http://www.greeningearthsociety.org/wca/2004/wca_17a.html
Assault From Above
It’s common knowledge that for twenty-five years the satellite-based temperature record of the lower atmosphere shows much less warming than surface observations or climate model predictions. The big question is, “Why?” A new study in Nature by University of Washington’s Qiang Fu and colleagues claims the satellite measurements are in error because they include a cooling effect from the atmospheric layer just above it. But in formulating their case, the authors assume the impossible.
The idea behind what Fu et al. report to Nature is simple enough. Satellite-based temperature measurements of the earth’s atmosphere are not collected from a single altitude. They represent the weighted average temperature of a rather large atmospheric layer, one so large that the measurements supposedly representing the middle to upper troposphere (from about five to eight miles in altitude) actually include part of the lower stratosphere (from about eight to twelve miles up), as Figure 1 shows. The extra few miles potentially are problematic because ozone loss in the stratosphere leads to a cooling trend in that layer.
By way of background, ozone absorbs incoming solar radiation. The radiation warms the air, meaning the less ozone there is, the less warming there can be. The stratosphere’s cooling trend to some degree contaminates the temperature trend observed in the mid- to upper troposphere and perhaps masks the warming from an enhanced greenhouse effect. So what happens if the stratospheric cooling component is removed from the troposphere’s temperature record? That’s what Fu and colleagues’ set out to do.
Figure 1. Atmospheric contribution to the satellite-based temperature observations of the stratosphere (dashed red line) and mid- to upper- troposphere (solid blue line). Notice the region of overlap (shaded pink) that indicates some portion of the stratosphere contributes to observations of the troposphere’s temperatures. They set out to use satellite temperature measurements of the stratosphere as an indicator of how much cooling has worked its way into the history of tropospheric temperatures. By combining the two datasets in such a way as to remove stratospheric cooling contamination, Fu concludes true temperatures in the troposphere have been warming at a greater rate than that reported to date, one which brings the satellite measurements more in line with surface observations and climate model projections. In this way, they believe they have solved the riddle concerning the large discrepancy between the trends in surface temperatures and those of the troposphere — a discrepancy climate models are not able replicate.
As in Paul Simon’s immortal lyric, “Slow down, you move too fast.”
Long ago, the co-founders of the original satellite-based temperature history (University of Alabama-Huntsville scientists John Christy and Roy Spencer) recognized that stratospheric temperatures were contaminating satellite measurements of the middle and upper troposphere. Spurred by a desire to produce a “true” tropospheric-only temperature dataset, in the late 1980s and early 1990s Spencer and Christy extensively examined various methodologies, including one like Fu et al. describe. They quickly realized that such a technique is infeasible because it produces a situation that violates a basic law of physics — namely, energy must be positive (Figure 2).
Figure 2. The stratospheric contribution to the satellite-based measurements of tropospheric temperatures. Solutions that don’t violate the basic laws of physics must have a weighting that is greater than or equal to zero.
The stratospheric contribution to raw tropospheric temperature observations is represented by the red line (top). The best, most physically realistic adjustment to the raw observations uses data in the stratosphere to adjust tropospheric temperatures and is represented by the green line (middle).
The solution used by Fu et al. appears as the blue line (bottom). The physically impossible negative contribution (shaded region with hatching) roughly equals the amount of positive contribution (shaded region without hatching), such that the total contribution from the stratosphere equals zero.There are three curves in Figure 2. The top, red line represents the stratosphere’s contribution to temperature measurements of the troposphere. Ideally, this value would be zero, indicating no contribution. By combining the tropospheric measurements with actual measurements of stratospheric temperature, it is possible to “subtract out” some of the stratosphere’s impact. But this must be carefully done or else a non-physical result is produced. You can’t remove more than you have at the outset.
The “zero” (black, horizontal) line represents the limit of removal. A result below this line relies on “negative” energy, which defies a law of physics. The middle (green, dashed) line shows the best, physically possible solution that can be produced using data from the stratosphere to adjust temperatures in the troposphere. It still makes a sizeable stratospheric contribution.
The blue (bottom) line represents Fu et al.’s solution. They succeed in setting the stratospheric contribution roughly equal to zero (the average of the area above and below zero), but in doing so they create an unrealistic representation of the real atmosphere. Though the portions in the stratosphere appear to cancel each other, the problem is this: The atmosphere in the negative portion is cooling rapidly (because of ozone depletion) while the atmosphere in the positive portion is changing very little.
In terms of net effect, Fu et al. multiply the rapid-cooling-trend layer by a too-large negative factor (making it appear to warm), while multiplying a near-zero-trend layer by a positive factor (which will have a very minor impact). Rather than canceling the stratospheric influence, Fu’s method adds a spurious warming trend to the net result. Simply put, Fu et al. overcorrect for stratospheric cooling.
In the interest of good science, Spencer and Christy abandoned this technique years ago because even the best acceptable solution (the middle curve in Figure 2) still has too much stratospheric influence. Instead, they developed a clever solution that makes use of different viewing angles by a single channel aboard the satellite. In this way, they measure temperatures occurring lower in the atmosphere and are able to produce the now famous lower tropospheric temperature history, one that essentially is free of stratospheric effects, and one that shows only about half as much global warming during the past twenty-five years as does the network of surface thermometers scattered across the globe.
Just a few weeks ago, another in a long string of publications demonstrated anew why we can have confidence in Spencer and Christy’s dataset (see http://www.co2andclimate.org/wca/2004/wca_15f.html for details). But here we are, more than a dozen years down the road and a technique discarded as physically implausible rears its head again. How Fu et al.’s physically-impossible result made it through Nature’s peer review process is unfathomable. Yet again, the reviewers Nature relies on to ensure scientifically sound results fail the needs of the publication.
Are Nature’s editors intentionally allowing articles on certain topics to be published despite a lack of scientific worthiness? Are the editors underqualified to judge the scientific merit of the research or are they unable to assign competent reviewers to do so? Whichever the case may be, the result is the same: When it comes to climate change, Nature’s reputation is becoming severely degraded.References:
Christy, J. R., and W. B. Norris, 2004: What may we conclude about global tropospheric temperature trends? Geophysical Research Letters, 31, L06211, doi:10.1029/2003GL019361.Fu, Q., et al., 2004. Contribution of stratospheric cooling to satellite-inferred tropospheric temperature trends. Nature, 429, 55-58.
Spencer, R.W., and J.R. Christy, 1990. Precise monitoring of global temperature trends from satellites. Science, 247, 1558-1562.
