PLANETARY MUSINGS

Entries for July 2005

Unbalanced climate?

Hansen, J, L Nazarenko, R Ruedy, M Sato, J Willis, A Del Genio, D Koch, A Lacis, K Lo, S Menon, T Novakov, J Perlwitz, G Russell, GA Schmidt, N Tausnev, Earth's energy imbalance: confirmation and implications, Science, 308, 1341-1435, (2005), doi:10.1126/science.1110252.

I've been a little busy to post lately, but we discussed this interesting paper by Hansen et al recently in our journal club. Basically, these authors take a climate model that includes forcing by human-made greenhouse gases, compares it to some data for validation (in particular recent global ocean heat content change) and then analyzes and discusses apparent disequilibrium in the present climate system. A major conclusion is that without an future changes in the composition of the atmosphere (levels of greenhouse gases stay constant instead of continually increasing like expected) that global climate will warm by another 0.6C because the climate system lags behind the current chemistry of the atmosphere. Therefore it is reasonable to expect that we will see at least 0.6C increase in the future, and probably more. The author's also discern from their model that the Earth absorbs 0.85 W/m^2 more than it emits back to space, a situation that if continued long-term could drastically raise ocean temps.

What I am curious about is what would happen if all greenhouse gas emissions were zeroed out today... how long would it take for the atmospheric chemistry to requilibrate (a residence time / feedback problem)? That's an unrealistic exercise, but I think it would give a sense of the maximum that we could do. Certain things cannot be changed (e.g., surface area covered in ice), but the problem is interesting nonetheless. It has also been a while since a major volcanic eruption which can result in global cooling, so what is the long-term energy imbalance? Is there such a thing?