Sun blocking only needed if Earth warms easily with added greenhouse gases

Washington: Some scientists are studying ways to temporarily reduce the amount of sunlight reaching the earth to potentially stave off some of the worst effects of climate change.

But a new computer analysis of future climate change that considers emissions reductions together with sunlight reduction shows that such drastic steps to cool the earth would only be necessary if the planet heats up easily with added greenhouse gases. The analysis might help future policymakers plan for a changing climate. The study by researchers at the Department of Energy’s Pacific Northwest National Laboratory explored sunlight reduction methods, or solar radiation management, in a computer model that followed emissions’ effect on climate. The analysis shows there is a fundamental connection between the need for emissions reductions and the potential need for some sort of solar dimming. “The conditions under which policymakers might want to manage the amount of sun reaching earth depends on how sensitive the climate is to atmospheric greenhouse gases, and we just don’t know that yet,” said Steven Smith with the Joint Global Change Research Institute in College Park, Md,, a joint venture between PNNL and the University of Maryland. The analysis started with computer-based virtual worlds, or scenarios, that describe different potential pathways to reduce greenhouse gas emissions, which limits the amount of heat in the earth system due to greenhouse gas accumulation. The researchers combined these scenarios with solar radiation management, a type of geoengineering method that might include shading the earth from the sun’s heat by either brightening clouds, mimicking the atmospheric cooling from volcanic eruptions or putting mirrors in space. How much sun blocking might be needed depends on an uncertain factor called climate sensitivity. Scientists measure climate sensitivity by how many degrees the atmosphere warms up if the concentration of carbon dioxide doubles. Smith said if the climate has a medium sensitivity of about 3 degrees Celsius (5.4 degrees Fahrenheit) per doubling of carbon dioxide, “it’s less likely we’d need solar radiation management at all. We’d have time to stabilize the climate if we get going on reducing emissions. But if it’s highly sensitive, say 4.5 degrees Celsius (8.1 degrees Fahrenheit) per doubling, we’re going to need to use solar radiation management if we want to limit temperature changes.” According to NOAA’s August report, the earth’s temperature has already risen about 0.62 degrees Celsius (1.12 degrees Fahrenheit) since the beginning of the 20th century as the carbon dioxide in the atmosphere has grown from 290 parts per million to 379 parts per million. But the atmosphere hasn’t reached equilibrium yet — even if humans stopped putting more carbon dioxide into the air, the climate would still continue to change for a while longer. Scientists do not know what temperature the earth will reach at equilibrium, because they don’t know how sensitive the planet is to greenhouse gases. Further, the study showed that, when coupled with emission reductions, the amount of solar radiation management needed could be far less than the amount generally considered by researchers so far. “Much of the current research has examined solar radiation management that is used as the sole means of offsetting a doubling of carbon dioxide concentrations. What we showed is that when coupled with emissions reductions, only a fraction of that amount of ‘solar dimming’ will be needed. This means that potential adverse impacts would be that much lower,” said Smith. “This is all still in the research phase. We do not know enough about the impacts of potential solar radiation management technologies to use them at this time,” he noted. The study will also help decision-makers evaluate solar reduction technologies side-by-side, if it comes to that. The analysis was reported in the journal Climatic Change. ANI