04 March, 2006

Misusing “measurement”

Stoat (recently assimilated by the Scienceblog collective) has an excellent post up about the new estimate of ice accumulation in the Antarctic, which is based on a very clever idea: using the GRACE satellites to measure the changing gravity field (and hence mass) over the Antarctic region. The original paper in Science uses three years of data to suggest that, although some previous altimeter studies have shown some parts of the ice sheet are thickening, there is an overall loss of 70-230 km3 of ice per year for the whole Antarctic ice sheet, equivalent to a global sea level rise of 0.2-0.6 mm/year.

Stoat points out that many write-ups, such as the one in The Guardian, miss an important consideration when examining this result:

…note that the results depend very heavily on the adjustment for isostatic rebound. This is because Antarctica is smaller now than at the height of the last glacial; hence lighter; hence the rock underneath is slowly moving upwards (post-glacial-rebound; PGR). GRACE measures gravity anomalies; ie weight of rock and snow (errm, and actually also weight of nearby water masses, which could be a problem on so short a scale as 3 years). So if the rock is going up, that needs to be subtracted to get mass of ice. The "problem" is that this is not a small term; see the papers figure 2, which shows that the trend is flat, without the correction for the rebound term.

Not every media organisation missed this (the BBC didn’t, to their credit), but it occurs to me that this sort of thing is quite common. When talking about our work, we scientists should be a bit more careful about saying “measure” when we mean something more like “infer”. Particularly in historical sciences like geology, we are often examining the effects of events rather than the events themselves, and we are making assumptions about how the value of the thing we are measuring relates to the thing we want to measure. Sometimes (as in this case) the effects of other factors are not completely clear. Sometimes we come to realise that our assumptions were not quite right, which changes our interpretation.

Scientists understand all this, of course; in fact, we spend most of our time questioning and testing those assumptions, which is why they do sometimes change. I just worry that in our eagerness to make our results understandable to the wider public, we sometimes leave out too much of this process; and in contentious issues like climate change, where sceptics love to play up the whole, “so now they’re seeing this, when last week they said the complete opposite!” angle, our carelessness can come back to haunt us.

No comments: