We’ve all heard that along with global warming, there is a phenomenon called global dimming. Of course, we’d expect it to counteract the warming and offer us some hope of mitigating the effects of climate change, and this has motivated newspaper writers to play on the idea of doubt in climate models, creating ambiguity and uncertainty where it really isn’t justified.
When Gerry Stanhill published his work in 2001, coining the phrase “global dimming”, he was trying to explain the results of his study of the amount of sunlight reaching the earth over the previous fifty years. He needed to explain the drop of around 22%.
He realised that the drop in sunlight corresponded to the increase in atmospheric pollutants, which caused the formation of water droplets, increasing the earths reflective potential, called it’s albedo. This increase in reflectivity meant that significantly less energy was reaching the earth’s surface than previously.
It was known that the greenhouse effect works by trapping the energy reflected from the surface when it reaches the greenhouse gases in the atmosphere, and that carbon dioxide even in small concentrations has a disproportionate effect. So clearly, if less energy is reaching the surface then less will end up reflected and get trapped in the atmosphere by those greenhouse gases. It seemed as though they had found an effect which opposed global warming.
Some thought as a result that we could pump atmospheric pollutants such as sulphur dioxide into the upper atmosphere and cancel out the effect of global warming. But things weren’t quite so simple. It was clear that if global dimming had reduced the impact of global warming, then the earth’s thermal properties were actually more sensitive that had previously been thought. Predictions which didn’t include global dimming may not be an accurate reflection of the trend to global warming, particularly if governments were implementing plans to reduce atmospheric pollution.
It was already clear that the Montreal agreement to reduce the impact of CFCs (chlorofluorocarbons) in aerosols had made a difference and there was evidence that the ozone hole over Antartica was closing up. If particulate pollution was set to decline in the coming years, the impact of global warming would be greater, and the need to deal with carbon dioxide emissions would consequently be more urgent still.
But dimming and warming are actually related effects. Water vapour itself is the major greenhouse gas and so when particulates cause the formation of water droplets, they are increasing the greenhouse effect at the same time as increasing the reflective property of the atmosphere: they are at the same time increasing the warming effect and cooling the earth by reducing the energy arriving at the surface.
And when water droplets form, that causes cloud formation and precipitation which affects weather systems. That has an impact on rainfall, floods, and droughts. But whenever it rains, the particulates are washed out of the atmosphere reducing the dimming effect. The dimming effect goes up and down but differently in different parts of the atmosphere.
So we cannot simply counterpose these two influences. They have common causes and both significantly affect the water cycle which has a critical effect on our weather systems. Computer climate models take these factors into account in making their predictions of climate change and these models are dynamic and include many related variables. It is their complexity that allows so many people to claim that doubt about some details means we don’t know about the major trends.
When climate change doubters or deniers insist that there is controversy between climate scientists they are mistaking a scientific desire for maximum data and clarity, with not understanding the basic causes. We know far more about global warming than is required to inform sensible decision-making. There are questions remaining for be answered. Of course there are: if science knew everything it would stop. But we know that global dimming is not enough to counter to effects of rising carbon dioxide levels, and that increasing pollution to compensate would condemn us to acid rain, catastrophic weather systems, and a need to base our survival on contaminating our very atmosphere.
Climate systems are complex and it is difficult to assimilate all of the data, but the simplistic claim that because there is a counteracting tendency, global warming is therefore not a problem, is irresponsible and incorrect. There is no controversy amongst the leading scientific authorities about the reality of global warming and this takes into account global dimming along with other relevant phenomena. Computer models have been remarkably accurate in their prediction of trends which have since been observed, indicating that the coverage of the major factors in the models are correct. Global dimming is just one of those factors.
