Abstract
Using state-of-the-art observational datasets and results from a large archive of computer model simulations, a consortium of scientists from 12 different institutions has resolved a long-standing conundrum in climate science—the apparent discrepancy between simulated and observed temperature trends in the tropics. Research published by this group indicates that there is no fundamental discrepancy between modeled and observed tropical temperature trends when one accounts for: (1) the (currently large) uncertainties in observations; and (2) the statistical uncertainties in estimating trends from observations. These results refute a recent claim that model and observed tropical temperature trends “disagree to a statistically significant extent”. This claim was based on the application of a flawed statistical test and the use of older observational datasets.
This paper was published online in the International Journal of Climatology during the week of Oct. 6–10, 2008.
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Notes
- 1.
The troposphere is the lowest layer of the atmosphere, where most weather phenomena take place. In the tropics, the troposphere extends from the surface to a height of about 10 miles (16 km) above the Earth’s surface.
- 2.
Both climate models and the experiments performed with them have become more realistic over time. Since the mid-1990s, many climate model experiments have incorporated not only human-caused changes in GHGs, but also changes in other “forcing agents” that have effects on global or regional climate. Examples include human-caused changes in various aerosol particles (such as sulfate and soot aerosols), and natural changes in the Sun’s energy output and the amount of volcanic dust in the atmosphere.
- 3.
This prediction of larger warming aloft than at the surface holds for all factors that tend to warm the surface of the Earth—it is not unique to human-caused changes in GHGs.
- 4.
This agreement between models and observations was also found for complex geographical patterns of surface temperature changes—not simply for trends in temperature changes averaged over very large areas (such as the tropics).
- 5.
Douglass DH , Christy JR, Pearson BD, Singer SF. 2007. A comparison of tropical temperature trends with model predictions. International Journal of Climatology 27: https://doi.org/10.1002/joc.1651.
- 6.
Karl TR , Hassol SJ, Miller CD, Murray WL (eds). 2006. Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. National Oceanic and Atmospheric Administration, National Climatic Data Center, Asheville, NC, 164 pp.
- 7.
In their paper, Douglass et al. claim to be testing “the proposition that greenhouse model simulations and observations can be reconciled.” The model simulations of twentieth-century climate change that they used to test this proposition, however, include a variety of different human and natural forcing factors, such as changes in sulfate and soot aerosols, volcanic dust, the Sun’s energy output, and land surface properties. These so-called “20CEN” experiments are not just driven by human-caused increases in GHGs. Douglass et al.’s proposition that they are only testing the response of climate models to GHG increases is simply incorrect.
- 8.
For example, 1998 was unusually warm because of the effects of a very large El Niño .
- 9.
Irrespective of which one of four different observational datasets was used to characterize changes in tropical surface temperatures.
- 10.
Developed by John Christy (one of the coauthors of the Douglass et al. paper), Roy Spencer , and colleagues.
- 11.
See Table V in our paper.
- 12.
A third reason is that several studies published within the last 12 months provide independent evidence for substantial warming of the tropical troposphere. These studies have documented pronounced increases in surface specific humidity and atmospheric water vapor that are in accord with tropospheric warming.
- 13.
Several of the newer radiosonde and satellite datasets that exhibit pronounced tropospheric warming are based on novel approaches to the construction of homogeneous datasets. These approaches often involve bringing in data from new sources (such as hitherto unused satellite data, or data on the physical relationship between temperature and wind) in order to better constrain uncertainties in estimated tropospheric temperature changes.
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Santer, B. et al. (2018). Fact Sheet for “Consistency of Modeled and Observed Temperature Trends in the Tropical Troposphere”. In: A. Lloyd, E., Winsberg, E. (eds) Climate Modelling. Palgrave Macmillan, Cham. https://doi.org/10.1007/978-3-319-65058-6_4
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