TR: What uncertainties remain, and how might technology help resolve them?
RP: One [challenge] is observing the ocean below the surface. One key concern is [whether] the deep overturning of the ocean from top to bottom is going to slow down in the future. At the moment, the ocean is a sink for heat, so it’s slowing down the warming. That sink for heat is possible only because you can take surface water down to the depths and bring cold water up to replace it.
The temperature record and salinity record in the ocean is very poorly sampled because you have to lower things on cables to the bottom of the ocean and bring them back up. Cheap sensors of temperature, salinity, and depth would be a big step forward. Their initial use would be understanding the ocean as it’s working right now.
TR: One of the IPCC charts shows that there is large uncertainty about the role of clouds. Why is that?
RP: One of the big differences when you see these models differing from one another is the way they handle clouds and convection. This relates to human influence because of aerosols–things like sulfates. When you throw sulfate aerosols into a preexisting cloud, water evaporates off the droplets that are there and recondenses on the sulfuric acid. For the same volume of water in the cloud, you now produce more cloud particles, and that increases the total surface area. That means they’re more reflective. So this is a human-induced increase in the reflectivity of existing clouds.
The uncertainty is because to assess that, you have to know the distribution of these short-lived aerosols all around the world. And then you have to understand the physics and the microphysics of these clouds in detail that you just do not have at the present time. Part of the understanding of convection and clouds is also to measure the cloud properties droplet by droplet, and that requires special instrumentation and a lot of work in the lab … If you have a droplet of water and a particle of sulfuric acid, what are the things that determine the transfer of the water droplet to the particle?
TR: Can technology ever completely remove uncertainty about climate change?
RP: This is not a mathematical theorem that we can say at the end, QED. It will never be that. There will always be uncertainty, because it’s a highly uncertain system. But for the first time, [the IPCC authors] do work very, very hard to estimate the uncertainty on every key number.
TR: Given that this is not a mathematical theorem, how can you best deal with that uncertainty?
RP: Bring in lots of models, but those that you bring in have got to be credible models. One of the stipulations for getting your information into the assessment is, you have to have published it in a peer-reviewed journal, not in the Wall Street Journal op-ed or something like that. This is not credibly done with op-eds and in popular magazines. There is no legitimate role in the IPCC assessment, and for good reason, for things that are not published in the peer-reviewed literature. Some don’t like that, but that’s the nature of it, and it’s one of the rules of the game because it’s supposed to be a scientific assessment.
Nothing is purely scientific, of course, because every person has their own views and philosophies about how to do things. My own view of how to get around the uncertainties is to objectively estimate them. Do your best to estimate them at every point.