, which kills hundreds of thousands every year, they might actually contribute to global warming (if they don’t also cut greenhouse gas emissions); the cooling effect of aerosols would be removed, leaving greenhouse gases to warm the globe unimpeded.

This “Faustian bargain, ” as NASA climate scientist James Hansen terms it, is a big problem for climate scientists and policy makers. Predicting how the trade-off will affect both global mean and local temperatures is one of the tasks of the century, as made clear in research by a multinational team of researchers, including John Marshall at MIT, , became curious about a climate model used by a UK research group at the Met Office Hadley Centre to argue in the journal Nature that anthropogenic aerosols are a prime driver of twentieth-century North Atlantic climate variability, even influencing peaks in hurricane activity and the Sahel drought. Zhang and her colleagues decided to look deeper.

They found that the simulations of the “HadGEM2-ES” climate model could not replicate the actual observations of the North Atlantic in the 20th century. Substantial warming trends in the heat content of the upper ocean have been observed in most ocean basins since 1955. Yet, the UK group’s model ocean is much colder. The team discovered that the discrepancy in ocean heat content is influenced and largely caused by modeled aerosol cooling effects on ocean temperature.

Indeed, aerosols, via clouds, reduce energy reaching the ocean, contributing a net cooling effect to both the surface and subsurface ocean temperature. In the model, the aerosol’s cooling effect is so strong it even cancels out any greenhouse gas-induced warming. However, actual observations show a much warmer ocean. “The aerosol effect in their model is overestimated, ” says Zhang. She notes that there is probably an issue with how the model handles the exact way in which aerosols affect radiation.

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