Even as computer modeling of global climate change has continually improved, global warming doubters have criticized projections for the future as being little more than educated guesses. But the latest report from the Intergovernmental Panel on Climate Change, released this month, shows that even earlier and cruder models accurately predicted global temperature increases.

When the IPCC released its first report in 1990, the panel’s models, which are simple by today’s standards, correctly predicted the increase in global temperature measured over the last 17 years. Their latest report revealed a 0.36 degrees Fahrenheit (0.2 degrees Celsius) per decade increase during that time, which fits within the 0.27 to 0.54 F degrees per decade increase (0.15 to 0.3 degrees Celsius) they predicted.

“If you just look at the global mean temperature, [the projection] is relatively straightforward to do,” said climate modeler Reto Knutti, a visiting scientist at the National Center for Atmospheric Research in Boulder, Colo. who worked on the report.

Even John Christy, a climate scientist at the University of Alabama in Huntsville who has spoken out against the IPCC report in the past, admitted that global mean temperature was “as sure as anything in the prediction of climate.” While he thought that the IPCC’s reported warming over the last too decades was too high, he agreed that his own measurements also fit within the range of earlier projections.

The reason the projections are straightforward, according to Knutti, is that the way climate behaves in the next twenty or thirty years depends almost entirely on what humans have already done. The “committed warming” of short-term models, as he calls it, is largely independent of new policies or changes. “Because you know the past, this is, in some way, pre-determined,” he said.

Predicting global climate change for the next 20 years is actually easier than making a weekend weather forecast. “You don’t have to know the weather to predict the climate,” said Knutti.

“The difference between weather and climate is an important one,” he cautioned. Even if we can’t accurately predict if it will snow next week, said Knutti, “we can perfectly predict that next winter will be colder than next summer.” Similarly, climate change represents a change in averages over long time scales.

Although the 1990 models worked, they lacked many of the sophisticated features of today’s models, which include complex processes like the effect of aerosols. These tiny particles suspended in the air are especially important in understanding climate change, because they can both cool or warm the planet under different conditions.

“At that time [1990], climate models were just beginning to include aerosols,” said atmospheric physicist Surabi Menon, a contributing author to one of the chapters of the report who works in the Lawrence Berkeley National Lab in Berkeley, Calif.

Better observational data from satellites have driven much of the improvement in aerosol modeling. Now almost all models treat aerosols as a mixture that can interact rather than individual particles, according to Menon, who collaborates on climate models with the Goddard Institute of Space Studies in New York.

In addition to affecting the climate through their presence in the atmosphere, aerosols can alter the amount of precipitation released by clouds and the size of their droplets. “The clouds are really the most complicated part,” explained Knutti, noting that they are still responsible for much of the remaining uncertainty in projections.

“We’re getting much better observational data on the clouds,” said Knutti. “These observations just weren’t available twenty years ago.”

While models and observations have improved since the initial report, the primary conclusions have changed little. Scientists like Knutti and Menon are confident in the continued warming of 0.36 degrees Fahrenheit per decade for the next few decades predicted by the new report.

“It’s important to start making decisions today,” said Knutti. “If you wait for 20 years, you will not be able to change your path very dramatically.”

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Other Articles In Our Climate Change Series:

Polar Regions Lose Their Shine: Melting snow and ice allow global warming to gain more ground.

Taking the Climate’s Temperature: How scientists measure the sensitivity of our climate.

Polyp Apocalypse: As atmospheric carbon dioxide dissolves into increasingly acidic oceans, vibrant coral hamlets are fading into ghost towns.

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