In the last article – Opinions and Perspectives – 3 – How much CO2 will there be? And Activists in Disguise – one commenter suggested that RCP8.5 was actually “business as usual” and put forward some comments. So I’m posting this followup to identify some key points in that scenario, before going onto the next topic.
I’m trying to keep this series of articles brief and as non-technical as possible, but this one has to be a little more detailed.
It was excellent to find someone prepared to defend RCP8.5 as “business as usual”. We need more champions of this idea so it can be discussed. What key climate scientists should do is publish a paper “Why RCP8.5 is actually business as usual“. Or “How RCP8.5 has a serious likelihood of becoming reality“. So it can be discussed out in the open.
Science dies in the darkness.
The brief for the astrologers and soothsayers – no I’m kidding, future forecasters – in preparing a set of scenarios, later the four RCPs, was to produce internally consistent storylines that covered the range of CO2 concentrations (and other GHGs) covered in climate science papers.
From Emissions Scenarios, IPCC (2000):
..A set of scenarios was developed to represent the range of driving forces and emissions in the scenario literature so as to reflect current understanding and knowledge about underlying uncertainties. They exclude only outlying “surprise” or “disaster” scenarios in the literature. Any scenario necessarily includes subjective elements and is open to various interpretations. Preferences for the scenarios presented here vary among users. No judgment is offered in this Report as to the preference for any of the scenarios and they are not assigned probabilities of occurrence, neither must they be interpreted as policy recommendations..
[Emphasis added]
RCP8.5 was one of the four RCPs constructed because some simulations had covered quadrupling CO2 in the atmosphere (from pre-industrial levels). From van Vuuren et al (2011):
By design, the RCPs, as a set, cover the range of radiative forcing levels examined in the open literature and contain relevant information for climate model runs..
[Emphasis added]
Here are a few extracts from Riahi’s 2007 paper, which was the foundation for RCP8.5 in the 2011 paper (scenario A2r is similar to RCP8.5):
The task ahead of anticipating the possible developments over a time frame as ‘ridiculously’ long as a century is wrought with difficulties. Particularly, readers of this Journal will have sympathy for the difficulties in trying to capture social and technological changes over such a long time frame. One wonders how Arrhenius’ scenario of the world in 1996 would have looked, perhaps filled with just more of the same of his time—geopolitically, socially, and technologically. Would he have considered that 100 years later:
- backward and colonially exploited China would be in the process of surpassing the UK’s economic output, eventually even that of all of Europe or the USA?
- the existence of a highly productive economy within a social welfare state in his home country Sweden would elevate the rural and urban poor to unimaginable levels of personal affluence, consumption, and free time?
- the complete obsolescence of the dominant technology cluster of the day-coal-fired steam engines?
How he would have factored in the possibility of the emergence of new technologies, especially in view of Lord Kelvin’s sobering ‘conclusion’ of 1895 that “heavier-than-air flying machines are impossible”?
Also note that an “extremely high emissions scenario” (as I prefer to describe it), could also be constructed in other ways:
For reasons of scenario parsimony, our set of three scenarios does not include a scenario that combines high emissions (and hence high climate change) with low vulnerability (e.g., as reflected in high per capita incomes).
[Emphases added].
Here is the brief summary of the development of the A2 world – basically the RCP8.5 world:
The A2 storyline describes a very heterogeneous world. Fertility patterns across regions converge only slowly, which results in continuously increasing global population. The resulting ‘high population growth’ scenario adopted here is with 12 billion by 2100, lower than the original ‘high population’ SRES scenario A2 (15 billion). This reflects the most recent consensus of demographic projections toward lower future population levels as a result of a more rapid recent decline in the fertility levels of developing countries.
As in the A2 scenario, fertility patterns in our A2r scenario initially diverge as a result of an assumed delay in the demographic transition from high to low fertility levels in many developing countries. This delay could result both from a reorientation to traditional family values in the light of disappointed modernization expectations in this world of ‘fragmented regions’ and from economic pressures caused by low income per capita, in which large family size provides the only way of economic sustenance on the farm as well as in the city.
Only after an initial period of delay (to 2030) are fertility levels assumed to converge slowly, but they show persistent patterns of heterogeneity from high (some developing regions, such as Africa) to low (such as in Europe).
Economic development is primarily regionally oriented and per capita economic growth and technological change are more fragmented and slower than in other [scenarios]. Per capita GDP growth in our A2r scenario mirrors the theme of a ‘delayed fertility transition’ in terms that potentials for economic catch-up only become available once the demographic transition is re- assumed and a ‘demographic window of opportunity’ (favorable dependency ratios) opens (i.e., post-2030).
As a result, in this scenario, ‘the poor stay poor’ (at least initially) and per capita income growth is the lowest among the scenarios explored and converges only extremely slowly, both internationally and regionally. The combination of high population with limited per capita income growth yields large internal and international migratory pressures for the poor who seek economic opportunities. Given the regionally fragmented characteristic of the A2 world, it is assumed that international migration is tightly controlled through cultural, legal, and economic barriers. Therefore, migratory pressures are primarily expressed through internal migration into cities. Consequently, this scenario assumes the highest levels of urbanization rates and largest income disparities, both within cities (e.g., between affluent districts and destitute ‘favelas’) and between urban and rural areas.
Given the persistent heterogeneity in income levels and the large pressures to supply enough materials, energy, and food for a rapidly growing population, supply structures and prices of both commodities and services remain different across and within regions. This reflects differences in resource endowments, productivities, and regulatory priorities (e.g., for energy and food security).
The more limited rates of technological change that result from the slower rates of both productivity and economic growth (reducing R&D as well as capital turnover rates) translates into lower improvements in resource efficiency across all sectors. This leads to high energy, food, and natural resources demands, and a corresponding expansion of agricultural lands and deforestation.
The fragmented geopolitical nature of the scenario also results in a significant bottleneck for technology spillover effects and the international diffusion of advanced technologies. Energy supply is increasingly focused on low grade, regionally available resources (i.e., primarily coal), with post-fossil technologies (e.g., nuclear) only introduced in regions poorly endowed with resources.
[Emphases added]
Events that would prevent RCP8.5 occurring
- If sub-Saharan Africa stays mired in high levels of poverty – then it won’t be burning vast amounts of coal
- If sub-Saharan Africa moves out of high levels of poverty but goes through the “demographic transition” – women having less babies. This is something that has happened to every country so far, with cultures as different as Iran, Thailand and Taiwan – then the population won’t reach anything like 12bn and so it won’t be burning vast amounts of coal
- If sub-Saharan Africa moves out of high levels of poverty and doesn’t go through the “demographic transition” but adopts latest technology from more advanced countries, which is something that always happens especially with the internet, but also happened well before – then the energy efficiency of their economies will be much better than assumed
- If the above but for some reason sub-Saharan Africa doesn’t adopt more efficient technologies but the rest of the world makes lots of natural gas available at attractive prices – then not much coal will be burnt
It is a very unlikely world where RCP8.5 becomes a reality.
As a small backdrop, here are the population predictions from Wolfgang Lutz & Samir KC (2010). Lutz is a prolific figure in this field:
The total size of the world population is likely to increase from its current 7 billion to 8–10 billion by 2050. This uncertainty is because of unknown future fertility and mortality trends in different parts of the world. But the young age structure of the population and the fact that in much of Africa and Western Asia, fertility is still very high makes an increase by at least one more billion almost certain. Virtually, all the increase will happen in the developing world. For the second half of the century, population stabilization and the onset of a decline are likely.
[Emphasis added].
For people who don’t know much about development over the 20th century I very highly recommend The Great Escape, by Angus Deaton, Nobel Prize winner in Economics. He also covers the demographic transition.
Why This is Important
Some of the results of climate models for this “extremely high emissions scenario” (my term for it) contain very scary outcomes (I’ll be writing about climate models in this series).
The difference in outcomes (as predicted by models) between RCP8.5 and RCP6 (a more likely “business as usual” scenario) should be highlighted. If policymakers and the public are concerned that we might reach 12bn population in 2100 with sub-Saharan Africa lighting up coal fired power stations by the thousands then there are some straightforward steps to take.
- Keep the internet going so that technology for energy efficiency is available to sub-Saharan Africa, also teams of experts for technology transfer
- Encourage women’s education in that region (as Angus Deaton points out in his book this appears to be the key for the demographic transition)
- Encourage large scale gas extraction so that cheap gas is available instead of coal
I realise this last point goes against everything that climate activists believe in. But it seems like the rational response to the concern over RCP8.5 vs RCP6.
Surely this is why these scenarios were developed – so we can make some attempt to assess the outcomes and costs of different possible futures?
References
Scenarios of long-term socio-economic and environmental development under climate stabilization, Keywan Riahi et al, Technological Forecasting & Social Change 74 (2007)
RCP 8.5—A scenario of comparatively high greenhouse gas emissions, Keywan Riahi et al, Climatic Change (2011)
A special issue on the RCPs, van Vuuren et al (2011)
Read Full Post »
The Science of Doom 
Opinions and Perspectives – 7 – Global Temperature Change from Doubling CO2
Posted in Commentary on January 10, 2019| 118 Comments »
How much will global temperature rise if we double CO2 from pre-industrial levels? Based on current behaviour that’s roughly what we are on course to do by the end of the century (see 3 – How much CO2 will there be? And Activists in Disguise and 3.5 – Follow up to “How much CO2 will there be?”).
For this we need a model. But to begin with we can use a much simpler model than a current GCM (global climate model).
The key is to say “all other things remaining equal”. So we double CO2 but assume (in our model) that the vertical temperature structure of the atmosphere doesn’t change, clouds don’t change, water vapor doesn’t change, etc.
This allows us to calculate how the radiation balance gets disturbed. Then we can find the new surface temperature which brings everything back into balance. We don’t need a GCM that attempts to model turbulent flows of the atmosphere and ocean.
It turns out that the global temperature change will be about 1.2ºC from pre-industrial levels.
Well, kind of.
This is with the absolute amount of water vapor staying the same. Water vapor is the strongest “greenhouse” gas in the atmosphere and the amount of water vapor is one key to understanding future climate change.
If you stand next to the ocean in the tropics, barring a strong wind coming from inland, it’s pretty humid. If you stand next to the ocean in the Arctic it’s pretty dry. The reason is that the amount of water vapor that the air can hold depends strongly on temperature. Next to the ocean the air can be close to 100% relative humidity, but 100% relative humidity in the tropics has lots more water vapor than 100% in the Arctic.
So a slightly different simulation has relative humidity staying constant. The result is some amplification from the water vapor. The Earth’s surface gets a little hotter, so there’s more water vapor – which is also a greenhouse gas – so the surface gets hotter still.
In this experiment the global temperature change will be about 2.4ºC from pre-industrial levels. This second experiment is intuitively a better experiment than the first one – at least to get a “finger in the air” kind of result. It doesn’t mean it’s correct, but most people working in climate would expect relative humidity to be more likely to be constant than absolute humidity, when you increase the temperature.
So, our “no feedback” result is 1.2ºC, and our slightly more realistic “some feedback” result is 2.4ºC (currently the global temperature has increased about 0.8ºC from pre-industrial levels).
Both of these results can be obtained without relying on models that have “giant fudge factors” which is what you need to model the atmosphere and ocean “fluid flows” – see 6 – Climate Models, Consensus Myths and Fudge Factors. They only rely on being able to accurately calculate how radiation is absorbed and emitted by the atmosphere – an extremely well understood physics problem. (I can reproduce the results on my home computer using Matlab and the spectroscopic properties of CO2 and water vapor – see Visualizing Atmospheric Radiation).
The real story, of course, is more complicated. However, to understand anthropogenic global warming (AGW) – a better name than “climate change” – it’s useful to know these results (first calculated in the late 1960s by Manabe & Wetherald) – and to understand the difference between simple radiation models and global climate models.
Articles in this Series
Opinions and Perspectives – 1 – The Consensus
Opinions and Perspectives – 2 – There is More than One Proposition in Climate Science
Opinions and Perspectives – 3 – How much CO2 will there be? And Activists in Disguise
Opinions and Perspectives – 3.5 – Follow up to “How much CO2 will there be?”
Opinions and Perspectives – 4 – Climate Models and Contrarian Myths
Opinions and Perspectives – 5 – Climate Models and Consensus Myths
Opinions and Perspectives – 6 – Climate Models, Consensus Myths and Fudge Factors
Read Full Post »