Climate models get the basic patterns right

Myth No. 3: You can't trust climate models because they do a lousy job representing clouds and aerosols.

Climate models line graph
When researchers remove greenhouse gases from the climate models, the observed warming over the last century (in black) evaporates. The red line represents model output with greenhouse gases, the blue line represents model output without. Graph: Stott et al. 2006b

Climate modelers have traditionally had a hard time incorporating clouds because clouds are very complex. On the one hand, by reflecting sunlight, they tend to cool Earth. On the other, they tend to hold in heat from the surface, which is why cloudy nights tend to be warmer than clear nights. The models also divide the atmosphere up into blocks much larger than clouds, so it's difficult to create realistically sized clouds.

Aerosols -- microscopic particles produced by volcanoes, burning fossil fuels and other sources -- are a challenge to model too. They tend to have a cooling effect because they reflect sunlight and can also form the seeds for reflective water droplets. Perhaps the models are missing important effects from clouds and aerosols that would counteract the effects of greenhouse gases on global temperatures.

Charles Jackson acknowledged that climate models do have uncertainties and don't create perfect predictions about future climate. But despite their shortcomings, when used to simulate past climate, the models get the basic patterns correct. The differences tend to come in the amplitudes, not the general patterns.

One example is the pattern of global temperature increase of the past century. Jackson noted that when researchers remove man-made greenhouse gases from the climate models, leaving behind all the known natural sources of variability, the observed warming over the last century evaporates.

Another type of pattern is based on how temperatures change over time with respect to depth in the atmosphere and latitude. This is sometimes represented in color coded charts with latitude running along the bottom and depth in the atmosphere running up the side. Red indicates warming and purple indicates cooling. Each major driver of climate (solar activity, volcanoes, greenhouse gases, ozone and sulfate aerosols) has its own unique fingerprint on this kind of image. These fingerprints have been revealed by running the climate models and changing just one forcing while holding the other forcings constant. The fingerprint for greenhouse gases turns out to be very distinct from all the others -- warming of the upper troposphere near the equator and cooling in the upper atmosphere. Rong Fu later showed how this distinct fingerprint of greenhouse gasses shows up loud and clear in direct observations of the atmosphere from satellites, indicating that these gases are playing a major role in climate change.

Fingerprints graph
Each major driver of climate leaves its own unique fingerprint on atmospheric temperatures. The fingerprint for greenhouse gases turns out to be very distinct from all the others -- warming of the upper troposphere near the equator and cooling in the upper atmosphere. Red indicates warming and purple indicates cooling. View a larger version of this graph.Graph: Santer et al. 2003

Other patterns that climate models accurately recreate in past climate include warming of the oceans, rising sea levels, decreased sea ice and snow cover, retreating ice sheets and glaciers, changes in atmospheric circulation patterns, and increased heavy precipitation. The fact that climate models get these patterns right gives modelers some confidence in their forecasts of future climate.

Read the other myths in this series.

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