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Tuesday, April 2, 2013

The Economist on climate science: Exploring uncertainty, inviting confusion

The Economist’s new article and accompanying editorial on climate science (3/30/13) seek to explore current efforts to determine the drivers of global warming. The article makes clear that we are already experiencing the impacts of global warming and need to act now to avert worse to come. However, it places most of its emphasis on questioning the sensitivity of climate in light of a much-discussed short-term reduction in the rate of atmospheric warming. This is an important area of inquiry where much uncertainty remains, but it is relevant only to the exact timing of projections. As The Economist notes, extra time before warming reaches catastrophic levels is only useful if action is taken in the meantime.

Furthermore, The Economist acknowledges but minimizes four of the most important scientific factors in this discussion:

-       Ocean heatA growing number of studies document heat being redirected into the ocean instead of the atmosphere.

-       Studies have shown that 90% of all heat transmitted to Earth is absorbed by the oceans, and different layers of the ocean absorb heat differently. The Economist quotes a study showing a slowdown in heating of the upper ocean, but also mentions one that shows substantial heating of the very deep ocean. The underlying mechanisms behind these trends are very complex and still being uncovered, but they appear to be working to reduce atmospheric warming in the short-term at the cost of even greater atmospheric warming in the future, when scientists predict that the sunken heat will re-circulate back into the atmosphere.

-       Relevant time framesMany scientists say the 15-year period under question is too short to affect the clear long-term trend of rapid warming.

-       Climate science is focused on long-term trends (usually defined as at least 30 years), and many scientists have dismissed the hiatus entirely on the grounds of its relative brevity. Because of the understanding that short-term variation can mask even a robust long-term trend, climate predictions are made with built-in confidence intervals. Although they are near the lower boundary, atmospheric temperatures remain within the 95% confidence bounds of existing predictions. This means they are within the range of temperatures that are likely to occur as natural variation impacts the trend.

-       El Niño: Many argue that the selection of a 15-year time frame incorrectly compares current atmospheric temperatures with those of an anomalously warm El Niño period.

-       In the late 1990s man-made warming and warming due to natural variation added to each other, while today cooling due to natural variation is subtracting from the baseline warming. These natural variations give temperature trends the appearance of a wavy line or staircase rather than a stable upward line, and are part of the reason why climate scientists prefer to use trends of 30 years or more.

-       AerosolsOngoing research into aerosols (fine particulate matter in the atmosphere) show it may be obscuring atmospheric warming we would otherwise see.

-       Fine particulate matter in the atmosphere can cool the climate by reflecting sunlight back into space. This cooling effect could be obscuring the warming that is simultaneously occurring due to greenhouse gases. In their latest draft, the IPCC decreased their estimate of the cooling effect of aerosols, but a great deal of uncertainty remains due to China’s rapidly increasing but poorly measured output of these particulates, as well as output from natural sources like volcanoes. Unfortunately, all these cooling effects are very short-lived, and only serve to temporarily mask long-term warming.

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