The bottom line of my previous post was that the ‘next generation questions’ on climate change should focus on what do we do about it? I also mentioned what major aspects of climate science I thought are most uncertain:
- Regional climate effects
- Equilibrium climate sensitivity
- The role of aerosol and clouds
- Sea level rise (update: added after Heiko’s suggestion)
Tom Fuller asked me to elaborate, based on some specific questions he posed:
1. Regional climate effects
I think we can agree that both adaptation and mitigation (emission reduction) are needed, but letting mitigation play second fiddle puts us at risk of ever increasing climate damages, which will cost future generations a lot to adapt to (if at all possible). To limit the future risks to manageable levels, mitigation is of the utmost importance.
Helping countries become richer should likewise be coupled to helping them become more sustainable. Even apart from climate change, this is self-evident: 20% of the population consumes 80% of its resources. For many of these resources, there is not enough to go around for 6 billion people to have the American consumption pattern, plain and simple.
The strong link between wealth and pollution, resource depletion and climate change should be weakened. The only way that developing countries can reach our level of wealth in a sustainable way is if the ecological impact per unit wealth decreases at least as fast as their wealth increases (see eg Michael Tobis). Otherwise the impacts will continue to increase, above and beyond what we can reasonably adapt to.
2. Climate sensitivity
To estimate the equilibrium (Charney) sensitivity (change in temperature after a doubling of CO2), the climate forcing needs to be known as well as the climate response after it’s had time to settle into a new equilibrium. For a slowly changing forcing such as we’re experiencing now, the equilibration time is long but not accurately known, and neither is the aerosol forcing, so 20th century data are not particularly useful. Collecting better or more temperature data is not going to help.
Climate sensitivity is primarily constrained by paleo-climate data, while the climate response following a volcanic eruption is also a useful indicator from what I’ve understood. These constraints leave some more wiggle room at the upper end than at the lower end. Combining multiple constraints together leads to a most likely value of 3 deg for a doubling of CO2 (See eg James’ empty blog). Climate models also converge on this value (+/- 1). This has been a remarkable stable estimate over the course of decades, while the uncertainty hasn’t decreased significantly. Perhaps it won’t anytime soon. And perhaps that doesn’t matter too much as far as policy goes, because even with a realistic low estimate we’re still way behind in our policy response.
3. Aerosols and clouds
The short lifetime (days to weeks) of aerosols is an important reason for the uncertainty in their role in climate change. It causes their concentration to be highly variable in time and space, and it’s hard to even know what the global concentration is. Add to that their variability in size and chemical composition, and the poorly understood role of clouds, and it’s clear that the uncertainty in aerosol radiative forcing will remain a steady feature of climate science for some time to come.
Different clouds have different climate effects. They both cool (by reflection of sunlight) and warm (by their trapping of IR radiation, much like GHG) the atmosphere. Which effect dominates depends on the type and altitude of the cloud. Their large variability and myriad of interdependencies involved makes quantifying their global effect very difficult indeed. This won’t change any time soon.
I have some hope that better and more satellite measurements will drive the better quantification in the future, but that’s just a guess. Process based cloud physics studies are equally necessary to elucidate the interdepencies.
With the industrialization, SO2 emissions soared, causing the aerosol burden to increase. More recently we’ve started to clean up our act regarding SO2, so the aerosol burden (at least in Europe and North America) is decreasing again. That’s the reason for their ‘bridging effect’ halfway through the 20th century; it has nothing to do with a political desire. Scientists are a strange bunch; they just want to understand what’s happening.
4. Sea level rise (update)
The dynamics of sea level rise are very uncertain, but very important since they determine to a great extent the speed of sea level rise, which in turn strongly affects the risk posed to society. (Thanks to Heiko for bringing this omission to my attention) The magnitude and speed of sea level rise are amongst the most uncertain, yet also the potentially most dangerous effects of climate change. I have written more about sea level rise before.