How can you predict global warming if you can't predict rain?

Some say climate change is part of a complex natural cycle – so complex, in fact, that it can't be forecast. Are current climate models reliable?

from the October 18, 2007 edition

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In early models, researchers say, large adjustments were needed to keep climate models from spinning off into the twilight zone. The adjustments had no real-world climate counterpart; they were made to keep the simulations plausible. As models have improved, the need for such intervention has receded, and any tweaking has reflected real-world observation.

Such interventions have led some to say that modelers are merely telling the model to yield a specific result. Dr. Winton dismisses that charge. "People overestimate the control we have," he says.

With more powerful computers, scientists have been able to model climate behavior over shorter timespans. The need to intervene in the models is disappearing but not likely to vanish, says Caroline Katsman, a researcher at the Royal Netherlands Meteorological Institute in De Bilt. No computer can crunch numbers for every point on the globe.

One measure of a model's success is how well it captures the main features of natural climate variation. Assuming it can do that, researchers can then use the model to test ideas about atmospheric conditions and their plausible causes.

Last month, for example, researchers at NOAA's Earth Systems Research Laboratory in Boulder, Colo., concluded that slightly more than half the unusual warmth the United States experienced in 2006 was probably due to the buildup of greenhouse gases in the atmosphere.

The team, led by Martin Hoerling, took on the study when NOAA announced in January that 2006 had been nearly 2 degrees F. warmer than normal. NOAA couldn't say why. It may have been El Niño (a periodic warming of the eastern Pacific that has profound climatic effects); it may have been a rise in greenhouse gases.

How models pointed to CO2

Dr. Hoerling's team looked at historical data and calculated that the 2006 increase was unlikely to have occurred through natural fluctuations alone. When they looked at temperature data from 10 previous El Niño years, they found that average temperatures over the US had not changed or had cooled slightly. Could El Niño-like conditions cool the US? They ran a climate model, and found the answer was yes. So if El Niño was unlikely to have caused 2006's warming, did greenhouse gases? Using the modeling data the Intergovernmental Panel on Climate Change used in its latest report, the team found that more than half the increase could be attributed to greenhouse gases.

Scientists have used a similar approach to implicate greenhouse-gas emissions from human activities in warming over the past 30 years. In short, the only way to reproduce late 20th-century warming is to include the growth in greenhouse gases. Modeling results are not the only line of evidence, researchers say. But in combination with other lines of evidence, the case becomes more persuasive.

But why should we trust climate models any more than a three-week weather forecast? Roger Pielke Sr., a research scientist at the University of Colorado at Boulder who focuses on land-atmosphere interactions, notes that climate forecasting is more complicated than weather forecasting. Far more processes are involved.

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