New Study Asks Experts About Climate Change Uncertainties

What’s the biggest uncertainty in the magnitude of climate change? A recent paper in the Proceedings of the National Academy of Sciences (PNAS) asked 14 leading climate researchers. The answer might surprise you though it’s something you might see everyday, especially if you live in the Pacific Northwest: clouds.

Of the 14 researchers, each one identified clouds as the single biggest uncertainty in making predictions for future climate change. This isn’t a major surprise to those familiar with research on climate change. The Intergovernmental Panel on Climate Change’s (IPCC) last assessment report notes:

"The modeling of clouds processes and feedbacks provides a striking example of the irregular pace of progress in climate science…At the time of the TAR clouds remained a major source of uncertainty in the simulation of climate changes (as they still are at present…)." (Emphasis added)

There are a number of challenges to modeling clouds and their contribution to the climate system. First, they have both cooling and warming properties at different levels in the atmosphere. At higher altitudes, clouds tend to be more transparent to solar radiation, which means more warming energy is reaching the Earth’s surface.

At lower latitudes, clouds tend to be more opaque to solar radiation, reflecting more back into space. They also tend to radiate at warmer temperatures than high clouds, thus sending more long wave radiation from the Earth back into space as well.

In addition, in a warming world, models have shown two possible paths for cloud formation. Some models have shown that as the Earth heats up from increased greenhouse gas emissions, evaporation will increase. That means more water vapor in the atmosphere to form clouds at lower levels, which would have a net cooling effect.

On the flip side, some models have shown that there will be fewer low level clouds, especially if parts of the tropics become dryer. That means more shortwave radiation from the sun gets in, which would have a net warming effect.

This uncertainty about cloud cover also has major ramifications for the Earth’s reflectivitiy, or albedo. Clouds account for about 66% of the Earth’s albedo (ice accounts for most of the other 34%). In the Earth’s system, energy in has to equal energy out, and albedo is a big part of the keeping that balance.

If albedo is reduced, then there has to be an increase somewhere else in the system to account for the extra energy coming in. The area where that increase is most likely to be felt is surface air temperature.

The results would be dramatic. A decrease of just 1% would cause a corresponding temperature increase of 1 degree Celsius to account for the extra energy entering the system.

So you can see how uncertainty about the overall effects of climate change on clouds can lead to a wide range of projections on climate change. Though another recent study highlighted the near unanimity of climate researchers who accept the evidence that climate change is occurring due to human activities, there are still uncertainties as to the magnitude of that change. The new study shows where more research needs to be done to narrow the range of temperature changes.

Unfortunately, the 14 researchers interviewed expected only modest gains in reducing uncertainty over the next 20 years, even if funding for research was tripled. This is not to say that all aspects of climate change are uncertain or that we should wait to take action to reduce emissions. Rather, we should be aware of the uncertainties and work towards solutions accordingly.

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