Researchers determine global warming during the 20th century may be slightly larger than earlier estimates

July 03, 2001

LIVERMORE, Calif.--Researchers at the Lawrence Livermore National Laboratory who examined effects of gaps in temperature measurements during the 20th century have concluded that global warming during that time period may have been slightly larger than the previously estimated value of roughly 0.6 degrees Celsius. These findings contrast with claims by greenhouse skeptics who contend that the warming seen in the observational record is an error introduced by incomplete and changing geographical coverage of temperature measurements.

The measured increase in the Earth's surface temperature during the 20th century is based upon thermometer measurements, which become increasingly incomplete further back in time. For example, at the beginning of the 20th century, thermometer measurements covered only 20 percent of the Earth's surface, compared to more than 87 percent in 1987. Some greenhouse-warming dissenters have claimed that the gradual increase in coverage during the 20th century introduced an artificial warming trend into the temperature record, which accounts for most or all of the 20th century's measured warming.

In an article titled "Effect of Mission Data on Estimates of Near-Surface Temperature Change Since 1900," in the July 1 edition of the Journal of Climate, LLNL researchers Philip B. Duffy, Charles Doutriaux, Imola Fodor and Benjamin Santer studied effects of the incompleteness of surface thermometer records on the estimated 20th century warming by examining 16 climate model simulations of the surface temperature changes from 1899 to 1998.

The scientists compared temperature trends obtained from globally complete model output with temperature trends derived by sampling the model output at only those locations where temperature observations are actually available. The comparison enabled the researchers to assess the effect of missing observational data on the apparent temperature trend during the 20th century.

"We found no evidence to support the hypothesis that incomplete observational data has caused us to overestimate the true warming trend," said Duffy, lead author of the paper. "On the contrary, our results suggest that the actual warming during the 20th century may have been slightly larger than the warming estimated from the incomplete observational data of -about 0.7 degrees Celsius instead of 0.6 degrees Celsius."

Livermore scientists examined climate models that incorporated estimated historical changes in both greenhouse gases and anthropogenic sulfate aerosols. Scientists concluded that in 10 of the 16 climate change simulations, missing data led to significant underestimates of the true global warming trend. In the remaining six simulations, missing data had no significant impact on the 20th century's warming trend.

If the climate simulations are credible estimates of human effects on historical climate and of natural climate variations, it is extremely unlikely that missing observational data caused the 20th century's warming to be overestimated.

"I hope that we've laid to rest the theory that warming that occurred during the 20th century is an artifact of missing data," Duffy said. "Knowing the accurate amount of the 20th century's warming is important because if it were much less than we've thought all along, we would have to fundamentally rethink our ideas about global warming."
-end-
Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.

DOE/Lawrence Livermore National Laboratory

Related Global Warming Articles from Brightsurf:

The ocean has become more stratified with global warming
A new study found that the global ocean has become more layered and resistant to vertical mixing as warming from the surface creates increasing stratification.

Containing methane and its contribution to global warming
Methane is a gas that deserves more attention in the climate debate as it contributes to almost half of human-made global warming in the short-term.

Global warming and extinction risk
How can fossils predict the consequences of climate change? A German research team from Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), the Museum of Natural History Berlin and the Alfred Wegener Institute compared data from fossil and marine organisms living today to predict which groups of animals are most at risk from climate change.

Intensified global monsoon extreme rainfall signals global warming -- A study
A new study reveals significant associations between global warming and the observed intensification of extreme rainfall over the global monsoon region and its several subregions, including the southern part of South Africa, India, North America and the eastern part of the South America.

Global warming's impact on undernourishment
Global warming may increase undernutrition through the effects of heat exposure on people, according to a new study published this week in PLOS Medicine by Yuming Guo of Monash University, Australia, and colleagues.

Global warming will accelerate water cycle over global land monsoon regions
A new study provides a broader understanding on the redistribution of freshwater resources across the globe induced by future changes in the monsoon system.

Comparison of global climatologies confirms warming of the global ocean
A report describes the main features of the recently published World Ocean Experiment-Argo Global Hydrographic Climatology.

Six feet under, a new approach to global warming
A Washington State University researcher has found that one-fourth of the carbon held by soil is bound to minerals as far as six feet below the surface.

Can we limit global warming to 1.5 °C?
Efforts to combat climate change tend to focus on supply-side changes, such as shifting to renewable or cleaner energy.

Global warming: Worrying lessons from the past
56 million years ago, the Earth experienced an exceptional episode of global warming.

Read More: Global Warming News and Global Warming Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.