Climate uncertainties


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.

5 Responses to “Climate uncertainties”

  1. Heiko Gerhauser Says:

    I think the most important climate science uncertainties relate to the key impacts of sea level rise and agricultural output. 6C more does not have to be a problem as such, 0C can be a big problem if it comes with 20 m sea level rise in 20 years and agricultural output going down by 80%.

    I readily accept that even a minor forcing might be enough to melt all ice eventually due to feedbacks, after all pretty minor precipitating events are believed to have caused the ice ages, and though we know too little to be able to say for certain that 550 ppm CO2 will eventually melt all ice by analogy to climate 30 million years ago, it’s certainly a plausible case.

    The thing is, if it takes 10000 years to melt said ice and in the first 100 years it’s just 20 cm and the next 100 years another 50 cm, we’ve got a rather different situation when compared to 10 metres by the year 2050.

    We can quibble a bit about how bad 1 m in 100 years really is. That depends on estimates of how rich we’ll be then, and how expensive it is to build dikes or abandon some land, but the basic idea that the speed of sea level rise matters hugely should be obvious.

    And this is not just about adaptation being easier. At some stage stage, we’ll have to be 100% CO2 free in terms of our energy generation, and if we can produce that much energy CO2 free, we can also produce 105% of that and use the extra 5% to suck CO2 out of the air for hundreds of years, so that long, long before the year 3000 we are back to less than 300 ppm.

    While climate science is not directly about agricultural output, it should be able to tell us more about rain and regional climate events, like heat waves and early frosts. Again we can still argue about the technological and political response, eg I think India becoming like Saudi Arabia in climate does not have to be a disaster, if the place is rich then and can import agricultural goods from Siberia or Canada, and overall world agricultural output is enhanced. But, in any case, a big hit to agriculture is clearly a key worry about climate change and also a big uncertainty. 500 ppm might double or halve agricultural output (certainly regionally) and it’s rather important to get some clarity on which it’ll be.

    Finally, one other point, you write about it being impossible for the rest of the world to enjoy American living standards because of unsustainable resource use. Now look at the IPCC scenarios. They assume that by the year 2100, the rest of the world will have a greater GDP per capita than the US did in 2000.

    Much of the resource issue comes down to energy. Can we generate two to three times what we generate now, improve energy efficiency by a factor 4 or so and do it all CO2 free by the year 2100? If so, GDP can go up by a factor 10 overall and by considerably more than that in the developing world and simultaneously CO2 emissions can fall to zero (or below zero).

  2. Bart Says:


    There’s a lot of merit to your argument. And indeed, the dynamics of sea level rise ought to have been placed in the list of major climate uncertainties. I’ll update the post to reflect that. Agricultural output I may place under ‘regional climate impacts’, though it’s definitely a biggie in itself (about which I know too little to comment on however).

    The point is, the later our energy generation becomes 100% CO2 free (and the longer we remain on the business as usual path), the more risk we run in terms of dangerous climate effects (eg the melt of the major icesheets may be practically irreversible, so we have to prevent it from even starting).

    You have more faith in technology coming to our rescue in time than I do. Even now, in principle we have the technology we need. It’s just that it’s more expensive than conventional technology (at least in the current economic system where not all costs are included in the price; think of health and environmental costs for example). So the problem is mainly economic in nature, and socially in terms of getting enough people demanding stronger measures.

    If the risks associated with business as usual are very large –which they are according to the science- then it’s prudent to try to limit our emissions already now. As Michael Tobis recently noted “In order to justify inaction, though, you need to put the likelihood (of extremely dangerous climate outcomes) at about, what, 5% or less?”

    Ultimately, it’s a question of risk management.

    As to your last comment, if the same counts for all other resources and pollutants (that they decrease faster than the GDP increases), then indeed, there won’t be a problem. That’s a very big *if* however. In developing countries there may be possibility for such a development at least for a while, when the service-industry increase much more than production-industry. (I’m an economist however). I recommend you take up the discussion on economic growth and sustainability with Michael Tobis at his blog; he’s been writing a lot of intelligent stuff about it recently (not that I’m not interested in it –to the contrary- but it could keep the relevant discussion less scattered).

  3. Heiko Gerhauser Says:

    In another comment thread you point out that it’s important to find common ground. I think that the distinction between inaction/action can be polarising and counterproductive. I’d say that most actions involving climate change also have a non climate component. In addition there is the option value of many actions. I may think that there is a 60% likelihood of a minor benefit from CO2 at 500 ppm, a 30% of minor trouble, and 5% each for CO2 saves us from terrible catastrophe or respectively will be responsible for it, and on that basis, not think there is value in reducing emissions per se, but circumstances can change and obtaining an option to reduce CO2 is of value right now even with these probabilities. That’s like your position on geoengineering as I understand it. You hope it’ll never be required and think there are better options, but just in case circumstances change it is good to acquire the option.

    All the above means that for concrete action affecting climate you don’t need to convince people, let’s put it in imperfect economic terms, that CO2 reductions are worth 30 Euros per tonne now rather than zero. For most issues other considerations are worth at least that. You can have people against nuclear power or wind turbines in spite of them valuing the CO2 reduction at 30 Euros per tonne, and others for them, even if they value the CO2 reduction at 0.

    In many respects I think the debate about what we should do about climate change can be quite independent of the level of urgency. That is as long as we are talking 0 to 30 Euros per tonne. At 300 or 3000 Euros per tonne say, measures like immediately shutting down coal fired power plants or putting petrol taxes up to 50 Euros per litre and outlawing with immediate effect the production of cars using fossil fuels become worthwhile choices.

    I think it is far more important myself to design a policy that is effective at the 30 Euro per tonne implied level than to get stranded in debate over the fact that I think it really should be 0.

    In my opinion, there are some really poor measures, especially carbon markets, and others that score highly, in-feed tariffs in particular. And a lot can be learned from both sides of the political spectrum.

  4. Tom Fuller Says:

    Hi Bart,

    I do appreciate the conversation ongoing over at examiner–I’m learning quite a bit, and enjoy the way this is going. I’m happy to continue it.

    I’d like to put forward the actions that a ‘lukewarmer’ is prepared to endorse while we work to gain a better understanding of both of our questions.

    Bear in mind that I’m a liberal democrat and a strong supporter of President Obama, which puts me in a very different position than most other ‘critics’ of some activist AGW commenters.

    So it shouldn’t be surprising that I support our President’s energy policy overall. In fact, up until I saw the mess that our current legislation has become, I supported a cap and trade policy as well. Now, I think we would be better off with a low carbon tax–anywhere from $2 to $12 a tonne.

    I support deficit spending on research and development in alternative energy generation, distribution and research into more efficiencies in consumption, particularly in better lighting, heating and air conditioning strategies.

    I support nuclear power, and would like to see a sustained building effort in this regard.

    I also would like to see a foreign aid initiative oriented at bringing efficient and dependable light and heat to the poorest 1/3rd of the Earth’s population. The green benefit, which I regard as a side benefit as we should do it out of common humanitarian interests, is that it would reduce deforestation dramatically.

    Lastly I also support higher mileage requirements for automobiles and more investment in public transportation options, which as you may know are pretty dire in the U.S. of A.

    My question to you is, isn’t this enough to get us started? It would take over a decade to implement, it would cut consumption and emissions, it would reorient people’s thinking–I find it hard to imagine AGW activists proposing something more ambitious in terms of a concrete plan of action. Where would we differ?

  5. Bart Says:

    Hi Tom,

    Conversations such as these are direly needed I think. In the end the search for common ground is very important. After all, we share this world and its climate with everyone else.

    That said, engaging with the likes of ‘Kim’ on your site, who yell absurdities, sling accusations, put logic on its head and refuse to listen, is disheartening and a waste of time. I concur with Gavin’s view (as stated e.g. in his reply to you on RC): There is no meaningful conversation to be had with people who dismiss out of hand pretty much anything science has to say about the topic. The problem is, a lot of people don’t see the absurdities for what they are, so I’m still often tempted to engage (even though I know it’s a bottomless pit of mud I’m getting into). (Note that this refers to the more fanatic commenters on climate blogs in general; not to yourself.)

    As to your proposed actions:

    We share a lot of common ground and our views mainly differ in the extent to which we would pursue things. Because of the large inertia in the energy system as well as in the climate system, I think the risks with postponing strong measures are very large. (“inertia + uncertainty = danger”, after David Keith) Some of the ‘hot topics’ are nuclear, coal, carbon tax/cap ‘n trade and subsidies for currently available technologies. And perhaps most of all, how to share the responsibilities between developed and developing countries.

    I don’t have a clear cut strategy for how to deal with climate change, though I agree with a lot of your suggestions: I favour a carbon tax and dividend over cap and trade, mainly because of the many complexities and loopholes in trading schemes. An increasing tax over time, to let the economy adjust, makes most sense to me. Its height will probably have to be higher (eventually) than what you suggest in order to start affecting consumption patterns. The majority of the tax revenues should be funneled back to the citizens (as eg James Hansen suggests), so that the tax burden doesn’t increase for the average citizen. An alternative (as proposed eg by Roger Pielke Jr) is to funnel the tax revenues to R&D in sustainable energy technologies. I think there’s a limit to the amount of extra taxes that the public will accept, so a carbon tax that exceeds this limit has to be accompanied by a dividend and clear communication.

    R&D into sustainable energy and energy efficiency measures needs (much) more investments, as you say. Besides R&D, currently available technologies need to be implemented at a much larger scale. The macro-economic costs are manageable and warranted in light of the big risks involved. Taking one step at a time is fine, as long as we keep on walking. That would be my answer to your question, isn’t this enough to get us started?

    I’m ambivalent in my attitude towards nuclear energy. The unsolved waste problem and especially nuclear proliferation are tremendous drawbacks to my mind. On the other hand, the risks of unbridled climate change have to be weighted against the risks of more nuclear power. I don’t hold a dogmatic anti-nuclear stance, but sustainable technologies such as wind, solar, hydro, geothermal and wave energy are definitely to be preferred. After that, air capture, coal/gas with CCS and sustainable biomass are still better than nuclear.

    Sustainable technology transfer to the developing world and poverty relief are both very important. Deforestation should be halted, and is intricately linked to poverty and providing alternative means of livelihood.

    The automobile industry should be held to much higher standards regarding emissions (incrementally of course). Eventually we’ll have to move away from the internal combustion engine. Strong R&D into and support for electric vehicles and battery technology. More support for/investment in public transit, car sharing and cycling.

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